Mechanism of grain refinement in an equiatomic medium-entropy alloy CrCoNi during hot deformation

Journal of Alloys and Compounds 815 (2020) 152382

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Mechanism of grain refinement in an equiatomic medium-entropy alloy CrCoNi during hot deformation Guoai He a, b, Yifan Zhao a, b, Bin Gan c, Xiaofei Sheng d, Yu Liu a, b, Liming Tan e, * a

Light Alloy Research Institute, Central South University, Changsha, 410083, China State Key Laboratory of High Performance Complex Manufacturing, Central South University, Changsha, 410083, China c Central Iron & Steel Research Institute, Beijing, 100081, China d Hubei University of Automotive Technology, Shiyan, 442020, China e School of Metallurgy and Environment, 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 11 July 2019 Received in revised form 19 September 2019 Accepted 20 September 2019 Available online 20 September 2019

With promising mechanical properties, the newly developed medium-entropy alloy (MEA) has aroused widespread interest recently. Plastic deformation processes including hot working are usually adopted to refine the grain of many alloys and thereby enhance the strength, hardness, and wear resistance at low temperatures. So far, few discoveries have reported the mechanism of grain refinement during hot deformation on MEA, especially for CrCoNi. In this article, plastic deformation at different conditions was intentionally designed to uncover the underlying mechanism of grain refinement in CrCoNi MEA alloy. Unlike many other alloys with low stacking fault energy, the occurrence of continuous dynamic recrystallization during hot deformation of CrCoNi alloy has been unexpectedly discovered, which is proved as the dominating mechanism for grain refinement of the current alloy. © 2019 Elsevier B.V. All rights reserved.

Keywords: Medium entropy alloy Hot deformation Continuous dynamic recrystallization Grain refinement

1. Introduction The multi-component solid solution alloy originally defined as high-entropy alloy (HEA) by Yeh et al. has attracted much attention recently [1,2]. It is well-known that the traditional alloys, such as Fe-based alloy, Al-based alloy, Ni-based superalloy, etc., generally take a single principal element as the matrix, mixed with other alloying elements in relatively smaller amounts. In contrast, the HEA contains at least five matrix elements typically with identical molar fraction, yielding high mixing entropy in the state of liquid or high-temperature solid solution [3]. It was found that HEA owns excellent properties, in terms of cryogenic strength and toughness, thermal phase stability, wear resistance, damage tolerance, and so on [2,4e9]. More recently, in the concept of entropy, the mediumentropy alloy (MEA) containing only three equiatomic elements was found to present even prior strength-toughness balance and fracture toughness, in comparison with high-entropy alloys and many other multi-phase alloys [10e12]. However, both HEA and MEA with coarse grains usually present

* Corresponding author. School of Metallurgy and Environment, Central South University, #932 south Lushan road, Changsha, Hunan province, 410083, PR China. E-mail address: [email protected] (L. Tan). https://doi.org/10.1016/j.jallcom.2019.152382 0925-8388/© 2019 Elsevier B.V. All rights reserved.

undesirable yield strength, hardness, wear resistance at low temperatures [13e15]. Introducing second phase particles is a good way to synchronously improve the strength and ductility of HEA [9]. Beside that, refining grain size is commonly used to optimize the property, considering Hall-Petch law [3,15e19]. Thereby, the plastic deformation processes, such as hot/warm working and cold rolling with thereafter annealing, are widely adopted. For thermomechanical processing, the microstructure evolution during that has been broadly investigated on conventional metals [20e31]. It is generally acknowledged that discontinuous dynamic recrystallization (DDRX) plays a dominating role in grain refinement for alloys with low stacking fault energy (SFE), including Al alloys, airon and nickel-based alloys [21,32,33]. However, continuous dynamic recrystallization (CDRX), which is usually found in high SFE alloys, has also been observed in some alloys with low SFE [34,35]. Even though some efforts have been conducted to study the recrystallization mechanisms of the HEAs or MEAs [36,37], less works on the grain evolution enhanced by dynamic recrystallization during hot deformation on these alloys have been reported systematically, especially for the promising MEAs. In this work, the microstructure evolution during thermal deformation at different conditions was studied on a MEA, specifically the CrCoNi alloy. The recrystallization behavior behind that

