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Dynamic Strain Aging in a Newly Developed NieCo-Base Superalloy with Low

Stacking Fault Energy

Chenggang Tian, Chuanyong Cui*, Ling Xu, Yuefeng Gu, Xiaofeng SunSuperalloys Division, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

[Manuscript received April 18, 2012, in revised form June 7, 2012, Available online 18 April 2013]

Characteristics of dynamic strain aging (DSA) in a NieCo-base superalloy were studied by tensile tests at

temperatures ranging from 250 C to 550 C and strain rates ranging from 3 105 to 8 104 s1.Serrated flow in the tensile stress-strain curves was observed in the temperature range from 300 C to

500 C. Normal DSA behavior was found at temperatures ranging from 300 C to 350 C, while inverse DSA

behavior was observed at temperatures ranging from 400 C to 500 C. The yield strength, ultimate tensile

strength, elongation, work hardening index, and fracture features were not affected by temperature and strain

rates in DSA regime. Negative strain-rate sensitivity of flow stress was observed in DSA regime. The analysis

suggests that the ordering of the substitutional solutes around some defects like mobile dislocations and

stacking faults due to the thermal activated process may cause the serrations on the tensile curves.

KEY WORDS: NiLCo base superalloy; Dynamic strain aging (DSA); Activation energy; Substitutional solute; Stacking fault

1. Introduction

Ni-base superalloys are the primary materials for the blades

and disks in the advanced gas turbine engines[1e3]. Many nickel-

base superalloys[4e15] exhibit serrated ow related to dynamic

strain aging (DSA) over a range of temperatures and strain rates.

The serrated ow known as the Portevin-Le Chatelier (PLC)

effect[16] and the negative strain rate sensitivity (NSRS) are the

predominant characteristics in DSA regime. For example,

negative strain rate sensitivity has been found in Inconel 600[4],

aged Waspaloy[5,6], Inconel 718[7], CMSX-4[8], Alloy 625[9] and

U720Li[12]. The critical strain (c) of serrated ow, depending on

the tensile temperatures and strain rates, is a crucial factor. Based

on the critical strain changing, DSA can be divided into twogroups: the normal DSA and inverse DSA. The characteristic of

the normal DSA is that the critical strain increases with

decreasing test temperature or increasing strain rate. While the

characteristic of inverse DSA is that the critical strain decreases

with decreasing test temperature or increasing strain rate.

Many models[5,17e20] have been proposed to explain the

mechanism of DSA behaviors during tensile plastic deformation,

which are based on the interactions between diffusing solute

atoms and mobile dislocations over the range of plastic ow. The

activation energies used to determine which kind of solute

locking mobile dislocations are generally obtained through these

models. For example, Hayes et al.[5] arrived at a conclusion in

Inconel 600 and aged Waspaloy that low test temperature gave

normal DSA behavior due to the carbon atmosphere aging

mechanism and high test temperature gave inverse DSA

behavior because of the interaction between Ni3(Al,Ti) precipi-

tate and carbon atoms. Shankar et al.[9] noted that Mo atoms

caused the normal DSA behavior at low temperature in Alloy

625. However, the activation energy cannot be used to determine

where the inverse DSA behavior was observed. Nalawade

et al.[10] investigated the DSA behavior of Inconel 718 and

ascribed the serrated ow to the diffusion of substitutional Nbsolute atoms. Hale et al.[11] suggested that the diffusion of C

atoms was the reason for the serrated phenomenon at low tem-

perature and the interaction between mobile dislocations and Cr

atoms caused the serrated phenomenon at high temperature in

Inconel 718SPF. Gopinath et al.[12] noted that the substitutional

solutes locking the mobile dislocations are responsible for DSA

behavior in U720Li, which includes the normal DSA behavior at

low temperature and the inverse DSA behavior at high

temperature.

Recently, Cui et al.[14] found that the inverse DSA behavior

may be related to the occurrence of stacking faults from TEM

observation in a NieCo base superalloy. Lee et al.[21] concluded

that the DSA may be caused by the breaking away of stacking

faults from point defects or pointe

defect complex throughobserving the DSA behavior of twinning-induced plasticity

* Corresponding author. Prof., Ph.D.; Tel.: 86 24 83978292; E-mail

address: [email protected] (C. Cui).

1005-0302/$ e see front matter Copyright 2013, The editorial ofce of

Journal of Materials Science & Technology. Published by Elsevier

Limited. All rights reserved.http://dx.doi.org/10.1016/j.jmst.2013.04.012

Available online at SciVerse ScienceDirect

J. Mater. Sci. Technol., 2013, 29(9), 873e878


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(TWIP) steel. Similarly in TWIP steel, Lee et al. [22] proposed

that DSA occurs by a single diffusive jump of C atoms of the

point defect complex in the stacking fault region and DSA

cannot be observed until the C atom reorientation time is smaller

than the residence time of the stacking faults at the point defect

complex. It is assumed that the stacking faults may take effect inDSA regime in the alloy with low stacking fault energy.

