1 Confidential: not for distribution. Submitted manuscript to MSE A for peer review (it is formatted by me, so please be patient).

Please cite this article as: B.N. Mordyuk, et al., Structurally induced enhancement in corrosion resistance of Zr-2.5%Nb alloy in saline solution by applying ultrasonic impact peening, Materials Science & Engineering A 559 (2013) 453–461, http://dx.doi.org/10.1016/j.msea.2012.08.125

Structurally induced enhancement in corrosion resistance of Zr-2.5%Nb alloy

in saline solution by applying ultrasonic impact peening B.N.Mordyuk*, O.P.Karasevskaya, G.I.Prokopenko

Kurdyumov Institute for Metal Physics, 36 Academician Vernadsky blvd. 30142, Kyiv, Ukraine Abstract Near surface layers in Zr-2.5%Nb alloy are modified using severe plastic deformation process induced by ultrasonic impact peening (UIP). The effect of the UIP processing time on the microstructural formation, microhardness and corrosion is analyzed. XRD analysis and TEM observations allow establishing the links between microstructure in the surface layers formed at different extents of the effective strain e and their microhardness and corrosion resistance. Structural formation occurs under influence of deformation induced heating, which facilitates dynamic recrystallization and omega precipitation in beta grains. Subdivision of alpha grains is observed to be a dislocation-mediated process mainly. Deformation twinning may be effective in the early stages of the UIP process till the strain extents of approx. e ≈ 0.2. Generation and rearrangement of dislocations result in formation of elongated cell structure at e ≈ 0.43. Further straining to e ≈ 0.8 leads to formation of ultrafine-grained (UFG) structure through the subdivision of elongated subgrains into equiaxed ones and subsequent increase in their misorientation. Average grain size in the formed UFG structure was about 150-200 nm. XRD analysis reveals high compressive residual stresses and strong basal texture after the UIP process. It is suggested that the sliding character of the impact loads occurred at the UIP process plays an important role in the crystallites’ reorientation in superficial layer. Formed UFG surface layer with strong basal texture provides an essential increase in surface microhardness from 2 to 3.15 GPa and marked enhancement of the corrosion resistance of Zr-2.5%Nb alloy in saline solution due to the broadening of passivity region on the anodic potentiodynamic polarization curve. Decreased surface roughness and large compressive residual stresses also promote higher corrosion resistance.

Keywords: ultrafine-grained structure; texture; zirconium alloys; microhardness; corrosion resistance; ultrasonic impact peening

*Corresponding author. Tel/ fax: +38 044 424 0521 E-mail address:mordyuk@imp.kiev.ua

1. Introduction

It was demonstrated in recent decades that surface mechanical treatments can play an important role in enhancement in different properties of materials. A vast majority of these treatments, such as shot peening [1], laser shot peening [2-4], surface mechanical attrition treatment (SMAT) [5-8], sand blasting [9,10], and ultrasonic impact peening (UIP) [11-14] uses high rate straining via multiple impact loads. Beneficial influence on the materials’ properties is due to compressive residual stress and modified microstructure in subsurface layers of treated components. Above mentioned techniques were shown to be capable in generating ultrafine-grained (UFG) or nanocrystalline (NC) grain structures in the topmost surface layers of metallic materials. These UFG/NC microstructures promote essentially higher strength [7,10,11-16], fatigue [10,11,15], wear [17] and corrosion resistance [9,10,13,14,18,19] of the surface modified materials in comparison with their coarse grained counterparts.

Zirconium based alloys including the Zr-Nb alloys are widely used in nuclear plant systems. A number of recent studies were addressed to investigation the corrosion resistance of some Zr alloys in the conditions close to the operational ones [20]. Besides, the effects of the Nb concentration were also addressed as well as the heat treatment [21,22]. It was also pointed out that the microstructure of the surface layer predetermines the properties and structure of the oxide film formed [20,21,23-25]. The latter was shown to play an important and frequently even governing role in the overall material performance. Only recently the Zr alloys have been emerging as new potential materials in such area as biomedicine [26,27]. Pure Zr and Nb are known to have outstanding in vitro biocompatibility [28], though their mechanical properties and corrosion resistance need to be enhanced.

