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Controlling grain size via dynamic recrystallization in an advanced polycrystalline nickel base superalloy
Journal of Alloys and Compounds 701 (2017) 909e919
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Controlling grain size via dynamic recrystallization in an advanced polycrystalline nickel base superalloy Guoai He a, b, c, Feng Liu a, b, c, Lan Huang a, b, c, Zaiwang Huang a, b, c, *, Liang Jiang a, b, c a
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China Powder Metallurgy Research Institute, Central South University, Changsha 410083, China c High Temperature Materials Research Institute, Central South University, Changsha 410083, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 December 2016 Received in revised form 13 January 2017 Accepted 18 January 2017 Available online 19 January 2017
Controlling grain size in polycrystalline nickel base superalloy is of paramount importance in optimizing hot-working process and achieving the desirable mechanical properties. Typically, a uniform and fine grain size is required throughout forging process to realize the superplastic deformation. Unfortunately, the forging processing window is very narrow and needs to be synergistically controlled by deformation temperature, strain rate, as well as strain amount, and the failure would lead to the non-uniform grain size, abnormal grain growth and even cracking. During superplastic deformation, dynamic recrystallization (DRX) is the governing mechanism to be adopted to maintain the required grain size. Previous literature have extensively documented the separated effect of deformation parameters using monotonic-pass compression test. Herein, we perform the multi-pass compression experiments to investigate the flow stress behavior and grain size evolution. By conducting multi-pass tests over a range of deformation parameters, it was demonstrated that the flow stress and grain size response differed from that in single-pass tests. The multi-pass compression tests unambiguously uncover the roles of DRX and static recrystallization (SRX) in regulating grain size and affecting flow stress. The experimental results further show, comparing to the monotonic-pass compression, the multi-pass approach can provide versatile routes to attain the desirable grain size distribution by means of utilizing DRX and SRX principles. © 2017 Elsevier B.V. All rights reserved.
Keywords: Nickel base superalloy Hot compression Flow stress Dynamic recrystallization Grain size
1. Introduction Powder metallurgy nickel base superalloys are the most widely used high temperature materials in turbine disc of aero-engine owing to a superb balance of high strength, corrosion resistance as well as fatigue resistance under elevated temperature [1e4]. Previous results have extensively documented that these outstanding mechanical properties are primarily governed by both the grain size [5,6] and gamma prime (g0 ) precipitate distribution [7,8], which are regulated by a sequential combination of powder atomization, hot isostatic pressing and hot extrusion consolidation. Isothermal forging [9,10] is subsequently applied to attain the superplastic deformation and avoid abnormal grain growth. This process is generally performed at the sub-solvus temperature and
* Corresponding author. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China. E-mail address: [email protected] (Z. Huang). http://dx.doi.org/10.1016/j.jallcom.2017.01.179 0925-8388/© 2017 Elsevier B.V. All rights reserved.
low strain rate [11], wherein a uniform and fine grain size can be produced and maintained by virtue of successive DRX occurring [12]. To date, most of results [13e21] have focused on investigating the independent effect of deformation temperature, strain rate and strain amount on the flow stress behavior and microstructure evolution via the monotonic-pass compression experiment. The corresponding processing map can be established based on the dynamic materials model to label the domains of different deformation mechanisms, such as DRX, adiabatic shear band, cracking, dynamic recovery [22e25]. Unfortunately, in the monotonic-pass compression test the deformed microstructure is inclined to form the inhomogeneous grain size [26e29] and in extreme cases induce cracking [30,31], in particular under the low temperatures and high stain rates. Recently, multi-pass hot compression experiment [32,33] provides a new perspective to study the DRX/SRX behavior and the effect of hot working parameters on the grain size evolution. The documented works have shown that the flow stress and grain size is closely related to the dwell time [34], strain amount [35] and strain rate [36]. In this article, we employ the multi-pass
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development, including changing the strain amounts in the firstpass, changing the strain rate in the first-pass and the secondpass, imposing different dwell time and deformation temperature. The results show that the multi-pass compression is more versatile to control the grain size than the monotonic-pass experiments. The different deformation conditions imposed in the firstpass and the second-pass can generate the vastly variable grain size distribution and strain rate is the most prominent one. The research
Table 1 The nominal composition of a polycrystalline nickel base superalloy (wt. %). Element
Co
Cr
Ti
W
Mo
Al
Nb
Hf
C
B
Zr
Ni
Wt(%)
26
13
3.7
4
4
3.2
0.95
0.2
0.05
0.025
0.05
Bal.
experiments to investigate the influence of deformation parameters in the first-pass on DRX and SRX behaviors and the grain size Table 2 Deformation parameters under different multi-pass hot compression tests. ID
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
First pass
Dwell time/s
Temperature/ C
Strain rate/s1
True Strain
1100
0.001
0.2 0.3 0.4 0.3
0.01 0.1 0.01
0.001 0.01 0.1
30
120 360 30
Second pass Temperature/ C
Strain rate/s1
True Strain
1100
0.001
0.493 0.393 0.293 0.393
1050 1150 1100
0.01 0.1 0.01 0.1 0.01 0.1
Fig. 1. The pristine microstructure observation shows the hot extruded superalloy (after annealing treatment) dominantly consists of equiaxed grains and no distinct crystallographic texture is detected (a). A closer-up TEM observation (b) further demonstrates that grain boundaries are geometrically planar and dislocation density is quite low. (c) The average grain size of as-extruded alloy is measured to be 10.67 mm by means of EBSD technique. All the grain size lies in the range below 40 mm. (d) A majority of grain boundaries are recognized as the high angle grain boundaries.
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results shed light on understanding the roles the DRX/SRX mechanism in controlling flow stress and microstructure evolution in the course of forging nickel base superalloy. 2. Experimental The material in this work was a newly designed nickel base superalloy and the nominal composition (weight percent) is shown in Table 1. The master alloy was prepared by vacuum induction melting and atomized by argon gas to obtain the nearly spherical powder. The powder with a particle size ranging from 50 mm to 150 mm was collected and transferred into a stainless steel container for hot isostatic pressing (HIP). The container was vacuum degassed at 400 C for 24 h before sealing. Subsequently, HIP consolidation was performed at 1100 C/140 MPa for 4 h. After HIP treatment, the stainless steel container was removed and the superalloy billet was canned for hot extrusion (HEX). The billet was soaked under 1100 C for 2 h and then extruded with an area reduction of 10:1. Prior to compression, all the specimens are soaked at 1050 C for 1 h to reduce the dislocation density. Isothermal multi-pass hot compression tests were carried out to evaluate grain size evolution under the deformation conditions, as shown in Table 2. The compression parameters were designed to investigate the effects of strain amount, deformation temperature, dwell time and strain rates on flow behavior and grain size distribution. Cylindrical specimens with 8 mm in diameter and 12 mm in height were machined from the same circumferential positions of hot extruded billet to ensure a uniform microstructure using wire electrical discharge machining (WEDM). The graphite foils with a thickness of 0.5 mm were applied at the ends of the specimen to
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reduce the friction with dies. Then, the hot compression tests were performed using Gleeble 3180 test system (Dynamic System Inc., NY) and a thermocouple was welded on the specimen surface to monitor the temperature. All the specimens were heated to preset temperature with a heating rate of 5 C s1 and are compressed to a total engineering strain of 50% using two-pass processes. After compression, all the specimens were manually quenched immediately to ambient temperature. The post-mortem specimens were cut parallel to compression direction using a low-speed diamond saw to avoid overheating. The specimens were ground using carbide silicon papers and finally polished with the colloidal silica to guarantee the surface quality. For EBSD specimens, vibration polishing was applied for about 8 h after standard metallographic procedure. A field-emission SEM (Quanta 650, FEI), equipped with an EBSD detector and Channel 5 software, was used to examine the microstructure evolution and measure the average grain size, the statistical results were determined based on three different areas. The high angle grain boundary and low angle grain boundary are discriminated by the misorientation angle of 15 . The transmission electron microscope (TEM) observation was performed on a field-emission TEM (Tecnai G2 F20, FEI) operating at 200 KV. 3. Results and discussions 3.1. Pristine microstructure prior to hot compression Fig. 1 demonstrates the EBSD micrograph that the hot extruded alloy after annealing treatment consists of equiaxed grains without obvious crystallographic texture (inset). A close-up TEM
Fig. 2. Compression true stress-strain curves and deformed microstructure. (a) Through imposing different strain amount at the first-pass compression, the second-pass compression behaviors varies differently. While the first-pass true strain is relatively small, i.e., 20%, the retained dislocation density becomes higher, corresponding to a low DRX nucleation rate. Through EBSD measurement (b, c, d), the smaller average grain size can be acquired via the higher first-pass strain (e).
