Development of microstructure and irradiation hardening of Zircaloy during low dose neutron irradiation at nominally 377–440°C

Journal of Nuclear Materials 449 (2014) 69–87

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Development of microstructure and irradiation hardening of Zircaloyduring low dose neutron irradiation at nominally 377–440 �C

http://dx.doi.org/10.1016/j.jnucmat.2014.03.0040022-3115/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +1 412 476 5647; fax: +1 412 476 5779.E-mail address: [email protected] (B.V. Cockeram).

B.V. Cockeram a,⇑, K.J. Leonard b, T.S. Byun b, L.L. Snead b, J.L. Hollenbeck a

a Bettis Laboratory, Bechtel Marine Propulsion Corporation, West Mifflin, PA 15122-0079, USAb Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6138, USA

a r t i c l e i n f o

Article history:Received 20 June 2013Accepted 2 March 2014Available online 11 March 2014

a b s t r a c t

Neutron irradiation of wrought Zircaloy-2 and Zircaloy-4 was performed in the Advanced Test Reactor(ATR) at irradiation temperatures of nominally 377–440 �C to relatively low neutron fluences between3 and 31 � 1024 n/m2 (E >1 MeV). The irradiation hardening was measured using tensile testing. For thisrelatively high application temperature (377–440 �C) saturation of hardening was observed at therelatively low dose of 3 and 8 � 1024 n/m2, but the magnitude of irradiation hardening is much less thanreported in the literature for lower irradiation temperatures of 260–326 �C. Examinations of microstruc-ture were used to show that a lower number density of hai loops is present that results in the lower levelof irradiation hardening. The lower irradiation hardening for the higher irradiation temperature isconsistent with literature data. The amorphization of Zr(Fe,Cr)2 precipitates and resulting change inprecipitate composition during irradiation is characterized, and the potential role of these effects onhai loop and hci loop formation and irradiation hardening is discussed.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Zircaloy-2 and Zircaloy-4 are two zirconium-base alloys used innuclear applications because of their low capture cross-section andenhanced corrosion resistance [1–15]. Both alloys consist primarilyof a hexagonal alpha Zr-phase containing Sn in solid-solution.Zircaloy-4 contains small additions of Fe and Cr that are tied upas Laves phase precipitates having either a hexagonal close packed(hcp) Zr(Fe,Cr)2 or cubic Zr(Fe,Cr)2 crystal structure [1–11].Zircaloy-2 contains small additions of Ni, Fe, and Cr tied up as pri-marily Laves precipitates and body centered tetragonal Zr2(Fe,Ni)phases [1–11]. Neutron irradiation of Zircaloy produces complexchanges in both the microstructure and mechanical behavior thatdepend on irradiation temperature, irradiation flux, and startingmicrostructure [1–15]. The hexagonal structure of Zircaloy resultsin an anisotropic distribution of point defects produced by irradia-tion and the formation of two different types of dislocation loops. Ahigh number density of small hai loops (4–30 nm diameter) areproduced with a Burgers vector bhai ¼ 1

3 h11 �20i that are formedon the prism planes (f10 �10g) [1–7,11–26]. These hai loops areobserved to be visible at a neutron fluence of 0.3–1.1 � 1024 n/m2

(E >1 MeV) and then rapidly form to saturation at a fluence ofabout 1–5 � 1025 n/m2 depending on the irradiation temperature

[1–3,12–15]. These hai loops may be both interstitial or vacancyin nature, and the fraction of each is a strong function of irradiationtemperature with about 50% and 70% vacancy hai loops being ob-served at irradiation temperatures of 350 �C and 400 �C, respec-tively [5,12,22,24]. Irradiation at 300 �C is reported to result inmost of the hai loops being of the interstitial type but vacancyloops are still being present in a measureable percentage [1,5,12].Increasing the irradiation temperature is shown to result in a largerhai loop size and lower hai loop number density (ND), with hai loopsizes of 7–22 nm and ND of 5 � 1020–5 � 1022 #/m3 observed attemperature of about 350 �C and fluence of greater than1 � 1025 n/m2 [1,12,15,24]. At temperatures of about 400 �C thehai loop sizes are larger (16–23 nm) and the reported range ofND are generally lower (2 � 1021–2 � 1022 #/m3) [1,2,18]. No hailoops are observed to form at irradiation temperatures above500 �C [1,2,18]. Since plasticity in unirradiated Zircaloy mainlyoccurs by slip on the prism planes (f10 �10g), the formation of hailoops on these planes during irradiation is believed to be theprimary mechanism for irradiation hardening. The irradiationhardening generally follows the trends for hai loop size and NDwith less hardening observed at higher irradiation temperatures.Other slip planes observed in irradiated Zircaloy have been thebasal ({0001}) and pyramidal (f10 �11g or f10 �12g) [1,2,18].

At higher fluences on the order of 3–5 � 1025 n/m2, significantnucleation of vacancy hci loops with a Burgers vectorbhci ¼ 1

2 h0001i or b ¼ 1=6h20 �23i are formed on the basal planes

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({0001}) [1,2,5,8–11,27–31]. The formation of the hci loops is ob-served to be coincident with changes in the structure of the Lavesphase precipitates from a crystalline to amorphous state with dis-solution of Fe and Cr during irradiation [1–5,24,26–32]. The size,number density, and fluence for the formation of hci loops arestrongly dependent on the irradiation temperature and resultingsolute effects produced by the change in structure of the Laves pre-cipitates. Irradiation at lower temperatures (300 �C) is reported toresult in change in the Laves phase precipitate structure, whileirradiation at higher temperatures (350 �C and greater) may resultin little change in the crystal structure of the Laves phase precipi-tates [1–5,24,26–32]. There is believed to be a strong interplay be-tween the size and ND of the hai and hci loops and the change inthe structure of precipitates that is likely driven by irradiationtemperature.

Since the processes of loop nucleation and change in precipitatestructure are driven by solid-state diffusion, the change inmechanical properties are also highly dependent on the irradiationtemperature [1,2,15,33–36]. Irradiation of Zircaloy at higher tem-peratures (326–450 �C) has been shown to result in nominally25–45% less irradiation hardening (Dr) than for irradiation at low-er temperatures of 260–326 �C [15,35,36]. The lower irradiationhardening observed at higher irradiation temperatures was alsoconsistent with higher values for uniform elongation and dimin-ished tendency for flow localization [15]. Zircaloy-2 exhibits theunusual result of very high initial irradiation hardening at lowerfluences (1.1–5.5 � 1020 n/cm2, E >1 MeV) followed by a decreasein irradiation hardening to result in comparable levels of irradia-tion hardening for both Zircaloy-2 and Zircaloy-4 at fluences of29.3 � 1020 n/cm2 [15]. The decrease in irradiation hardening ob-served for Zircaloy-2 at higher fluences results from a coarseningof the hai loop distribution at higher fluence [15]. The lowerhardening observed for the 358 �C irradiations of Zircaloy-2 andZircaloy-4 in the High Flux Isotope Reactor (HFIR) has been shown[15] to be the result of a coarser distribution of hai loops than re-ported for Zircaloy-4 and Zircaloy-2 irradiations at 260–326 �C[1–4,12–14,16–18,37–44]. Little change in the structure of theLaves precipitates with a limited extent of dissolution was re-ported for the 358 �C irradiations at fluences of 29.3 � 1024 n/m2

[15]. This result is consistent with observations from the literaturefor irradiation at temperatures of 358 �C and higher [1–6,16–20].The small extent of precipitate dissolution and change in structurelikely contributes to the difference in loop structure and resultingchanges in mechanical properties following irradiation at highertemperatures. One view of the cause of the decreased irradiationhardening observed at higher irradiation temperatures of 320–360 �C is the enhancement of the coarsening/recovery of hai loopsduring irradiation [33,34].

The purpose of this work is to understand and quantify some ofthe effects of higher irradiation temperature on the irradiationhardening and changes in the microstructure of Zircaloy by exam-ination of Zircaloy-2 and Zircaloy-4 irradiated in the Advanced TestReactor (ATR) at temperatures of nominally 377–440 �C. Althoughirradiation temperature and starting microstructure is reported tohave a strong effect on the change in microstructure and resultinghardening during irradiation, flux is reported to have little effect onthe irradiation hardening of Zircaloy [1,2]. In this work, alpha-an-nealed Zircaloy-4 and alpha-annealed and beta-treated Zircaloy-2

Table 1Nominal chemical composition for alpha-annealed Zircaloy-4, alpha-annealed Zircaloy-2,

Element Zr Sn Fe

Alpha-annealed Zircaloy-4 Bal 1.52 0.22Alpha-annealed Zircaloy-2 Bal 1.50 0.15Beta-treated Zircaloy-2 Bal 1.56 0.15

are irradiated in the ATR at 377 �C to 440 �C to relatively low neu-tron fluences between 3 and 31 � 1024 n/m2 using a flux of about afactor of 3 or more lower (0.94–8.6 � 1017 n/m2/s) than used inprevious experiments in HFIR (2.9–3.2 � 1018 n/m2/s) [15].Irradiation hardening is determined by tensile testing. DetailedTEM examinations of microstructure are performed to determinethe hai and hci loop distributions and the evolution of precipitatemicrostructure.

2. Materials and experimental procedure

The nominal composition of the Zircaloy-4 and Zircaloy-2wrought products produced by warm rolling and annealing[4,15,45–47,51] that were used for testing are given in Table 1.The alpha-annealed processing results in an equiaxed alpha-Zrgrain structure with texture in the wrought processing orientation[4,15,45–47]. Beta-treated material is given a heat treatment afterwrought processing that produces a Widmanstätten microstruc-ture characterized by full conversion of the beta to alpha structure.The microstructure for beta-treated material consists of alignedlaths of alpha-Zr in colonies within prior beta grains that are mac-roscopically random [4,14,45–47]. An alpha-Zr phase with a hcpcrystal structure is present in both alpha-annealed and beta-trea-ted materials.

The irradiations were performed in the Advanced Test Reactor(ATR) using two separate capsule locations. Each capsule containedsubsized SS3 tensile specimens (25.4 mm � 4.95 mm � 0.76 mmthick with a gage length of 7.98 mm, width of 1.52 mm, and thick-ness of 0.76 mm [62]) machined in the transverse orientation tothe rolling direction. Prior to irradiation the tensile specimenswere laser engraved and then pickled at room temperature usinga solution of nitric acid (30–39 vol%), hydrofluoric (HF) acid (0.5–3 vol%) and water to remove about 25–100 lm total thickness,and then oxidized to produce a film of about 0.6 lm thick by expo-sure in a flowing atmosphere of 20 vol% oxygen/80% Argon in atube furnace at 765 �C for 8 h.