This paper describes our attempts on tailoring microhardness and the corrosion behavior of a Zr-2.5%Nb alloy in a 3.5% NaCl solution by means of the UIP processing. The main purpose of this work was to retrace microstructural evolution and corrosion behavior of the Zr-2.5%Nb alloy at the UIP process. Particular focus was to determine the microstuctural features responsible for the observed microhardness and corrosion behaviors of the surface layer formed. A mechanism of grain refinement at UIP is also discussed.

2. Experimental details

Received ingots of a Zr-2.5%Nb alloy contained (in wt.%) 2.5 Nb, 0.12 Fe, 0.02 Cu, 0.08 Ni, and < 0.1 O were hot forged and rolled to obtain strips of 10 mm thick. Then, after removal of surface oxidized layer of 1 mm thick and vacuum annealing (1023 K, 2 hrs, 10-3 Pa) the strips were cold rolled to the thickness reduction of about 75%. Plane disks of 10 mm in diameter and 2 mm in thickness were then cut out (Fig.1a). These disks were mechanically polished, annealed in vacuum at 873 K for 1 hour and quenched. Grain size of the material after cold rolling followed by stress recovery annealing and quenching was approx. 12 µm.

The equipment for the UIP process is described in detail in Refs. [12-14,29,30]. Here, we briefly give the process principle. The energy in this technique is supplied by an ultrasonic generator with a frequency (fUS) of 21.7 kHz and a power output of 0.3 kW. At this loading scheme shown in Fig.1b the specimen settled in the reciprocating with low frequency (fLFV = 50 Hz) holder is exposed to repetitive sliding impacts by pins positioned between it and an ultrasonic horn in a special impact head. The induced frequency of impacts fI in this loading scheme is about 3 kHz [11-14]. Principal direction of the impact loads (viz. load direction – LD) differs from the normal direction (ND) significantly. It is tilted to the treated surface by the angle that can be changed dependently on the ratio between the normal and transversal constituents of load, and this angle is around 45o if both the constituents are

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equal. In this study, an impact velocity of the pin caused by ultrasonic energy ( usυυ =⊥ = 2πfUSa = 2.99 m/s) was equal to a shift velocity of the specimen surface ( lfvυυ =// = 2πfLFVA= 2.98 m/s) provided with low-frequency vibrations (Table 1). The inclination of the LD appears important in the sense of the operated deformation modes (viz. prismatic slip, tensile twinning, compressive twinning or pyramidal slip) and associated reorientation of crystallites in the surface layer during the UIP process that will be discussed in the next sections. Naturally, the largest strain occurs in the topmost surface layer of the treated specimen. Inset in Fig.1a shows changes in dimensions of the layer. Diameter d and thickness h were taken into account to estimate the effective strain of the layer using the following formula [13,14,31]:

( ) ( ) ( )[ ]21213

232

2213

2 εεεεεε −+−+−=e (1)

where ε1 = (h0-hp)/h0, ε2 = ε3 = (d0-dp)/d0 are the principal strains, h0 and d0 are the initial layer thickness and diameter, and hp and dp are the same for the deformed layer after the UIP process. The evaluated magnitudes of the effective strain at certain UIP regimes used and sample markings are listed in Table 1.

Deformation heating, which is known to be an important parameter in structural evolution and hardening process in the surface layer, can affect both to structural and phase transformations and to the material ductility. Assuming adiabatic heating, it is possible to estimate the temperature rise within the topmost surface layer. Taking into account that about 90-95% of the deformation work done is converted into heat (β ≈ 0.9-0.95) the temperature rise ∆T can be estimated using the following equation [14,32]:

∆T = β σY e / ρ CP, (2)

where e is the effective strain, ρ and CP are the density and the specific heat capacity of the specimen material, σY ≈ HV/3 is the yield stress that correlates to the Vickers hardness through the Tabor’s relation. Using equation (2) and accounting for the following physical properties of Zr-2.5%Nb alloy (ρ=6558 kg m–3, CP = 0.28 kJ kg–1 K–1) and appropriate magnitudes of σY and e the temperature rise ∆T in the outermost surface layer of the UIP treated specimens can be roughly estimated (Table 1). Experimentally, the temperature at the UIP process was registered using a thermocouple welded to the side of the central section of the specimen treated (Fig.1b).