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was hereafter interpreted, in addition to dynamic recovery (DRV) and grain growth. The uncovered competition mechanism of these processes is helpful for tailoring the microstructure of this specific kind alloy.

average grain size are determined by averaging the equivalent size of all detected grains. To assure the reliability of data, the fraction of modified fraction by cleanup was controlled under 9%. Specially, the local misorientation maps were adopted to identify the hardening state of every individual grain, as specifically described in previous work [38].

2. Materials and methods The composition of the investigated medium-entropy alloy (MEA) with nominal composition of CoCrNi (1:1:1 in atomic ratio) was initially produced by vacuum induction melting using highpurity raw materials (purity of 99.95 wt%). To obtain refined and homogeneous grains, the ingot was then forged at 1100
C, and heat treated at 850
C for 1 h, followed by water quenching, resulting in uniform grains with average grain size of about 20 ± 0.8 mm, as indicated in Fig. 1(a). The cylinder specimens in diameter of 8 mm and height of 12 mm were cut from the billet, after which the surfaces were polished before being thermally compressed at Gleeble thermosimulator system (DSI, 3180D). To dwindle the end-friction influence and make the deformation more uniform during compression, the carbon sheet and lubricant were positioned at specimen-die interface. Before compression, the specimens were heated to the desired temperature with rate of 5
C/s and held for 2 min to homogenize the temperature field with in specimens. Then, the specimens were thermally compressed to 50% height reduction at temperatures of 900e1000
C and strain rates in rang of 0.01e1 s1. Finally, the specimens were water-quenched rapidly to ‘freeze’ the deformed microstructure after deformation, as schematically illustrated in Fig. 1(b). To characterize the microstructure evolution during deformation, the deformed samples were sectioned along the compression direction, and mechanically polished by abrasive paper and colloidal silica, followed by vibration polishing in 0.03 mm colloidal silica for about 3 h. The grains in the MEA superalloy were characterized using electron backscatter diffraction (EBSD) technique, on the field-emission SEM FEI Quanta FEG 650 equipped with EBSD detector (Oxford Instrument plc, NordlysMax2). In order to avoid possible microstructure inhomogeneity in the compressed specimens caused by nonuniform deformation, the EBSD observation was focused on center regions of the semi-sections. In addition, the step size of EBSD scanning was set as 0.10 mm to ensure adequate pixels in each grain. To balance the accuracy and time costing, the charge-coupled device was set as 320  240, which was further assured by the results that all the average mean angular deviations were under 0.8
. The yielded EBSD data were thereafter postanalyzed by HKL Channel5 pffiffiffiffiffiffiffiffiffi software and the equivalent grain size was calculated by 2 A=p, where A was the area of a grain, and the

3. Results and discussion 3.1. Flow behaviors of CrCoNi at hot deformation Fig. 2 illustrates the true stress-strain curves of the MEA alloy compressed at different conditions. In general, the MEA alloy presents higher deformation resistance at higher strain rates and lower temperatures during thermal compression. As shown in Fig. 2, different from the flow behaviors of some HEAs [39,40], the stress of MEA increases gradually at the initial status, then the stress reaches a steady state without significant dynamic softening and clear peak, which is generally thought as character of continuous dynamic recrystallization (CDRX) [41]. In comparison with other low SFE alloys, the flow stress will decrease after reaching a peak stress, with a significant period of dynamic softening, which is consider as the discontinuous dynamic recrystallization (DDRX) [22,42]. In specific, typical hardening rate q is used to further investigate the discontinuous hardening and softening processes based on the flow curves, which is defined by Refs. [22,43,44]:

q ¼ ds/dε

(1)