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observation (Fig. 1b) further reveals that the grain boundaries are geometrically planar and dislocation density is quite low. The EBSD measurements shows that the grain size is uniform and the average size is determined to be 10.67 mm (Fig. 1c). Through further analysis, up to 94.2% of grain sizes are below 20 mm and the grain size larger than 40 mm is not detected. The misorientation measurement indicates that most of grain boundaries belong to the high angle grain boundaries (Fig. 1d). Prior to compression, no obvious DRX nucleus are detected under TEM examination. 3.2. Flow behaviors and microstructure evolution in response to multi-pass compression 3.2.1. Effects of first-pass strain on final microstructure Fig. 2a shows the compression true stress-strain behaviors under a total true strain amount of 69.3% (corresponding to the engineering strain of 50%). The compression consists of two-pass strain accumulation with the first-pass true strain of εfir ¼ 0:2; 0:3; 0:4, respectively. The true stress-strain curves show that the yield strengths are in the vicinity of 90 MPa regardless of different first-pass strain amounts imposed. Once the preset strain is reached, the load is completely removed but the high temperature is maintained within the dwell time of 30 s. When the compression load is recovered with the same strain rate, the stress loss is dependent on the imposed first-pass strain amount. Let us examine the deformation process in detail, at strain of εfir ¼ 0:2, the compression stress loses 12 MPa comparing to the equivalent before unloading. As increasing the first-pass strain higher, the stress drop becomes smaller and even nearly fully recovered to previous level under εfir ¼ 0:4: Microscopic examinations
demonstrate (Fig. 2b, c, d) that grain micro architectures vary vastly in spite of the same total strain input on the basis of EBSD orientation image microscope map. The EBSD micrographs clearly reveal that the fraction of DRX nucleation is closely related to the imposed first-pass strain. Fig. 2b reveals that there is a small quantity of grown DRX grains with the size of a few micrometers and situated in the vicinity of the grain boundaries under εfir ¼ 0:2. As the firstpass strain is increased, the percentage of recrystallization grains further increases and corresponding grain size decreases (Fig. 2e). The DRX and SRX nucleation rates are even higher with increasing the first-pass strain, leading to the smallest average grain size. When the first-pass strain is relatively high, i.e. εfir ¼ 0:4, the percentage of DRX nucleation is higher and a finer average grain size is obtained compared to the counterparts at the lower first-pass strain. During dwell treatment at high temperature, the accumulated dislocations are more annihilated by means of static recrystallization SRX and results in a larger stress drop. 3.2.2. Effects of dwell time on final microstructure Following with the previous results, a question is raised to be answered: can we change dwell time to revise the microstructure through SRX process? Indeed, different dwell time is intentionally applied to investigate the effect of SRX on the microstructure development. The first-pass parameters are set up as: Tfir ¼ 1100 C, εfir ¼ 0:3, and a relatively higher strain rate (0.01 s1) is, with a purpose, adopted to induce the dislocation density increase during the first-pass deformation. Fig. 3a depicts true stress-strain curves under different dwell time. At the first-pass compression, three curves are reproduced till to 30% compressive true strain, yet, comparing to the equivalents upon 0.001 s1 (Fig. 2a), compression
Fig. 3. Dependence of grain size on the dwell time. (a) After the first-pass compression under strain rate of 0.01 s1, a relatively high DRX nucleation rate takes place and retained dislocations induced by the first-pass deformation can be annihilated by virtue of dwell treatment. This leads to different second-pass compression responses. The EBSD micrographs (b, c, d) show that, with increasing the dwell time, the degree of DRX accomplishment becomes higher and a finer grain size is accordingly achieved (e).
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hardening behavior occurs after yielding and points to the dislocation density increasing. After an unloading-loading cycle with different dwell time, such as 30s, 120s, 360s, the recovered stresses are obviously lower than the values before unloading. The results show that, with increasing the dwell time, the stress drop becomes larger. Furthermore, the EBSD micrographs (Fig. 3b, c, d) exhibit that the average grain size is the smallest upon 360s dwell treatment and directly points to the working role of SRX mechanism. Upon the first-pass strain rate of 0.01 s1, deformation-induced dislocations are rapidly multiplied because diffusion ability lags behind the imposed strain rate [11]. During the dwell process, high temperature (1100 C) and high dislocation density work together to promote the SRX occurring and a longer dwell time corresponds to a higher SRX nucleation rate and a finer grain size (Fig. 3e). Since the dislocations are consumed by the SRX mechanism, a higher stress drop occurs under the longer dwell period. 3.2.3. Effects of strain rates on the grain size evolutions Moreover, the strain rate is believed to have critical importance to influence the flow stress behavior and microstructure evolution. Under both the first-pass and the second-pass compressions, different strain rates ranging from 0.001 s1 to 0.1 s1 are implemented and see what happened for the flow stress responses and grain size evolutions. Let us start with the case of a constant strain rate (strain rate ¼ 0.001 s1) from the two-pass compressions, three representative strain rates, i.e., 0.001 s1, 0.1 s1, 0.1 s1, are applied in the first-pass compression. The typical true stress-strain curves are demonstrated in Fig. 4a, where stronger strain rate sensitivity exists for the yield strength and plastic behavior. For example, at the low strain rate such as 0.001 s1, the alloy exhibits
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rapid softening after yielding owing to the DRX occurring. With the first-pass strain rate is accelerated to 0.01 s1, the dislocations annihilation driven by DRX nucleation can not keep up with the imposed strain rate, leading to the work hardening after yielding point. Subsequently, stress starts to gradually decrease in virtue of high DRX nucleation rate up to 30% true strain. Furthermore, let us see what will happen while the strain rate is increased to 0.1 s1, the yield strength is much higher than those of low strain rates and immediately followed by a sharp stress drop. After that, stress undergoes a gradual increasing and decreasing process until to 30% true strain. Why does it manifest differently? Prior to yielding, high strain rate loading rapidly promotes the dislocation multiplication and reaches the critical condition for DRX nucleation. The high nucleation rate leads to the formation of DRX nucleus in a very short time scale, which consumes the accumulated dislocations and triggers the stress drop. Owing to the competition mechanism between dislocation multiplication induced by high strain rate and dislocation annihilation driven by DRX nucleation and DRV, a serrated flow stress behavior can be clearly observed for the whole deformation process. Similarly, obvious stress drop as many as 121 MPa is also found after dwell treatment. Since the second-pass strain rate is changed to 0.001 s1, the stress drop can be explained by two factors: 1) dislocations annihilation induced by SRX occurring; 2) the decreasing strain rate. In the course of the second-pass compression, the stress gradually decreases and indicates that DRX/DRV nucleation governs the deformation process. The EBSD examinations (Fig. 4b, c, d) reveal that the percentage of DRX nucleus/grains increases as the first-pass strain rate enhanced, and finally a very uniform and fine grain size distribution can be achieved by virtue of DRX nucleation at ε_ fir ¼ 0.1 s1.
Fig. 4. Effect of strain rate on the grain size evolution. (a) Under three typical strain rates at the first-pass compression, the compression stress-strain curves behave differently, demonstrating strong rate-dependent yielding strength and plastic behavior. Upon high strain compression, the dislocation density increases rapidly and is accommodated by a high DRX nucleation rate. Clearly, increasing the first-pass strain rate can refine the grain size via a high DRX nucleation rate (b, c, d), an average grain size of 3.36 mm is achieved under 0.1 s1 strain rate.
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We then change the second-pass strain rate from 0.001 s1 to 0.01 s1 and see the difference in flow stress behavior and microstructure. The manifestation of the first-pass compression stressstrain behaviors (Fig. 5a) is reproduced with Fig. 4a. The different compression responses begin from the dwell process. While the first-pass strain rate is 0.001 s1, dislocation density remains to be a low level after the first-pass compression and further decreases through SRX process. A close-up view indicates that the recovered stress slightly exceeds the value before unloading, which can be attributed to the strain rate-induced stiffening from 0.001 s1 to 0.01 s1. After yielding, a remarkable stress drop is immediately followed due to the DRX nucleation. Subsequently, the stress slightly increases until to the 69.3% true strain, which can be related to the elevation of dislocation density by high strain rate. The EBSD micrograph (Fig. 5b) shows that there is a small fraction of DRX nucleus/grains located on the pristine grain boundaries. If the strain rates remain to be 0.01 s1 during two-pass compressions, dislocation density rapidly increases before reaching the maximum compression stress and then decreases due to the DRX occurring in the first-pass deformation. Through the dwell treatment, the dislocations are further consumed for SRX process, which can be reflected by the stress drop after the unloading-loading cycle. Once loaded in the second-pass, the compression stress-strain curve demonstrates the similar behavior with that in the first-step process. The post-mortem microscopic examination (Fig. 5c) reveals that the average grain size is much smaller than the equivalents under 0.001 s1 strain rate. A higher strain rate in the first-step can result in a higher density of DRX nucleation rate and develop to the finer grain size. It is highly expected to obtain the smaller grain size
if the first-step strain rate is accelerated to 0.1 s1. Similarly, the first-pass compression behavior (Fig. 5a) experiences a series of yielding, sharp stress softening and stress climb under strain rate of 0.1 s1. After dwell treatment, the stress drop reaches 101 MPa as a result of SRX occurring and strain rate decrease. In the second-pass compression, the flow stress suffers from an abrupt decrease at the initial stage and stress-strain curve remains to be zigzag during whole second-pass process, which is believed to be controlled by the competition mechanism between DRX nucleation and dislocation multiplication induced by strain rate. A very uniform and fine grain size distribution is obtained through EBSD measurement (Fig. 5d) and clearly demonstrates that increasing the first-pass strain rate can acquire a finer grain size distribution (Fig. 5e). Accordingly, the effect of the second-pass strain rate of 0.1 s1 on the compression performance and microstructure evolution is further investigated through changing the first-step strain rate. Similar with Figs. 4a and 5a, the compression response (Fig. 6a) within the first-step and dwell treatment are fully repeated. The different scenario comes from the second-step compression. The recovered stresses are substantially enhanced in comparison with the counterparts under 0.001 s1 and 0.01 s1strain rates. After reloaded, the stresses decrease after peak force and three curves during the second-step process are serrated, indicating high DRX nucleation rates. By means of microscopic observations, the grain sizes for three different deformation conditions are very uniform (Fig. 6b, c, d), wherein the smallest grain size is attained from the high strain rate in two-pass (Fig. 6e) deformation. To shed more light on the deformation mechanisms, a detailed observation is employed to demonstrate the deformed
Fig. 5. Effect of strain rate on the grain size evolution. (a) The first-pass compression behavior is highly reproduced from the counterparts of Fig. 4a. The difference comes from the second-pass response under strain rate of 0.01 s1. At strain of 0.001 s1, owing to the dislocation consumption for DRX nucleation in the first-pass and SRX during dwell treatment, the dislocation density is quite low and gradually increases with the second-pass deformation proceeding, and corresponding to the slight stress increment. On the contrary, the high strain rate, i.e., 0.1 s1, can initiate high DRX nucleation rate and further refine the grain sizein combination with the second-pass strain rate 0.01 s1 compression. The EBSD measurements (b, c, d) show that a more uniform and finer grain size is produced by a higher strain rate used the first-pass compression (e).