Structural studies of the deformed surface layer were carried out by means of X-ray diffraction (XRD),

transmission electron microscopy (TEM) and selective area electron diffraction (SAED) analyses. The XRD θ–2θ and texture analyses as well as the sin2ψ based analysis of residual stresses were conducted using a DRON-3M diffractometer with monochromatic Cu Kα irradiation. The TEM and SAED investigations were performed using a JEM 100 CX-II microscope. The plane-view TEM foils prepared from the top sub-surface layer (~5 – 10 µm) were mechanically polished from the un-treated side to the thickness about 30 µm followed by one-side electro-polishing using a twin-jet technique at –35oC in a solution of 400 ml CH3OH + 240 ml C6H14O2 + 40 ml HClO4. Microhardness of the top surface was evaluated using a PMT-3 Vickers microhardness tester at a load of 100 g and a dwell time of 10 s. Surface roughnesses of the annealed and UIP-processed specimens were characterized with the arithmetic mean roughness Ra of the surface profile (Table 1) measured using a P-201 profilograph-profilometer.

The corrosion behaviors of stress relieved (SR) and UIP-treated specimens were evaluated using potentiodynamic polarization in a 3.5% NaCl solution at room temperature. A saturated calomel electrode (SCE) and a platinum plate were used as a reference electrode and as the counter one, respectively. For characterization of corrosion behavior, the following electrochemical quantities were measured and estimated: the corrosion potential (CP), the corrosion current, the breakdown potential, and the passivity region (PR).

3. Results 3.1. Microhardness and corrosion behaviors The UIP process leads to significant strain hardening

of the treated surface layer (Fig.2). Microhardness increases with the processing time (curve 1 in Fig.2) demonstrating a sharp growth from the very beginning of the UIP process till the strain extent ≈e 0.22 and marked deceleration of the microhardness increase when the effective strain exceeds ≈e 0.43. Still, the latter stage is characterized with the ascending hardening rate. Distinct difference between these two stages of the observed dependence allows assuming operation of several hardening mechanisms that have different efficiency during various stages of the deformation process. The possible mechanisms of the strain hardening, which could

operate at the UIP process, will be analyzed in the next section with taking into account dislocation activity, deformation twinning, and phase transformations. Another important feature worth to be mentioned here is a

Fig. 1. Scheme for the UIP loading cell (a) and scheme for the specimens cut out from the rolled strip (b): holder (1), specimen (2), pins (3), impact head (4), ultrasonic horn (5). Insert shows dimensions of surface layer before (d0,h0) and after (dP,hP) the UIP process.

Fig. 2. Microhardness (1) and residual stresses (2) in the UIP-treated specimens of Zr-2.5%Nb alloy against the UIP processing time and the effective strain.

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significant rise of compressive stresses observed in the deformed specimens with ongoing straining (curve 2 in Fig.2). The compressive residual stresses formed at the UIP process are effectual cause for hardening and integrity of the treated surface as they complicate the cracks’ initiation and propagation at external loads and/or in aggressive environment, particularly in the outermost layers. Decreased surface roughness after the UIP process (Table 1) also promotes higher corrosion resistance since the higher the roughness is, the larger the specific surface area is exposed to the corrosive attack [14].

Electrochemical properties of stress relieved (SR) and UIP-treated specimens of Zr-2.5%Nb alloy are shown in Fig.3 and in Table 2 as compared to some literature data [14,28,32-34]. Results of potentiodynamic polarization indicate several features in the corrosion behavior. Anodic polarization curves of the UIP-treated surfaces slightly shift to a lower current density in comparison to that of the SR specimen. This shift corresponds to a lower dissolution rate or higher corrosion resistance. On the other hand, the corrosion potential (CP) demonstrates gradual diminution with increase of the UIP processing time (Fig.3, Table 2). The materials with more negative potentials are known to have a tendency to undergo more significant corrosion, while the others (with positive potential values) will generally suffer less attack. In this sense, the UIP-treated specimens appear slightly deteriorated. Besides for all the specimens studied, the active-passivation behaviors show stable passivity regions (PR). Breakdown potential Ebd becomes markedly higher for the UIP-treated surfaces, causing appreciable broadening of the PR (inset in Fig.3, Table 2). From the polarization curves, the features common to the three specimens presented are the similar startups of passivity regions, which indicate to their similar passivation capability in the medium and weak influence of the UIP process (in the regimes used).

