J. Cent. South Univ. (2021) 28: 338-350

DOI: https://doi.org/10.1007/s11771-021-4606-0

Effect of processing parameters on flow behaviors and microstructure during high temperature deformation of GH4586 superalloy

LUO Jiao(罗皎), LI Xiang-yang(李向阳), LI Cong(李聪), LI Miao-quan(李淼泉)

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University,Xi’an 710072, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract: The apparent activation energy for deformation (Q) and strain rate sensitivity (m) of GH4586 superalloy are calculated and the variation trend is reasonably explained by the microstructure observations. Constitutive modelling of this superalloy is established and the processing maps at different strains are constructed. The results show that the Q value is in the range of 751.22-878.29 kJ/mol. At a temperature of 1060 °C, strain rate of 0.001 s^-1, and strain of 0.65, the m value of GH4586 superalloy reaches a maximum of 0.42. The optimal processing parameter of GH4586 superalloy is at a deformation temperature of 1050 °C and a strain rate of 0.001 s^-1. The domains of flow instability notably expand with increasing strain during high temperature deformation of GH4586 superalloy.

Key words: GH4586 superalloy; apparent activation energy for deformation; strain rate sensitivity; constitutive model; processing maps

Cite this article as: LUO Jiao, LI Xiang-yang, LI Cong, LI Miao-quan. Effect of processing parameters on flow behaviors and microstructure during high temperature deformation of GH4586 superalloy [J]. Journal of Central South University, 2021, 28(2): 338-350. DOI: https://doi.org/10.1007/s11771-021-4606-0.

1 Introduction

GH4586 superalloy was a typical precipitation strengthening nickel based wrought superalloy which had been widely applied for the aerospace field, such as the turbine rotor of rockets and multiple airspace engines [1]. The chemical component of this superalloy was similar to the René 88DT superalloy, and it exhibited homologous deformation behavior characteristics as other nickel based wrought superalloys during high temperature deformation, including Inconel 718 [2], René41 [3], UNS10276 [4], Incoloy 901 [5], etc. However, GH4586 superalloy was a type of difficulty- deformation material with high alloying elements, high applied stress, limited deformation temperature range and strong microstructure sensitivity. Therefore, it is very difficult to acquire the desired workpiece shape and control microstructure during deformation of GH4586 superalloy.

In recent years, numerous researches on the flow behaviors of the nickel-based superalloys were continuously reported in the open literatures [6-10]. For instance, XU et al [2] investigated the flow behaviors of Inconel 718 superalloy with a fined grain by compression tests. Constitutive modelling and processing maps of this superalloy were established so as to optimize the processing parameters. PAN et al [3] discussed the flow behaviors of René41 superalloy by isothermal compression and established a hyperbolic-sine type constitutive model, the optimal deformation parameters were concluded according to the processing maps and microstructure. WANG et al [11] investigated the deformation characteristics and deformation activation energy of GH3535 superalloy, and finally optimized the hot processing parameters of this superalloy. And, the investigations of GH4586 superalloy mostly focused on the microstructure evolution at various working processes. For example, HUA et al [12] investigated the effect of a surface treatment technology named laser shock processing (LSP) on the microstructure of GH586 (same with GH4586 in Chinese series) superalloy, and drew a conclusion that there was an obvious grain refinement on the surface layer of GH586 superalloy. ZHANG et al [13] investigated the effect of deformation temperature, strain rate, and strain on the flow behaviors and the microstructure of GH4586 superalloy at the deformation temperature ranging from 950 °C to 1150 °C and strain rate ranging from 0.001 to 1 s^-1. The result showed that dynamic recrystallization process was effectively promoted by increasing deformation temperature, and ideal microstructure could be achieved by controlling processing parameters. Although many researchers had devoted to studying the flow behaviors of GH4586 superalloy, relatively little further discussion on the strain rate sensitivity, constitutive modelling and processing maps at different processing parameters was reported.