wherein s and ε correspond to the true stress and true strain respectively. Basically, the q value becomes positive, as that true stress increases with true strain, and relates with work hardening (WH). On the contrary, the negative q value associates with dynamic softening. In addition, the stress keeps constant during compression if q ¼ 0, where dynamic softening induced by dynamic recovery (DRV) and DRX is balanced with work hardening caused by dislocation pileup. As illustrated in Fig. 3, there is not obvious dynamic softening period where q is negative, different from DDRX happened in other SFE alloys like Ni-base superalloy [22]. The q achieves the peak rapidly at the initial deformation stage, and the degree of WH is more significant at relatively lower temperatures and higher strain rates. Then the q drops to zero at true strain of around 0.1, corresponding to the peak strain (εP) of true strain and true stress curves in Fig. 2. Thereby, the emergence of DRX can be judged from it, as critical strain to initiate DRX (εc) is found 0.5e0.8 times of peak

Fig. 1. (a) initial microstructure and (b) schematic diagram of hot compression and EBSD observation on the CrCoNi MEA alloy.

G. He et al. / Journal of Alloys and Compounds 815 (2020) 152382

Fig. 2. Typical true stress-true strain curves of CrCoNi MEA alloy compressed at temperatures of (a) 900
C; (b) 1000
C; (c) 1100
C.

Fig. 3. The relationship of strain hardening rate (q) and flow strain (s) of the CrCoNi MEA alloy compressed at temperatures of (a) 900
C; (b) 1000
C; and (c) 1100
C.

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Fig. 4. EBSD IPF images of specimens deformed at different conditions.

strain (εP) [45]. Thereafter, the competition between WH and dynamic softening reaches a balanced period, as the q fluctuates around zero when the strain reaches at a specific level. 3.2. Microstructure after hot deformation The EBSD inverse pole figures (IPFs) in Fig. 4 present the grains of MEA specimens deformed at different conditions, from which it can be observed that the texture at these deforming conditions is not significant. In general, grains in the CrCoNi alloys deformed at

higher temperatures are more uniform than these of their counterparts. As found in many other works [38,41,45], the typical necklace structure is also detected in the CrCoNi specimens compressed at 900
C, especially at strain rate of 0.01 s1. With temperature rising, the necklace structure disappears gradually and more equiaxed grains are observed, indicating the degree of recrystallization for NiCoCr alloy is promoted at higher temperature, which is more apparent under 1100
C. Moreover, at a specific strain rate, the CrCoNi alloy deformed at higher temperature presents larger grain size generally, which can be attributed to the fact

Fig. 5. (a) equivalent grain size distribution, and (b) the average grain size of the CrCoNi alloy compressed at different conditions.

G. He et al. / Journal of Alloys and Compounds 815 (2020) 152382

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Fig. 6. EBSD local misorientation images of the CrCoNi MEA samples deformed at different conditions, wherein the average values of local misorientation are indexed.

that the grain growth of recrystallized grains is enhanced as temperature arise, as illustrated in Fig. 5(b). For example, the average grain size increases from 2.2 mm to 5.1 mm at temperatures of 900e1000
C. While for strain rates of 1 s1 and 0.1 s1, the average grain sizes of samples deformed at 1000
C are smaller by 1.3% and 8.1% than that compressed at 900
C, in these cases the grain refinement is more adequate at higher temperature, which can be judged from the appearance of necklace structure. For a specific temperature, the strain rate imposes ambiguous effects on the grain evolution, as indicated in Fig. 5. For example, decreasing the strain rate can refine the average grain size at 900
C, while the grain size drops slightly from 2.27 mm to 2.2 mm as the strain rate increases from 0.01 s1 to 1 s1 at 1000
C. As the temperature increases to 1100
C, the grain size declines from 5.1 mm to 3.9 mm as the strain rate increases from 0.01 s1 to 0.1 s1, and climbs to 4.7 mm when the strain rate increases to 1 s1. It is broadly accepted that the grain evolution is dominated by the interactions among dislocation, DRX, DRV, and grain growth [24,45]. Considering these factors, the detailed mechanism of grain evolution in the MEA alloy will been discussed in the following parts. Basically, to obtain homogenous and refined grains, compressing the CrCoNi MEA alloy at temperature in range of 900e1100
C and strain rate within 0.01e1 s1, or 1000
C with strain rates of 1 s1 and 0.1 s1 is preferred at present experimental condition. EBSD local misorientation image is considered as a favorable indication of work hardening degree, as presented in Fig. 6, illustrating the hardening status of grains for CrCoNi samples

