稀有金属(英文版) 2019,38(07),675-682

Flow behavior and microstructural evolution in nickel during hot deformation

Wen-Li Gao Shi-Zhen Lai Jie Teng Zhen-Tao Du Xue-Sheng Liu Yao-Wei Chang

College of Materials Science and Engineering, Hunan University

Jinchuan Nickel and Cobalt Research and Engineering Institute,Jinchuan Group Nickel Alloy Co.Ltd.

作者简介：*Wen-Li Gao,e-mail: wenligaohd@163.com;

收稿日期：23 March 2016

基金：financially supported by the National Natural Science Foundation of China (Nos. 51271076 and 51474101);

Flow behavior and microstructural evolution in nickel during hot deformation

Wen-Li Gao Shi-Zhen Lai Jie Teng Zhen-Tao Du Xue-Sheng Liu Yao-Wei Chang

College of Materials Science and Engineering, Hunan University

Jinchuan Nickel and Cobalt Research and Engineering Institute,Jinchuan Group Nickel Alloy Co.Ltd.

Abstract：

The hot deformation behavior of pure nickel with coarse,columnar grains in the temperature range of 950-1150℃ at intervals of 50℃ and in the strain rate range of 0.001-10.000 s^-1 at intervals of one order of magnitude was investigated by isothermal hot compressive testing with the compression ratio of 70%.The results reveal that the strain rate and the temperature strongly affect the flow stress during hot deformation and that flow stress increases with the increase in strain rate while decreases with temperature increasing.Moreover,the relationship among flow stress,strain rate and temperature can be represented by the Zener-Hollomon parameter with the calculated apparent activation energy of 312.403 kJ ·mol^-1,and the variation of activation energy is sensitive to strain rate rather than temperature.In addition,the dynamic recrystallization(DRX)analysis reveals that the DRX behavior of nickel is evidently affected by both deformation temperature and strain rate and that the distinct mechanisms of nucleation are the bulging of serrated grain boundaries and the development of twinning.

Keyword：

Pure nickel; Hot compression; Flow behavior; Microstructure; Mechanism;

Received： 23 March 2016

1 Introduction

Generally,the dynamic recovery and dynamic recrystallization (DRX) are the main softening mechanisms during hot deformation.However,for metallic materials with low or medium stacking fault energies such as nickel,copper and lead,dynamic recovery and polygonization do not occur readily due to common occurrence of DRX [ 1, 2, 3] .The general characteristics of DRX in the flow curves are as follows:The stress-strain curve for the material which undergoes DRX generally exhibits a peak.Under conditions of low Zener-Hollomon parameter,multiple peaks may be exhibited at low strains.The flow stress and grain size are almost independent of the initial grain size,but the kinetics of DRX are accelerated in specimens with smaller initial grain sizes.A critical deformation is necessary in order to initiate DRX,and the critical deformation decreases steadily with stress or Zener-Hollomon parameter decreasing.Therefore,the flow curves exhibit a peak at a critical strain followed by either a steady-state increase in strain rate or nmultiple peaks at low strains.It can be concluded that DRX was an operative softening process in this case and was controlled by nucleation and growth process.It also showed that these two types of flow curves can result in grain refinement and coarsening,respectively [ 4] .Up to now,the characteristics of DRX of pure nickel with fine grains shaped by rolling,forging,extrusion,or by other methods have been studied over a wide range of temperature and strain rate by hot tensile,fatigue,torsion and compression tests [ 5, 6, 7, 8] .However,limited information is available on the flow behavior and dynamically recrystallized microstructure in the as-cast industrial materials with coarse,columnar grains during isothermal hot compression [ 9, 10, 11, 12, 13] .The evolution of the dynamically recrystallized micro structure is associated with nucleation and growth of new grains.DRX is usually initiated at preexisting grain boundaries even at the conditions of very low strain and large initial grain size.The initiation of DRX is preceded by growing fluctuations of the grain boundary shape.serrations and bulges development,and eventually new grains are generated along these prior grain boundaries.Discontinuous DRX (DDRX) has clear nucleation and growth stages,and it is associated with a bulging mechanism which operates in materials with relatively low stacking fault energies [ 14] .The local migration of grain boundaries leads to the formation of nuclei,and the nucleation dominated by the bulging mechanism is assisted by the formation of twins [ 15, 16] .The constitutive equation can relate stress and strain to the related conditions of temperature and strain rate [ 17] .Hyperbolic sine constitutive equations have ever been established to predict the elevated temperature flow behavior in 42CrMo steel [ 18] ,2124-T851 aluminum [ 19] and pure titanium [ 20] .Sellars and McTegart [ 21] proposed constitutive equations to describe the relationship between flow stress,strain rates and deformation temperatures.