In the present study, isothermal compression was performed to study the flow stress of GH4586 superalloy at different processing parameters. The apparent activation energy for deformation and strain rate sensitivity of this superalloy were analyzed with the microstructure observations. A constitutive modelling modified from Arrhenius equation was established based on the flow stress-strain curves of GH4586 superalloy. The processing maps of GH4586 superalloy at different strains were developed to analyze the optimal deformation domains and flow instability domains. Ultimately, the optimized processing parameters were confirmed according to the processing maps of GH4586 superalloy.

2 Experimental material and procedures

GH4586 alloy is a nickel-chromium-cobalt- based superalloy, and its chemical composition is shown in Table 1. This superalloy was supplied as hot forged bar consisting of equiaxed γ phase with a grain size of 7.8 μm, precipitated hardening γ′ phase and carbides, as shown in Figure 1(a). The γ′ precipitates with a size of 500 nm are extremely tiny and uniformly distributed in γ matrix phase. Moreover, some carbides are distributed in γ grain boundaries and/or triangle grain boundaries.

Table 1 Chemical composition of as-received GH4586 superalloy (mass fraction, %)

Figure 1 Microstructure of as-received GH4586 superalloy using scanning electron microscopy (SEM) (a) and transmission electron microscopy (TEM) (b)

Figure 1(b) shows the TEM micrograph of as-received GH4586 superalloy.

From Figure 1(b), it is seen that the twins are detected in the as-received GH4586 superalloy. HUMPHREYS and HATHERLY [14] have reported that twins may form during recovery, primary recrystallization or during grain growth following recrystallization in low stacking fault energy FCC alloys.

The hot forged bars of GH4586 superalloy were machined to cylindrical specimens with d8×12 mm. Isothermal compression was conducted on a Thermecmaster-Z simulator at the deformation temperatures of 1040, 1060, 1080, 1100 °C, constant strain rates of 0.001, 0.01, 0.1, 1 and strains of 0.2, 0.35, 0.5, 0.65. Each specimen was heated up to a given deformation temperature with a heating rate of 10 °C/s, and kept the temperature for 5 min so as to acquire a uniform deformation temperature. The flow stress-strain curves were obtained in isothermal compression of GH4586 superalloy.

To study the effect of processing parameters on the microstructure, the axial section of all compressed specimens was mechanically polished and chemical etched with a solution of 3 g CuSO₄+40 mL HCl+80 mL C₂H₅OH. And, the microstructure observations were examined by the Leica DMI3000M optical microscope and the FEI NOVA scanning electron microscope. TEM samples were cut by electrical-discharge machining and mechanically ground to a thickness of 30-40 μm. Then, these samples were twin-jet electro-polished in a solution of 9% perchloric acid and 91% methanol at -25 °C. TEM observations of each sample were performed at 300 kV in a FEI Tecnai F30G2 TEM.

3 Results and discussion

3.1 Flow behaviors

The flow curves during isothermal compression of GH4586 superalloy are illustrated in Figure 2. At the strain rates of 0.001 and 0.01 s^-1,the flow curves of GH4586 superalloy tend to be steady at a lower strain. As well known, it is a competing process between the working hardening and recovery softening mechanisms during deformation of metals or alloys. The steady state of GH4586 superalloy at a lower strain implies that the work hardening is sufficient to balance the recovery softening. Possible reason is that lower strain rates (0.001 and 0.01 s^-1) provide enough time for the dynamic recovery and softening. The normal increase of dislocation density, i.e., working hardening, is counteracted by a softening process, i.e., the dislocation rearrangement and/or even the formation of subgrains. Therefore, the steady state is reached at a lower strain and the strain rates of 0.001 and 0.01 s^-1 during isothermal compression of GH4586 superalloy. And, it is seen that the overall shapes of flow curves at the higher strain rates of 1 and 0.1 s^-1 are noticeably different from those at the lower strain rates of 0.001 and 0.01 s^-1.

Figure 2 Flow stress-strain curves during isothermal compression of GH4586 superalloy at different deformation temperatures:

At the strain rates of 1 and 0.1 s^-1, the flow curves rapidly increase and reaches a peak stress at a critical strain, which is attributed to a relatively rapid increase of dislocation density, i.e., working hardening. After that, a significant yield drop could be observed when the applied stress is sufficiently high to impel the dislocations to cross the grain boundaries. Then the flow curves continuously decrease with increasing strain and the steady state is not presented above a strain of 0.65. Remarkably dynamic softening is mainly related to three aspects: 1) the deformation heating effect at the higher strain rates, 2) the recovery-type softening and 3) the dynamic recrystallization (DRX) phenomenon. DRX is confirmed by the microstructure observations in the following Section 3.3.

