The present study is aimed to investigate two classical phenomena in the low stacking fault energy (SFE) materials with face centered cubic (FCC) crystal lattice. The first one is the transition of the copper-type texture into the brass-type texture during the cold rolling. Although it is a well-known phenomenon in deformation behavior of low SFE material, the role of deformation twinning is not quite certain. The difficulty arises due to length scale of deformation twinning which is in nanometers. The second phenomenon is related to texture evolution during primary recrystallization of cold rolled materials with low SFE. Despite the final annealed texture is known for long times, the contribution of annealing twins has remained enigma even after decades of research. An equi-atomic CoCrFeMnNi alloy, also known as Cantor alloy, was chosen for the current study as it has low SFE and exist in single FCC phase. This alloy belongs to a class of materials termed as high entropy alloys (HEAs) due to their high configurational entropy. The study of deformation and annealing behaviors was carried out with state-of-the-art experimentation such as electron channeling contrast imaging (ECCI), transmission Kikuchi diffraction (TKD), electron back-scattered diffraction (EBSD) techniques etc. Furthermore, deformation behavior was investigated with crystal plasticity simulations while annealing behavior was simulated using Monte Carlo simulations as well as phase field simulations to understand the microstructure and texture evolution comprehensively. The main objective was to gain new perspectives regarding the role of twinning on microstructure and texture evolution during both the deformation and the annealing.
The deformation behavior during cold rolling of the equiatomic CoCrFeMnNi high entropy alloy (HEA) was studied by imposing thickness reductions of 20, 40, and 60%. Important aspects of deformed microstructures such as the activation of multiple twin variants and the formation of shear bands in the matrix were captured using electron back-scatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) techniques. Twin trace analysis (TTA) was performed in conjunction with resolved shear stress (RSS) analysis for the identification of active twin variants. The RSS ratio, which is a ratio of the maximum RSS values for corresponding twin and slip systems, was used to reveal the orientation dependence of deformation twinning. Visco-plastic self-consistent (VPSC) simulations were carried out to predict the evolution of the crystallographic texture, the transition routes of ideal orientations subjected to activation of multiple variants of deformation twins. Experimental and simulation results substantiated the key finding of the deformation twinning of a Brass orientation, which established new perspectives concerning the evolution of microstructure and texture. One twin variant of the Copper orientation was moved to a Goss orientation by dislocation slip while the other two variants were rotated towards Brass and S orientations. Meanwhile, twin variants of the S and Brass orientations primarily transitioned to a Brass orientation. The Goss orientation showed great resistance to the twinning mode. Furthermore, dislocation slip and the formation of shear bands contributed to strengthening of deformation texture while deformation twinning delayed the transition of copper-type texture to brass-type texture.
The annealing behavior of the HEA was studied by subjecting the 80% cold rolled specimens to annealing heat treatment at 700° C for various time ranging between 5 minutes to 1 hour. During annealing of cold rolled HEA, the occurrence of multiple generations of annealing twins (MGAT) led to significant texture weakening with the evolution of some new prominent texture components. However, the exact correlation between MGAT and texture evolution in low SFE materials is a long-standing enigma. The presence of MGAT in HEAs was confirmed by transmission Kikuchi diffraction (TKD) and high resolution electron backscattered diffraction (EBSD) in partially (700℃/5min) and fully (700℃/1h) recrystallized specimens of equiatomic CoCrFeMnNi high entropy alloy, respectively. Further, the twin clusters in the vicinity of the potential nucleation sites (Brass, Goss and S orientations) were investigated to gain insights on the contribution of MGAT on texture development. The newly developed (φ₁=90°, φ=30°, φ₂=45°) orientation showed its roots originating from 2nd generation twin of the Brass orientation while Cube orientation was evolved from the 1st and 5th generation twins of the S orientation.
Lastly, the annealing behavior of the HEAs was modeled through simulation techniques such as Monte-Carlo (MC) simulations and multi-phase field (MPF) simulations. Both type of models were incorporated with annealing twinning scheme to gain insights on the texture randomization effect of multiple twinning during recrystallization. As MC simulations are probabilistic in nature, it could not follow complex grain growth phenomena such as misorientation based grain boundary mobility, curvature driven growth. These phenomena can be modeled in MPF modeling. However, this leads to higher computational cost in the MPF model as compared to MC simulations. The stored energy due to plastic deformation was driving mechanism for the grain growth and was modeled by calculating the grain boundary energy of sub-grain structures. Both models were able to reproduce experimentally observed multiple generations of annealing twinning. However, the twin variant selection plays in an import role in the texture evolution of low SFE materials. The exact mechanism of twin variant selection is still unknown. This limits the exact prediction of experimental texture through annealing simulation techniques.