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microstructures in details before and after the dwell treatment. The first-pass strain rate of 0.1 s1 is selected and the SEM and TEM examinations are shown in Fig. 7a and b. After the first-pass compression at a relatively high strain rate, a significant fraction of DRX nucleus is formed in the vicinity of pristine grain boundaries. The TEM measurement indicates that the grain sizes are in the range of tens to hundreds of nanometers (7b). After dwell treatment, the accumulated dislocation density close to grain boundaries is still high and readily triggers the SDX occurring. Fig. 7c shows that a much higher percentage of recrystallization grains emerge at the expense of the disappearance of pristine grains. A closer examination (Fig. 7d) reveals that recrystallization nucleation is very high and the nucleus lies nearby the pristine grains, which verifies the role of SRX deformation mechanism. 3.2.4. Effects of deformation temperature on the flow stress and grain size development To investigate the role of deformation temperature in the flow behavior and microstructure, three typical temperatures, i.e., 1050 C, 1100 C, 1150 C, are imposed during dwell treatment and the second-pass deformation, respectively. Fig. 8a shows the true stress-strain curves that the first-pass compression behavior is fully reproduced from Fig. 3a till to 30% compressive true strain. Subsequently, a relatively slow strain rate (0.001 s1) is executed in order to provide more time for dynamic recovery and three different temperatures are immediately applied after unloading. Upon different temperatures, the stress drops manifest vastly and show that the higher stress loss is consistent with the higher temperature. The results further indicate that the dwell process serves a critical role in controlling the microstructure development.
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At 1050 C (Fig. 8b), the stress is recovered to the previous level before unloading, which can be explained by three aspects: 1) stress drop induced SRX process; 2) strain rate is changed from 0.01 s1 to 0.001 s1; 3) temperature change from 1100 C to 1050 C. The stress loses 34 MPa if the temperature and strain rate in two-pass remain to be constant (Please see Fig. 4a). When we consider the effects of both SRX and strain rate (Fig. 5a), the stress drop reaches 55 MPa. In the first-pass compression, dislocation density increases to the critical value and induces DRX nucleation. Upon the dwell treatment, dislocations are further consumed in a manner of SRX mechanism. Once loaded, dislocation density increases again prior to the second-pass peak force. After that, the stress gradually decreases owing to the DRX nucleation but the decreasing rate is relatively rapid than those under 1100 C and 1150 C. At 1100 C (Fig. 8c), the first-pass compression behavior is same with that at 1100 C. After an unloading-loading cycle, the stress is recovered to 62 MPa in contrast to 115 MPa prior to unloading. The stress drop is ascribed to two aspects: 1) dislocations annihilation by SRX; 2) strain rate decrease in the second pass. Likewise, a short period of strain hardening occurs and followed by deformation softening in the presence of DRX nucleation. The scenario is different when the temperature is 1150 C, the stress dives to 20 MPa and remains to be nearly constant for the whole second-pass process. This clearly indicates that the retained dislocations from the first-pass deformation are depleted during 30 s dwell treatment, and reaching a balance between DRX and DRV. The successive nucleation results in a smaller grain size distribution.
Fig. 6. Effect of strain rate on the grain size evolution. (a) The first-pass compression curves are replicated with the equivalents in Figs. 4a and 5a. Under high strain rate of 0.1 s1 in the first-pass compression, the true stress-strain curves are serrated and indicate the competition between DRX nucleation and dislocation multiplication. The EBSD examinations indicate that the grain sizes from three loading modes are very uniform, yet, the high strain rate during two-pass compressions is consistent with a finer grain size (b, c, d, e).
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Fig. 7. Deformed microstructure before and after dwell treatment. (a) Under the first-pass compression with a strain rate 0.1 s1, the high DRX nucleation rate is triggered by high strain rate, which produces a high density of DRX nucleus and grown grains in the vicinity of the pristine grains. (b) Because of the dwell treatment at 1100 C, the SRX occurs and induces the formation of nanometer-sized nucleus, meanwhile, in accompany with the grain growth of DRX nucleus. Prior to the dwell treatment, the DRX grain size is in the range of tens of nanometers nearby the grain boundaries (c). After that, the DRX grains grow and the density of nano-sized nucleus increases (d).
3.3. Discussion Based on the experimental observation, a sequential microstructure evolution process is schematically depicted in Fig. 9. Prior to hot compression, the alloy undergoes high temperature annealing and the dislocation density decreases to a low level and most of grain boundaries belong to high angle grain boundaries. As the hot deformation proceeding, compression strain will increase and the dislocation density is several orders of improvement. Once the compression strain reaches the critical strain, the piled-up dislocations nearby the grain boundaries can be consumed to facilitate the DRX nucleation. The further compression will induce DRX nucleus growth and is accompanied with a new generation of DRX nucleus occurring prior to the end of the first-pass deformation. During dwell treatment, the size of DRX grain grows bigger and SRX grains emerges under the assistance of high temperature and retained strain, where the dislocations are further consumed. While entering the second-pass compression, the dislocations density increases and new DRX nucleation occurs, this triggers the size refinement of the pristine grains. After a series of DRX and SRX processes, the average grain size is refined and the two-pass deformation substantially lowers the cracking tendency. In contrast, the scenario is different for the flow behavior and microstructure upon the monotonic-pass tests. It can be clearly observed that strong strain rate sensitivity of flow stresses can be found upon three typical strain rates (Fig. 10a). At the strain rate of 0.1 s1, a sharp stress drop emerges immediately after yielding, indicating a high DRX nucleation rate. Owing to the high strain rate, dislocations are rapidly multiplied and there exists a competition mechanism between DRX nucleation and DRV, which is reflected
by the serrated flow stress (zoom-in view). The corresponding microscopic examination shows that the pristine grains are elongated normal to the compression direction and surrounded by a high density of tiny DRX grains (Fig. 10b). While the strain rate is changed to be 0.01 s1, an obvious strain hardening stage occurs before reaching the peak stress, in accompany with the substantial increase of dislocation density. Once the critical value is touched, both DRX and DRV mechanisms take over the deformation process and the flow stress gradually decreases by the end of the test. Owing to the relatively low strain rate, the time available for DRV is enough and simultaneously the DRX nucleation rate is low, which leads to the grain growth for both the pristine grains and DRX grains (Fig. 10c). Furthermore, let us see what happened if the strain rate is decreased to be 0.001 s1, stress softening rapidly emerges after yielding due to the DRX and DRV occurring. As the plastic deformation proceeding, the competition between DRX and DRV reaches a balance, wherein the grain size is continuously refined by means of DRX nucleation. Since the strain rate is slow, the growth of the pristine grains is striking and the grain size presents the bimodal distribution (Fig. 10d). It is concluded that inhomogeneous grain size or substantial grain growth is obtained upon monotonicpass results, which is undesirable in the engineering practice (Fig. 10e). The classical Avrami equation describes the effect of dwell time and strain rate on the SRX kinetics:
t n Xsta ¼ 1exp ln2 t0:5
(1)
Wherein, Xsta is the static recrystallization fraction, t is the dwell
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Fig. 8. Compression behaviors and microstructure development under different deformation temperatures. (a) After the first-pass compression under 1100 C/0.001 s1 and dwell treatment, DRX nucleation and grain growth consume the dislocations to a low level. At 1050 C, the temperature decrease results in a higher flow stress and increases the resistance for diffusion. Hence, the retained dislocation density is correspondingly high and triggers the more DRX nucleation. In contrast, while the second-pass deformation temperature is increased to 1150 C, the high temperature facilitates the diffusion process and the flow stress is quite flat, exhibiting no obvious DRX occurring. As demonstrated by EBSD measurements (b, c, d), the average grain size decreases with increasing the second-pass deformation temperature (e).