The tensile specimens were irradiated in seven different holderswithin each capsule with a gradient in temperature that resultedfrom the axial height in the capsule with respect to the neutronflux profile in the core, see Table 2. Capsule 01 was located inthe central elevation of the ATR where the flux is relatively consis-tent over the length of the capsule (8.3–8.6 � 1017 n/m2/s, E>1 MeV, for each holder), and the result is the final fluence valuesfor specimens in each holder ranging from 30 to 31 � 1024 n/m2 (E>1 MeV). Capsule 02 was located at the top edge of the core wherea larger gradient in flux was present, ranging from 0.94–4.9 � 1017 n/m2/s for each holder, which resulted in final fluencesranging from 3.4 to 18 � 1024 n/m2 for the seven holders. The cap-sules remained at the same elevation during irradiation, producinga consistent irradiation fluence and temperature for each holder.Both capsules were sealed. Capsule 01 contained only a heliumgas while capsule 02 contained a gas mixture of 55% He and 45% Ar.

Each holder had two passive SiC temperature monitors that aresticks of high purity CVD SiC having a cross-section of0.508 mm � 1.016 mm (nominally 31.75 mm in length). The nom-inal irradiation temperature for the passive temperature monitorswere measured (post-irradiation) using a series of isochronal

and beta-treated Zircaloy-2 (weight%).

Cr Ni Hf C O

0.11 <.0035 <0.008 0.012 0.150.11 0.06 0.0035 0.0087 0.150.11 0.06 0.0055 0.0074 0.10

Table 2Summary of the nominal flux, neutron fluences, and irradiation temperatures for Zircaloy irradiations.

Alloy type specimens Nominal flux(n/m2/s, E >1 MeV)

Nominal fluence(n/m2, E >1 MeV)

Actual calculated specimentemperature (�C)a,d

Capsule 01b

Alpha-annealed Zircaloy-4,Alpha-annealed Zircaloy-2,Beta-treated Zircaloy-2 in each location

8.4 � 1017 30 � 1024 402 �C/380 �C–423 �C8.5 � 1017 31 � 1024 408 �C/389 �C–428 �C8.6 � 1017 31 � 1024 410 �C/392 �C–429 �C8.6 � 1017 31 � 1024 410 �C/392 �C–429 �C8.5 � 1017 31 � 1024 408 �C/389 �C–427 �C8.4 � 1017 30 � 1024 406 �C/387 �C–424 �C8.3 � 1017 30 � 1024 401 �C/382 �C–419 �C

Capsule 02c

Alpha-annealed Zircaloy-4 4.9 � 1017 18 � 1024 440 �C/421 �C–461 �CAlpha-annealed Zircaloy-2 4.3 � 1017 16 � 1024 421 �C/403 �C–439 �CAlpha-annealed Zircaloy-4 3.6 � 1017 13 � 1024 418 �C/402 �C–434 �CBeta-treated Zircaloy-2 3.0 � 1017 11 � 1024 407 �C/394 �C–422 �CAlpha-annealed Zircaloy-4 2.3 � 1017 8.1 � 1024 400 �C/387 �C–413 �CAlpha-annealed Zircaloy-2 1.5 � 1017 5.5 � 1024 387 �C/376 �C–399 �CBeta-treated Zircaloy-2 0.94 � 1017 3.4 � 1024 377 �C/363 �C–389 �C

a The specimen temperatures reported at the nominal value for the average reactor power and the maximum value for the maximum reactor power that occurred duringthe irradiation. The nominal temperature is assumed to be the specimen temperature for the holder.

b For Capsule 01, all holder elevations each contained a specimen of alpha-annealed Zircaloy-4, alpha-annealed Zircaloy-2, and beta-treated Zircaloy-2.c In the case of capsule 02, the holders all contained specimens of the same alloy type.d The temperatures were determined from the SiC monitor temperature analysis based on a thermal analysis calculation.

B.V. Cockeram et al. / Journal of Nuclear Materials 449 (2014) 69–87 71

anneals followed by post-anneal measurements of electrical resis-tivity [15,48]. A significant decrease in the electrical resistivity ofthe SiC monitor results from the recovery of irradiation defectsto produce a pre-irradiated structure in the SiC monitor, which isused to determine the irradiation temperature of the monitorwithin a temperature increment of ±20 �C [15,48]. The SiC temper-ature monitors provide a measure of the last nominal irradiationtemperature that occurred for a long enough period time to set arepresentative population of defects in the SiC temperature moni-tor [48]. Based on the measured irradiation temperatures for themonitors, the average, maximum, and minimum irradiation tem-perature values were determined for the tensile specimens foreach holder location in each capsule in Table 2 using thermal anal-ysis methods to model and calculate the temperatures and heattransfer in the capsule, and based on the location of the tempera-ture monitor in the holder the irradiation temperature for the spec-imens are calculated. The variation in specimen temperatureaccounts for the variation in reactor power over the course of a cy-cle in the ATR. The average specimen temperatures for each holderrepresent the nominal temperature for the tensile specimens overthe span of the irradiations. For Capsule 01, the average tempera-ture for the 7 holders ranged from 401 �C to 410 �C with an averagetemperature of 407 �C with a ±20 �C temperature uncertainty band,so the irradiation temperature for all specimens in capsule 01 islabeled as 407 �C with a fluence of 31 � 1024 n/m2. For Capsule02, the gradient in flux and resulting gamma heating resulted intemperatures ranging from nominally 377 �C to 440 �C with anominal temperature uncertainty of ±20 �C and the temperaturefor each holder is used, see Table 2 for details.

After irradiation, the capsules were disassembled and the ten-sile specimens were tested at room-temperature using ASTM E8methods [29]. All tensile testing was performed at a cross-headspeed of 0.38 mm/min (nominal strain-rate of 0.05 min�1). Appliedload and crosshead displacement were used to determine the fol-lowing engineering properties: 0.2% offset yield strength, ultimatetensile strength, fracture stress, uniform elongation, and total elon-gation. Engineering stress/strain curves were measured from theload and crosshead displacement data with no correction madefor the compliance of the load train [15]. Following tensile testing,SEM fractography was performed on selected specimens to charac-terize the fracture surfaces. The 3-mm discs needed for Transmis-sion Electron Microscopic (TEM) examinations of microstructure

were taken from the gauge region of the tested tensile specimens.All TEM examinations were performed using a CM200-FEG micro-scope that was operated at 200 kV using two-beam and weak beamimaging conditions at a variety of magnifications to determine thedislocation loop sizes. The irradiated specimens were mechanicallypolished to a thickness less than 0.254 mm followed by electropol-ishing to perforation in a Tenupol twin-jet electropolisher using a10 vol% HClO4 in methanol solution at 5 �C at near 100 mA. EDSanalysis of precipitates was also performed in Scanning TEM(STEM) mode to characterize the change in precipitate structureduring irradiation using line scans and spot analysis. Thicknessmeasurements from each viewed location were performed usingthe Kossel–Möllenstedt fringe spacing technique in the zero orderLaue zone convergent beam electron diffraction pattern for a giveng-vector imaged under a two-beam condition.

3. Results and discussion: post-irradiation tensile testing

The yield strength, change in yield strength (DrYS), total elonga-tion, and uniform elongation values determined from post-irradi-ated tensile testing are given in Fig. 1. These values weredetermined from the engineering stress–strain curves for alpha-annealed Zircaloy-4, alpha-annealed Zircaloy-2, and beta-treatedZircaloy-2 given in Figs. 2–4, respectively. Tensile tests reportedin this work were performed in duplicate and triplicate, and the re-sults are shown in Fig. 1.

3.1. Yield strength and irradiation hardening

For the irradiations at nominally 377–410 �C, the values foryield strength and DrYS for all materials are shown in Fig. 1a andb to exhibit a rapid increase at low fluences (3.4–8.1 � 1024 n/m2) to values that are relatively close to results reported in the lit-erature for Zircaloy irradiations at lower temperatures between260–326 �C [1–4,30–42]. The lower fluence results (3.1–13 � 1024 n/m2) for irradiations at 377–410 �C are shown inFig. 1 to also generally be consistent with the results previously re-ported for HFIR irradiations at the lower temperature of 358 �C[15]. All tensile specimens tested in this work and in [15] werein the transverse orientation where the average Kearns parametersare closest for alpha-annealed (0.243) and beta-treated (0.320) to

Fluence [X 1024 n/m2, E > 1 MeV]

0 5 10 15 20 25 30

Yiel

d St

reng

th [M

Pa]

300

350

400

450

500

550

600

650

700

750

800

alpha Zircaloy-2 (387C to 421C)beta Zircaloy-2 (377C to 410C)alpha Zircaloy-4 (400C to 417C)alpha Zircaloy-4 (440C)Zircaloy Literature (260C to 326C)Zircaloy Literature (326C to 450C)alpha Zircaloy-2: 358C [15]beta Zircaloy-2: 358C [15]alpha Zircaloy-4: 358C [15]beta Zircaloy-4: 358C [15]

418ºC 421ºC 440ºC

Fluence [X 1024 n/m2, E > 1 MeV]0 5 10 15 20 25 30

σΔYS

, Cha

nge

in Y

ield

Stre

ngth

[MPa

]

-50

0

50

100

150

200

250

300

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400

450


418ºC 421ºC 440ºC


0 5 10 15 20 25 30

Tota

l Elo

ngat

ion

[%]

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25

30


418ºC 421ºC 440ºC

Fluence [X 1024 n/m2, E > 1 MeV]0 5 10 15 20 25 30

Uni

form

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ngat

ion

[%]

0

1

2

3

4

5

6

7

8

9

10


418ºC 421ºC 440ºC

(a) (b)

(c) (d)

Fig. 1. Comparison of post-irradiated tensile data for Zircaloy irradiated nominally in this work at temperatures of 377–440 �C compared with Zircaloy literature results fromirradiations in HFIR at 358 �C [15], and other literature for irradiations at 260–326 �C [1,2,4,30,31,37–42] and 326–450 �C [35,43]: (a) 0.2% yield strength, (b) irradiationhardening data (DrYS = rYS(irad) – rYS(non-irad)), (c) total elongation, and (d) uniform elongation. For this work, the points that apply to irradiations at 418–440 �C areidentified, while all other points apply to irradiations at 377–410 �C.