3.2. Microstructural observations

Figure 4 shows XRD spectra and partial (0002) pole figures for the Zr-2.5%Nb specimens. The diffractogram of the SR specimen displays sharp and narrow hcp-Zr peaks (Fig. 4a). Low intensity peak corresponded to a bcc Zr-Nb β-phase are visible indicating that the volume fraction of the β-phase is just above the sensitivity limit of the XRD

method (~5 %). The (0002) peak is slightly favored in the XRD pattern of the SR specimen indicating that a lot of basal planes are parallel to the specimen surface after the chosen thermal treatment of the cold rolled semis. Further crystallite rotation and texture development in the surface layer of specimens due to the effect of the pin collisions occur at the UIP process. The (0002) and (0004) Zr diffraction lines become more intensive and (0002) texture gets much stronger (Fig. 4d). The latter can be quantitatively estimated using the peak intensity ratio I0002/I10ī1. The estimated ratios for the SR, UIP-2 and UIP-4 disks were obtained to be 1.28, 1.88 and 2.27, respectively. Clearly, an intensification of the (0002) plane texture occurs after the UIP process. However, it is essentially lower than that observed in a Zr-1%Nb alloy processed with UIP for the same duration (4 minutes) [14], where approximately triple increase in this ratio was registered.

Figure 5 shows typical microstructures of UIP-05 specimens observed using TEM analysis as compared to initial SR state (Fig.5,a,b) characterized with equiaxed dislocation free grains of α-phase separated by the stringers of β-phase. TEM observations of the UIP-05 specimen show that the deformation process is accompanied with intensive growth of the dislocation density (Fig.5c). Rearrangement of dislocations appears in formation of elongated subgrains with thick low-angle subboundaries, which contain high density of dislocations (Fig.5d). At the same time, regions with the twins’ related structure can be frequently observed. Boundaries are thin here (Fig.5e) as they are formed as a result of the twinning process.

With ongoing deformation, a number of β-grains appears undeformed as if they being much harder than α-grains (Fig.6). Network of extremely tangled dislocations is visible in the β-grains in bright field images (Fig.6,a,c) and uniformly distributed nano-precipitates of ω-phase in the respective dark field images (Fig.6,b,d,f). Higher magnification of the β-grain allows observing moiré fringe patterns in bright field images (Fig.6,e). The area fraction of these nanosized ω-precipitates in the β-grain shown in

Fig.6f is approx. 6%. Additional diffraction spots of ω-phase in 1/3 and 2/3 {110}β positions are also appeared in <111> β zone SAED pattern. With ongoing deformation, harder β-grains squeeze the α-grains provoking their further straining in the constrained conditions. Gradual

Fig. 3. Potentiodynamic polarization curves of stress-relieved (1), UIP-2 (ē = 0.62) (2) and UIP-4 (ē = 0.78) (3) specimens of Zr-2.5%Nb alloy obtained in 3.5% NaCl solution. Inset shows behaviors of the passivation region (PR) and the corrosion potential (CP).

Fig. 4. X-ray spectra (a, c) and pole figures (b, d) for the specimens of Zr2.5Nb alloy in SR state (a, b) and after the UIP-4 process (ē = 0.79) (c, d).

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enlargement of misorientation subdivides elongated subgrains into smaller equiaxed grains. As a result, an UFG structure is formed in the surface layer of the Zr-2.5%Nb alloy after the UIP-4 process, that is, at the strain extent of 0.78 (Fig.6g,h). Appropriate SAED pattern contains significantly azimuthally dispersed spots that catenulate in almost continuous rings. The most of the rings are shown to correspond to the main crystallographic planes of hcp Zr (Table 3). Still, the UFG structure contains high density of dislocation both in the grains’ interiors and boundaries. In the structural formation, dynamic recrystallization intensified with deformation heating of the outmost surface layer seems to play an essential role. The dynamic recrystallization occurs at large strain extents (ē ≈ 0.6–0.8) and elevated temperatures by means of rotation of the subgrains/grains with concomitant absorption of the dislocations approaching the moving subboundaries. Average grain size diminishes down to approx. 100-150 nm. Similar mechanism consisting in the rotation of subgrains was discussed earlier for α-zirconium processed by dynamic deformation [35], α-titanium deformed with SMAT [6] and VT3-1 titanium alloy [36], Zr-1%Nb alloy [14,34] or Zr-2.5%Nb alloy [37] processed with UIP.

