Effect of heavy cryo-rolling on the evolution of microstructure and texture during annealing of equiatomic CoCrFeMnNi high entropy alloy
Introduction
High entropy alloys (HEAs) are newly developed multicomponent alloys based on the novel alloy design concept of mixing five or more constituent elements in equiatomic or near equiatomic proportion [1]. Despite having the presence of a large number of components the HEAs often show rather simple crystal structures such as, FCC (e.g. equiatomic CoCrFeMnNi [2]), BCC and FCC + BCC [1]. The research on HEAs has gained considerable attention in recent years and many interesting and unique properties of these materials have been reported [3], [4], [5], [6], [7], [8], [9]. The unique properties of the HEAs are attributed to the core effects of multicomponent solid solution formation, namely distorted lattice structure [3], cocktail effect [3], [4], sluggish diffusion [3], [10] and formation of nanoscale deformation twins [5], [11].
A major area of research in widening the potential applications of HEAs is to understand the thermo-mechanical processing (TMP) behavior of HEAs [12], [13], [14], [15], [16], [17], [18]. Appropriate understanding of the TMP response of HEAs can significantly affect the microstructure and mechanical properties of HEAs [11], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. TMP routes usually involve heavy deformation and annealing, which leads to the refinement of microstructure and development of crystallographic texture [33], [34]. Consequently, microstructure and texture of TMP processed materials are crucial aspects which have been intensely researched in different materials [35], [36] and remain topics of much interest in case of HEAs.
Cryo-rolling (i.e. rolling at the liquid N2 temperature) of materials constitutes an interesting TMP route which can introduce significant microstructural refinement and enhance mechanical strength. Consequently, the effect of cryo-rolling has been investigated in a wide variety of materials [37], [38], [39], [40], [41], [42], [43], [44], [45]. Recently, the effect of cryo-rolling has also been investigated in HEAs and significant enhancement in strength has been reported by Stepanov et al. [46]. However, the annealing behavior of the cryo-rolled HEAs has not been reported in the research.
In the present work, we are investigating the effect of cryo-rolling on microstructure and texture formation during annealing for cryo-rolled FCC CoCrFeMnNi HEA which has not been attempted earlier. For comparison purpose the alloy has been cold-rolled and cryo-rolled at room temperature and at the liquid N2 temperature, respectively.
Section snippets
Processing
The equiatomic CoCrFeMnNi alloy used in the present research work was prepared by vacuum arc melting. The as-cast alloy was subjected to a homogenization treatment at 1100 °C for 6 h (hrs) to enhance the chemical homogeneity. The as-cast and homogenized alloy showed a rather coarse microstructure. In order to generate a wrought microstructure suitable for the present research, samples with dimensions 25 mm (length) × 8 mm (width) × 5 mm (thickness) were extracted from the homogenized material
Microstructure and texture evolution during deformation
The microstructure of the starting recrystallized material (Fig. 1(a)) shows the presence of recrystallized grains separated by high angle grain boundaries (HAGBs defined by misorientation angle (θ) ≥ 15° and highlighted in black in Fig. 1(a)) [12]. The average grain size (neglecting annealing twins as separate grains) of the starting material is ∼7 μm. The microstructure also shows the presence of profuse annealing twin boundaries (highlighted in red).
The microstructure of the 90% cold-rolled
Discussion
The bulk textures of the two materials processed by the two different routes show very similar strong brass type. The development of strong brass type texture after heavy deformation is characteristic of low SFE materials [48] and also consistent with the low SFE of this alloy obtained by theoretical calculations [49] and recent experimental observations [50]. Although the precise mechanism of low SFE of the present HEA has not been clarified, the highly distorted whole-solute matrix of the HEA
Acknowledgment
The authors acknowledge the financial support of DST, India (Grant no. SB/S3/ME/47/2013). The authors also acknowledge Professor I. Samajdar, IIT Mumbai, India for kindly providing access to the national facility for Texture-OIM (DST-IRPHA) at I.I.T. Bombay, India.
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