From Figure 2, it is also seen that the flow stresses of GH4586 superalloy decrease with increasing deformation temperature. It is ascribed to the enhancement of dislocation motion, and the increasing degree of dynamic recovery (DRV) and DRX at higher deformation temperatures. The noticeable DRX could be observed above 1060 °C in the following Section 3.3. Similarly, FU et al [15] proposed that the degree of the DRX increased as the deformation temperature raised during isothermal compression of directionally solidified superalloy Rene88DT. In the present study, the flow stress during isothermal compression of GH4586 superalloy significantly decreases with decreasing strain rate because of a decrease in the rate of dislocation accumulation and multiply.

3.2 Effect of strain on apparent activation energy for deformation (Q)

Kinetic analysis is usually carried out to investigate the flow behaviors of metals and alloys. The Q signifying the workability of metals and alloys is the energy barrier of atom transition during high temperature deformation. The values of Q are highly correlated to the microstructure evolution of alloys [16]. Its expression can be derived from the equation:

                   (1)

where A is the material constant;is the strain rate (s^-1); m is the strain rate sensitivity; R is the gas constant (8.3145 J·mol^-1·K^-1); T is the absolute deformation temperature (K); and Q can be calculated as follows:

                            (2)

From Eq. (2), it is observed that Q is dependent on the slope of lnσ-1/T, R and the reciprocal of m. m is expressed by the slope of lnσ-ln:

                               (3)

As shown in Figure 3(a), the slopes can be obtained from the plots of lnσ-ln at different deformation temperatures via the method of linear fitting. The value of m at a strain of 0.4 is an average value of those slopes, which is 0.22. The average value of the slopes of lnσ-1000/T is calculated from the plots shown in Figure 3(b), which is 21.68. Finally, the value of Q is 837.78 kJ/mol at a strain of 0.4 using Eq. (2). The values of Q at the strain ranging from 0.1 to 0.65 with an interval of 0.05 are calculated by the same method, and are within the scope from 751.22 kJ/mol to 878.29 kJ/mol. In addition, WU et al [17] studied the activation energy of FGH100 superalloy, which was found to be 866.7 kJ/mol.

It is seen in Figure 4 that the Q values of GH4586 superalloy firstly decrease with increasing strain in the early deformation stage and then increase in the later deformation stage and finally tend to be steady state. The minimal Q value is present at a strain of 0.2. The phenomenon could be explained by the flow behaviors and the microstructure of GH4586 superalloy. At the beginning of deformation, the dynamic softening mechanisms (DRV and DRX) are induced by the accumulated distortion energy. The driving force of dynamic softening is enhanced with increasing strain and gives reason to the decrease of Q values, being consistent with the flow softening behavior shown in Figure 2. Moreover, the microstructure observations for a heat-treated sample at a temperature of 1080 °C for 5 min and a deformed sample at 1080 °C, 0.01 s^-1, and 0.2 (Figure 5) support this opinion. It is seen in Figure 5(a) that coarse and equiaxed γ grains with a size of 14.0 μm are observed. In Figure 5(b), it is observed that the complete recrystallized and equiaxed grains with a size of 10.3 μm appear, indicating the obvious refinement of DRX at a strain of 0.2. Therefore, the dynamic softening mechanisms are dominated below 0.2, which supports the decrease of Q values. Similarly, COURTNEY [18] also pointed out that the microstructure remarkably affected the Q values of metals or alloys. As the strain further increases, the dislocation accumulation and multiply occur again in the DRX grains, resulting in the enhancement of work hardening in this stage. Therefore, the values of Q increase in the later deformation stage of GH4586 superalloy. Afterwards, as the increasing dislocation density achieves a critical value, the dynamic softening effect is enhanced by the accumulation of distortion energy. And, the dynamic balance is reached between work-hardening effect and flow softening effect during hot deformation. So, the values of Q reach a steady state in the last deformation stage of GH4586 superalloy.