Detailed analysis of the microstructural evolution in the topmost surface layers of UIP-treated specimens is carried out in the next section accounting for the respective effective strains and microstructural hardening mechanisms that might be operative. But, the histograms for SR and UIP-4 specimens (Fig.5b and Fig.6h) clearly indicate essential grain refinement despite of broad grain-size distribution.

4. Discussion Brief analysis of literature data shows that hcp Zr

is one of the most corrosion resistant and vital material among transition metals [26,27]. Alloying with Nb results in further improvement in the performance of hcp Zr in corrosive media. The influence of Nb is beneficial in a wide concentration range. Dilute Zr-Nb alloys contained ≤ 0.6-1% of Nb form solid solution of Nb in hcp Zr, which possesses higher corrosion resistance as compared to pure Zr [25]. The alloys with higher Nb concentrations retain enhanced corrosion resistance albeit they already have heterogeneous phase composition, that is, they consist of the hcp Zr-Nb solid solution and a bcc β-phase [22,25]. Depending on the thermomechanical

treatment used, the β-phase may appear as continuous or interrupted interlayers between α-grains, as small precipitates or larger conglomerates [38-41]. Morphology of β-phase can affect to the corrosion behavior. The bcc β-phase may undergo β→ω or β→α|| transformations upon heat treatment or deformation [38,39]. Both β→α|| [42] and β→ω [43] were shown can be reversible albeit the presence of nanoscale β-grains is required in the case of ω→β transition [43].

Besides the above-mentioned metallurgical aspects of the corrosion behavior of Zr-Nb alloys, the microstructural state in surface layer plays an essential role in the control and improvement of the corrosion performance. According to the recently published data grain size is one of the most important factor affecting corrosion resistance [2,9,10,13,19,40,41,44,45]. Besides, preferred orientation of crystallites [14,24,45,46] and the nature of grain boundaries, that is, appearance of coincidence site lattice (CSL) boundaries [14,23,24], are shown to be determinative in the sense of improvement in the corrosion behavior. Therefore, let’s build our discussion of the obtained data on the analysis of the microstructural evolution occurred in the UIP-treated surface layers with ongoing deformation. The formation of UFG structure and texture development are the most important microstructural features worth to be discussed in detail.

4.1. Grain subdivision

It is well known that dependently on the lattice type and quantity of operative slip systems a number of different mechanisms of grain subdivision can be involved to accommodate plastic deformation. The grain refinement process may occur through formation of dislocation cell structures evolving with ongoing deformation into highly misoriented fragmented structure, or by involving martensitic transformations or deformation twining. Both latter mechanisms are frequently operative at the deformation processes of the materials with low stacking fault energy (fcc – austenitic stainless steels) or limited slip

Fig. 6. Bright filed (a,c,e,g) and dark field (b,d,f) TEM observations of microstructures in Zr-2.5Nb alloy after the UIP-1 (ē = 0.43) (a,b), UIP-2 (ē = 0.62) (c-f) and UIP-4 (g,h) (ē = 0.79) processes and histogram (h) of grain size distribution in Zr-2.5%Nb alloy.

Fig. 5. Bright filed TEM observations of grain/ dislocation structures in Zr-2.5Nb alloy in SR (a,b), UIP-05 (ē = 0.22) (c,d,e) and histogram of grain size distribution (b).

5 systems (hcp zirconium or titanium). In hcp-Zr, along with <a> prismatic slip the {10-12} tensile twinning or {11-22} compressive twinning as well as <c+a> pyramidal slip were reported to be the likely mechanisms accommodated deformation along the <c> axis [47-49]. However, elevated temperature (room temperature and above) or high strain rate can activate many slip systems in hcp Zr [50], and suppress deformation twinning. Besides, the suppression of twinning in some hcp materials can be caused by high stress and/or small grain size. The UIP process is the case when all the above-mentioned factors play against twinning, particularly after the strain extent exceeds 0.22 (see curve 1 in Fig.2), – the increasing stresses and elevated temperature in thin surface layer that is deformed with the concomitant reduction in the grain size. However, from the very beginning of the UIP process, twinning seems to be operative because of high strain rate at the impact load. Associated reorientation of basal planes (Fig.4c,d) may explain essential hardening (Fig.2).