In this work, the newly developed NieCo-base superalloy[23]

with stacking fault energy of 24.9 mJ/m2 was selected to study

the role of stacking faults in DSA regime. In addition, the alloy

contains substitutional (Co, Cr, Mo, Al, Ti, W) solutes and

interstitial (B, C) solutes, which provides a chance to study how

these solutes affect the characteristics of DSA in the alloy.

2. Experimental

Table 1 lists the chemical composition of the alloy. The ingot

was homogenized at 1200 C for 10 h and subsequently hot

extruded to 30 mm in diameter. The alloy was treated by the

following heat treatment: 1100 C for 4 h followed by air cooling

and then rst aging at 650 C for 24 h followed by air cooling

and then second aging at 760 C for 16 h followed by air

cooling. The specimens for tensile tests were machined from the

heat-treated rods. The samples with a gage section of 3 mm in

diameter and 23 mm in length were used in the tensile tests.

Tensile tests were carried out on a machine at temperatures

ranging from 250 C to 550 C and strain rates from 3 105 to8 104 s1. All the tensile tests were started after keeping thespecimens at the test temperature for 10 min. The samples for

optical microscopy (OM) observation were etched in a solution

of modied Kalling reagent (100 ml HCl, 100 ml methanol and

50 g CuCl2). The samples for scanning electron microscopy

(SEM) were electronically etched in an 80 ml H2O5 ml glacialacetic acid 15 ml nitric acid solution at 1.5 V, normally for30 s. Thin foils for transmission electron microscopy (TEM)

observation were prepared by means of a standard twin jet

polishing technique in a solution of 10% perchloric acid and

90% ethanol at about 16 mA and 20 C.

3. Results

3.1. Microstructures

Fig. 1 shows the microstructure of the alloy. The average grain

size of the alloy was 33 mm (Fig. 1(a)). The inset SEM image in

Fig. 1(a) reveals that the irregularly shaped particles at the grain

boundary were TiC conrmed by EDS. At higher magnication(Fig. 1(b)), primary g 0 (300 nm), secondary g 0(about 100 nm)and tertiary g 0 precipitations (about 20 nm) were observed in the

g grain.

3.2. Mechanical properties

3.2.1. Tensile stressestrain plots.Fig. 2 shows the plots of true

stress vs. true strain from 250 C to 550 C at a strain rate of

3 104 s1. The serrated ow of stress was observed at the test

temperatures ranging from 300 C to 500 C. While, at the

temperatures of 250 C and 550 C, the stress-strain curves were

relatively smooth. The critical strain (c) of serrated ow indi-

cated that the initial strain of serrations depended on temperature

and strain rate.

3.2.2. DSA behaviors. Fig. 3 shows the variation of critical

strain with strain rates and temperatures. There were two trends

in Fig. 3. One is associated with the increase in c

with

increasing strain rate and decreasing T, which is called normal

DSA; and another one is associated with the increase in c

with

decreasing strain rate and increasing T, which is called inverse

Table 1 Chemical composition of the NieCo based superalloy (wt%)

Cr Co Mo W Ti Al B C Zr Ce Ni

14 23 2.8 1.2 5.6 2.3 0.02 0.02 0.03 0.01 Bal.

Fig. 1 Microstructures of the NieCo superalloy: (a) optical micrograph

of the microstructure and SEM image (inset) showing TiC at the

grain boundary and in the grain; (b) dark-eld TEM image

showing the primary, secondary and tertiary g 0 precipitations.

Fig. 2 True stressetrue strain curves of the alloy tested at different

temperatures with a strain rate of 3 104 s1.

874 C. Tian et al.: J. Mater. Sci. Technol., 2013, 29(9), 873e878


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DSA. The normal DSA behaviors occurred in the temperature

range from 300 C to 350 C and the inverse DSA behaviors

corresponded to the temperature range from 400

C to 500

C.

3.2.3. Tensile properties.Fig. 4 shows the variation of the yield

strength, the ultimate tensile strength, the elongation and the

work hardening index with temperature changing under different

strain rates. The work hardening index (n) can be obtained from

following relation [24,25]:

s Kn (1)

wheresis the true stress, is the true strain,Kis a constant and nis the work hardening index. All the factors except n can be

obtained directly from the stress-strain curves. It can be seen

that the four tensile properties seemed unaffected by the

temperature and strain rates from Fig. 4.

