Effect of grain size on the tensile behavior of V10Cr15Mn5Fe35Co10Ni25 high entropy alloy
Introduction
High-entropy alloys (HEAs) are a class of advanced materials with a unique alloy design concept and remarkable properties that have drawn substantial attention from researchers in the field of materials science [1], [2], [3], [4], [5], [6], [7]. Unlike the design concept of conventional alloys, which are based on one or two major elements, HEAs are designed with multi-principal elemental contents ranging from 5 to 35 at% each [8]. There have been many efforts to design new HEAs for specific applications, which have resulted in the development of a variety of HEAs [3], [9], [10].
Among these HEAs, alloys based on the five elements Cr, Mn, Fe, Co, and Ni with a single face-centered cubic (FCC) structure have been intensively studied. These HEAs show exceptional mechanical properties such as superior fracture toughness, especially at cryogenic temperature [4], [11]. Several studies have been focused on the effect of temperature on the mechanical properties of Cr-Mn-Fe-Co-Ni based HEAs [12], [13], [14]. It has been reported that the main reason for such outstanding mechanical properties at cryogenic temperature is the evolution of mechanical twins during cryogenic deformation. Laplanche et al. [4] explained that the formation of mechanical twins in equiatomic CrMnFeCoNi HEA at cryogenic temperature leads to a decrease in dislocation-mean-free-paths, resulting in a dynamic Hall-Petch effect. Subsequently, the formation of new twin boundaries enhances these mechanical properties by postponing the onset of necking and provides more uniform elongation [4], [15].
It is generally believed that the mechanical twinning initiates when the resolved shear stress exceeds the critical stress for twinning [16], [17]. In equiatomic CrMnFeCoNi HEA, the mechanical twinning is considered to occur at room temperature because of its low stacking fault energy (SFE = 21 mJ/m2 [18]). However, a high critical stress of ~720 MPa is needed in the tensile process for the onset of mechanical twinning at room temperature [4]. Therefore, because of insufficiently resolved shear stress, it is difficult for mechanical twinning to occur at room temperature and to contribute to the deformation mechanism [19].
In addition to the effect of temperature on mechanical twinning, there is another intrinsic parameter (grain size) that controls the transition of deformation mechanisms from dislocation slip to mechanical twinning [16], [20]. The effect of grain size on twinning behavior is mainly noted in steels exhibiting the twinning-induced plasticity (TWIP) effect [21], [22]. Meyers et al. [16] proposed that the grain size has a Hall-Petch effect on the critical stress for twinning. They concluded that the twinning capability decreases with decrease in the grain size due to the increased critical stress for twinning onset. Sun et al. [23] investigated the influence of grain size on the deformation behavior of equiatomic CrMnFeCoNi HEA at room temperature. They observed that there is a critical grain size, above which mechanical twins can be formed in the grains. Moreover, Wu et al. [24] explained that the twinning activity of fully recrystallized Al0.1CoCrFeNi HEA is strongly prohibited by a decrease in the grain size, resulting in decreased tensile ductility.
Recently, Jo et al. [25] studied the mechanical properties of fully and partially recrystallized V10Cr15Mn5Fe35Co10Ni25 (at%) non-equiatomic HEAs at room and cryogenic temperatures. It was demonstrated that the dominant deformation mechanism at cryogenic temperature is mechanical twinning in addition to dislocation slip. In contrast, after deformation at room temperature, the fully recrystallized sample with a mean grain size of ~5.2 µm showed a high-density of dislocations without any mechanical twinning. However, in the partially recrystallized sample, several mechanical twins were detected inside the coarse non-recrystallized regions. These researchers mainly focused on the twinning behavior of the mentioned HEA at cryogenic temperature, so the effect of grain size on the twinning capability at room temperature needs further investigation.
Thus, the aim of this work was to study the effects of grain size on work-hardening behavior and deformation mechanisms in the HEA V10Cr15Mn5Fe35Co10Ni25 during plastic deformation at room temperature. In addition, the mechanical properties and the evolution of microstructure during tensile deformation at room temperature were investigated in detail.
Section snippets
Experimental
The V10Cr15Mn5Fe35Co10Ni25 (at%) HEA was fabricated in a vacuum induction furnace using pure elements (purity higher than >99.9%). The as-cast ingot was homogenized at 1100 °C for 6 h under argon gas, followed by water quenching. The homogenization temperature was selected based on the thermodynamic calculation reported in the previous work [25]. The homogenized sample was rolled at room temperature with the rolling reduction ratio of ~70% (from 5.5 to 1.5 mm). The rolled plate was annealed at
Results and discussion
Fig. 1(a–c) shows EBSD grain boundary maps of V10Cr15Mn5Fe35Co10Ni25 HEA after annealing under different conditions. The high-angle grain boundaries (HAGBs) and annealing twin boundaries (TBs) are represented by black and red colors, respectively. All the annealed microstructures show equiaxed fully recrystallized grains with a high fraction of annealing twins. The average grain size was estimated using a linear intercept method in which high-angle boundaries were counted without considering
Conclusions
In this study, a fully recrystallized V10Cr15Mn5Fe35Co10Ni25 (at%) HEA with a single phase FCC structure was fabricated at different grain sizes, and the effects of grain size on the work hardening behavior and deformation mechanisms were investigated. The following conclusions are drawn from the results presented and discussed in this study.
- 1.
The Hall-Petch equation derived for the present HEA showed that the value is higher than that of other typical HEAs, indicating that the alloy may have
Acknowledgments
This research was supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (2016M3D1A1023384). S. Praveen is supported by Korea Research Fellowship program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2017H1D3A1A01013666). We also acknowledge funding from POSCO TJ Park Foundation through POSCO Asia Fellowship.
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