Analysis of the TEM data presented in Fig.5, Fig.6, Fig.7 shows that the dislocation activity plays a major role in the structural formation after the strain extent exceeds ē =0.22. However, some outcomes of twinning were also registered in the specimen treated with UIP for the lowest time (0.5 minute). This observation is in concordance to a number of studies reported that in SMATed titanium [6] or zirconium [50], multisystem twinning occurs at low to moderate strains, above which it is observed to expire being replaced with dislocation activities. In our case, indirect evidence for participation of twinning in the deformation process is the reorientation of crystallites in the surface layer and the development of basal texture (Fig.4).

Thus, the dislocation activities sequentially appears in to the growth in dislocation density, arrangement of dislocations in new subboundaries, increase in misorientation of subboundaries till they form high-angle grain boundaries, further mutual rotation of the neighbor equiaxed subgrains that provokes breakups of elongated subgrains into equiaxed grains. Elevated temperature initiates dynamic recrystallization, which seems to occur during the UIP process. As a result, an UFG structure contained equiaxed grains of nanometer regime is formed. Suggested mechanism of the grain refinement at UIP is similar to the sequence of microstructural evolution reported for hcp metals being processed with high strain rate deformation using Split Hopkinson bar [35] or SMAT [6,7].

It is also important that the UIP process could initiate ω-precipitation in β-phase. It is suggested that it can be caused by essential deformation heating of the topmost surface layer during the UIP process. The isothermal ω-phase is reported to form in a β-phase matrix of relatively concentrated Zr-Nb alloys upon ageing at temperatures below about 773 K [38]. According to [38,39] α| martensitic or ω phases can be formed in β-phase of the deformed Zr-2.5%Nb alloy specimens upon heating in the temperature range of 423-773 K, and these precipitates may explain increased hardness of the β-phase as well as a whole alloy. Occurrence of ω-precipitates in Zr-Nb alloys of near eutectoid composition is usually accompanied with significant increase in hardness [38,52]. In our case, due to the strain induced heating of the outmost surface layer at the UIP process the temperature of the treated specimen

falls just into the above-mentioned temperature interval – according to the estimations based on Eq.(2) the temperature exceeds 473K already after the UIP for 1 minute (Table 1). Thus, the ascending hardening in the late stages of experimental curve (Fig.2) may be partially explained by the precipitation hardening of β-phase. Besides, harder β-grains squeeze α-grains provoking their further straining in the constrained conditions. Nanohardness of Zr2.5Nb alloy was also reported to increase significantly, when the ω-precipitates had formed in the β-phase [40].

4.2. Texture development

Fig. 4 shows that pins’ collisions at the UIP process lead to the crystallite rotations in the surface layer. Two key features should be taken into account while considering the observed texture development. First, it is initial texture that consists in preferential orientation of <c> axis of superficial crystallites tilted by 25-40o from the ND (Fig.4b, Fig.7a). Second, the sliding impacts have a large transversal component of compressive load, that is, the resultant load direction always differs from the ND

(Fig.7a). Thus, the compressive load produced by the inclined impacts acts along the <c> axis of a large number of crystallites. The crystallites with <c> axis inclined to the ND on the angle 45 o < φ < 90 o seem to contribute mainly to the development of basal texture (Fig.7a). According to [47-49] in the deformed Zr tensile twinning, compressive twinning and <c+a> pyramidal slip can mainly cause reorientations of the <c> axis (up to 90o). Therefore, it is believed that these mechanisms might be involved in the texture development observed in our case (Fig.7b). Twinning induced reorientations of crystallites that lead their basal planes to align parallel to the specimen surface

Fig. 7. Scheme for crystallites’ reorientation as a result of pyramidal slip (10-11)<11-23> (pyr), tensile twinning (10-12)<10-11>(ttw) and compressive twinning (11-22)<11-23>(ctw). LD, ND and C are the load direction, normal direction and <C> axis direction, and n is a normal to the basal plane.

6 is shown schematically in Fig.7c. At the same time, crystallites that already acquired the (0001) basal texture seem to stop changing their orientation. The latter suggestion is consistent with the recent study of texture development in fully recrystallized Zircaloy 2 at cold compression [53]. Insignificant twinning and texture developments were observed in samples having starting (0001) basal textures unlike those with predominant non-basal texture, which had gone through strong texture developments with progressive deformation via noticeable twinning till about 15% compression. The Zr-2.5%Nb alloy studied here can be compared to the single phase Zircaloy 2 because they undergo rather similar structural changes. Based on the obtained structural data the UIP processed specimens demonstrate pronounced texture development and contain discontinuous β-phase (Fig.4, Fig.5). Discontinuous β-phase promoted texture development in the α-matrix unlike the absence of texture in the α-grains separated with continuous β-interlayers [54,55]. Rotation of basal planes in cold rolled Zr-2.5%Nb alloy in the normal direction of a rolling plane was reported also in [56], where deformation twinning and associated texture variation were observed in the range of 5 to 15 % strains. It was also shown that basal texture of Zr-1%Nb alloy becomes stronger after the UIP process performed in similar regimes [14].