The aim of the present work is to study the flow stress associated with microstructure by hot compression tests of polycrystalline nickel,in which no phase change is involved.The dependence of flow behavior on deformation temperature and strain rate was represented by introducing the Zener-Hollomon parameter.In addition,the activation energy was also studied at various temperatures and strain rates in nickel,and nucleation mechanism of new grains and microstructure observations of DRX nucleation under various deformation parameters were discussed in detail.

2 Experimental

Cylindrical cast ingots with 80 mm in diameter and2000 mm in length used in the present investigation were prepared by horizontal continuous casting process,and the chemical compositions of the as-cast ingots are shown in Table 1.Prior to deformation,the ingots were annealed for2 h at 900℃and then cooled to room temperature in the air.The microstructure contains a large amount of coarse and columnar grains,and the average grain size is about650μm in cross section and about 2000μm in longitudinal section by optical microscope (OM,ZEISS Axiovert 40MAT)(Fig.1).

In order to investigate isothermal compression behavior of this material,cylindrical specimens with 10 mm in diameter and 15 mm in height were prepared from the annealed ingot for single-hit hot compression tests.All specimens were quickly heated to 1150℃and held for about 5 min,and then they were cooled to the hot compression temperature with a cooling rate of 10℃·s^-1 and held for 3 min so as to eliminate the thermal gradients of specimens [ 22] .And then a series of isothermal compression tests were conducted on a computer controlled Gleeble-3500thermal simulator at various temperatures of 950-1150℃under strain rates of 0.001-10.000 s^-1.All the specimens were compressed up to a true strain of 0.7,followed by an immediate water quench in order to preserve the hot deformed structures.The deformed specimens were sectioned parallel to the compression axis along the direction of centerline and the cut surfaces of the specimens after a series of solve procedures were etched in a solution of 25%HF and75%HNO₃ prepared for optical microstructure observations.Samples for transmission electron microscope (TEM,Tecnai G2 F20 S-TWIN) were prepared and observed by conventional nickel techniques [ 6] .

Fig.1 OM image of annealed ingots

3 Results and discussion

3.1 Stress-strain behavior

A series of flow curves obtained from the hot compression tests in nickel are presented in Fig.2.It is widely known that the hot deformation process is a competing process of work hardening and dynamic softening.At the beginning of deformation,the impact of work hardening exceeds that of the dynamic softening due to the rapid multiplication of dislocations,thereby leading to the increase in flow stress.As the strain increases,dynamic softening mechanisms such as DRX play a bigger role.If the rate of dynamic softening is higher than that of work hardening,it can offset or partially offset the effect of work hardening and result in the gradual decrease in flow stress [ 23] .At the strain rate of 0.001 s^-1,multiple peaks are observed during the early stage of deformation at temperatures of higher than 1050℃(Fig.2a),and the same phenomenon is also observed at temperature of 1150℃and strain rate of0.010 s^-1 (Fig.2b).In addition,transition from multiple peaks associated with grain growth to single peak happens with the decrease in the temperatures.At temperatures of lower than 1050℃,the stress-strain curves exhibit single peak,followed by softening and reaching a steady state after critical strain.This means that a new balance between softening and hardening is obtained,which shows a typical DRX behavior.It can also be seen that softening occurs only at lower strain rates and higher temperatures (Fig.2a,b).By contrast,the effect of work hardening far exceeds that of softening at high strain rates and low temperatures.The effect of strain rate is rationalized considering the formation of tangled dislocation structures which are considered as barriers to the dislocation movement,thereby increasing the flow stress [ 24, 25] .And it is also rationalized considering the effect of the temperature on restoration processes,as these processes are thermally activated ones [ 26] .Therefore,flow stress cannot attain a steady state and exhibits continuous increase.In addition,flow curves show broad oscillations at strain rates of higher than1.000 s^-1(Fig.2e).It means that the influence of adiabatic heating should be taken into account and the curves should be corrected for adiabatic heating [ 27] .In general,it can be seen from Fig.2 that the flow stress increases with the increase in strain rate and decreases with temperature increasing,indicating the sensitivity of flow stress to the variations of strain rate and temperature during the deformation,and the effect of strain rate is larger than that of temperature.

  下载原图

Table 1 Chemical compositions of as-cast ingots (wt%)

Fig.2 Flow curves in nickel under different deformation temperatures with different strain rates:a 0.001 s^-1,b 0.010 s^-1,c 0.100 s^-1,d 1.000 s^-1 and e 10.000 s^-1 (corrected for adiabatic heating)

Many hot deformation mechanisms such as flow instabilities,cracking and DRX may result in flow softening and show similar shapes of the flow curves [ 28] .So,it is not appropriate to analyze the deformation mechanisms only on the basis of the shapes of the flow curves.Further analysis is required to verify these mechanisms by other methods.

