The effect of prior FCC grain size on lamellar microstructure in Al₁₀(CoFeNi_1.5)₉₀ high entropy alloy

In high entropy alloys (HEAs), it is very difficult to achieve a suitable composition for eutectoid lamellar microstructure. In this work, Thermo-Calc (TCHEA3 database) was used to determine the phase transformation temperatures (solid solution and eutectoid reaction) for the Al₁₀(CoFeNi_1.5)₉₀ (at%) HEA. The effect of heat treatment on grain growth and lamellar structure transformation was thoroughly investigated. The results showed that the lamellar structure formation is very fast at the initial stage and then becomes slower until it reaches the fraction defined by the lever rule. Furthermore, the transformation kinetics and the activation energy for lamellar structure were determined by using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) and Arrhenius equations, correspondingly. The suitable prior grain size of face centred cubic structure for fast formation of lamellar microstructure is around 18 µm. The activation energy for the grain growth for the temperatures range of 1010 ∼ 1170 °C is 378.43 KJ/mol. The intrinsic hardness H₀, and Hall-Petch co-efficient K_H were calculated from the slope of the Hall-Petch curve and are 114.59 H_v and 149.48 H_v.µm^0.5, respectively. The activation energy for lamellar structure of two solid solutions (1050 °C for 10 min and 1130 °C for 15 mins) are 123.21 and 162.61 KJ/mol, respectively.