minimize the role of texture [45,46]. The yield strength values foralpha-annealed Zircaloy-4 and alpha-annealed Zircaloy-2 irradi-ated at 377–410 �C are generally higher than for beta-treated Zir-caloy-2 over the range of fluences studied (3.1–31 � 1024 n/m2)with the highest values observed for alpha-annealed Zircaloy-2.Beta-treated Zircaloy has a larger grain size (average of 23.6 lm[45,46]) than for alpha-annealed (average grain size of 15.2 lm[45,46]) and random texture, which likely explains the lowerstrength. Following the increase in yield strength at low fluences,only a slight increase is observed at higher fluences of 30–31 � 1024 n/m2 in this work for alpha-annealed Zircaloy-4 and Zir-caloy-2 and beta-treated Zircaloy-2. Although the irradiation tem-peratures used in this work are higher (377–410 �C), the yieldstrength and irradiation hardening for all materials irradiated to

the highest fluences of 30–31 � 1024 n/m2 are shown in Fig. 1 tobe at the high end of the range of values from the higher fluenceresults for the HFIR irradiations at the lower temperature of358 �C [15]. Irradiation of Zircaloy at higher temperatures of326–450 �C is known to result in a tendency for less irradiationhardening than at lower temperatures [1–12,15,35,43], but thehigh irradiation temperature of 407 �C used in this work resultsin yield strength and DrYS values at the high fluence of 30–31 � 1024 n/m2 that are not much less than observed for Zircaloyliterature data for irradiations at 260–326 �C and are consistentlyat the high end of the range for literature data for the higher irra-diation temperatures of 326–450 �C [1–4,15,30–43]. Variability ex-ists between the various datasets that could be both a function oflocal texture, the differences in texture and microstructure

Strain [%]0 2 4 6 8 10 12 14 16 18 20 22 24

Stre

ss [M

Pa]

0

100

200

300

400

500

600

700

800

L353 : non-irrad L290 : irrad 407C : 3.0X1025 n/m2

L304 : irrad 407C : 3.0X1025 n/m2

L322 : irrad 400C : 8.1X1024 n/m2

L323 : irrad 400C : 8.1X1024 n/m2

L324 : irrad 400C : 8.1X1024 n/m2

Strain [%]0 2 4 6 8 10 12 14 16 18 20 22 24

Stre

ss [M

Pa]

0

100

200

300

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L304 : irrad 407C : 3.0X1025 n/m2

L333 : irrad 418C : 1.3X1025 n/m2

L335 : irrad 418C : 1.3X1025 n/m2

L337 : irrad 418C : 1.3X1025 n/m2

(a)

(b)

Strain [%]0 2 4 6 8 10 12 14 16 18 20 22 24 26

Stre

ss [M

Pa]

0

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L304 : irrad 407C : 3.0X1025 n/m2

L346 : irrad 440C : 1.8X1025 n/m2

L347 : irrad 440C : 1.8X1025 n/m2

L348 : irrad 440C : 1.8X1025 n/m2

(c)

Fig. 2. Summary plot of engineering stress–strain curves comparing results fornon-irradiated testing and the post-irradiated testing of alpha-annealed Zircaloy-4segregated by irradiation fluence showing non-irradiated and irradiated in capsule01 at 407 �C to a fluence of nominally 3.1 � 1025 n/m2 compared to specimensirradiated in capsule 02 at: (a) 400 �C to a fluence of 8.1 � 1024 n/m2, (b) 418 �C to afluence of 1.3 � 1025 n/m2, and (c) 440 �C to a fluence of 1.8 � 1025 n/m2. All stress–strain curves were determined from the load versus actuator displacement with nocorrection made for the compliance of the load train. Many of the curves werepurposely off-set from the origin by an increment of strain in order of increasedfluence. The numbers given are serial numbers for the specimens.

Strain [%]0 2 4 6 8 10 12 14 16 18 20 22 24 26

Stre

ss [M

Pa]

0

100

200

300

400

500

600

700

800


L155 : irrad 407C : 3.1X1025 n/m2

L175 : irrad 421C : 1.6X1025 n/m2

L176 : irrad 421C : 1.6X1025 n/m2

L177 : irrad 421C : 1.6X1025 n/m2

Strain [%]0 2 4 6 8 10 12 14 16 18 20 22 24 26

Stre

ss [M

Pa]

0

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L155 : irrad 407C : 3.1X1025 n/m2

L163 : irrad 387C : 5.5X1024 n/m2

L164 : irrad 387C : 5.5X1024 n/m2

L165 : irrad 387C : 5.5X1024 n/m2

(a)

(b)

Fig. 3. Summary plot of engineering stress–strain curves comparing results fornon-irradiated testing and the post-irradiated testing of alpha-annealed Zircaloy-2segregated by irradiation fluence showing non-irradiated and irradiated in capsule01 at 407 �C to a fluence of nominally 3.1 � 1025 n/m2 compared to specimensirradiated in capsule 02 at: (a) 387 �C to a fluence of 5.5 � 1024 n/m2, and (b) 421 �Cto a fluence of 1.6 � 1025 n/m2. All stress–strain curves were determined from theload versus actuator displacement with no correction made for the compliance ofthe load train. Many of the curves were purposely off-set from the origin by anincrement of strain in order of increased fluence. The numbers given are serialnumbers for the specimens.


between the alpha-annealed and beta-treated microstructure andthe irradiation temperature, as discussed in the following sections.

The irradiated results for alpha-annealed Zircaloy-4 are shownin Fig. 1a and b to exhibit the highest yield strength and DrYS atthe lowest fluence of 8.1 � 1024 n/m2 followed by a slight decreasein irradiation hardening and yield strength at the intermediate flu-ence of 13 � 1024 n/m2 for the higher irradiation temperature of418 �C, and then an increase in irradiation hardening at the highestfluence of 30–31 � 1024 n/m2. The slight decrease and then in-crease in yield strength observed in the results for alpha-annealedZircaloy-4 could represent data scatter or decreased irradiationhardening due to the higher irradiation nominal irradiation tem-perature of 418 �C at the fluence of 13 � 1024 n/m2. The yieldstrength and DrYS results for alpha-annealed Zircaloy-4 irradiatedat 440 �C to a fluence of 18 � 1024 n/m2 show only small changesfrom the non-irradiated condition at this irradiation temperature,Fig. 1a and b. In fact, 1 of the 3 specimens irradiated at 440 �C

Strain [%]0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Stre

ss [M

Pa]

0

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L224 : non-irrad L242 : irrad 407C : 3.0X10 25 n/m2

L250 : irrad 407C : 3.1X10 25 n/m2

L221 : irrad 407C : 1.1X10 25 n/m2

L222 : irrad 407C : 1.1X10 25 n/m2

L223 : irrad 407C : 1.1X10 25 n/m2

Strain [%]0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Stre

ss [M

Pa]

0

100

200

300

400

500

600

700

800


L250 : irrad 407C : 3.1X1025 n/m2

L232 : irrad 377C : 3.4X1024 n/m2

L233 : irrad 377C : 3.4X1024 n/m2

L234 : irrad 377C : 3.4X1024 n/m2

(a)

(b)

Fig. 4. Summary plot of engineering stress–strain curves comparing results fornon-irradiated testing and the post-irradiated testing of beta-treated Zircaloy-2segregated by irradiation fluence showing non-irradiated and irradiated in capsule01 at 407 �C to a fluence of nominally 3.1 � 1025 n/m2 compared to specimensirradiated in capsule 02 at: (a) 377 �C to a fluence of 3.4 � 1024 n/m2, and (b) 407 �Cto a fluence of 1.1 � 1025 n/m2. All stress–strain curves were determined from theload versus actuator displacement with no correction made for the compliance ofthe load train. Many of the curves were purposely off-set from the origin by anincrement of strain in order of increased fluence. The numbers given are serialnumbers for the specimens.


actually had a lower yield strength than the non-irradiated control.These results suggest that irradiation of Zircaloy-4 at temperaturesas high as 440 �C results in nearly full recovery of irradiation dam-age due to coarsening of the hai loop structure from the high tem-perature [60,61] so that little or no irradiation hardening isobserved. It is noted that data here is limited and this result is onlyobserved for a single fluence that is relatively low. Greater accu-mulation of irradiation damage that leads to irradiation hardeningcould occur at higher fluences. The minimal levels of irradiationhardening for the irradiation of alpha-annealed Zircaloy-4 at440 �C, and the lower level of irradiation hardening observed forthe irradiation of alpha-annealed Zircaloy-4 at 418 �C suggests anupper bound for the maximum irradiation temperature wherehardening may be observed to be around 440 �C. The lower yieldstrength observed for alpha-annealed Zircaloy-4 at 418 �C than

for the results at 400–407 �C shows the role of irradiation temper-ature on lessening irradiation hardening may be very significant inthis temperature range, for this fluence range.

The irradiated yield strength and DrYS results in this work foralpha-annealed and beta-treated Zircaloy-2 show high values atthe lower fluences of 3.4–5.5 � 1024 n/m2 in Fig. 1a and b, but theseare less than the respective values determined from the HFIR irra-diations at fluences less than 5.5 � 1024 n/m2 and 358 �C [15]. Theyield strength and irradiation hardening measured in this work atthe higher fluences of 30–31 � 1025 n/m2 are within the range ofvalues for the lower fluence results (3.4 � 1024 n/m2) for alpha-an-nealed Zircaloy-2 or slightly higher than the lower fluence results(5.5 � 1024 n/m2) for beta-treated Zircaloy-2. The fluence depen-dence on yield strength and DrYS for beta-treated Zircaloy-2 in thiswork exhibits an increase at the fluence of 3.4 � 1024 n/m2 fol-lowed by slightly lower values at the fluence of 11 � 1024 n/m2,but then a slight increase in yield strength is observed at the high-est fluences of 30–31 � 1024 n/m2. These higher fluence results foralpha-annealed and beta-treated Zircaloy-2 do not exhibit the sig-nificant decrease in yield strength and DrYS that was observed forthe HFIR irradiations and resulted from the coarsening of the hailoop distributions [15]. The yield strength and DrYS for alpha-an-nealed Zircaloy-2 irradiated in this work at the fluence of13 � 1024 n/m2 at 421 �C is significantly below the range of valuesfor irradiations at 387–407 �C at fluences of 5.5–31 � 1024 n/m2.This shows a strong effect of irradiation temperature on yieldstrength and DrYS between 387–410 �C and 418–421 �C for bothalpha-annealed Zircaloy-2 and Zircaloy-4 at these fluences.

3.2. Stress–strain curves, ductility, and rractography

The stress–strain curves for alpha-annealed Zircaloy-4 and Zir-caloy-2, and beta-treated Zircaloy-2 are shown in Figs. 2–4, respec-tively, to generally exhibit a peak in stress at low values of strain(or yield drop), but this peak in stress is not sharp and occurs overa range of strain so that the use of the 0.2% offset method results ina yield strength that is generally only 7–14 MPa less (about 1–2%difference) than the ultimate tensile stress for most tests. Thestress–strain curves show in Figs. 2–4 that there is less tendencyfor beta-treated Zircaloy-2 and alpha-annealed Zircaloy-2 and Zir-caloy-4 materials irradiated at 418–440 �C to exhibit a promptyield point and higher values of uniform elongation are observedin comparison to the results for alpha-annealed Zircaloy-2 and Zir-caloy-4 irradiated at 377–410 �C. The ultimate tensile strength wasreported as equal to the yield strength for the cases where the ulti-mate tensile strength occurred before the 0.2% off-set yield valuedue to the peak in the stress–strain curve. Irradiation hardeningcan be determined from the ultimate tensile strength values, butthe yield strength values are believed to be the more consistentmeasure of the transition from elastic to plastic behavior that pro-vides the best representation of irradiation hardening effects whenthe yield point is low [15].