Figure 3 Plots of lnσ-ln(a) and lnσ-1000/T (b) at a strain of 0.4

Figure 4 Effect of strain on apparent activation energy for deformation of GH4586 superalloy

Figure 5 Microstructure of GH4586 superalloy at temperature of 1080 °C for 5 min (a) and at a deformation temperature of 1080 °C, strain rate of 0.01 s^-1, strain of 0.2 (b)

3.3 Effect of processing parameters on strain rate sensitivity (m)

In recent years, the m values have been used for analyzing the flow behaviors and the evolution mechanisms of metals or alloys. Many factors, such as deformation temperature, strain rate, strain, grain size, have great effect on the m values. In general, the grain refinement and the temperature rise will lead to the increase of the m values, but the grain coarsening will contribute to the decrease of m values.

The m values of GH4586 superalloy at a strain of 0.65 are illustrated in Figure 6. It is observed that the deformation temperature and strain rate have an obviously effect on the m values of GH4586 superalloy. As seen in Figure 6, the m value for a deformed sample at 1060 °C, 0.001 s^-1 reaches a maximum of 0.42. This implies the possible superplastic deformation behavior [19]. The microstructure morphology at this condition shows homogeneous and fine equiaxed DRX grains in Figure 7, which is beneficial to the grain boundary sliding and accommodation of GH4586 superalloy. So, the m value for a deformed sample at 1060 °C, 0.001 s^-1 is high. After that, the m value sharply decreases to 0.14 and reaches a minimum at 1080 °C, 0.001 s^-1. The m values below 0.20 imply that the plastic deformation of GH4586 superalloy is controlled by climb-limited glide processes [20].

Figure 6 Effect of deformation temperature on strain rate sensitivity of GH4586 superalloy at a strain of 0.65

Figure 7 Microstructure evolution of GH4586 superalloy during isothermal compression at a strain rate of 0.001 s^-1 and strain of 0.65:

Subsequently, it increases to 0.27 at 1100 °C, 0.001 s^-1. A similar change trend of m values is observed at the strain rates of 0.01 and 1 s^-1, as illustrated in Figure 6. The variation of m value could be reasonably explained by the microstructure of GH4586 alloy. At a strain rate of 0.001 s^-1 and strain of 0.65, the microstructure of GH4586 alloy at different deformation temperatures is shown in Figure 7. The microstructure is comprised of coarse and elongated γ grains in Figure 7(a), implying a typical of dynamic recovery structure. Elongated γ grains are distributed along the direction perpendicular to the compression axis. Some tiny DRX nuclei are present at elongated γ grain boundaries. This implies that the DRV mechanism is dominated and DRX is not completely activated at 1040 °C. Coarse and elongated γ grains are not good for the grain boundary sliding and accommodation, so the m value of GH4586 superalloy is lower than that at 1060 °C. As above-mentioned analysis, complete DRX grains are present at 1060 °C (Figure 7(b)). And the deformed grains are not observed at this condition. Therefore, homogeneous and equiaxed DRX grains result in a sharp increase of m value. At a deformation temperature of 1080 °C (Figure 7(c)), It is seen that equiaxed DRX grains begin to grow up with increasing deformation temperature. The size of equiaxed γ grains is measured to be 16.0 μm. The growth of γ grains leads to a decrease of m value. At a deformation temperature of 1100 °C (Figure 7(d)), the microstructure is comprised of equiaxed γ grains with a size of 16.3 μm, and some coarse γ grains are also observed at this condition due to the enhanced diffusion process of grain boundaries at higher deformation temperature (1100 °C). The occurrence of γ grain coarsening will result in a decrease of m value. But, an increasing deformation temperature is made for the grain boundary sliding. Therefore, the γ grain coarsening and the increase of deformation temperature finally result in a slight increase of m value during isothermal compression of GH4586 superalloy.