3.2 Kinetic analysis

Constitutive equations are significant mathematical models,and they are commonly used to calculate the flow stress for further analyzing the flow behavior during deformation.The relationship between flow stress,strain rates and deformation temperatures can be established by the constitutive equations proposed by Sellars and McTegart [ 21] .

For low stresses,

For high stresses,

For all stress levels,

where Z is Zener-Hollomon parameter,εis the strain rate(s^-1),σis the flow stress (MPa),Q is the apparent activation energy (kJ·mol^-1) for deformation,R is the gas constant,T is the temperature (K),A,A₁,A₂,βandαare material constants,and n is the stress exponent.Considering flow stress under a wide range of deformation conditions in nickel,therefore,Eq.(3) is the best form to calculate the constitutive equation.In order to calculate the values of material constants by a sequence of solve procedures,Eq.(3) can be expressed as:

The plot of Inεversus Insinh(ασ) for different temperatures is shown in Fig.3a,whose slopes give the average stress exponent (n).It can be seen that n is dependent on the strain rates and it is considered to be a constant under a certain strain rate.The average value of the slopes of lnsinh(ασ)-1/T curves can be obtained from Fig.3b.Consequently,the apparent activation energy(Q) can be calculated as 312.403 kJ-mol^-1 by Eq.(4).

The activation energies at different temperatures and strain rates are calculated and shown in Fig.4.It can be seen from Fig.4a that the activation energy increases gradually with temperature increasing,and it increases sharply when the temperature reaches 1100℃.At all given temperatures,the activation energy rarely changes at the strain rate of 0.001 and 0.010 s^-1.In addition,the Q value increases at a faster pace at strain rates of0.001-10.000 s^-1 (Fig.4a).Likewise,with strain rates increasing,the activation energy increases,and it increases sharply when strain rate reaches 0.010 s^-1 (Fig.4b).At all given strain rates,the activation energy increases with the increase in temperature,and it rapidly increases with temperature from 1100 to 1150℃.However,the activation energy at 1050℃is very close to that at 1100℃.According to Fig.4,the variation of activation energy is sensitive to strain rate rather than temperature,and this further confirms that the effect of strain rate on flow stress is greater than that of temperature.In addition,the average value of activation energy at various temperatures and strain rates is calculated as 311.602 kJ·mol^-1,which is very close to 312.403 kJ.mol^-1.

Fig.3 Plots of flow stress at different strain rates and temperatures in nickel:a lnε-lnsinh(ασ) and b lnsinh(ασ)-1000/T

Fig.4 Plot of activation energy at different temperatures and strain rates:a Q versus T and b Q versus lgε

Fig.5 Plot of lnZ versus lnsinh(ασ)

Some factors can exert marked effect on measured activation energy,and it is impossible to make a direct comparison between the widely pergent values of activation energy reported in different grades of nickel,initial grain size and hot working methods.The apparent value is slightly larger than the activation energy of 308 kJ·mol^-1reported by Sakai and Ohashi [ 5] .The value is also larger than the activation energy of 274 kJ·mol^-1 during torsion testing of a higher purity nickel with smaller initial grain size [ 29] .It is also much larger than self-diffusion of about279 kJ·mol^-1 in nickel reported by Schaffer et al. [ 30] and207 kJ·mol^-1 determined for recrystallization during creep.The activation energies of peak stress and steady state during high temperature deformation of higher purity swaging polycrystalline nickel are 282 and 265 kJ·mol^-1,respectively,which are less than the measured activation energy in this study [ 31] .

It can be seen from Eq.(5) that the value of lnA is the intercept of the plot lnsinh(ασ) versus lnZ shown in Fig.5.Consequently,the value of material constant A can be calculated as 3.991×10¹⁰.The plot of lnZ versus lnsinh(ασ) also shows the relationship between the peak stress and Z.Apparently,the peak stress increases with the increase in Z parameter.The peak flow stress can be represented by the Zener-Hollomon parameter.The resultant regression equation with various constants can be expressed as:

3.3 Microstructural observations

The microstructures of the deformed samples at the temperature of 1000℃and the strain rates of0.001-10.000 s^-1 are illustrated in Fig.6.It can be seen that partial or complete recrystallization occurs and equiaxed grains form during hot deformation.The size of equiaxed grain is several orders of magnitude finer than that of the original micro structure.In addition,the grain coarsening also occurs corresponding to multiple peaks(Fig.6a) [ 4] .Hence,too small strain rate does not favor the formation of recrystallized fine grains.The microstructure in Fig.6b contains a large number of small equiaxed recrystallized grains,which implies that it is prone to recrystallization at a given strain rate.Figure 6 also indicates that the degree of recrystallization is reduced with the increase in strain rate.It can be attributed to the insufficient time for multiplication of dislocations,thereby limiting the process of DRX.It is interesting that the grain size obtained at higher strain rate is much larger and the number of twins also increases.From the analysis above,too low or too high strain rate is not conducive to recrystallization process.

