Microstructure and texture development in CoCrNi medium entropy alloy processed by severe warm cross-rolling and annealing
Introduction
High entropy alloys (HEAs) are multicomponent alloys synthesized based on the novel alloy design strategy of mixing a large number (usually greater than five) of constituents with concentrations varying from 5 to 35 at.% [1]. Defying the classical metallurgical expectation of microstructures dominated by various intermetallic phases, several HEAs have simple solid-solution phases such as FCC [2], BCC [3], FCC + BCC dual structure [1], or even HCP [4,5]. It has been presumed that the decrease in free energy is affected by the large configurational entropy ( of mixing of a large number of elements could stabilize these solid solution phases [1]. Consequently, the new paradigm in alloy design based on the configurational entropy evolved, resulting in the grouping of alloys into high (HEAs with ), medium (MEAs with , and low entropy ( alloys. HEAs [[6], [7], [8], [9], [10], [11], [12], [13], [14], [15]] and MEAs [[16], [17], [18]] have attracted unprecedented attention due to their intriguing structural and functional properties.
It can hardly be overemphasized that thermo-mechanical processing (TMP) treatments can considerably enhance the microstructure and properties of a wide range of metallic materials. Consequently, research focusing on tailoring microstructure and properties of single- and multiphase HEAs using TMP treatments has gained considerable momentum [[19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. Towards this direction, TMP studies of FCC equiatomic CoCrNi MEA by different routes, including cold- [31,32], cryo- [33], and warm-rolling [31], have been initiated. Among these different treatments, warm-rolling is featured by lower flow stress than cold-rolling (due to the elevated deformation temperature) and superior surface finish than hot-rolling (due to lower temperature of deformation). The efficacy of warm-rolling in attaining fine-grained microstructure, controlling texture, and enhancing mechanical properties has already been reported [31]. In effect, these results encourage researchers to study the effect of different processing parameters on microstructure and texture evolution during warm-rolling.
A key thermo-mechanical processing parameter affecting microstructure and texture formation is strain path, apart from imposed strain, starting grain size, and deformation temperature. The effect of strain path change can be executed by cross-rolling, i.e., by mutually interchanging the rolling direction (RD) and transverse direction (TD) by the rotation of the sample around the normal direction (ND) [34]. The effect of cross-rolling has been highlighted in cold-rolled FCC materials [[34], [35], [36], [37], [38], [39], [40], [41]], which show significant differences with conventional unidirectional rolled counterparts. Development of ND-rotated brass texture after cross-rolling has been observed in cross-rolled aluminum and copper alloys [[35], [36], [37], [38], [39]], and also in nickel [40,42]. Development of unusual recrystallization texture (ND//<111>) fiber texture in nickel [42]), weakening, and even complete randomization of texture [43] are possible in cross-rolled materials. Despite these interesting outcomes, the effect of cross-rolling has been investigated only to a limited extent in HEAs, which nonetheless reveal considerable influence on microstructure and texture formation [44,45].
In the current study, we investigate the effect of different cross-rolling routes on the evolution of microstructure and texture in FCC equiatomic CoCrNi MEA after severe warm-rolling and subsequent recrystallization. The purpose of this study is two-fold; firstly, to clarify the effect of cross-rolling on the MEAs and secondly, to complete the landscape of various processing variables on microstructure and texture formation of MEAs during warm-rolling.
Section snippets
Processing
The equiatomic CoCrNi MEA was synthesized by vacuum arc melting in a controlled argon-filled inert atmosphere starting with high purity elements (≥99.9% purity). The arc-melted buttons (∼100 gm weight) were flipped and subjected to multiple remelting and solidification cycles (4–5 times) before being finally cast into a water-cooled copper mold. The as-cast ingots were subsequently homogenized at 1100 °C for 6 hours (h)).
To attain a wrought starting microstructure, samples having dimensions of
Effect of cross-rolling on microstructure and texture evolution
The microstructure of the starting material (obtained after 50% cold-rolling and treated at 800 °C/1 h) (Fig. 2(a)) shows fully recrystallized grains bounded by high angle boundaries (HAGBs; misorientation angle (θ)≥15°; highlighted in black) containing annealing twin boundaries (TBs; highlighted in red). The grain size distribution (Fig. 2(b)) shows a large fraction of fine grains with average grain size (excluding the TBs) ∼5–7 μm. A comparison of the (111) PF (Fig. 2(c)) with the ideal (111)
Warm-rolled microstructure and texture
The microstructural development in UWR and different cross-rolled materials is featured by the gradual evolution of elongated microstructures, further grain refinement, and finally, forming deformation-induced ultrafine microstructures. The grain subdivision mechanism mainly contributes to the ultrafine microstructure formation in all the differently processed materials. The starting equiaxed recrystallized grains are progressively subdivided on an increasingly finer scale [46]. Remarkably, the
Conclusions
The CoCrNi MEA was processed by unidirectional and cross-rolling routes and further annealed at different temperatures to understand the microstructure and texture formation. The following conclusions may be drawn.
- (i)
The MEA processed by the UWR and different cross-rolling routes developed fine-scale deformation-induced microstructures. The different cross-rolled materials showed a greater propensity for forming complex intersecting shear bands than the UWR specimen.
- (ii)
Greater hardness in the
Author statement
1. J. Saha: Investigation, Analysis, Validation, Writing-Original draft.
2. R. Saha: Investigation (Bulk texture measurement).
3. P.P. Bhattacharjee*: Methodologies, Writing-Original draft, Review and Editing, Supervision, Fund acquisition, Project administration.
Data availability
The dataset analyzed in this work will be made available by the corresponding author upon reasonable requests. Some dataset may not be shared he raw/processed data required to reproduce these findings cannot be shared at this time as the dataset also forms part of an ongoing study.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. (We declare no conflict of interest).
Acknowledgment
The financial supports of DST-SERB, India (CRG/2020/00665), the DST-FIST program (SR/FST/ETI-421/2016), and the DRDO (ER&IPR) (ERIP/ER/2002002/M/01/1773) are sincerely acknowledged.
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2023, Acta MaterialiaCitation Excerpt :The distinct microstructure and texture components are responsible for different deformation and fatigue behaviors. Recent attempts have been focused on clarifying the evolution of microstructures and crystallographic orientations of grains during different processing and annealing conditions in the HEAs [29–31]. However, understanding the deformation-texture orientation-dominated fatigue-crack growth in the HEAs upon cyclic loading has not been acquired yet.