At a strain rate of 0.01 s^-1 and strain of 0.65, the deformation temperature has a negligible effect on the m value of GH4586 superalloy in the deformation temperature of 1040-1080 °C. And, the m value of GH4586 superalloy is in the range of 0.25-0.26. But it slightly increases to 0.29 at 1100 °C. The corresponding microstructure for a deformed sample at 0.01 s^-1, 0.65 is shown in Figure 8. Figure 8(a) shows that the original γ grains with a size of 16.5 μm are elongated during deformation, presenting the structure of dynamic recovery at 1040 °C. And, there are a few tiny DRX nuclei at the boundary of initial elongated grains, as shown in Figure 8(b). It is seen that these DRX nuclei with an approximately size of 100 nm are distributed at the boundary of elongated γ grains. In addition, some twinning boundaries (marked with red arrows) are observed in the γ grains, and many dislocations and dislocation tangles are present in the γ twin grains. The phenomenon is possibly attributed to the hindered effect of twin boundaries on the dislocation slip and motion, resulting in the dislocation accumulation, eventually local dislocation tangles. Comparing Figure 8(a) to Figure 7(a), it is seen that elongated γ grains at 0.01 s^-1 are coarser than that at 0.001 s^-1. This leads to a lower m value of GH4586 superalloy at 0.01 s^-1. In Figure 8(b), complete DRX occurs at 1100 °C. The microstructure of deformed samples consists of fine and equiaxed γ grains with a size of 12.3 μm. So, the m value slightly increases to 0.29 at this condition. Comparing Figure 8(b) to 7(d), it is seen that equiaxed γ grains at 0.01 s^-1 are finer than that at 0.001 s^-1, leading to a higher m value at 0.01 s^-1, as shown in Figure 6.

Figure 8 Microstructure evolution of GH4586 superalloy during isothermal compression at a strain rate of 0.01 s^-1 and strain of 0.65:

The m value for a deformed sample at 1040 °C, 0.1 s^-1, and 0.65 is 0.11 during the isothermal compression of GH4586 superalloy, suggesting that the deformation is controlled by climb-limited glide processes. As the deformation temperature increases, the m value continually increases to 0.21 at 1080 °C. At 1100 °C, it slightly decreases to 0.20. The microstructure evolution of GH4586 superalloy at 0.1 s^-1, 0.65 is shown in Figure 9. As seen in Figure 9(a), coarse γ grains with a size of 15.9 μm are elongated and presented a remarkably oriented characteristic at 1040 °C. And, the microstructure is inhomogeneous. So, the m value of GH4586 superalloy is very low. At a deformation temperature of 1060 °C (Figure 9(b)), the DRX nucleation occurs at the boundary of elongated γ grains. And, fine and equiaxed γ grains are observed at this condition. The microstructure is extremely inhomogeneous. The size of γ grains slightly decreases, leading to a subtle increase of m value. Microstructure of deformed specimen at 1080 °C is shown in Figure 9(c), where there is no sign of elongated γ grains, and equiaxed γ grains with a size of 6.9 μm are obtained by complete DRX. The DRX grain refinement and the increase of deformation temperature give reason to a significant increase of m value. At 1100 °C (Figure 9(d)), the microstructure of GH4586 superalloy is still comprised of equiaxed γ grains, but the growth of equiaxed γ grains occurs and the size is measured to be 11.3 μm. So, the m value of GH4586 superalloy slightly decreases due to the occurrence of grain coarsening at this condition.

3.4 Constitutive modelling

Arrhenius equation is one of the phenomenological models and has been used to describe the flow behaviors of Hastelloy C-276 [21], Co-27Cr-5Mo alloy [22], Ti-6242S alloy [23]. In the present work, the constitutive modelling is established to correlate the processing parameters (i.e., deformation temperature, strain rate and strain) to the flow stress during high temperature deformation of GH4586 superalloy based on Arrhenius equation and Zener-Hollomon parameter. The constitutive model is described as follows:

lnσ=A+B₁lnZ+B₂lnZ²+C₁lnε+C₂lnε²+C₃lnε³        (4)

where Z is the Zener-Hollomon parameter [24], is the strain rate (s^-1); A, B₁, B₂, C₁, C₂ and C₃ are the material constants. The material constants are determined via the method of polynomial fitting based on the experimental flow curves. Table 2 shows the material constants of GH4586 superalloy in constitutive modelling.