Fig.6 OM images of nickel deformed at temperature of 1000℃with different strain rates:a 0.001 s^-1,b 0.010 s^-1,c 0.100 s^-1,d 1.000 s^-1and e 10.000 s^-1

Fig.7 OM images of nickel deformed at strain rates of 0.010 s^-1 with different temperatures:a 950℃,b 1000℃,c 1050℃,d 1100℃and e 1150℃

Figure 7 shows the deformation microstructure of nickel deformed at different temperatures with a strain rate of0.010 s^-1.As shown here,the microstructure is closely related to the deformation temperature.At low temperatures,fine recrystallized grains form readily whereas the recrystallized grains grow larger as the deformation temperature increases.This might correlate to the quick migration of DRX grain boundaries at higher deformation temperatures.In general,DRX process is sensitive to strain rate and deformation temperature in nickel.

The microstructure close to the sample surface of nickel deformed at 1000℃and strain rate of 0.010 s^-1 is shown in Fig.8.The serrated and bulging grain boundaries are observed along the original coarse grain boundaries,and it may be attributed to the DRX during hot deformation.However,the center of the sample contains a large number of fine recrystallized grains.During hot deformation,markedly serrated boundaries at the preexisting boundaries can easily bulge out,thereby leading to the formation of new fine recrystallized grains which initiate from the grain boundaries [ 32, 33] .Typically,the initiation of DRX is normally accompanied by the appearance of serrations along the grain boundaries known as bulging mechanism,which is one of the mechanisms involved in the nucleation of new grains in nickel.

Nucleation often takes place at the bulging of serrated grain boundaries,and the evolution can be accelerated by the development of twinning during grain boundary migration under hot deformation [ 34] .Typical TEM image shown in Fig.9 demonstrates the formation of twins during hot deformation at temperature of 950℃and strain rate of10.000 s^-1.It was reported that twinning played an important role in the nucleation process during DRX and refined the initial grains and commonly occurred in materials with low stacking fault energy [ 35] .In addition,higher grain boundary mobility facilitates the nucleation of the twins.This can be attributed to the assistance of migrating grain boundaries on the formation of new grains and leaves twin boundaries behind them,which acts as the main activated nucleation mechanism involved in the generation of new grains at elevated temperatures.The effect of twins can accelerate the bulging and the separation of bulged parts from preexisted grains,thereby resulting in fine grains.This phenomenon implies that the formation of twins can initiate the nucleation of DRX in nickel.The model proposed by Montheillet and Thomas [ 36] revealed the nucleation by twinning process,as shown in Fig.10.The degree of twinning varies with the change of strain and repeats in flow curve during deformation process.With the migration of twinning boundaries (TB),TB are converted into“ordinary”grain boundaries (GB),and simultaneously recrystallization process takes place at these boundaries.Furthermore,the average diameters of the recrystallized grains depend strongly on the presence of twins.When the twins are removed,the average size of recrystallized grains increases with strain increasing [ 37] .Therefore,it can be concluded from the results described above that the boundary bulging and repeated (growth) twinning of grain are very important mechanisms of nucleation for new grains.

Fig.8 OM image of nickel close to specimen surface after defor-mation at 1000℃and 0.0100 s^-1

Fig.9 TEM image of nickel after deformation at 950℃and10.000 s^-1 showing twins

Fig.10 Schematic illustration of nucleation process by twinning

4 Conclusion

Isothermal hot compression experiments were performed in nickel with coarse and columnar grains.The stress-strain behavior,kinetic analysis and microstructure of deformed nickel were investigated.The flow curves are prone to peak or even multiple peaks at the condition of high temperature and low strain rate while do not exhibit peak due to the effect of work hardening far exceeding that of softening at high strain rate.The strain rate and the temperature strongly affect the flow stress in nickel during hot deformation.With the deformation temperature increasing or the strain rate decreasing,the flow stress decreases.The apparent activation energy can be calculated as313.803 kJ·mol^-1,and the variation of activation energy is sensitive to strain rate rather than temperature.The relationship among flow stress,deformation strain rates and temperatures can be established by the constitutive equations:

The DRX process is dependent sensitively on strain rate and deformation temperature in nickel,and recrystallized grains gradually grow with deformation temperature increasing.Too low or too high strain rate is not beneficial for recrystallization process.Markedly serrated and bulging grain boundaries lead to the formation of new fine recrystallized grains.In addition,higher grain boundary mobility results in the nucleation of the twins which play an important role in the generation of new grains during DRX and refine the initial grains.The boundary bulging and repeated (growth) twinning of grains are very important and distinct mechanisms of new grains nucleation in nickel.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos.51271076 and51474101).

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