The yield point peak in the stress–strain curves shown inFigs. 2–4 results in low uniform elongation values for alpha-an-nealed Zircaloy-2 and Zircaloy-4 from this work that are shownin Fig. 1d to be at the lower end of the range in values for literaturedata for irradiations at 260–326 �C [1–4,30–43]. Higher values ofuniform elongation were observed for beta-treated Zircaloy-2 inFig. 1d because the yield point in the stress–strain curves inFig. 4 were either lower or absent (little prompt yield point behav-ior). Beta-treated Zircaloy-2 has a random texture that makes itless susceptible to the formation of a yield drop than for alpha-an-nealed Zircaloy-2 and Zircaloy-4 [1–4,49], which have texture[45,46]. Low values of uniform elongation and higher peaks inthe stress–strain curve are observed in the literature for Zircaloyirradiated at nominally 260–326 �C due to a dislocation channeling


mechanism [1–4,49]. The higher irradiation temperatures in thiswork (377–410 �C) result in peaks in the stress–strain curve thatare less sharp, exhibit a smaller increase in stress and occur overa wider range of strain than observed for irradiations at lower tem-peratures of 260–326 �C. The uniform elongation values from liter-ature data for irradiations at temperatures of 326–450 �C[15,35,43] are shown in Fig. 1d to be higher than typically mea-sured in this work for irradiations at 377–410 �C.

The total elongation values determined in this work are shownin Fig. 1c to be within the range of literature data values for irra-diations at 260–326 �C [1–4,30–42] and our previous data forHFIR irradiations at temperatures of 358 �C [15]. For the irradia-tion of alpha-annealed Zircaloy-2 and Zircaloy-4 at the highertemperatures of 418–440 �C where no prompt yield point was ob-served, the uniform and total elongation values are higher andcloser to literature data values for irradiations at the tempera-tures of 326–450 �C [15,35,43]. There is a direct correlation be-tween the observation of a yield drop in the stress–straincurves with low uniform elongation, and higher post-irradiationyield strength and irradiation hardening. The high post-irradiatedyield strength and irradiation hardening observed for alpha-an-nealed Zircaloy-2 and Zircaloy-4 irradiated at 377–410 �C in this

Fig. 5. SEM fractography of tensile specimens irradiated at nominally 407 �C to a fluenceand b), alpha-annealed Zircaloy-2 (c and d), and beta-treated Zircaloy-2 (e and f).

work and results reported for HFIR irradiations [15] for alpha-an-nealed and beta-treated Zircaloy-2 irradiated at fluences of 1.1–10.6 � 1024 n/m2 correlate with the observed yield points withlow uniform elongation. The lower post-irradiated yield strengthsand irradiation hardening observed for beta-treated Zircaloy-2and alpha-annealed Zircaloy-2 and Zircaloy-4 irradiated at 418–440 �C in this work and literature data for irradiations at 326–450 �C [15,35,43] also correlates with the higher uniform elonga-tion and lack of a yield point. High post-irradiation yield strengthand irradiation hardening are also correlated with low uniformelongation values for literature data for Zircaloy irradiated at260–326 �C that are also correlated with the presence of a promptyield point [1–4,30–42,49]. Higher post-irradiated yield strengthand irradiation hardening occur under conditions where the de-fect structure produces a yield point with a low value for uniformelongation, and are associated with a dislocation channelingmechanism where local channels are cleared of hai loops and dis-location motion is concentrated in these regions [1–4,30–42,49].Irradiation conditions that result in little or no yield points withhigher uniform elongation typically produce a defect structurethat results in lower values of post-irradiated strength and irradi-ation hardening.

of 3.1 � 1025 n/m2 showing a planar and side view for alpha-annealed Zircaloy-4 (a


Representative SEM fractography shows in Fig. 5 the ductilefailure mode that was observed for all tensile specimens that isconsistent with the measurable elongation and ductile behaviorshown in the stress–strain curves in Figs. 2–4. This is consistentwith the ductile behavior reported in the literature for irradiatedZircaloy [1–15,30–44,49]. For the alpha-annealed Zircaloy-4 andZircaloy-2 (Fig. 5a–d), the fracture surfaces consist of dimplesand tearing ridges inhomogeneously distributed on the fracturesurface that are separated by smoother regions that appear to belarge dimples. The fracture surface for beta-treated Zircaloy-2(Fig. 5e and f) consists of fine voids and tearing ridges that are sep-arated by smoother regions generally having the same shape as theacicular lath shaped grains of the starting microstructure. Highmagnification images show the tearing ridges present in both al-pha-annealed and beta-treated Zircaloy-2 and Zircaloy-4 consistof fine voids. Similar types of fracture surfaces were also observedfor alpha-annealed and beta-treated Zircaloy-2 and Zircaloy-4 fol-lowing HFIR irradiations at 358 �C [15]. These types of fracture sur-faces are similar to those observed for non-irradiated Zircaloy-2and Zircaloy-4, and are shown to result from a void nucleation,growth, and coalescence mechanism that originates from secondphase precipitates, grain boundaries, slipbands, and slipband inter-sections [45,46]. However, the irradiated fracture surfaces exhibitthe following three features that were not observed for the non-irradiated materials that indicate the fracture mechanism for irra-diated Zircaloy has some differences that require further investiga-tion [15]. First, small cracks were observed on the fracture surfacethat run into the plane of the fracture surface. Features appearingto be spherical pores that are relatively deep were observed insome cases. Planar steps were also observed on the side of the ten-sile specimens that are likely slipband intersections with the sur-face of the specimen.

Table 3Summary of TEM characterization results for specimens taken from post-irradiated tensile01 at 410 �C to a nominal fluence of 31 � 1024 n/m2 and capsule 02 at 400 �C to a nomina

Specimennumber/capsule

Irrad T/neutronfluence [�1024 n/m2

(E >1 MeV)]

hai Loops

Averagediameter ± St.deviation (nm)

Max–Mindiameter(nm)

Average loodensity ± Smin (#/cm

L324/02 400 �C/8.1 3.92 ± 1.74 12.75–0.88 1.63 � 1016

3.28–0.82 �L304/01 410 �C/31 4.50 ± 1.36 10.93–1.36 1.09 � 1016

1.29–0.86 �

Note: N/A means the hci loops were not observed.

Table 4Summary of TEM characterization results for specimens taken from post-irradiated tensinominally 407 �C in capsule 01 to a nominal fluence of 31 � 1024 n/m2 and capsule 02 at tem2, respectively. Each line is the average for each specimen.



(E >1 MeV)]

hai Loops



Average Lodensity ± StMin (#/cm

L163/02 387 �C/5.5 4.42 ± 1.55 10.66–1.55 1.67 � 1016

2.40–1.12 �L176b/02 421 �C/16 6.38 ± 2.81 19.40–1.98 5.63 � 1015

9.06–4.33 �6.39 ± 2.38 14.38–1.98 8.07 � 1015

11.62–4.93L149/01 402 �C/30 4.70 ± 1.51 11.00–1.29 1.87 � 1016

2.04–1.55 �L155/01 410 �C/31 4.53 ± 1.93 11.56–1.23 1.50 � 1016

1.93–1.19 �

Notes:a N/A means the hci loops were not observed.

b For specimen L176, two values are provided to represent measurements made from

The fracture surfaces for the alpha-annealed Zircaloy-4 and Zir-caloy-2 specimens are shown in Fig. 5a–d to generally occur alongdefined planes near the surface that are orientated about 45� to theaxis of loading. For beta-treated Zircaloy-2, the fracture surfacewas less continuous with angled planes being observed (Fig. 5e–f). The observed fine steps, assumed to be slip bands, could be evi-dence of dislocation channeling that is known to occur for irradi-ated Zircaloy [1–4,30–42,49] where the strain is localized onshear planes that have been swept clear of barriers to slip. Suchplanes are less defined and organized for beta-treated Zircaloy-2specimens due to the more random texture [45,46] that likely re-sults in more crossing of slip planes. Alpha-annealed material hastexture [45,46] and is shown to exhibit more organized and clearlydefined slip planes on the side of the fracture surface. Fracture inbeta-treated Zircaloy generally occurs along lath boundaries dueto void coalescence from the Laves phases along these boundaries[45,46], and this may also disrupt the planar-type fracture mode.

4. TEM examinations of irradiated microstructure

The loop size and number densities determined from the TEMexamination of alpha-annealed Zircaloy-4 and Zircaloy-2 andbeta-treated Zircaloy-2 are provided in Tables 3–5, respectively.Representative TEM images of hai loops, hci loops, and precipitatesfor the irradiated materials are provided in Figs. 6–10. Plots of theaverage hai loop size and Number Density (ND) as a function of flu-ence are compared with literature data [1–4,15,30–42] in Fig. 11.The hai loops are shown in Figs. 6–10 to form as circular discs thatare typical of those observed for irradiated Zircaloy [1–18,23]. Thehci loops were not observed for irradiations at the lower fluences of3.4–8.1 � 1024 n/m2, but were present in very low number

specimens following testing of Alpha-annealed Zircaloy-4 after irradiation in capsulel fluence of 8.1 � 1024 n/m2. Each line is the average for each specimen.

hci Loops

p numbert. deviation/max–3)


Max–mindiameter(nm)

Loop number density(#/cm3)

± 1.15 � 1016/1016

N/A N/A N/A

± 0.19 � 1016/1016

110.4 ± 46.4 243.2–39.7 2.21 � 1013 ± 1.22 � 1013/3.87–1.11 � 1013

le specimens following testing for alpha-annealed Zircaloy-2 following irradiation atmperatures of either 387 �C or 421 �C to nominal fluences of either 5.5 or 16 � 1024 n/

hci Loops

op number. deviation/Max–

3)




± 0.50 � 1016/1016

N/A N/A N/A

± 1.96 � 1015/1015

115.3 ± 45.9 209.2–71.2 4.43 � 1012 ± 0.89 � 1012/5.06–3.80 � 1012

± 2.91 � 1015/� 1015

135.2 ± 58.0 280.0–53.5 3.32 � 1012 ± 3.75 � 1012/7.59–0.58 � 1012

± 0.23 � 1016/1016

137.4 ± 63.0 276.6–51.1 2.36 � 1013 ± 2.05 � 1013/3.81–0.91 � 1013

± 0.31 � 1016/1016

106.0 ± 40.1 205.8–48.2 2.74 � 1012 ± 0.21 � 1012/2.88–2.59 � 1012

2 separate specimens.