Fig. 5 shows the fracture surfaces of the specimens tested in

the normal DSA regime (350 C, 8 104 s1) and the inverseDSA regime (450 C, 8 104 s1). The dimples were the maincharacteristic of the fracture surfaces which indicated ductile

fracture in both cases. These observations suggested that the

characteristic of fracture in the normal DSA regime was the sameas that in the inverse DSA regime.

3.2.4. Negative strain rate sensitivity (m).Strain rate sensitivity

of the ow stress (s) at a given Tand was estimated using thedata from the stressestrain curves through following relation[5]:

m log s2=s1

log _1= _2 ;T(2)

wheres1 and s2 are the ow stresses at _1 and _2, respectively.Fig. 6 shows values ofmat a strain of 5% determined at different

stress levels corresponding to different strain rates from the

stress-strain curves. Negative strain rate sensitivity of the owstress was observed in DSA regime and a minimum value ofmat

450 C was obtained.

3.3. Microstructures after tensile test

It had been known that the alloy in this experiment has a low

stacking fault energy of 24.9 mJ/m2[23]. As a result, the process

of super-dislocations decomposing into partial dislocations and

stacking faults became easier when the plastic deformation

prevailed, as shown in Fig. 7(a). When the formed stacking faults

extended on the active slip planes, the partial dislocations

changed their slip planes, rather than their slip directions, after

their interaction with some obstacles, as shown in Fig. 7(b).

Fig. 3 Variation of critical strain (c) for serrations with strain rate ( _)

and temperature (T).

Fig. 4 Variation of the tensile properties with temperature (a) yield strength; (b) ultimate tensile strength; (c) work hardening index; (d) elongation.

C. Tian et al.: J. Mater. Sci. Technol., 2013, 29(9), 873e878 875


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4. Discussion

4.1. Activation energy

The evaluation of activation energy plays an important role

in understanding the DSA mechanism. Based on reports in

literature[5,17,2628], four different methods were employed to

calculate the activation energy of serrated ow. The activation

energies determined from all the methods are summarized in

Table 2. It is generally accepted that when the exponent

(m b) values range from 0.5 to 1, interstitial solutes areresponsible for serrated ow, whereas when (m b) valuesrange from 2 to 3, substitutional solutes are responsible[29,30].

The values of (m

b) can be determined in the followingrelation[26].

mbC K_exp Q=RT (3)

where m is an exponent related to the vacancy concentration

with plastic strain (CVfm) and b is an exponent related to

the density of mobile dislocations with plastic strain (rmfb),

K is a constant, _ is the strain rate, Q is the activation energy,

R is the gas constant and Tis the absolute temperature.

In Table 2, the mean value of the activation energies in the

normal DSA regime was calculated to be 46 kJ/mol, which is

close to the activation energies for serrations reported for

Inconel 600 (58 kJ/mol)[6], Inconel 718 (42 kJ/mol)[7], and theactivation energy for diffusion C in Ni (69 kJ/mol)[31]. In

Inconel 600 and Inconel 718, C atoms interacting with mobile

dislocations were thought to be responsible for the appearance

of serrations. However, the serrated ow in NiC system wasobserved at a low temperature of25 C[32], then the diffusionrates of C atoms were too high to produce an atmosphere to lock

mobile dislocations at T300 C in the experiment. Addition-ally, the (m b) value determined to be 2.87 implies the normalDSA behavior is the result of the interaction between substitu-

tional solutes with mobile dislocations. The mechanism for the

inverse DSA behavior is also attributed to substitutional solutes

pinning up mobile dislocations because the temperatures for the

inverse DSA behavior are higher for interstitial solutes forming

an atmosphere. The mean value of activation energy for inverseDSA behavior was estimated to be 143 kJ/mol, which is much

larger than the value (46 kJ/mol) for normal DSA behavior. But

the phenomenon may coincide with the trend of the stress drop

indicating the height of individual serration with the increasing

temperature in Fig. 8 which shows the stress drop increases with

the increasing temperature at 5% when _ 3 104 s1.Obviously, the change of stress drop and activation energy from

the normal DSA region to the inverse DSA region with

increasing temperature is related to the thermal activated pro-

cess. It is possible that the diffusion rates of substitutional sol-

utes are high enough to make them arrange more orderly along

mobile dislocations when temperature changes from 350 C to

400 C, which produces a sharp stress drop and a sharp acti-

vation energy increase. Therefore, the ordering of substitutionalsolutes along mobile dislocations may cause the serrated ow in

this alloy.