Fig. 9. Schematic showing of microstructure evolution during multi-pass compression. (a) After annealing treatment upon HEX alloy, the dislocation density is very low. (b) As plastic deformation proceeding, dislocations multiply and pile up near the grain boundaries. (c) Once reaching the critical strain, DRX nucleation occurs and nanometer-sized nucleus form in accompany with grain growth. (d) At the end of the first-pass compression, the pristine grains are surrounded by a continuously-generated layer of DRX grains. (e) During dwell treatment, SRX emerges and triggers new SRX nucleus, meanwhile, the DRX grains further grow at the expense of the refinement of the pristine grains. (f) While entering the second-pass compression, the DRX nucleation rate may become even higher because of the increasing fraction of grain boundaries. (g) After a series of DRX, SRX, DRV, SRV processes, a more uniform and finer grain size distribution can be obtained.
time and t0.5 is the time corresponding to the half of the recrystallization volume. t0.5 depends on the variables involved in the hot deformation and can be expressed as [37]:
t0:5 ¼ Aεp ε_ q Ds exp
Q RT
(2)
A, p, q and s are the materials constants, ε is the strain, ε_ the
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Fig. 10. Monotonic-pass compression behavior and microstructure observation. (a) At the relatively low strain rate, i.e., 0.001 s1, the flow stress gradually decreases after yielding and reaches steady till to 69.3% true strain. This deformation process triggers the formation of necklace-like grain microarchitectures, where the grown pristine grains are surrounded by a high percentage DRX grains/nucleus (b). Under the strain rate of 0.01 s1, an obvious working hardening process occurs after yielding, and flow stress slowly decreases after peak stress. Through EBSD examination, the pristine and DRX grains have grown up and a small quantity of recrystallization nucleus are situated near the grain boundaries (c). A higher strain rate, such as 0.1 s1, can generate a higher yielding stress followed by a sudden stress softening. The flow stress progressively declines up to 69.3% true strain. The DRX nucleation rate is very high and the pristine grains becomes elongated normal to the compression axis (d). The EBSD measurement shows that the finest average grain size is obtained by the highest strain rate, and the coarsest grain size is related to the 0.01 s1. Interestingly, the average grain size inversely increases with decreasing the strain rate to 0.001 s1.
strain rate, D the average grain size, Q the activation energy, T the absolute temperature and R is the gas constant. Based on the above relationship, it can be on quantity obtained that, increasing dwell time t, the recrystallization percentage is higher. An increasing strain rate can obtain more recrystallization fraction.
4. Conclusions The flow behaviors and grain size of a nickel based superalloy under various combinations of deformation conditions using multipass compression are investigated. The following conclusions can be drawn: 1) Comparing to the monotonic-pass hot compression, the multipass compression provides a versatile approach to control the flow stress and grain size, the cracking tendency is substantially reduced. 2) The effect of the first-pass strain amount, dwell time, the second-pass deformation temperature, strain rates in two-pass processes on the compression behavior are investigated, the flow behavior and grain size evolution are evaluated and corresponding deformation mechanisms are discussed in details. 3) The higher first-pass strain generates the finer grain size owing to the SRX occurring. The post-mortem examination of grain size after two-pass compression is closely associated with the second-pass processing conditions. A finer and more uniform grain size can be attained by the higher strain rate, longer dwell time and higher deformation temperature.
Acknowledgments Z. H. and L. J. appreciate the financial support from The National Key Research and Development Program of China (2016YFB0700300). G. H. is grateful for the support from the Fundamental Research Funds for the Central Universities of Central South University (2015zzts031) and the outstanding graduate project of Advanced Non-ferrous Metal Structural Materials and Manufacturing Collaborative Innovation Center. F. L. and L. H. are thankful for the funding sponsorship of the Natural Science Foundation of China (51401242, 51301209). We would like to acknowledge the help from Hong JIANG, Liming TAN and Longfei ZHANG for the helpful technical discussion. References [1] G. He, F. Liu, L. Huang, L. Jiang, Hot deformation behaviors of a new hot isostatically pressed nickel based powder metallurgy superalloy, J. Mater. Res. 31 (2016) 3567e3579. [2] R.C. Reed, The Superalloys: Fundamentals and Applications, Cambridge University Press, 2008, pp. 1e28. [3] M. Donachie, S. Donachie, Superalloysea Technical Guide, ASM International, Materials Park, OH, 2002. [4] M. Detrois, R.C. Helmink, S. Tin, Microstructural stability and hot deformation of geg0 ed Ni-Base superalloys, Metall. Mater. Trans. A 45 (2014) 5332e5343. [5] A.K. Koul, J.P.A. Immarigeon, Modelling of plastic flow in coarse grained nickel-base superalloy compacts under isothermal forging conditions, Acta Metall. 35 (1987) 1791e1805. [6] T. Osada, Y. Gu, N. Nagashima, Y. Yuan, T. Yokokawa, H. Harada, Optimum microstructure combination for maximizing tensile strength in a polycrystalline superalloy with a two-phase structure, Acta Mater. 61 (2013) 1820e1829. [7] S. Babu, M. Miller, J. Vitek, S. David, Characterization of the microstructure evolution in a nickel base superalloy during continuous cooling conditions, Acta Mater. 49 (2001) 4149e4160.
G. He et al. / Journal of Alloys and Compounds 701 (2017) 909e919 [8] T. Osada, N. Nagashima, Y. Gu, Y. Yuan, T. Yokokawa, H. Harada, Factors contributing to the strength of a polycrystalline nickelecobalt base superalloy, Scr. Mater. 64 (2011) 892e895. [9] J.W. Brooks, Forging of superalloys, Mater. Des. 21 (2000) 297e303. [10] N.A. Wilkinson, Technological considerations in the forging of superalloy rotor parts, Met. Technol. 4 (1977) 346e359. [11] M.G. Benz, E.L. Raymond, R.D. Kissinger, E.S. Huron, C.P. Blankenship Jr., M.F. Henry, Grain size control in nickel base superalloys: U.S. Patent No. 5,759,305. 1998. [12] F. Montheillet, O. Lurdos, G. Damamme, A grain scale approach for modeling steady-state discontinuous dynamic recrystallization, Acta Mater. 57 (2009) 1602e1612. [13] S.L. Semiatin, F. Zhang, J.S. Tiley, D.U. Furrer, A comparison of the precipitation behavior in PM g-g0 nickel-base superalloys, Mater. High. Temp. 33 (2016) 301e309. ramo, A. Salas Zamarripa, M. de la Garza, M.P. Guerrero-Mata, [14] L.A. Reyes, P. Pa Influence of processing parameters on grain size evolution of a forged superalloy, J. Mater. Eng. Perform. 25 (2016) 179e187. [15] V.M. Miller, A.E. Johnson, C.J. Torbet, T.M. Pollock, Recrystallization and the development of abnormally large grains after small strain deformation in a polycrystalline nickel-based superalloy, Metall. Mater. Trans. A 47 (2016) 1566e1574. [16] S.S.S. Kumar, T. Raghu, P.P. Bhattacharjee, G.A. Rao, U. Borah, Strain rate dependent microstructural evolution during hot deformation of a hot isostatically processed nickel base superalloy, J. Alloys Compd. 681 (2016) 28e42. [17] R.C. Buckingham, C. Argyrakis, M.C. Hardy, S. Birosca, The effect of strain distribution on microstructural developments during forging in a newly developed nickel base superalloy, Mater. Sci. Eng. A 654 (2016) 317e328. [18] S.L. Semiatin, P.N. Fagin, R.L. Goetz, D.U. Furrer, R.E. Dutton, Characterization of plastic flow pertinent to the evolution of bulk residual stress in powdermetallurgy, nickel-base superalloys, Metall. Mater. Trans. A 46 (2015) 3943e3959. [19] G. He, F. Liu, J. Si, C. Yang, L. Jiang, Characterization of hot compression behavior of a new HIPed nickel-based P/M superalloy using processing maps, Mater. Des. 87 (2015) 256e265. [20] S.L. Semiatin, J.M. Shank, A.R. Shiveley, W.M. Saurber, E.F. Gaussa, A.L. Pilchak, The effect of forging variables on the supersolvus heat-treatment response of powder-metallurgy nickel-base superalloys, Metall. Mater. Trans. A 45 (2014) 6231e6251. [21] D. Collins, B. Conduit, H. Stone, M. Hardy, G. Conduit, R. Mitchell, Grain growth behaviour during near-g0 solvus thermal exposures in a polycrystalline nickelbase superalloy, Acta Mater. 61 (2013) 3378e3391. [22] K.P. Rao, Y.V.R.K. Prasad, K. Suresh, Materials modeling and simulation of isothermal forging of rolled AZ31B magnesium alloy: anisotropy of flow,