Table 5Summary of TEM characterization results for specimens taken from post-irradiated tensile specimens following testing for beta-treated Zircaloy-2 following irradiation testing atnominally 407 �C in capsule 01 to a nominal fluence of 31 � 1024 n/m2 and capsule 02 at temperatures of either 377 �C or 407 �C to nominal fluences of 3.4 or 18 � 1024 n/m2,respectively. Each line is the average for each specimen.



(E >1 MeV)]

hai Loops hci Loops



Average Loop numberdensity ± St. deviation/Max–Min (#/cm3)




L232/02 377 �C/3.4 2.93 ± 1.03 8.06–0.93 1.45 � 1016 ± 0.29 � 1016/1.70–1.13 � 1016

N/A N/A N/A

L221/02 407 �C/11 6.77 ± 2.38 14.54–2.24 7.33 � 1015 ± 1.32 � 1015/8.84–6.38 � 1015

111.2 ± 47.8 245.5–30.5 8.40 � 1012 ± 2.67 � 1012/13.0–5.49 � 1012

L222/02 407 �C/11 5.15 ± 1.85 15.32–1.75 6.97 � 1015 ± 3.04 � 1015/9.97–2.94 � 1015

N/A N/A N/A

L242/01 402 �C/30 5.24 ± 2.20 13.04–1.72 1.30 � 1016 ± 0.46 � 1016/1.82–0.94 � 1016

N/A N/A N/A

L250/01 410 �C/31 3.96 ± 1.67 11.43–1.21 1.95 � 1016 ± 0.24 � 1016/2.23–1.78 � 1016

124.3 ± 51.3 274.3–21.6 1.29 � 1013 ± 0.57 � 1013/2.18–0.63 � 1013

Note: N/A means the hci loops were not observed.


densities at fluences between 11 and 31 � 1024 n/m2. The hci loopsare shown in Fig. 6d, Fig. 7e, Fig. 8c, Fig. 9d, and Fig. 10e to bestraight dislocation lines under g = 0002 imaging conditions. Hy-dride precipitates were observed in some cases that are artifactsof specimen preparation and were distinguishable from the hciloops in that they did not show the contrast inversion of the hciloops when g is varied between ±002. The hci loops were non-uni-formly distributed in the Zircaloy samples. No hci loops were de-tected for duplicate conditions for beta-treated Zircaloy-2(Table 5). Variation in the hci loop number density was also ob-served for alpha-annealed Zircaloy-2 specimens irradiated undersimilar conditions as noted in Table 4. This variation in hci loopslikely results from the low ND and non-uniform distribution ofthese features. No change in the crystal structure of the Zr2(Fe,Ni)precipitates was observed, but loss of iron and partial amorphiza-tion of the Laves type (Zr(Fe,Cr)2) precipitates is observed. Greaterloss of iron was measured when amorphization of the Zr(Fe,Cr)2

precipitates was observed, but even crystalline Zr(Fe,Cr)2 precipi-tates exhibited a decrease in the Fe:Cr ratio from a starting valueof about 1.6–2.0 to values of about 0.6–1.0. Such a loss of iron fromZr(Fe,Cr)2 precipitates is typical during irradiations at tempera-tures of 350 �C and greater [1–3,5–14,22–28,32,42–44,50,51,55–59].

4.1. hai Loop size and ND evolution with fluence

For the Zircaloy materials evaluated in this work, the fluencedependence for the increase in hai loop ND and size is assumed tofollow a (fluence)1/n or (/t)1/n dependence where n = 2, 4, or 6 thathas been observed in the literature for irradiated Zircaloy, molybde-num, stainless steel, and other materials [1,2,5,6,11–18,25–28,39–

44,50–54,7,55–58,65]. An incubation term /tð Þxwx

oþð/tÞx

h i� �was also used

that varies from 0 to 1, where wo and x are material constants [15].The fluence dependence for the change in hai loop diameter (D) andNumber Density (ND) was then expressed as the following equa-tions [15].

D ¼ ð/tÞx

wxo þ ð/tÞx

� �� ½aþ bð/tÞ1=n� ð1Þ

ND ¼ ð/tÞx

wxo þ ð/tÞx

� �� c þ dð/tÞ1=nh i

ð2Þ

where a, b, c, and d are material constants obtained from regressionfits to the TEM data in Tables 3–5. The best fit that represented the

fluence dependence for the limited data in this work was found forn = 2. The fits shown in Fig. 11 are intended to provide a basis forcomparison of the change in hai loop diameter (D) and ND as a func-tion of fluence at a constant irradiation temperature.

For the irradiations at 377–410 �C, the hai loops formed in al-pha-annealed Zircaloy-4 and Zircaloy-2 and beta-treated Zirca-loy-2 are shown in Fig. 11a to have a consistent size withaverage diameters of 3.9–5.2 nm observed at fluences between5.5 and 31 � 1024 n/m2. A very small increase in hai loop diameteris observed going from fluences of 3.4–8.1 � 1024 n/m2 to 30–31 � 1024 n/m2. The hai loop diameters were generally comparablein size for the three Zircaloy materials in this work. Slightly largerhai loop diameters of 6.4 nm were observed for alpha-annealed Zir-caloy-2 irradiated at 421 �C that are attributed to the higher irradi-ation temperature. The slightly larger hai loop diameter of 6.8 nmobserved for one beta-treated Zircaloy-2 specimen irradiated at407 �C to a fluence of 11 � 1024 n/m2 is difficult to understand, asa duplicate specimen had a value of 5.2 nm. This could representsome inherent variation in the hai loop distributions in the irradi-ated materials. The hai loop distributions are shown in Fig. 12 togenerally have a symmetric distribution that is centered at diame-ter value of about 5 nm with the exception of the two cases previ-ously discussed. For alpha-annealed Zircaloy-2 irradiated at 421 �C,a lower hai loop ND is observed in Fig. 11b that corresponds to thecoarser hai loop distribution shown in Fig. 12b. The beta-treatedZircaloy-2 specimen irradiated at 407 �C to a fluence of11 � 1024 n/m2 shows a dual peak curve, which suggests some lo-cal variation in the hai loop sizes are present that results in the lar-ger average diameter of 6.8 nm compared to 5.2 nm for theduplicate specimen. More investigation is needed to determinethe reason for this local hai loop coarsening.

Similar to the trends observed for hai loop diameter, littlechange in ND is shown in Fig. 11b from values of 1.5–1.6 � 1016 cm�3 at low fluences of 3.4–8.1 � 1024 n/m2 to ND val-ues of 1.1–2.0 � 1016 cm�3 at the highest fluences of 30–31 � 1024 n/m2. In the case of alpha-annealed Zircaloy-4, a smalldecrease in hai loop ND is observed at higher fluence that couldbe data scatter or variations in the observed hai loop ND. The hailoop ND for alpha-annealed and beta-treated Zircaloy-2 irradiatedto the higher fluences of 30–31 � 1024 n/m2 are shown in Fig. 11bto be much higher than observed for alpha-annealed Zircaloy-4,but the lower fluence values are less for beta-treated Zircaloy-2.Irradiation of alpha-annealed Zircaloy-2 at the higher temperatureof 421 �C is shown in Figs. 11b and 12b to result in a much lowerhai loop ND that is consistent with the lower irradiation hardeningshown in Fig. 1a and b. The lower hai loop ND for beta-treated Zir-

8.1 X 1024 n/m2

[110]

3.1 X 1025 n/m2

(a) (b)

(d) (c)

3.1 X 1025 n/m2

(e)

Fig. 6. Representative TEM images of loops and precipitates from alpha-annealed Zircaloy-4 following irradiation at 400 �C to a dose of 8.1 � 1024 n/m2 showing (a) hai loopsand (b) a cubic Zr(Fe,Cr)2 precipitate and diffraction pattern showing an amorphous rim and crystalline central region; and following irradiation at 407 �C to a fluence of3.1 � 1025 n/m2 showing (c) hai loops, (d) showing hci loops, and (e) showing Zr(Fe,Cr)2 precipitates without a resolvable amorphous rim.


caloy-2 irradiation at 407 �C to the fluence of 11 � 1024 n/m2 inFig. 11b also corresponds to the lower irradiation hardening shownin Fig. 1a and b, but the exact reason for this is not clear.

Literature data has shown that a coarsening of the hai loop struc-ture is expected at higher temperatures [60,61], and coarser hai loop

distributions are reported in the literature for irradiations at tem-peratures >316 �C [1,2,12,15,18,24]. Although the irradiation tem-peratures for this work are relatively high (377–421 �C), theaverage hai loop diameters (3.9–5.2 nm) and hai loop ND are shownin Fig. 11 to be within the range of values (but at the lower end)

5.5 X 1024 n/m2

(e) (f)

1.6 X 1025 n/m2

(d)

(b) (c) (a)

Fig. 7. Representative TEM images of loops and precipitates from alpha-annealed Zircaloy-2 following irradiation in capsule 02 at 387 �C to a dose of 5.5 � 1024 n/m2 showing(a) hai loops, (b) a Zr(Fe,Cr)2 with an amorphous rim, and (c) a Zr(Fe,Cr)2 precipitate without a resolvable amorphous rim; following irradiation at 421 �C to a fluence of1.6 � 1025 n/m2 showing (d) hai loops, (e) hci loops with hydride precipitates (an artifact) indicated, and (f) a Zr(Fe,Cr)2 precipitate without a resolvable amorphous rim.


reported in the literature for irradiations at temperatures <316 �C.Larger hai loop diameters of 7–22 nm are reported in the literaturefor the irradiation of Zircaloy at temperatures of 300–358 �C thatare at the high end of the range of values determined for these irra-diations [1,2,12,15,18,24]. Some of these results are plotted asT > 316 �C and HFIR data [15] in Fig. 11. The hai loop ND valuesobserved in the work (1.1–2.0 � 1016 #/cm3) are at the upper endof the range of values reported in the literature (0.05–5 � 1016 #/cm3) for irradiations at 300–358 �C [1,2,12,15,18,24] with some ofthese results plotted as T > 316 �C and HFIR data [15] in Fig. 11.Although the hai loop size and ND observed in this work are similarto those typically observed for irradiations at lower temperatures(T < 316 �C), the irradiation hardening at fluences of 30–31 � 1024 n/m2 is shown in Fig. 1a and b to be lower than reportedin the literature for irradiations at 260–326 �C [1,2,4,30–37,42]. Thissuggests the irradiation hardening of Zircaloy is not exclusivelyrelated to the hai loop size and ND, but other factors are likely atwork such as loop strengthening from solutes or the presence ofpoint defect clusters below the TEM size detection limit [15].