Considering the magnitudes of estimated and registered temperature they can hardly affect the crystallite rotation. The temperature should be above approx. 873 K to initiate the rotation of the c-axis toward the ND turning (001) basal texture stronger [47]. It is believed however that a local increase in temperature at the UIP process can be much higher than the registered one (Table 1). According to the Eq. (2) the strain hardening observed with ongoing deformation leads to the increase of the temperature rise as it is proportional both to the applied stress and to the accumulated strain. Thus, at the late stages of the UIP process the crystallites reorientation process in thin surface layer could be intensified with increasing strain hardening and temperature rise.

4.3. Structurally determined microhardness and corrosion behaviors

Based on the microstructural evidences three main factors can affect mechanical properties. Deformation twinning seems to operate intensively from the very beginning of the UIP process. A lot of twins (Fig.5e) cause quick hardening (Fig.2) and crystallites reorientations during first minute of treatment (ē ≤ 0.22). The formed basal texture participates essentially in the hardening process since deformation along <c>axis requires higher stresses to initiate stronger deformation modes as (10-12) tensile twinning, (11-22) compressive twinning and <c+a> pyramidal slip. Precipitation strengthening of the beta grains influences on the hardness at strains ē ≥ 0.43, and it appears in the ascending hardening. Significant increase in dislocation density also contribute to hardening (Fig.5c,d,e). Besides, the microhardness behavior (curve 1 in Fig.2) correlates with the diminution of the grain size in the surface layer (Fig.7). Some deceleration in the strain hardening at the strain extents higher than ≈e 0.45 could be a result of the deformation heating, depletion of deformation twinning, formation of cell structure, equiaxed

subgrains/grains and their mutual rotation that involves the sliding of the grain boundaries.

The observed corrosion behavior could be conditioned by several reasons. First one is the pronounced texture with the most close packed lattice planes (basal planes) parallel to the specimens’ surface. It is consistent with the results obtained on pilgered α-Zr [57], α-Ti processed by ECAP [46] and on Mg–Al–Zn alloy processed by cryogenic burnishing [26] showing that the corrosion resistance can by significantly enhanced due to the combined effects of grain refinement and strong basal texture. It was shown in [58] that in textured materials are characterized with minimal energy of grain boundaries and high frequency of occurrence of coincident site lattice (CSL) misorientations. The CSL model is known to be the base for the effective method used to improve various properties including corrosion resistance and called as ‘grain boundary engineering’ [23]. One of the most effective path of thermomechanical treatment (TMP) was reported to consist of low to moderate deformations in the range of 5 to 30 pct followed by annealing for short duration at relatively high temperatures [21,23]. Moreover, iterative, rather than single step, deformation and annealing treatments have lately shown an increase in the volume fraction of the CSL boundaries, which possess “special” properties. Fortunately at the UIP process, thin surface layer is in the conditions very similar to the above described ones – induced by the repeated impacts accumulation of moderate to low strains alternates with short-time recovery during inter-impact time, when the treated layer is at elevated temperature caused by the deformation heating. Thus, formation of ‘special’ boundaries at the UIP process seems to be plausible albeit it is not investigated yet. The properties of surface layer are determined by the quantities of ‘general’ and ‘special’ grain boundaries, which are naturally controlled by the grain size. The latter is diminished significantly at the UIP process.