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Mater. Des. 32 (2011) 2545e2553. [23] Y. Prasad, H. Gegel, S. Doraivelu, J. Malas, J. Morgan, K. Lark, D. Barker, Modeling of dynamic material behavior in hot deformation: forging of Ti6242, Metall. Trans. A 15 (1984) 1883e1892. [24] M. Somani, E.B. Rao, N. Birla, M. Bhatia, V. Singh, Y. Prasad, Processing map for controlling microstructure in hot working of hot isostatically pressed powder metallurgy NIMONIC AP-1 superalloy, Metall. Trans. A 23 (1992) 2849e2857. [25] M. Somani, K. Muraleedharan, Y. Prasad, V. Singh, Mechanical processing and microstructural control in hot working of hot isostatically pressed P/M IN-100 superalloy, Mater. Sci. Eng. A 245 (1998) 88e99. [26] O. Underwood, J. Madison, R.M. Martens, G.B. Thompson, S. Welsh, J. Evans, An examination of abnormal grain growth in low strain Nickel-200, metallography, Microstruct. Analysis (2016) 1e11. [27] J. Chen, P. Zhang, J. Dong, M. Zhang, Z. Yao, Relevance of primary g0 dissolution and abnormal grain growth in UDIMET 720LI, Mater. Trans. 56 (2015) 1968e1976. , J.-M. Franchet, J. Laigo, N. Bozzolo, Selective [28] A. Agnoli, M. Bernacki, R. Loge growth of low stored energy grains during d sub-solvus annealing in the inconel 718 nickel-based superalloy, Metall. Mater. Trans. A 46 (2015) 4405e4421. , Mater. Sci. Forum, Trans. [29] N. Bozzolo, A. Agnoli, N. Souai, M. Bernacki, R.E. Loge Tech. Publ. (2013) 321e324. [30] S.L. Semiatin, K.E. McClary, A.D. Rollett, C.G. Roberts, E.J. Payton, F. Zhang, T.P. Gabb, Plastic flow and microstructure evolution during thermomechanical processing of a PM nickel-base superalloy, Metall. Mater. Trans. A 44 (2013) 2778e2798. [31] B. Fang, Z. Ji, M. Liu, G. Tian, C. Jia, T. Zeng, B. Hu, C. Wang, Study on constitutive relationships and processing maps for FGH96 alloy during two-pass hot deformation, Mater. Sci. Eng. A 590 (2014) 255e261. [32] G. Kugler, R. Turk, Modeling the dynamic recrystallization under multi-stage hot deformation, Acta Mater. 52 (2004) 4659e4668. [33] D.S. Weaver, S.L. Semiatin, Recrystallization and grain-growth behavior of a nickel-base superalloy during multi-hit deformation, Scr. Mater. 57 (2007) 1044e1047. [34] S. Gardner, W. Li, M. Coleman, R. Johnston, The effects of thermomechanical history on the microstructure of a nickel-base superalloy during forging, Mater. Sci. Eng. A 668 (2016) 263e270. [35] N.K. Park, I.S. Kim, Y.S. Na, J.T. Yeom, Hot forging of a nickel-base superalloy, J. Mater. Process. Technol. 111 (2001) 98e102. [36] B. Fang, Z. Ji, M. Liu, G. Tian, C. Jia, T. Zeng, B. Hu, Y. Chang, Critical strain and models of dynamic recrystallization for FGH96 superalloy during two-pass hot deformation, Mater. Sci. Eng. A 593 (2014) 8e15. Martin, The Avrami kinetics of dynamic [37] J.J. Jonas, X. Quelennec, L. Jiang, E. recrystallization, Acta Mater. 57 (2009) 2748e2756.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Controlling grain size via dynamic recrystallization in an advanced polycrystalline nickel base superalloy Guoai He a, b, c, Feng Liu a, b, c, Lan Huang a, b, c, Zaiwang Huang a, b, c, *, Liang Jiang a, b, c a
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China Powder Metallurgy Research Institute, Central South University, Changsha 410083, China c High Temperature Materials Research Institute, Central South University, Changsha 410083, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 December 2016 Received in revised form 13 January 2017 Accepted 18 January 2017 Available online 19 January 2017
Controlling grain size in polycrystalline nickel base superalloy is of paramount importance in optimizing hot-working process and achieving the desirable mechanical properties. Typically, a uniform and fine grain size is required throughout forging process to realize the superplastic deformation. Unfortunately, the forging processing window is very narrow and needs to be synergistically controlled by deformation temperature, strain rate, as well as strain amount, and the failure would lead to the non-uniform grain size, abnormal grain growth and even cracking. During superplastic deformation, dynamic recrystallization (DRX) is the governing mechanism to be adopted to maintain the required grain size. Previous literature have extensively documented the separated effect of deformation parameters using monotonic-pass compression test. Herein, we perform the multi-pass compression experiments to investigate the flow stress behavior and grain size evolution. By conducting multi-pass tests over a range of deformation parameters, it was demonstrated that the flow stress and grain size response differed from that in single-pass tests. The multi-pass compression tests unambiguously uncover the roles of DRX and static recrystallization (SRX) in regulating grain size and affecting flow stress. The experimental results further show, comparing to the monotonic-pass compression, the multi-pass approach can provide versatile routes to attain the desirable grain size distribution by means of utilizing DRX and SRX principles. © 2017 Elsevier B.V. All rights reserved.
Keywords: Nickel base superalloy Hot compression Flow stress Dynamic recrystallization Grain size
1. Introduction Powder metallurgy nickel base superalloys are the most widely used high temperature materials in turbine disc of aero-engine owing to a superb balance of high strength, corrosion resistance as well as fatigue resistance under elevated temperature [1e4]. Previous results have extensively documented that these outstanding mechanical properties are primarily governed by both the grain size [5,6] and gamma prime (g0 ) precipitate distribution [7,8], which are regulated by a sequential combination of powder atomization, hot isostatic pressing and hot extrusion consolidation. Isothermal forging [9,10] is subsequently applied to attain the superplastic deformation and avoid abnormal grain growth. This process is generally performed at the sub-solvus temperature and
* Corresponding author. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China. E-mail address: [email protected] (Z. Huang). http://dx.doi.org/10.1016/j.jallcom.2017.01.179 0925-8388/© 2017 Elsevier B.V. All rights reserved.
low strain rate [11], wherein a uniform and fine grain size can be produced and maintained by virtue of successive DRX occurring [12]. To date, most of results [13e21] have focused on investigating the independent effect of deformation temperature, strain rate and strain amount on the flow stress behavior and microstructure evolution via the monotonic-pass compression experiment. The corresponding processing map can be established based on the dynamic materials model to label the domains of different deformation mechanisms, such as DRX, adiabatic shear band, cracking, dynamic recovery [22e25]. Unfortunately, in the monotonic-pass compression test the deformed microstructure is inclined to form the inhomogeneous grain size [26e29] and in extreme cases induce cracking [30,31], in particular under the low temperatures and high stain rates. Recently, multi-pass hot compression experiment [32,33] provides a new perspective to study the DRX/SRX behavior and the effect of hot working parameters on the grain size evolution. The documented works have shown that the flow stress and grain size is closely related to the dwell time [34], strain amount [35] and strain rate [36]. In this article, we employ the multi-pass
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development, including changing the strain amounts in the firstpass, changing the strain rate in the first-pass and the secondpass, imposing different dwell time and deformation temperature. The results show that the multi-pass compression is more versatile to control the grain size than the monotonic-pass experiments. The different deformation conditions imposed in the firstpass and the second-pass can generate the vastly variable grain size distribution and strain rate is the most prominent one. The research
Table 1 The nominal composition of a polycrystalline nickel base superalloy (wt. %). Element
Co
Cr
Ti
W
Mo
Al
Nb
Hf
C
B
Zr
Ni
Wt(%)
26
13
3.7
4
4
3.2
0.95
0.2
0.05
0.025
0.05
Bal.
experiments to investigate the influence of deformation parameters in the first-pass on DRX and SRX behaviors and the grain size Table 2 Deformation parameters under different multi-pass hot compression tests. ID
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
First pass
Dwell time/s
Temperature/ C
Strain rate/s1
True Strain
1100
0.001
0.2 0.3 0.4 0.3
0.01 0.1 0.01
0.001 0.01 0.1
30
120 360 30
Second pass Temperature/ C
Strain rate/s1
True Strain
1100
0.001
0.493 0.393 0.293 0.393
1050 1150 1100
0.01 0.1 0.01 0.1 0.01 0.1
Fig. 1. The pristine microstructure observation shows the hot extruded superalloy (after annealing treatment) dominantly consists of equiaxed grains and no distinct crystallographic texture is detected (a). A closer-up TEM observation (b) further demonstrates that grain boundaries are geometrically planar and dislocation density is quite low. (c) The average grain size of as-extruded alloy is measured to be 10.67 mm by means of EBSD technique. All the grain size lies in the range below 40 mm. (d) A majority of grain boundaries are recognized as the high angle grain boundaries.