Figure 9 Microstructure evolution of GH4586 superalloy during isothermal compression at a strain rate of 0.1 s^-1 and strain of 0.65:

Table 2 Material constants of GH4586 superalloy in constitutive modeling

Figure 10 shows the comparison between the experimental and calculated flow stress of GH4586 superalloy. It is observed that the calculated flow stress agrees well with the experimental flow stress. The value of the mean relative error (MRE) is 8.2%, which indicates that the constitutive modelling has well predictability in modeling the high temperature flow behaviors of GH4586 superalloy.

3.5 Effect of strain on processing maps

Processing maps were established based on the dynamic material model (DMM). It was widely used to optimize the hot processing parameters as well as control microstructure during hot deformation of superalloys [25, 26]. Based on the DMM, the total power P absorbed in the system is described as follows [27]:

P=G+J                       (5)

where G is the power dissipation by plastic work; J is related to the energy through the microstructure evolution. The efficiency of power dissipation η is calculated as follows [27]:

Figure 10 Comparison between calculated and experimental flow stress of GH4586 superalloy at deformation temperatures of 1060 °C (a) and 1080 °C (b)

                          (6)

The η values at different strain rates and deformation temperatures constitute a power dissipation map of GH4586 superalloy, in which specific mechanisms of microstructure evolution are preliminarily judged. According to the principles of the maximum rate of entropy production, the parameter of flow instability is expressed as [28, 29]:

                   (7)

whereis the parameter of flow instability, and its negative values imply the flow instability of metals or alloys. The values at different strain rates and deformation temperatures constitute an instability map of GH4586 superalloy. Finally, the processing maps of GH4586 superalloy are developed by the superimposing of the power dissipation map on the instability map.

Figure 11 shows the processing maps of GH4586 superalloy at the strains of 0.2, 0.35, 0.5 and 0.65. The value of contour plots in these maps represents the efficiency of power dissipation η and the grey regions show the flow instability of GH4586 superalloy.

As seen from Figures 11(a)-(d), two domains with high η values are illustrated: 1) one marked using domain I is at the deformation temperature ranging from 1080 °C to 1100 °C and the strain rate ranging from 0.01 to 0.1 s^-1; 2) the other marked using domain II is at the deformation temperature ranging from 1040 °C to 1060 °C and at a strain rate of 0.001 s^-1. According to the research of WANG et al [11], the higher η value is always associated with the safe deformation mechanisms during hot deformation of superalloys, such as DRX and superplasticity. In the present work, the detailed mechanisms in the domains of high η value are discussed with the help of microstructure observation. At a strain of 0.65 (Figure 11(d)), the η value as high as 0.45 in domain I occurs at 1100 °C, 0.01 s^-1. The microstructure of this domain is shown in Figure 8(c). It is seen that complete DRX occurs and the microstructure consists of fine and equiaxed γ grains. As well known, DRX process is beneficial for enhancing the intrinsic workability of material. Therefore, the higher η domain corresponds to a desirable processing parameter of GH4586 superalloy. In addition, the η value as high as 0.66 in domain II occurs at 1050 °C, 0.001 s^-1, corresponding to an optimal processing parameter and probably represents superplastic deformation behaviors.

At a strain of 0.2 (Figure 11(a)), a relatively narrow domain of flow instability occurs at the deformation temperature ranging from 1070 to 1100 °C and the strain rate ranging from 0.3 to 1 s^-1, which is marked as domain A. At a strain of 0.35 (Figure 11(b)), there are two flow instability domains: 1) one marked using domain A is at the deformation temperature ranging from 1070 to 1100 °C and the strain rate ranging from 0.2 to 1 s^-1; 2) the other marked using domain B is at the deformation temperature ranging from 1040 to 1060 °C and the strain rate ranging from 0.0012 to 1 s^-1. This indicates that the flow instability domains expand notably as the strain increases, and then the flow instability domains contact with each other at the strains of 0.5 and 0.65 (Figures 11(c) and (d)). This combined instability region marked as domain C can be expressed as two parts: 1) one is in the deformation temperature range of 1070-1100 °C and strain rate range of 0.12-1 s^-1; 2) the other is in the deformation temperature range of 1040-1070 °C and strain rate range of 0.006-1 s^-1. The microstructure in the instability domain C (1040 °C, 0.1 s^-1, and 0.65) is shown in Figure 9(a). It is seen that coarse γ grains are obviously elongated and the microstructure is inhomogeneous. This is a typical DRV microstructure. The microstructure is unstable for the high temperature deformation of GH4586 superalloy due to strongly oriented characteristics. Therefore, the hot working should not be performed in the domains of flow instability during isothermal compression of GH4586 superalloy to guard against the appearance of microstructure-based defects. It is concluded that the strain has an obvious influence on the domains of flow instability during high temperature deformation of GH4586 superalloy.