4.2. Precipitate structure changes and hci loops

For alpha-annealed Zircaloy-4 irradiated to the lower fluence of8.1 � 1024 n/m2 at 400 �C, the Zr(Fe,Cr)2 precipitates were ob-served to be partially amorphous with an amorphous rim observedat the precipitate interface in Fig. 6b. The amorphous rim is ob-served to be depleted in iron, which is consistent with literaturedata for the amorphization of Zr(Fe,Cr)2 precipitates [1–3,5–14,22–28,32,42–44,50,51,55–58]. However, all Zr(Fe,Cr)2 precipi-tates examined were observed to be crystalline for alpha-annealedZircaloy-4 irradiated at 410 �C to higher fluences of 31 � 1024 n/m2, see Fig. 6e. Irradiation to higher fluences would be expectedto result in a greater fraction of the Zr(Fe,Cr)2 precipitatesbecoming amorphous [1–3,5–14,22–28,32,42–44,50,51,55–58].

The absence of amorphous regions in the Zr(Fe,Cr)2 precipitatesobserved for alpha-annealed Zircaloy-4 irradiated to higher flu-ences likely results from local variation in precipitate amorphiza-tion that could occur at these higher irradiation temperatures.The Zr(Fe,Cr)2 precipitates in the Zircaloy materials were observedto have both the cubic and hexagonal crystal structures and varia-tion in the initial ratio of Fe:Cr for the Zr(Fe,Cr)2 precipitate alongwith the initial crystal structure may contribute to the local varia-tion in amorphization [1–3,5–14,22–28,32,42–44,50,51,55–58].

Local variation in the amorphization of Zr(Fe,Cr)2 precipitateswas also observed for alpha-annealed Zircaloy-2 irradiated at387 �C to a low fluence of 5.5 � 1024 n/m2 with an amorphousrim observed for some Zr(Fe,Cr)2 precipitates (Fig. 7b), while otherZr(Fe,Cr)2 precipitates were shown to be completely crystallinewith no amorphous region detected (Fig. 7c). Irradiation ofalpha-annealed Zircaloy-2 at the higher temperature of 421 �C tothe fluence of 16 � 1024 n/m2 is shown in Fig. 7e and f to resultin only crystalline Zr(Fe,Cr)2 precipitates. The hai loops for alpha-annealed Zircaloy-2 irradiated at 421 �C are shown in Figs. 7 and11 to be much coarser in size, which could be related to the higherirradiation temperature and lower fraction of iron solute in thealpha-Zr matrix from the still crystalline Zr(Fe,Cr)2 precipitates.

Irradiation of alpha-annealed Zircaloy-2 to higher fluences of30–31 � 1024 n/m2 at 402–410 �C is shown in Fig. 8d and e to re-sult in a higher fraction of the Zr(Fe,Cr)2 precipitates becomingamorphous with a wider rim compared to the lower fluence irradi-ation shown in Fig. 7b. The higher fraction of Zr(Fe,Cr)2 precipitateamorphization is likely the result of the higher fluence. The EDStrace across the Zr(Fe,Cr)2 precipitate in Fig. 8e shows significantdepletion of iron going from a composition of Zr–30.9%Fe–32.7%Cr (atom%) in the center of the Zr(Fe,Cr)2 precipitate to Zr–7.6%Fe–35.0%Cr in the amorphous rim. Full amorphization ofZr(Fe,Cr)2 precipitates is reported to occur for Zircaloy irradiationsat lower temperatures of 260–326 �C [1–3,5–14,22–28,32,42–

3.1 X 1025 n/m2

Fig. 8. Representative TEM images of loops and precipitates from alpha-annealed Zircaloy-2 following irradiation in capsule 01 at nominally 407 �C to a nominal fluence of3.1 � 1025 n/m2: (a) and (b) hai loop images from both specimens in Table 4, (c) hci loop image, (d) Zr(Fe,Cr)2 precipitates with an amorphous rim, (e) EDS traces across theprecipitate showing a composition of Zr–30.9Fe–32.7Cr (atom%) in the center of the precipitate (point A) and amorphous rim showing a composition of Zr–7.6Fe–35.0Cr(point B), and (f) grain boundary shown to be enriched in about 0.5 at% Fe.


(b) (a)

3.4 X 1024 n/m2

1.1 X 1025 n/m2

(d) (c)

1.1 X 1025 n/m2

(f) (e)

Fig. 9. Representative TEM images of loops and precipitates from beta-treated Zircaloy-2 following irradiations in capsule 02 at 377 �C to a dose of 3.4 � 1024 n/m2 showing(a) hai loops and , (b) Zr(Fe,Cr)2 precipitates on lath boundaries with an amorphous rim; following irradiation at 407 �C to 1.1 � 1025 n/m2 showing (c) hai loops, (d) hci loops,(e) Zr(Fe,Cr)2 precipitates on lath boundaries with a wider amorphous rim, and (f) images from a second specimen of a Zr(Fe,Cr)2 precipitate without a resolvable amorphousrim.


44,50,51,55–59] that is also associated with diffusion of iron fromthe precipitate and into the alpha-Zr. The partial amorphization ofZr(Fe,Cr)2 precipitates observed in this work for alpha-annealedZircaloy-4 and Zircaloy-2 is more typical of results for irradiationat higher temperatures. Enrichment of iron at the alpha-Zr grain

boundary was observed in some cases, such as the 0.5 atom% ironenrichment for alpha-annealed Zircaloy-2 irradiated to31 � 1024 n/m2 in Fig. 8f. However, the grain boundary enrichmentwith iron was not uniform, as grain boundaries with no detectableiron enrichment were also observed in the microstructure.

Fig. 10. Representative TEM images of loops and precipitates from beta-treated Zircaloy-2 following irradiation in capsule 01 at nominally 407 �C to a nominal fluence of3.1 � 1025 n/m2: (a) image of hai loops, (b) image of hci loops, (c) Zr(Fe,Cr)2 precipitates shown to be crystalline, (d) Zr(Fe,Cr)2 precipitates at lath boundary with an amorphousrim, and (e) Zr(Fe,Cr)2 precipitate at lath boundary shown to be fully amorphous.


Although the hai loop sizes observed for beta-treated Zircaloy-2were similar to those observed for alpha-annealed Zircaloy-2 andZircaloy-4, the hai loop ND are slightly lower for beta-treated Zir-caloy-2 and hci loops were observed less consistently for beta-trea-ted Zircaloy-2. The Zr(Fe,Cr)2 precipitates are primarily located atlath boundaries for beta-treated Zircaloy-2 prior to irradiation,and the finer size and more irregular shape for the Zr(Fe,Cr)2 pre-cipitates [59], and higher surface energy and faster diffusion atthe boundaries could be a reason for the greater degree and greatervariation of Zr(Fe,Cr)2 precipitate amorphization observed for beta-

treated Zircaloy-2 than observed for alpha-annealed Zircaloy-2.Irradiation of beta-treated Zircaloy-2 at 377 �C to even the low flu-ence of 3.4 � 1024 n/m2 is shown in Fig. 9b to result in partialamorphization of Zr(Fe,Cr)2 precipitates at lath boundaries forthe regions examined. Local variation in the degree of Zr(Fe,Cr)2

precipitate amorphization is also observed for beta-treated irradi-ated to the higher fluence of 11 � 1024 n/m2 at 407 �C with a wideramorphous rim observed for a Zr(Fe,Cr)2 precipitate in one region(Fig. 9e), but a Zr(Fe,Cr)2 precipitate without an amorphous regionis shown in Fig. 9f for a duplicate specimen. This variation in

Fluence [ X 1024 n/m2 , E > 1 MeV]

0 5 10 15 20 25 30 35

Dia

met

er [n

m]

0.0

5.0

10.0

15.0

20.0

25.0

Alpha Zircaloy-4: HFIR Ref [15]Beta Zircaloy-4: HFIR Ref [15]Alpha Zircaloy-2: HFIR Ref [15]Beta Zircaloy-2: HFIR Ref [15]Alpha Zircaloy-4: This workAlpha Zircaloy-2: This workBeta Zircaloy-2: This workFit - alpha-Zr4 - (pt)1/2

Fit - beta-Zr4 - (pt)1/2

Fit - alpha-Zr2 - (pt)1/2

Zircaloy Literature T < 316CZircaloy Literature T > 316C

421º C

Fluence [ X 1024 n/m2 , E > 1 MeV]

0 5 10 15 20 25 30 35

Num

ber D

ensi

ty [#

/cm

3 ]

5.0e+15

1.0e+16

1.5e+16

2.0e+16

2.3e+16

Alpha Zircaloy-4: HFIR Ref [15]Beta Zircaloy-4: HFIR Ref [15]Alpha Zircaloy-2: HFIR Ref [15]Beta Zircaloy-2: HFIR Ref [15]Alpha Zircaloy-4: This workAlpha Zircaloy-2: This workBeta Zircaloy-2: This workFit - alpha-Zr4 - (pt)1/2

Fit - alpha-Zr2 - (pt)1/2

Fit - beta-Zr2 - (pt)1/2

Zircaloy Literature T < 316CZircaloy Literature T > 316C

421º C

(a)

(b)

Fig. 11. Summary plot of average hai loop measurements determined from TEMcharacterization as a function of fluence for alpha-annealed Zircaloy-4 and Zircaloy-2 and beta-treated Zircaloy-2 from this work for irradiations at 387–418 �C with acurve fit to the data and comparisons to literature data for alpha-annealed andbeta-treated Zircaloy-4 and Zircaloy-2 irradiated at nominally 358 �C from HFIRirradiations [15] and additional literature data [5,11–13,16–18,23]: (a) hai loop

diameters with a fit to D ¼ ð/tÞxwx

oþð/tÞx

h i� ½aþ bð/tÞ1=n� with n = 2, and (b) hai loop

number density values with a fit to ND ¼ ð/tÞxwx

oþð/tÞx

h i� ½c þ dð/tÞ1=n� with n = 2. For this

work, the points that apply to irradiations at 421 �C are identified, while all otherpoints apply to irradiations at 377–410 �C.