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results shed light on understanding the roles the DRX/SRX mechanism in controlling flow stress and microstructure evolution in the course of forging nickel base superalloy. 2. Experimental The material in this work was a newly designed nickel base superalloy and the nominal composition (weight percent) is shown in Table 1. The master alloy was prepared by vacuum induction melting and atomized by argon gas to obtain the nearly spherical powder. The powder with a particle size ranging from 50 mm to 150 mm was collected and transferred into a stainless steel container for hot isostatic pressing (HIP). The container was vacuum degassed at 400 C for 24 h before sealing. Subsequently, HIP consolidation was performed at 1100 C/140 MPa for 4 h. After HIP treatment, the stainless steel container was removed and the superalloy billet was canned for hot extrusion (HEX). The billet was soaked under 1100 C for 2 h and then extruded with an area reduction of 10:1. Prior to compression, all the specimens are soaked at 1050 C for 1 h to reduce the dislocation density. Isothermal multi-pass hot compression tests were carried out to evaluate grain size evolution under the deformation conditions, as shown in Table 2. The compression parameters were designed to investigate the effects of strain amount, deformation temperature, dwell time and strain rates on flow behavior and grain size distribution. Cylindrical specimens with 8 mm in diameter and 12 mm in height were machined from the same circumferential positions of hot extruded billet to ensure a uniform microstructure using wire electrical discharge machining (WEDM). The graphite foils with a thickness of 0.5 mm were applied at the ends of the specimen to
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reduce the friction with dies. Then, the hot compression tests were performed using Gleeble 3180 test system (Dynamic System Inc., NY) and a thermocouple was welded on the specimen surface to monitor the temperature. All the specimens were heated to preset temperature with a heating rate of 5 C s1 and are compressed to a total engineering strain of 50% using two-pass processes. After compression, all the specimens were manually quenched immediately to ambient temperature. The post-mortem specimens were cut parallel to compression direction using a low-speed diamond saw to avoid overheating. The specimens were ground using carbide silicon papers and finally polished with the colloidal silica to guarantee the surface quality. For EBSD specimens, vibration polishing was applied for about 8 h after standard metallographic procedure. A field-emission SEM (Quanta 650, FEI), equipped with an EBSD detector and Channel 5 software, was used to examine the microstructure evolution and measure the average grain size, the statistical results were determined based on three different areas. The high angle grain boundary and low angle grain boundary are discriminated by the misorientation angle of 15 . The transmission electron microscope (TEM) observation was performed on a field-emission TEM (Tecnai G2 F20, FEI) operating at 200 KV. 3. Results and discussions 3.1. Pristine microstructure prior to hot compression Fig. 1 demonstrates the EBSD micrograph that the hot extruded alloy after annealing treatment consists of equiaxed grains without obvious crystallographic texture (inset). A close-up TEM
Fig. 2. Compression true stress-strain curves and deformed microstructure. (a) Through imposing different strain amount at the first-pass compression, the second-pass compression behaviors varies differently. While the first-pass true strain is relatively small, i.e., 20%, the retained dislocation density becomes higher, corresponding to a low DRX nucleation rate. Through EBSD measurement (b, c, d), the smaller average grain size can be acquired via the higher first-pass strain (e).
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observation (Fig. 1b) further reveals that the grain boundaries are geometrically planar and dislocation density is quite low. The EBSD measurements shows that the grain size is uniform and the average size is determined to be 10.67 mm (Fig. 1c). Through further analysis, up to 94.2% of grain sizes are below 20 mm and the grain size larger than 40 mm is not detected. The misorientation measurement indicates that most of grain boundaries belong to the high angle grain boundaries (Fig. 1d). Prior to compression, no obvious DRX nucleus are detected under TEM examination. 3.2. Flow behaviors and microstructure evolution in response to multi-pass compression 3.2.1. Effects of first-pass strain on final microstructure Fig. 2a shows the compression true stress-strain behaviors under a total true strain amount of 69.3% (corresponding to the engineering strain of 50%). The compression consists of two-pass strain accumulation with the first-pass true strain of εfir ¼ 0:2; 0:3; 0:4, respectively. The true stress-strain curves show that the yield strengths are in the vicinity of 90 MPa regardless of different first-pass strain amounts imposed. Once the preset strain is reached, the load is completely removed but the high temperature is maintained within the dwell time of 30 s. When the compression load is recovered with the same strain rate, the stress loss is dependent on the imposed first-pass strain amount. Let us examine the deformation process in detail, at strain of εfir ¼ 0:2, the compression stress loses 12 MPa comparing to the equivalent before unloading. As increasing the first-pass strain higher, the stress drop becomes smaller and even nearly fully recovered to previous level under εfir ¼ 0:4: Microscopic examinations
demonstrate (Fig. 2b, c, d) that grain micro architectures vary vastly in spite of the same total strain input on the basis of EBSD orientation image microscope map. The EBSD micrographs clearly reveal that the fraction of DRX nucleation is closely related to the imposed first-pass strain. Fig. 2b reveals that there is a small quantity of grown DRX grains with the size of a few micrometers and situated in the vicinity of the grain boundaries under εfir ¼ 0:2. As the firstpass strain is increased, the percentage of recrystallization grains further increases and corresponding grain size decreases (Fig. 2e). The DRX and SRX nucleation rates are even higher with increasing the first-pass strain, leading to the smallest average grain size. When the first-pass strain is relatively high, i.e. εfir ¼ 0:4, the percentage of DRX nucleation is higher and a finer average grain size is obtained compared to the counterparts at the lower first-pass strain. During dwell treatment at high temperature, the accumulated dislocations are more annihilated by means of static recrystallization SRX and results in a larger stress drop. 3.2.2. Effects of dwell time on final microstructure Following with the previous results, a question is raised to be answered: can we change dwell time to revise the microstructure through SRX process? Indeed, different dwell time is intentionally applied to investigate the effect of SRX on the microstructure development. The first-pass parameters are set up as: Tfir ¼ 1100 C, εfir ¼ 0:3, and a relatively higher strain rate (0.01 s1) is, with a purpose, adopted to induce the dislocation density increase during the first-pass deformation. Fig. 3a depicts true stress-strain curves under different dwell time. At the first-pass compression, three curves are reproduced till to 30% compressive true strain, yet, comparing to the equivalents upon 0.001 s1 (Fig. 2a), compression
Fig. 3. Dependence of grain size on the dwell time. (a) After the first-pass compression under strain rate of 0.01 s1, a relatively high DRX nucleation rate takes place and retained dislocations induced by the first-pass deformation can be annihilated by virtue of dwell treatment. This leads to different second-pass compression responses. The EBSD micrographs (b, c, d) show that, with increasing the dwell time, the degree of DRX accomplishment becomes higher and a finer grain size is accordingly achieved (e).
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hardening behavior occurs after yielding and points to the dislocation density increasing. After an unloading-loading cycle with different dwell time, such as 30s, 120s, 360s, the recovered stresses are obviously lower than the values before unloading. The results show that, with increasing the dwell time, the stress drop becomes larger. Furthermore, the EBSD micrographs (Fig. 3b, c, d) exhibit that the average grain size is the smallest upon 360s dwell treatment and directly points to the working role of SRX mechanism. Upon the first-pass strain rate of 0.01 s1, deformation-induced dislocations are rapidly multiplied because diffusion ability lags behind the imposed strain rate [11]. During the dwell process, high temperature (1100 C) and high dislocation density work together to promote the SRX occurring and a longer dwell time corresponds to a higher SRX nucleation rate and a finer grain size (Fig. 3e). Since the dislocations are consumed by the SRX mechanism, a higher stress drop occurs under the longer dwell period. 3.2.3. Effects of strain rates on the grain size evolutions Moreover, the strain rate is believed to have critical importance to influence the flow stress behavior and microstructure evolution. Under both the first-pass and the second-pass compressions, different strain rates ranging from 0.001 s1 to 0.1 s1 are implemented and see what happened for the flow stress responses and grain size evolutions. Let us start with the case of a constant strain rate (strain rate ¼ 0.001 s1) from the two-pass compressions, three representative strain rates, i.e., 0.001 s1, 0.1 s1, 0.1 s1, are applied in the first-pass compression. The typical true stress-strain curves are demonstrated in Fig. 4a, where stronger strain rate sensitivity exists for the yield strength and plastic behavior. For example, at the low strain rate such as 0.001 s1, the alloy exhibits
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rapid softening after yielding owing to the DRX occurring. With the first-pass strain rate is accelerated to 0.01 s1, the dislocations annihilation driven by DRX nucleation can not keep up with the imposed strain rate, leading to the work hardening after yielding point. Subsequently, stress starts to gradually decrease in virtue of high DRX nucleation rate up to 30% true strain. Furthermore, let us see what will happen while the strain rate is increased to 0.1 s1, the yield strength is much higher than those of low strain rates and immediately followed by a sharp stress drop. After that, stress undergoes a gradual increasing and decreasing process until to 30% true strain. Why does it manifest differently? Prior to yielding, high strain rate loading rapidly promotes the dislocation multiplication and reaches the critical condition for DRX nucleation. The high nucleation rate leads to the formation of DRX nucleus in a very short time scale, which consumes the accumulated dislocations and triggers the stress drop. Owing to the competition mechanism between dislocation multiplication induced by high strain rate and dislocation annihilation driven by DRX nucleation and DRV, a serrated flow stress behavior can be clearly observed for the whole deformation process. Similarly, obvious stress drop as many as 121 MPa is also found after dwell treatment. Since the second-pass strain rate is changed to 0.001 s1, the stress drop can be explained by two factors: 1) dislocations annihilation induced by SRX occurring; 2) the decreasing strain rate. In the course of the second-pass compression, the stress gradually decreases and indicates that DRX/DRV nucleation governs the deformation process. The EBSD examinations (Fig. 4b, c, d) reveal that the percentage of DRX nucleus/grains increases as the first-pass strain rate enhanced, and finally a very uniform and fine grain size distribution can be achieved by virtue of DRX nucleation at ε_ fir ¼ 0.1 s1.