corresponding to different conditions. With the decrease of temperature at a specific strain rate, the dynamic hardening gets more significant as the average values of local misorientation are increased on the whole, which is closely related to the dislocation accumulation. At temperature of 900
C, small DRX grains with local misorientation approximating to zero locate around coarse grains with high local misorientation, indicating that DRX grains prefer to nucleate along the initial grain boundaries. Interestingly, the size and area fraction of the grains below 5 mm in specimens deformed at higher strain rate are relatively larger under deformation temperature of 900
C, and the fraction of the severely deformed grains shrinks as a result, which can also be observed from Figs. 5 and 7. As the temperature reaches 1000
C, the dislocation accumulation is relieved to some extends, as the average local misorientation decreases and the fractions of deformed grains are less than their counterparts, especially at low strain rates of 0.01 s1 and 0.1 s1. When the deformation temperature further increases to 1100
C, the recovery of dislocation is more obvious, specifically at strain rates of 0.1 s1 and 1 s1. The fraction of recrystallized grains dramatically increases from 15.2% to 73.0%, as strain rate increases from 0.01 s1 to 0.1 s1 at 1100
C. This phenomenon can be understood as faster strain rate at that high temperature would enhance the dislocation accumulation which can facilitate the occurrence of DRX. While as the strain rate comes up to 1 s1, the number of recrystallized grains decreases sharply and the percentage of sub-structured grains reaches 59.3% due to the short deformation time for accomplishment of DRX.

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Fig. 7. EBSD images showing the recrystallized, substructured, and deformed grains in the CrCoNi MEA samples deformed at different conditions, wherein the pie charts reflect the area fractions of these grains.

Additionally, a large amount of S3 annealing twins are detected in the DRX grains as showed in Fig. 8. In general, the existence of annealing twins in low SFE metallic materials indicates that the DRX process is facilitated by the migration of grain boundaries [46]. The S3 annealing twins prefer to stay in DRX grains, indicating that many of the them are newly formed during the DRX process. Moreover, S3 boundaries in the deformed grains are barely observed, as most of the original twin boundaries is broken during hot compression, which is mainly induced by the crystal rotation and changes of the orientation [46]. Thus, considering the relationship between twins and recrystallization, in addition to the finer grains comparing with initial status, the dynamic recrystallization has already happened in the MEA alloy deformed at 1100
C. Since the dislocation prefers to accumulate along grain boundaries or triple junction because of the local strain concentration and plastic incompatibility, the DRX nucleus may forms therein via coalescence of dislocation networks induced subgrains or strain induced boundary migration (SIBM) [38,47e49]. In another way, strain induced dislocations tend to organize themselves in grain interiors to form subgrains with the progress of compression. The local angular misorientations within grains (marked with the dark arrows in Fig. 4) is detected by EBSD, as presented in Fig. 9. Basically, at temperatures of 900
C and 1000
C and strain rates within 0.01e1 s1, the local misorientations over 3
are frequently observed at grain interior, and cumulative misorientations keep increasing from grain boundary to grain interior, with many of them exceed 10
or even 15
, which indicates that the progressive subgrain rotation occurs and facilitates the transformation of low angle boundaries (LAGBs) into high-angle boundaries (HAGBs)