4.2. Stacking fault

Recently, some researchers investigated the effect of stacking

faults on DSA[14,21,22], as presented in the section of introduc-

tion. It is inferred from their investigations that the stacking

faults may contribute to the appearance of serrations through its

interaction with some solutes. As shown in Fig. 7(b), partial

dislocations changed their slip planes after their interaction with

some obstacles in DSA regime. As reported before, the inter-

stitial solutes diffused too fast to form an obstacle in the tem-

perature regime (T 300 C) where the serrated ow occurred.Therefore, the occurrence of the serrated ow can be attributed to

Fig. 5 Fractographs of specimen tested at (a) 350 C, 8 104 s1 (normal DSA regime); (b) 450 C, 8 104 s1 (inverse DSA regime).

Fig. 6 Variation of strain rate sensitivity (m) with temperature at a strain

of 5%.



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the diffusion of substitutional solutes. The substitutional solutes

diffused along dislocations to the stacking faults and reoriented

in the stacking fault region, which led to be an obstacle for the

stacking faults moving. The interaction of this short ordered

obstacle with the moving stacking faults was the reason for the

serrated ow occurrence.

Based on the discussion in section 4.1 and 4.2, the serrated

ow in this alloy may be resulted from the interaction between

the substitutional solutes and some defects such as mobile dis-

locations and stacking faults. The movement of the defects in thealloy is controlled by the thermal activated process.

4.3. Negative strain rate sensitivity (m)

The strain rate sensitivity of the ow stress was negative in

DSA regime and a minimum value ofm at 450 C was obtained

(Fig. 6). Mulford and Kocks[4] contended that the rate sensitivity

becomes negative after a critical strain, which causes serrated

ow. Sleeswyk[18] suggested that solute atmospheres form on

the forest dislocations and then drain through pipe diffusion

from the forest dislocations to the mobile dislocations when the

mobile dislocations are waiting at the forest dislocations. Basing

on this model, Mulford and Kocks[4] demonstrated that the so-

lute atmosphere does not need pin the entire dislocation but a

portion of the dislocation line at the forest dislocation which

makes little diffusion produce obstacles strength with waiting

time resulting in a negative rate sensitivity. A recent model [33]

favored that the strength of the forest-mobile dislocation junc-

tion increases with the size and the strength of the solute at-

mosphere clustering on the forest dislocation. Gopinath et al.[12]

considered that the increasing resistance to the dislocation mo-

tion by the serrated ow facilitates diffusion of the solute atoms

to dislocations and a resultant negative strain rate sensitivity. All

the assumptions pointed out that the negative strain rate sensi-

tivity is related to the interaction between mobile dislocations

and solute atmospheres corresponding to serrated ow. From theconclusion obtained for explaining the mechanism causing

serrated ow earlier, it is believed that the mechanism also in-

uences the trend of the value of m corresponding to the

increasing temperature: the reorientation of order of the sub-

sutitional solutes around the defects will reduce the value ofm,

and contrarily the destruction of order of the subsutitional sol-

utes around the defects will increase the value of m. It can be

seen that the reorientation of subsutitional solutes around defects

is predominant when T 450 C and the destruction of sub-sutitional solutes around defects is the leading factor when

T 450 C, as shown in Fig. 6. As a result, the value of mreaches minimum at 450 C.

Fig. 7 TEM image showing (a) the stacking faults at 450 C, 8 105 s1; (b) stacking faults interacting with obstacles at 400 C, 3 104 s1. SFmeans stacking fault.

Table 2 Summary of activation energy for the DSA behavior

Calculating method Q for Normal

DSA (kJ/mol)

Q for Inverse

DSA (kJ/mol)

(1) ln _ vs. 1/Tplots[23] 46 (m b 2.87)

142 (mb 2.17)

(2) McCormick method[14] 49 138

(3) Intercept method[24] 50 na

(4) Stress drop method[2,25]

40 149Average Q 46 4 143 5 Fig. 8 Variation of stress drop with temperature at a stain of 5% and

strain rate of 3 104 s1.

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5. Conclusions

(1) The alloy exhibited normal DSA behavior at temperatures

ranging from 300 C to 350 C and inverse DSA

behavior at temperatures ranging from 400 C to 500 C.

(2) Negative strain rate sensitivity was observed and thevariation of its value with temperature related to the

mechanism controlling DSA behavior.

(3) The yield strength, ultimate tensile strength, elongation,

work hardening index and fracture surfaces did not

change noticeably in DSA regime.

(4) The ordered arrangement of substitutional solutes around

some defects such as mobile dislocations, stacking faults

may cause the appearance of serrations in the NiCo basedsuperalloy.

Acknowledgments

This work was partly supported by Hundred of Talents

Projects, the National Basic Research Program (973 Program)of China under grant No. 2010CB631206 and the National

Natural Science Foundation of China (NSFC) under Grant Nos.

51171179, 51128101 and 51271174.

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