Inte

rval

Num

ber D

ensi

ty [#

/cm

3 ]

0.0

5.0e+14

1.0e+15

1.5e+15

2.0e+15

2.5e+15

3.0e+15

3.5e+15

4.0e+15

4.5e+15

5.0e+15

5.5e+15

6.0e+15

Alpha-Zr2: L163: irrad 387C : 5.5X1024 n/m2




Loop Size [nm]0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Inte

rval

Num

ber D

ensi

ty [#

/cm

3 ]

0.0

5.0e+14

1.0e+15

1.5e+15

2.0e+15

2.5e+15

3.0e+15

3.5e+15



Loop Size [nm]0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Inte

rval

Num

ber D

ensi

ty [#

/cm

3 ]

0.0

5.0e+14

1.0e+15

1.5e+15

2.0e+15

2.5e+15

3.0e+15

3.5e+15

4.0e+15

4.5e+15

Beta-Zr2: L232: irrad 377C : 3.4X1024 n/m2





Loop Size [nm]0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

(a)

(b)

(c)

Fig. 12. Measured hai loop size distributions where the number of loops is definedin terms of a loop number density by dividing by interval hai loop number by thetotal volume used for measurement taken from TEM characterization followingirradiation at nominally 377–421 �C to nominal fluences of 3.4–31 � 1024 n/m2

(E >1 MeV) for: (a) alpha-annealed Zircaloy-4, (b) alpha-annealed Zircaloy-2, and (c)beta-treated Zircaloy-2.


amorphization of Zr(Fe,Cr)2 precipitates was also observed for theirradiation of beta-treated Zircaloy-2 at 402–410 �C to the higherfluences of 30–31 � 1024 n/m2 with Zr(Fe,Cr)2 precipitates presentat lath boundaries observed to be crystalline (Fig. 10c), partiallyamorphous with an amorphous rim (Fig. 10d), and fully amor-phous (Fig. 10e). The variation and reason for variation in amorph-ization of the Zr(Fe,Cr)2 precipitates observed for beta-treatedZircaloy-2 requires further study to quantify and understand.

Variation in hci loops was also much greater for beta-treatedZircaloy-2 (Table 5) than for alpha-annealed Zircaloy-2 (Table 4)with hci loops not observed at all in some beta-treated specimensfor irradiations at both 11 and 30–31 � 1024 n/m2 while they were

observed in other samples under near duplicate irradiation condi-tions. However, there is no clear evidence that the variation in thedegree of Zr(Fe,Cr)2 precipitate amorphization and presence of hciloops is related, or how these two observations might be related.The hci loop ND for these irradiations are very low, and there isinherent variation in the characterization of features at this lowND [15]. The nucleation of hci loop ND on the order of 3–6 � 1014 cm�3, which is more than an order of magnitude greaterthan the hci loop ND in this work (0.22–3.2 � 1013 cm�3), are re-ported to occur at higher fluences of 5 � 1025 n/m2 for Zircaloyirradiations at lower temperatures of 260–326 �C [1–14,25–28].The higher irradiation temperatures in this work (377–421 �C)


likely result in hci loop nucleation to a high enough ND that can beresolved, but the ND is lower than reported for irradiations at 260–326 �C to higher fluences [1–15,25–28]. Irradiation to higher flu-ences would need to be pursued to determine if irradiation at hightemperatures produces a similar hci loop ND at fluences of5 � 1025 n/m2 and greater.

The differences in the size and ND of the hai and hci loops ob-served in this work and for irradiations at similar temperaturesof 326–450 �C [1,15,26,35,43] could be related to slight differencesin the irradiation temperature and flux, but may originate from dif-ferences in the amorphization of Zr(Fe,Cr)2 precipitates. For theHFIR irradiations at 358 �C, all Zr(Fe,Cr)2 precipitates were ob-served to be crystalline, and only a limited release of iron and chro-mium for the Zr(Fe,Cr)2 precipitates on the order of 1–3% wasobserved [15,47]. The partial amorphization of Zr(Fe,Cr)2 precipi-tates observed in this work for irradiations at 377–421 �C is moreconsistent with literature data for irradiations in this temperaturerange [1–3,5–14,22–28,32,42–44,50,51,55–58]. This partial amor-phization does result in a greater release of iron from the Zr(Fe,Cr)2

precipitates and into the alpha-Zr alloy matrix during irradiation.This greater release of iron from the amorphization of Zr(Fe,Cr)2

precipitates may provide nucleation sites that result in the forma-tion of a finer distribution of hai and hci loops [1,5,67,68]. The ironmay also segregate to the hai and hci loops so that coarseningduring the higher temperature irradiation is slowed and the finerdistribution of loops are maintained over the range of fluencesexamined. A finer hai loop distribution resulting from a greaterdegree of Zr(Fe,Cr)2 precipitate amorphization could explain whythe irradiation hardening observed in Fig. 1a and b for the irradia-tions at 377–410 �C in this work is at the high end of values for lit-erature data for irradiations at similar temperatures of 326–450 �C[15,35,43]. The reason for the greater extent of Zr(Fe,Cr)2 precipi-tate amorphization observed in this work is currently unknown.The HFIR irradiations [15] occurred at a lower temperature(358 �C versus 377–410 �C in this work) and a higher fast flux(2.9–3.2 � 1018 n/m2/s versus 0.94–8.6 � 1017 n/m2/s). Both ahigher irradiation temperatures and lower flux have been observedto decrease the degree of amorphization of Zr(Fe,Cr)2 precipitates[1–3,5–14,22–28,32,42–44,50,51,55–59,65,66]. Comparisons ofthese results with the HFIR data [15] are showing the opposite.


0 5 10 15 20 25 30

Inve

rse

<a>

loop

spa

cing

[cm

-1]

0.0

1.0e+4

2.0e+4

3.0e+4

4.0e+4

5.0e+4

6.0e+4

7.0e+4

8.0e+4

9.0e+4

1.0e+5

1.1e+5

alpha Zircaloy-2beta Zircaloy-2alpha Zircaloy-4

Fig. 13. Summary plot of measured inverse hai loop spacing determined by averagehai loop values measured from TEM characterization following irradiation atnominally 377–421 �C to nominal fluences of 3.4–31 � 1024 n/m2 (E >1 MeV) foralpha-annealed Zircaloy-4, and alpha-annealed and beta-treated Zircaloy-2.

5. Irradiation hardening and microstructure

The relationship between the measured irradiation hardeningand microstructure is quantified using standard models for irradi-ation hardening that are described in terms of a generalized modelfor Orowan hardening [1–3,15,52–54]. The increase in the yieldstress (Dry) required to move the dislocations in this model is gi-ven by the following general expression [52]

Dry ¼ aMGbffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðDÞðNDÞ

pð3Þ

where M is the Taylor factor, G is the shear modulus, b the Burgersvector, ND the loop number density and D the loop diameter. Anaverage Taylor factor for randomly orientated grains has been cal-culated to be 3.06 for fcc metals and 2.65–3.06 for bcc metals[53,61,63], but M = 2 is used as a bounding value in this work basedon the assumption that local deformation occurred in grains thatwere optimally orientated for slip [15]. The parameter a is thestrength of the individual barriers. A hard barrier such as a void thatcannot be cut by a dislocation or precipitate that can only be over-come by dislocation bow has a value of a = 1 [52]. A softer barrierthat could be cut by a dislocation would have a value of a < 1. Typ-ical values of a for dislocation loops range from 0.25 to 0.5 [53] withthe value of a = 0.25 most generally applied to a dislocation loop[53].

The main terms that control the irradiation hardening in Eq. (3)are the spacing of the loop barriers, defined as

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðDÞðNDÞ

p, and the

strength of the barriers, defined as a. Assuming the irradiationhardening is primarily controlled by hai loops, the inverse hai loopspacing term in Eq. (3), defined as


p, would be expected to

predict the increase in yield strength. The fluence dependence forthe


pterms determined from the hai loop characterization

results from Tables 3–5 are shown in Fig. 13 to exhibit a significantincrease at the low fluences of 3.4–8.1 � 1024 n/m2 followed by agradual increase between fluences of 3.4–8.1 � 1024 n/m2 to 30–31 � 1024 n/m2 that generally mirrors the experimentally mea-sured irradiation hardening values shown in Fig. 1b. This indicateshai loops have a strong effect on irradiation hardening that may bedescribed using Eq. (3). However, use of Eq. (3) to calculate irradi-ation hardening results in values that are too low by a factor of 1.3–7.7 if the barrier strength is assumed to be the typical value ofa = 0.25 for a dislocation loop with G = 32.3 GPa and b = 3.23 Å[1,2,15]. The significantly lower calculated strength indicates thehai loops must either be much stronger barriers than indicatedby a typical loop hardening model, or fine clusters smaller thanthe detection limit for TEM are present that dominate the harden-ing. It was previously shown that even using the hai loop


presults from the Zircaloy literature for irradiations at temperaturesless than 316 �C (T < 316 �C) shown in Fig. 11 [1,2,4,30–37,42], thecalculated loop hardening values are still a factor of 3.4–6.1 lowerthan the experimentally measured irradiation hardening values[15].

Assuming the hai dislocation loops are hard barriers like voidswith a = 1 [15] is shown in Fig. 14a and c to result in an improvedcorrelation between the measured and calculated irradiation hard-ening values due to hai loops using Eq. (3). The ratio of measuredversus predicted values range from 0.33 to 1.94 higher or lowerthan the measured values with an average of 1.01 and standarddeviation of 0.36. This is close to the ratio of measured versus pre-dicted values ranges determined for the previous HFIR irradiationsalso plotted in Fig. 14c, which range from 0.31 to 2.5, with an aver-age of 1.32 and standard deviation of 0.66 [15]. Segregation of sol-utes to loops is one possible reason for the high barrier strength.Assuming the barrier strength of loops is controlled by the solutecontent, the barrier strength of the loops would be expected tovary with the level of solute segregation, such as observed forthe presence of solutes and interstitials for materials such as vana-dium [64]. One reason for the differences in the calculated andmeasured irradiation hardening for the irradiated Zircaloy-2 andZircaloy-4 could result from variation in the influence of solute

alpha Zircaloy-2: HFIR [15]beta Zircaloy-2: HFIR [15]alpha Zircaloy-4: HFIR [15]beta Zircaloy-4: HFIR [15]Perfect Fit Linealpha Zircaloy-2: This workbeta Zircaloy-2: This workalpha Zircaloy-4: This work

alpha Zircaloy-2: HFIR [15]beta Zircaloy-2: HFIR [15]alpha Zircaloy-4: HFIR [15]beta Zircaloy-4: HFIR [15]Perfect Fit Linealpha Zircaloy-2: This workbeta Zircaloy-2: This workalpha Zircaloy-4: This work

Fluence [X 1024 n/cm2, E > 1 MeV]

0 5 10 15 20 25 30

σΔYS

, Cal

cula

ted

Valu

e [M

Pa]

0

50

100

150

200

250

300

350


Fluence [X 1024 n/cm2, E > 1 MeV]

0 5 10 15 20 25 30

σΔYS

, Cal

cula

ted

Valu

e [M

Pa]

0

50

100

150

200

250

300

350


σΔ YS, Calculated Value [MPa]0 50 100 150 200 250 300 350

σΔYS

, Mea

s. C

hang

e in

Yie

ld S

treng

th [M

Pa]

0

50

100

150

200

250

300

350

σΔ YS, Calculated Value [MPa]0 50 100 150 200 250 300 350

σΔYS

, Mea

s. C

hang

e in

Yie

ld S

treng

th [M

Pa]

0

50

100

150

200

250

300

350

(a)

(b)

(c)

(d)

Fig. 14. Comparison of calculated irradiation hardening values determined for hai loops only and hai + hci loops using the summation void hardening equation for alpha-annealed Zircaloy-4, alpha-annealed Zircaloy-2, and beta-treated Zircaloy-2 irradiated at nominally 377–421 �C to nominal fluences of 3.4–31 � 1024 n/m2 (E >1 MeV)compared with previous results for the 358 �C HFIR irradiation of alpha-annealed and beta-treated Zircaloy-2 and Zircaloy-4 [15]: (a) calculated hardening as a function offluence for hai loops only, (b) calculated hardening as a function of fluence for hai + hci loop hardening using the summation method, (c) plot of measured versus calculatedhardening using hai loops with the HFIR data [15], and (d) plot of measured versus calculated hardening using hai + hci loops with the HFIR data [15].


atoms on the barrier strength of the loops and the level of solutesegregation [15], but further investigation is needed to verify thistheory. Fig. 14a shows the calculated irradiation hardening valuesalso follow the same trend as determined for the measured hard-ening values shown in Fig. 1a and b with initial hardening at lowfluences followed by a gradual increase at the higher fluences.