Fig. 4. Effect of strain rate on the grain size evolution. (a) Under three typical strain rates at the first-pass compression, the compression stress-strain curves behave differently, demonstrating strong rate-dependent yielding strength and plastic behavior. Upon high strain compression, the dislocation density increases rapidly and is accommodated by a high DRX nucleation rate. Clearly, increasing the first-pass strain rate can refine the grain size via a high DRX nucleation rate (b, c, d), an average grain size of 3.36 mm is achieved under 0.1 s1 strain rate.
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We then change the second-pass strain rate from 0.001 s1 to 0.01 s1 and see the difference in flow stress behavior and microstructure. The manifestation of the first-pass compression stressstrain behaviors (Fig. 5a) is reproduced with Fig. 4a. The different compression responses begin from the dwell process. While the first-pass strain rate is 0.001 s1, dislocation density remains to be a low level after the first-pass compression and further decreases through SRX process. A close-up view indicates that the recovered stress slightly exceeds the value before unloading, which can be attributed to the strain rate-induced stiffening from 0.001 s1 to 0.01 s1. After yielding, a remarkable stress drop is immediately followed due to the DRX nucleation. Subsequently, the stress slightly increases until to the 69.3% true strain, which can be related to the elevation of dislocation density by high strain rate. The EBSD micrograph (Fig. 5b) shows that there is a small fraction of DRX nucleus/grains located on the pristine grain boundaries. If the strain rates remain to be 0.01 s1 during two-pass compressions, dislocation density rapidly increases before reaching the maximum compression stress and then decreases due to the DRX occurring in the first-pass deformation. Through the dwell treatment, the dislocations are further consumed for SRX process, which can be reflected by the stress drop after the unloading-loading cycle. Once loaded in the second-pass, the compression stress-strain curve demonstrates the similar behavior with that in the first-step process. The post-mortem microscopic examination (Fig. 5c) reveals that the average grain size is much smaller than the equivalents under 0.001 s1 strain rate. A higher strain rate in the first-step can result in a higher density of DRX nucleation rate and develop to the finer grain size. It is highly expected to obtain the smaller grain size
if the first-step strain rate is accelerated to 0.1 s1. Similarly, the first-pass compression behavior (Fig. 5a) experiences a series of yielding, sharp stress softening and stress climb under strain rate of 0.1 s1. After dwell treatment, the stress drop reaches 101 MPa as a result of SRX occurring and strain rate decrease. In the second-pass compression, the flow stress suffers from an abrupt decrease at the initial stage and stress-strain curve remains to be zigzag during whole second-pass process, which is believed to be controlled by the competition mechanism between DRX nucleation and dislocation multiplication induced by strain rate. A very uniform and fine grain size distribution is obtained through EBSD measurement (Fig. 5d) and clearly demonstrates that increasing the first-pass strain rate can acquire a finer grain size distribution (Fig. 5e). Accordingly, the effect of the second-pass strain rate of 0.1 s1 on the compression performance and microstructure evolution is further investigated through changing the first-step strain rate. Similar with Figs. 4a and 5a, the compression response (Fig. 6a) within the first-step and dwell treatment are fully repeated. The different scenario comes from the second-step compression. The recovered stresses are substantially enhanced in comparison with the counterparts under 0.001 s1 and 0.01 s1strain rates. After reloaded, the stresses decrease after peak force and three curves during the second-step process are serrated, indicating high DRX nucleation rates. By means of microscopic observations, the grain sizes for three different deformation conditions are very uniform (Fig. 6b, c, d), wherein the smallest grain size is attained from the high strain rate in two-pass (Fig. 6e) deformation. To shed more light on the deformation mechanisms, a detailed observation is employed to demonstrate the deformed
Fig. 5. Effect of strain rate on the grain size evolution. (a) The first-pass compression behavior is highly reproduced from the counterparts of Fig. 4a. The difference comes from the second-pass response under strain rate of 0.01 s1. At strain of 0.001 s1, owing to the dislocation consumption for DRX nucleation in the first-pass and SRX during dwell treatment, the dislocation density is quite low and gradually increases with the second-pass deformation proceeding, and corresponding to the slight stress increment. On the contrary, the high strain rate, i.e., 0.1 s1, can initiate high DRX nucleation rate and further refine the grain sizein combination with the second-pass strain rate 0.01 s1 compression. The EBSD measurements (b, c, d) show that a more uniform and finer grain size is produced by a higher strain rate used the first-pass compression (e).
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microstructures in details before and after the dwell treatment. The first-pass strain rate of 0.1 s1 is selected and the SEM and TEM examinations are shown in Fig. 7a and b. After the first-pass compression at a relatively high strain rate, a significant fraction of DRX nucleus is formed in the vicinity of pristine grain boundaries. The TEM measurement indicates that the grain sizes are in the range of tens to hundreds of nanometers (7b). After dwell treatment, the accumulated dislocation density close to grain boundaries is still high and readily triggers the SDX occurring. Fig. 7c shows that a much higher percentage of recrystallization grains emerge at the expense of the disappearance of pristine grains. A closer examination (Fig. 7d) reveals that recrystallization nucleation is very high and the nucleus lies nearby the pristine grains, which verifies the role of SRX deformation mechanism. 3.2.4. Effects of deformation temperature on the flow stress and grain size development To investigate the role of deformation temperature in the flow behavior and microstructure, three typical temperatures, i.e., 1050 C, 1100 C, 1150 C, are imposed during dwell treatment and the second-pass deformation, respectively. Fig. 8a shows the true stress-strain curves that the first-pass compression behavior is fully reproduced from Fig. 3a till to 30% compressive true strain. Subsequently, a relatively slow strain rate (0.001 s1) is executed in order to provide more time for dynamic recovery and three different temperatures are immediately applied after unloading. Upon different temperatures, the stress drops manifest vastly and show that the higher stress loss is consistent with the higher temperature. The results further indicate that the dwell process serves a critical role in controlling the microstructure development.
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At 1050 C (Fig. 8b), the stress is recovered to the previous level before unloading, which can be explained by three aspects: 1) stress drop induced SRX process; 2) strain rate is changed from 0.01 s1 to 0.001 s1; 3) temperature change from 1100 C to 1050 C. The stress loses 34 MPa if the temperature and strain rate in two-pass remain to be constant (Please see Fig. 4a). When we consider the effects of both SRX and strain rate (Fig. 5a), the stress drop reaches 55 MPa. In the first-pass compression, dislocation density increases to the critical value and induces DRX nucleation. Upon the dwell treatment, dislocations are further consumed in a manner of SRX mechanism. Once loaded, dislocation density increases again prior to the second-pass peak force. After that, the stress gradually decreases owing to the DRX nucleation but the decreasing rate is relatively rapid than those under 1100 C and 1150 C. At 1100 C (Fig. 8c), the first-pass compression behavior is same with that at 1100 C. After an unloading-loading cycle, the stress is recovered to 62 MPa in contrast to 115 MPa prior to unloading. The stress drop is ascribed to two aspects: 1) dislocations annihilation by SRX; 2) strain rate decrease in the second pass. Likewise, a short period of strain hardening occurs and followed by deformation softening in the presence of DRX nucleation. The scenario is different when the temperature is 1150 C, the stress dives to 20 MPa and remains to be nearly constant for the whole second-pass process. This clearly indicates that the retained dislocations from the first-pass deformation are depleted during 30 s dwell treatment, and reaching a balance between DRX and DRV. The successive nucleation results in a smaller grain size distribution.
Fig. 6. Effect of strain rate on the grain size evolution. (a) The first-pass compression curves are replicated with the equivalents in Figs. 4a and 5a. Under high strain rate of 0.1 s1 in the first-pass compression, the true stress-strain curves are serrated and indicate the competition between DRX nucleation and dislocation multiplication. The EBSD examinations indicate that the grain sizes from three loading modes are very uniform, yet, the high strain rate during two-pass compressions is consistent with a finer grain size (b, c, d, e).
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Fig. 7. Deformed microstructure before and after dwell treatment. (a) Under the first-pass compression with a strain rate 0.1 s1, the high DRX nucleation rate is triggered by high strain rate, which produces a high density of DRX nucleus and grown grains in the vicinity of the pristine grains. (b) Because of the dwell treatment at 1100 C, the SRX occurs and induces the formation of nanometer-sized nucleus, meanwhile, in accompany with the grain growth of DRX nucleus. Prior to the dwell treatment, the DRX grain size is in the range of tens of nanometers nearby the grain boundaries (c). After that, the DRX grains grow and the density of nano-sized nucleus increases (d).