Figure 11 Processing map of GH4586 superalloy at different strains:

4 Conclusions

Effect of processing parameters on the Q values, m values, processing maps and microstructure is investigated during high temperature deformation of GH4586 superalloy. And, a constitutive modelling is established using Arrhenius equation and Zener-Hollomon parameter. The following conclusions are obtained in the present study:

1) The Q values of GH4586 superalloy are in the range of 751.22-878.29 kJ/mol. It firstly decreases and then increases with increasing strain, finally reaches an equilibrium state in the last deformation stage.

2) The m value for a deformed sample at 1060 °C, 0.001 s^-1, and 0.65 reaches a maximum of 0.42, in which the microstructure is homogeneous and equiaxed DRX grain. This implies the possible superplastic deformation behavior of GH4586 superalloy. In general, inhomogeneous microstructure morphology, coarse and elongated γ grains both result in a decrease of m value.

3) A constitutive modelling during high temperature deformation of GH4586 superalloy is described as:

lnσ=-49.53747+1.3369lnZ-0.00803lnZ²-0.25309lnε-0.01749lnε²+0.00982lnε³.

The average relative error between the experimental and calculated flow stress is 8.2%, indicating that the constitutive modelling has well predictability in modeling the flow behaviors of GH4586 superalloy.

4) The optimal processing parameter of GH4586 superalloy is at 1050 °C, 0.001 s^-1. The domains of flow instability notably expand with increasing strain. The hot working should not be performed in the domains of flow instability during isothermal compression of GH4586 superalloy.

Contributors

LUO Jiao provided the overarching research goals and edited the draft of manuscript. LI Xiang-yang conducted the literature review and wrote the first draft of the manuscript. LI Cong analyzed the calculated results. LI Miao-quan provided the concept. All authors revised the final version.

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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(Edited by HE Yun-bin)

中文导读

GH4586合金高温变形过程中工艺参数对流动行为和微观组织的影响

摘要：本文计算了GH4586高温合金的表观变形激活能(Q)和应变速率敏感性指数(m)，并基于微观组织观察分析了其变化的原因。本文还建立了GH4586高温合金的本构模型和不同应变下的热加工图。研究结果表明：表观变形激活能值(Q)是751.22~878.29 kJ/mol。当变形温度为1060 °C、应变速率 为0.001 s^-1、应变为0.65时, GH4586高温合金的应变速率敏感性指数(m)达到最大值0.42。该合金的最优加工参数是变形温度1050 °C和应变速率 0.001 s^-1。GH4586合金高温变形过程中的非稳定区域随着应变的增加而显著增加。

关键词：GH4586高温合金；表观变形激活能；应变速率敏感性指数；本构模型；加工图

Foundation item: Project(2020JC-17) supported by the Science Fund for Distinguished Young Scholars from Shaanxi Province, China; Project(51705425) supported by the National Natural Science Foundation of China; Project(2019-QZ-04) supported by the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China; Projects(3102019PY007, 3102019MS0403) supported by the Fundamental Research Funds for the Central Universities, China

Received date: 2020-04-20; Accepted date: 2020-11-18

Corresponding author: LUO Jiao, PhD, Professor; Tel: +86-29-88460465; E-mail: luojiao@nwpu.edu.cn; ORCID: https://orcid.org/ 0000-0003-3734-7975