X-Ray Diffraction Analysis of Sigma-Phase Evolution in Equimolar AlCoCrFeNi High Entropy Alloy

Novel equimolar AlCoCrFeNi high entropy alloy (HEA), as a promising candidate for demanding engineering applications has recently received considerable attention due to its random solid solution structure. It confers its synergy of high strength, ductility, high-temperature structure stability and corrosion resistance. Thermally assisted spinodal dissolution from BCC random solid solution (B1) to ordered BCC (B2) and precipitation of brittle σ-phase renders its ability to undergo as an alloy for thermomechanical processing. Therefore, it was necessary to manifest σ-phase nucleation and growth from homogenization temperature (1250oC) at different cooling rates, i.e., furnace cooled and water quenched along with variation in σ-phase crystallite size at transformation temperature (800oC) in Fibonacci time frame (1h, 2h, 4h, 8h, and 16h) to know hot working temperature range and post treatment. An equimolar AlCoCrFeNi high-entropy alloy fabricated via vacuum induction melting was subjected to ICP-MS and XRD analysis to confirm its chemical composition and crystal structure distribution. The XRD results were further analyzed by using Scherrer’s equation and Williamsons-Hall plot to calculate the crystallite size of σ-phase and its associated microstrain. The work aimed to determine the σ-phase reversibility from homogenization temperature 1250oC and crystallite size growth at transformation temperature (800oC) to reveal the importance of thermomechanical processing parameters of equimolar AlCoCrFeNi HEA. In the present study, remarkable retention of the FCC phase was witnessed during furnace cooling and its traces appeared during rapid cooling. The vital phenomenon of FCC-phase nucleation signifies the key importance of the transformation zone, revealing its effect on the post-treatment cooling rate. However, its coupled phenomenon of the tetragonal σ-phase formation remained suppressed during cooling from homogenization temperature (1250οC). It determined the wide hot workability range of equimolar AlCoCrFeNi HEA to make it a suitable candidate for bulk engineering applications.