Observed grain refinement is therefore another reason responsible for the improved corrosion behavior of the UIP-treated Zr-2.5%Nb specimens. This improvement is mainly related to the increase of the breakdown potential on the anodic polarization curve as the UFG structure supposedly promotes more protective ZrO2 oxide film (Fig.8). Remarkable improvement of the corrosion resistance of Zr the grain refinement to nano-scale level was shown in [19] on the bases of the developed computation model of corrosion rate versus grain size. Experimentally, enhanced corrosion resistance of UFG Ti in hydrochloric and sulfuric acids was revealed in [44] and explained with more rapid and readily formation of passive film onto UFG Ti as compared to its coarse-grained counterpart. Analysis of TEM observations in conjunction with the known texture data and the orientation relationships between monoclinic ZrO2 and the α-Zr substrate was carried out in [24], and it allowed concluding that the special grain boundaries hindered transport through the zirconia film. Significant difference in dislocation densities is also crucial – being the lowest for the SR specimen it provides the highest corrosion potential (Fig.3). The UIP process increases dislocation density (Figs. 5, 6, 7) leading to some deterioration of the corrosion potential (inset in Fig.3, Table 2). Similar behavior of the sand blasted titanium was explained with

7 higher dislocation density beneath the oxide film in comparison to that in the specimen annealed after the sand blasting [10]. Some deterioration in active corrosion performance might also be related to the formation of discontinuous beta phase and/or omega precipitates in the specimen after the UIP-4 process. However, this deterioration is compensated with much higher strength of the formed oxide film and higher breakdown potential (Fig.3, Table 2) on the anodic potentiodynamic polarization curve that provides the overall improvement of the corrosion behavior of the UIP-treated Zr-2.5%Nb specimens.

5. Summary

An ultrafine-grained surface layer with strong basal texture was produced during UIP-induced surface SPD of bulk Zr-2.5%Nb alloy. Average grain size is approx. 180 nm in the topmost surface layer of 10 µm thick. Considering structural evolution observed with TEM at different strain extents a mechanism for the grain refinement is discussed. The mechanism for subdivision of α-grains is dislocation-mediated mainly. However, from the very beginning of the UIP process up to the strain extents ē = 0.22 twinning seems to play a significant role. With ongoing UIP process (increase in the effective strains), accommodation of deformation in the topmost surface layer occurs through sequential changes in dislocation structures that can be described as follows: (i) significant increase in dislocation density; (ii) rearrangement of dislocations facilitating formation of elongated cell structures; (iii) subdivision of the cells/twins into equiaxed subgrains subsequently evolved into equiaxed grains through the increase in misorientation of neighbor subgrains. At the effective strains ē ≥ 0.45, discontinuous β-phase undergoes precipitation hardening due to formation of nanosized ω-precipitates promoted by the strain induced temperature rise (∆T ≥ 200K). Then, these hard β-grains facilitate further straining of α-grains in constrained conditions. The strain induced adiabatic heating and respective temperature rise seem to provoke dynamic recrystallization that facilitates grains’ rotation accompanied with some decrease in dislocation density.

XRD analysis reveals high compressive residual stresses and strong basal texture after the UIP process. The load scheme used provides the situation when the sliding impacts (which resultant load direction is around 45o from the ND) act along <c> axis of a large number of crystallites, which <c> axes tilted by 25-40o from the ND. Such compressive load produces reorientation of some crystallites to align their basal planes parallel to the specimen surface. The crystallites with <c> axis inclined to the ND on the angle 45 o < φ < 90 o seem to contribute mainly to the development of basal texture. At the same time, crystallites that already have acquired the (0001) basal texture seem to stop changing their orientation.

Formed UFG surface layer with strong basal texture provides an essential increase in surface microhardness from 2 to 3.15 GPa and marked enhancement of the corrosion resistance of Zr-2.5%Nb alloy in saline solution due to the broadening of passivity region. It is suggested that sufficiently high fraction of the low energy ‘special’ boundaries formed in such UFG textured surface layer may explain the observed corrosion behavior. As compared to the stress relieved specimen the UIP-processed ones are

characterized with lower corrosion current. Despite of some decrease in the corrosion potential that is supposedly connected with appearance of discontinuous β-phase with ω-precipitates the UIP-treated specimens demonstrate the increased breakdown potential that expands the passivity region on the anodic potentiodynamic polarization curve essentially. The latter seems to relate to more protective Zr2O oxide films formed due to development of strong (0001) basal texture in the surface layer.

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Please cite this article as: B.N. Mordyuk, et al., Structurally induced enhancement in corrosion resistance of Zr-2.5%Nb alloy in saline solution by applying ultrasonic impact peening, Materials Science & Engineering A 559 (2013) 453–461, http://dx.doi.org/10.1016/j.msea.2012.08.125