3.3. Discussion Based on the experimental observation, a sequential microstructure evolution process is schematically depicted in Fig. 9. Prior to hot compression, the alloy undergoes high temperature annealing and the dislocation density decreases to a low level and most of grain boundaries belong to high angle grain boundaries. As the hot deformation proceeding, compression strain will increase and the dislocation density is several orders of improvement. Once the compression strain reaches the critical strain, the piled-up dislocations nearby the grain boundaries can be consumed to facilitate the DRX nucleation. The further compression will induce DRX nucleus growth and is accompanied with a new generation of DRX nucleus occurring prior to the end of the first-pass deformation. During dwell treatment, the size of DRX grain grows bigger and SRX grains emerges under the assistance of high temperature and retained strain, where the dislocations are further consumed. While entering the second-pass compression, the dislocations density increases and new DRX nucleation occurs, this triggers the size refinement of the pristine grains. After a series of DRX and SRX processes, the average grain size is refined and the two-pass deformation substantially lowers the cracking tendency. In contrast, the scenario is different for the flow behavior and microstructure upon the monotonic-pass tests. It can be clearly observed that strong strain rate sensitivity of flow stresses can be found upon three typical strain rates (Fig. 10a). At the strain rate of 0.1 s1, a sharp stress drop emerges immediately after yielding, indicating a high DRX nucleation rate. Owing to the high strain rate, dislocations are rapidly multiplied and there exists a competition mechanism between DRX nucleation and DRV, which is reflected
by the serrated flow stress (zoom-in view). The corresponding microscopic examination shows that the pristine grains are elongated normal to the compression direction and surrounded by a high density of tiny DRX grains (Fig. 10b). While the strain rate is changed to be 0.01 s1, an obvious strain hardening stage occurs before reaching the peak stress, in accompany with the substantial increase of dislocation density. Once the critical value is touched, both DRX and DRV mechanisms take over the deformation process and the flow stress gradually decreases by the end of the test. Owing to the relatively low strain rate, the time available for DRV is enough and simultaneously the DRX nucleation rate is low, which leads to the grain growth for both the pristine grains and DRX grains (Fig. 10c). Furthermore, let us see what happened if the strain rate is decreased to be 0.001 s1, stress softening rapidly emerges after yielding due to the DRX and DRV occurring. As the plastic deformation proceeding, the competition between DRX and DRV reaches a balance, wherein the grain size is continuously refined by means of DRX nucleation. Since the strain rate is slow, the growth of the pristine grains is striking and the grain size presents the bimodal distribution (Fig. 10d). It is concluded that inhomogeneous grain size or substantial grain growth is obtained upon monotonicpass results, which is undesirable in the engineering practice (Fig. 10e). The classical Avrami equation describes the effect of dwell time and strain rate on the SRX kinetics:
t n Xsta ¼ 1exp ln2 t0:5
(1)
Wherein, Xsta is the static recrystallization fraction, t is the dwell
G. He et al. / Journal of Alloys and Compounds 701 (2017) 909e919
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Fig. 8. Compression behaviors and microstructure development under different deformation temperatures. (a) After the first-pass compression under 1100 C/0.001 s1 and dwell treatment, DRX nucleation and grain growth consume the dislocations to a low level. At 1050 C, the temperature decrease results in a higher flow stress and increases the resistance for diffusion. Hence, the retained dislocation density is correspondingly high and triggers the more DRX nucleation. In contrast, while the second-pass deformation temperature is increased to 1150 C, the high temperature facilitates the diffusion process and the flow stress is quite flat, exhibiting no obvious DRX occurring. As demonstrated by EBSD measurements (b, c, d), the average grain size decreases with increasing the second-pass deformation temperature (e).
Fig. 9. Schematic showing of microstructure evolution during multi-pass compression. (a) After annealing treatment upon HEX alloy, the dislocation density is very low. (b) As plastic deformation proceeding, dislocations multiply and pile up near the grain boundaries. (c) Once reaching the critical strain, DRX nucleation occurs and nanometer-sized nucleus form in accompany with grain growth. (d) At the end of the first-pass compression, the pristine grains are surrounded by a continuously-generated layer of DRX grains. (e) During dwell treatment, SRX emerges and triggers new SRX nucleus, meanwhile, the DRX grains further grow at the expense of the refinement of the pristine grains. (f) While entering the second-pass compression, the DRX nucleation rate may become even higher because of the increasing fraction of grain boundaries. (g) After a series of DRX, SRX, DRV, SRV processes, a more uniform and finer grain size distribution can be obtained.
time and t0.5 is the time corresponding to the half of the recrystallization volume. t0.5 depends on the variables involved in the hot deformation and can be expressed as [37]:
t0:5 ¼ Aεp ε_ q Ds exp
Q RT
(2)
A, p, q and s are the materials constants, ε is the strain, ε_ the
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Fig. 10. Monotonic-pass compression behavior and microstructure observation. (a) At the relatively low strain rate, i.e., 0.001 s1, the flow stress gradually decreases after yielding and reaches steady till to 69.3% true strain. This deformation process triggers the formation of necklace-like grain microarchitectures, where the grown pristine grains are surrounded by a high percentage DRX grains/nucleus (b). Under the strain rate of 0.01 s1, an obvious working hardening process occurs after yielding, and flow stress slowly decreases after peak stress. Through EBSD examination, the pristine and DRX grains have grown up and a small quantity of recrystallization nucleus are situated near the grain boundaries (c). A higher strain rate, such as 0.1 s1, can generate a higher yielding stress followed by a sudden stress softening. The flow stress progressively declines up to 69.3% true strain. The DRX nucleation rate is very high and the pristine grains becomes elongated normal to the compression axis (d). The EBSD measurement shows that the finest average grain size is obtained by the highest strain rate, and the coarsest grain size is related to the 0.01 s1. Interestingly, the average grain size inversely increases with decreasing the strain rate to 0.001 s1.
strain rate, D the average grain size, Q the activation energy, T the absolute temperature and R is the gas constant. Based on the above relationship, it can be on quantity obtained that, increasing dwell time t, the recrystallization percentage is higher. An increasing strain rate can obtain more recrystallization fraction.
4. Conclusions The flow behaviors and grain size of a nickel based superalloy under various combinations of deformation conditions using multipass compression are investigated. The following conclusions can be drawn: 1) Comparing to the monotonic-pass hot compression, the multipass compression provides a versatile approach to control the flow stress and grain size, the cracking tendency is substantially reduced. 2) The effect of the first-pass strain amount, dwell time, the second-pass deformation temperature, strain rates in two-pass processes on the compression behavior are investigated, the flow behavior and grain size evolution are evaluated and corresponding deformation mechanisms are discussed in details. 3) The higher first-pass strain generates the finer grain size owing to the SRX occurring. The post-mortem examination of grain size after two-pass compression is closely associated with the second-pass processing conditions. A finer and more uniform grain size can be attained by the higher strain rate, longer dwell time and higher deformation temperature.
Acknowledgments Z. H. and L. J. appreciate the financial support from The National Key Research and Development Program of China (2016YFB0700300). G. H. is grateful for the support from the Fundamental Research Funds for the Central Universities of Central South University (2015zzts031) and the outstanding graduate project of Advanced Non-ferrous Metal Structural Materials and Manufacturing Collaborative Innovation Center. F. L. and L. H. are thankful for the funding sponsorship of the Natural Science Foundation of China (51401242, 51301209). We would like to acknowledge the help from Hong JIANG, Liming TAN and Longfei ZHANG for the helpful technical discussion. References [1] G. He, F. Liu, L. Huang, L. Jiang, Hot deformation behaviors of a new hot isostatically pressed nickel based powder metallurgy superalloy, J. Mater. Res. 31 (2016) 3567e3579. [2] R.C. Reed, The Superalloys: Fundamentals and Applications, Cambridge University Press, 2008, pp. 1e28. [3] M. Donachie, S. Donachie, Superalloysea Technical Guide, ASM International, Materials Park, OH, 2002. [4] M. Detrois, R.C. Helmink, S. Tin, Microstructural stability and hot deformation of geg0 ed Ni-Base superalloys, Metall. Mater. Trans. A 45 (2014) 5332e5343. [5] A.K. Koul, J.P.A. Immarigeon, Modelling of plastic flow in coarse grained nickel-base superalloy compacts under isothermal forging conditions, Acta Metall. 35 (1987) 1791e1805. [6] T. Osada, Y. Gu, N. Nagashima, Y. Yuan, T. Yokokawa, H. Harada, Optimum microstructure combination for maximizing tensile strength in a polycrystalline superalloy with a two-phase structure, Acta Mater. 61 (2013) 1820e1829. [7] S. Babu, M. Miller, J. Vitek, S. David, Characterization of the microstructure evolution in a nickel base superalloy during continuous cooling conditions, Acta Mater. 49 (2001) 4149e4160.
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