within deformed grains. Thereafter, with the continuous formation of HAGBs within grains during hot deformation, the new DRX forms therein, which was interpreted as a CDRX mechanism [34,41]. Although the necklace structure has been broadly thought as typical indication of DDRX, the necklace-shape grains are also observed in some recrystallized alloys through CDRX [33,34]. This is mainly caused by the rapid development of strain gradients near grain boundaries without sufficient DRV, which thereby forms large misorientations in the vicinities of grain boundaries. Combining the flow characteristics of the MEA alloy, CDRX plays dominating roles in the grain refinement at present experimental conditions, which can be also verified by the features of local misorientation and strain-stress evolution. As the strain rate decreases from 1 s1 to 0.1 s1 at 1100
C, the average local misorientation within the grains decreases from 0.76
to 0.60
, and the fraction of DRX grains significantly increases from 28.7% to 73.0%, since the DRX is fully processed and enhanced at the condition of 1100
C/0.1 s1. While at 1100
C/0.01 s1, the DRX is retarded at lower strain rate, as the grain growth and dynamic recovery are enhanced and more dominating to consume the dislocations, then the stored strain energy is insufficient to trigger and produce adequate DRX grains.

4. Conclusion In summary, the flow curves and microstructure evolution during high thermal compression on the CoCrNi MEA have been characterized and analyzed in this work. Accordingly, several conclusions can be summarized as below:

G. He et al. / Journal of Alloys and Compounds 815 (2020) 152382

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Fig. 8. Grain boundary maps of the CrCoNi MEA samples deformed at different conditions, wherein lack and color lines represent grain boundaries with misorientation angles over P 15
and twin boundaries with overwhelming majority of 3 boundary marked by red. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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G. He et al. / Journal of Alloys and Compounds 815 (2020) 152382

Fig. 9. The misorientation profiles within grains of CrCoNi MEA samples deformed at different conditions: (a) 900
C/0.01 s1, (b) 1000
C/0.01 s1, (c) 1100
C/0.01 s1; (d) 900
C/ 0.1 s1, (e) 1000
C/0.1 s1, (f) 1100
C/0.1 s1; (g) 900
C/1 s1, (h) 1000
C/1 s1, (i) 1100
C/1 s1.

1. The features of true strain-stress and EBSD analyses indicate the CDRX apparently occurred during deformation at the current experiment conditions, which is different from the most other low SFE alloys dominated by DDRX. 2. Rapid development of strain gradients near grain boundaries at relatively low temperatures induced the occurrence of the necklace structure, wherein large misorientations forms along grain boundaries. 3. Progressive subgrain rotation was found and triggered the formation of the high-angle boundaries (HAGBs), thereafter, with the formation of HAGBs within grains continuously, the new DRX formed therein. 4. With significant effects of CDRX, compressing the CrCoNi MEA alloy at 1000
C with strain rates of 1 s1 and 0.1 s1 is suggested to obtain homogenously refined grains. Acknowledgment Prof. Guoai He was grateful for the financial support from the Project of State Key Laboratory of High Performance Complex

Manufacturing, Central South University (ZZYJKT2019-04), scientific research initial funding of Central South University (202045001) and National Natural Science Foundation of China (51901247). Zhao wants to thank the funding from Fundamental Reasearch Funds for the Central Univeristies of Central South University (2019zzts526). References [1] J.-W. Yeh, S.-K. Chen, S.-J. Lin, J.-Y. Gan, T.-S. Chin, T.-T. Shun, C.-H. Tsau, S.Y. Chang, Nanostructured high-entropy alloys with multiple principal elements: novel alloy design concepts and outcomes, Adv. Eng. Mater. 6 (2004) 299e303. [2] C.-Y. Hsu, J.-W. Yeh, S.-K. Chen, T.-T. Shun, Wear resistance and hightemperature compression strength of Fcc CuCoNiCrAl0.5Fe alloy with boron addition, Metall. Mater. Trans. A 35 (2004) 1465e1469. [3] S. Yoshida, T. Bhattacharjee, Y. Bai, N. Tsuji, Friction stress and Hall-Petch relationship in CoCrNi equi-atomic medium entropy alloy processed by severe plastic deformation and subsequent annealing, Scr. Mater. 134 (2017) 33e36. [4] F. Otto, A. Dlouhý, C. Somsen, H. Bei, G. Eggeler, E.P. George, The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy, Acta Mater. 61 (2013) 5743e5755. [5] C. Zhang, F. Zhang, H. Diao, M.C. Gao, Z. Tang, J.D. Poplawsky, P.K. Liaw,

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Al deformed to strains of 60 at 400
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