A comparison of hai loop hardening calculations using Eq. (3)with average values for


pis made with a calculation using

the summation over the hai loop distribution results given inFig. 12 to determine if the hai loops are accurately representedusing the average diameter and total number density [15,56]

Dry ¼ aMGbffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXi¼bins

ðDÞðNDÞr

ð4Þ

where a fractional summation is performed for the void size distri-bution [15]. For a = 1, the hai loop hardening values determinedfrom the average size and number density (Eq. (3)) are only slightlylower than for those determined from the size distribution (Eq. (4))by 2.0% to 8.5% with the average being 4.0% higher and a standarddeviation of 3.6%. The size distributions for the hai loops given inFig. 14 are close to normal and symmetric, which explains whythe use of average values for


pprovides nearly the same re-

sult as obtained using the size distribution (Eq. (4)).One other effect to consider is the role of the hci-loops on irra-

diation hardening in addition to the effects of hai loop hardening[15]. The effect of hci-loops is quantified by assuming they are alsohard barriers (b = 5.15 Å for hci loops) by a superposition approach

by a linear sum of squares value recommended for barriers thathave a similar strength [56], which yields

Dry ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiahaiMGbhai

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðDhaiÞðNDhaiÞ

q� �2

þ ahciMGbhciffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðDhciÞðNDhciÞ

q� �2r

ð5Þ

Direct linear addition of the hai and hci loop strengthening isconsidered as the maximum extent of calculated irradiation hard-ening contribution due to the combined effects of hai and hci loophardening [15].

Dry ¼ ahaiMGbhaiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðDhaiÞðNDhaiÞ

qþ ahciMGbhci

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðDhciÞðNDhciÞ

qð6Þ

The calculated hardening values using the summation approachare shown in Fig. 14b and d to be more of an over-prediction ofmeasured irradiation hardening for the irradiations in this workthan for consideration of hai loop hardening only using Eq. (3).For the summation approach (Eq. (6)), the calculated values forhardening using hai and hci loops are a factor of 0.31–1.72 differentthan for the measured irradiation hardening with an average of0.93 and standard deviation of 0.31. For this work, use of the super-position approach (Eq. (5)) gives a closer match than for Eq. (6), butconsideration of hai loop hardening alone without the contributionof the hci loops using Eq. (3) gives a better match to the measuredirradiation hardening. These comparisons indicate that the hciloops could have an effect on irradiation hardening that requiresadditional investigation. Since basal slip has been shown to be-come important for irradiated Zircaloy [1,2,18,36,44], the


formation of hci loops likely also effects the irradiation hardening.However, the hci loops do not lie on the typical slip planes for Zir-caloy, and may have an effect on the yield strength that is morecomplex than given by Eqs. (5) and (6).

6. Summary

The increase in yield strength resulting from irradiation harden-ing was determined for alpha-annealed Zircaloy-4 and Zircaloy-2,and beta-treated Zircaloy-2 irradiated in the ATR at nominally377–440 �C to fast fluences between 3.4 and 31 � 1024 n/m2. TheZircaloy materials are observed to initially harden very quickly atdoses of 3.4–8.1 � 1024 n/m2, but then display a reduced dosedependence with little increase in strength observed at the higherfluence of 31 � 1024 n/m2. The irradiation hardening (Dr) observedin this work was generally at the high end of literature data for thehigher irradiation temperatures of 326–450 �C [15,35,43], and wasless than for literature data for irradiations between 260 and326 �C [1,2,4,30,31,37–42]. Irradiation at nominally 377–440 �C isshown in this work to result in a 10–60% lower yield strength thanfor literature data for Zircaloy irradiated at lower temperatures of260–326 �C. A strong sensitivity to irradiation temperature was ob-served in this work. Irradiation hardening observed for the irradi-ations at 377–410 �C was only slightly less than that reported inthe literature for irradiations at the much lower temperatures of260–326 �C. However, much less irradiation hardening was mea-sured in this work for irradiations at 417–440 �C, consistent withthe irradiation hardening reported in the literature for Zircaloyirradiations at 326–450 �C [15,35,43]. Additionally, the decreasein uniform elongation observed in this work for the irradiationsat 377–410 �C was similar to that reported in the literature ob-served for the irradiation of Zircaloy at lower temperatures ofnominally 260–326 �C.

TEM examinations of the microstructure for alpha-annealed Zir-caloy-4 and Zircaloy-2 and beta-treated Zircaloy-2 show that hailoops are observed to form to a high ND at a fluence of 3.4–8.1 � 1024 n/m2 generally followed by a gradual increase in sizeand ND at higher fluences between 30–31 � 1024 n/m2. This is gen-erally consistent with the irradiation hardening results observed inthis work. The hai loop diameters were generally comparable insize for the Zircaloy materials irradiated in this work, but the hailoop ND values are generally higher for alpha-annealed and beta-treated Zircaloy-2 than for alpha-annealed Zircaloy-4 at the high-est fluences. Although the irradiation temperatures used in thiswork are relatively high (377–421 �C), the average hai loop diame-ter (3.9–5.2 nm) was within the range of values reported in the lit-erature for irradiations at temperature <316 �C, and are at the lowend of the range (7–22 nm) reported in the literature for Zircaloyirradiations at 300–358 �C [1,2,12,15,18,24]. The hai loop ND valuesobserved in this work for irradiations at 377–421 �C (1.1–2.0 � 1016 cm�3) are within the range of literature values reportedfor irradiations at temperatures <316 �C, but are at the upper endof the range in values reported in the literature (0.05–5 � 1016 cm�3) for irradiations at 300–358 �C [1,2,12,15,18,24].Although the irradiation temperature in this work is relatively high(377–410 �C), these results suggest that something has stabilizedthe finer hai loop distribution to produce irradiation hardeningonly slightly less than reported in the literature for irradiationsat nominally 260–326 �C. Furthermore, the irradiation hardeningof Zircaloy may not be exclusively related to the hai loop size andND, but other factors could contribute to the hardening such a loopstrengthening from solutes or possibly the presence of point defectclusters that are below the TEM size detection limit [15].

The solutes that contribute to irradiation hardening may be pro-duced by partial amorphization of Zr(Fe,Cr)2 precipitates with irondiffusion into the alpha-Zr observed in this work that is consistent

with literature data for irradiations at higher temperatures [1–3,5–14,22–28,32,42–44,50,51,55–58]. The partial amorphization ofZr(Fe,Cr)2 precipitates results in the release of iron into the al-pha-Zr matrix that may result in the finer distribution of the hailoops and hci loops that is at the high end in ND of the range re-ported in the literature for irradiations at 326–450 �C[1,15,26,35,43]. This amorphization may also provide iron thatstrengthens the hai loop barriers resulting in irradiation hardeningat the high end of the range of values reported for irradiations at326–450 �C. In previous HFIR irradiation at a lower temperatureof 358 �C at a higher flux [15], the Zr(Fe,Cr)2 precipitates were crys-talline and the lower fraction of iron solute may explain the greaterhai loop coarsening and lower irradiation hardening. The greaterdegree of amorphization of Zr(Fe,Cr)2 precipitates observed in thiswork suggests that irradiation at a lower flux at higher tempera-tures of 358–410 �C may actually increase the amorphization ofZr(Fe,Cr)2 precipitates, but this is opposite the trend generally seenunder lower temperature irradiation conditions. Furthermore,these results suggest a strong interplay between the amorphiza-tion of Zr(Fe,Cr)2 precipitates and the hai loop distribution andstrength of the loop barriers that affects the irradiation hardening.However, the impact of such segregation on loop hardening needsto be characterized and understood.

The irradiation hardening observed in this work is characterizedusing a generalized or modified Orowan hardening model with theTEM results to give calculated hardening results that are correlatedwith the measured irradiation hardening values. The calculatedirradiation hardening results for hai loop hardening matches thetrends in measured irradiation hardening as a function of fluence.Differences between the calculated and measured irradiation hard-ening values are believed to result from the change in the barrierstrength terms for the loops due to the segregation of alloying ele-ments to the loops during irradiation [15]. The coarser distributionof hai loops formed at the higher irradiation temperature is the rea-son for the lower irradiation hardening than observed for Zircaloyirradiated at temperatures of 260–326 �C. The formation of hciloops may also have an influence on irradiation hardening that re-quires further study [15].

7. Conclusions

The following conclusions are made based on this work

1. Irradiation at high temperatures (377–440 �C) results in lesshardening than reported in the literature for irradiations at260–326 �C.

2. hci Loops form at very low fluences for irradiations at hightemperatures.

3. A slight extent of amorphization and dissolution of the Lavesphase precipitates is observed following high temperatureirradiations.

4. A complex interaction between the observed irradiation hard-ening, hai loop formation, and amorphization and dissolutionof the Laves phase precipitates is observed that requires addi-tional investigation to fully understand.

Acknowledgments

This work was supported by USDOE. The authors are gratefulfor the review and comments provided by J.E. Hack and B.F. Kam-menzind. Thanks also to the following ORNL personnel for their ef-forts in completing the testing (A.W. Williams), a Department ofEnergy Office of Science User Facility. Research sponsored in partby ORNL’s Shared Research Equipment (ShaRE) user facility, which


is sponsored by the Office of Basic Energy Sciences, U.S. Depart-ment of Energy.

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Development of microstructure and irradiation hardening of Zircaloy during low dose neutron irradiation at nominally 377–440°C