Phase formation and thermal stability of CoCrFeNi and CoCrFeMnNi equiatomic high entropy alloys

The present work focuses on the phase evolution and thermal stability of CoCrFeNi and CoCrFeMnNi equiatomic high entropy alloys in the temperature range 1073-1373 K. For comprehensive understanding, phase stability of ternary CoFeNi has also been studied in the same temperature domain. These alloys have been prepared by vacuum arc melting followed by homogenization at 1473 K for 50 h. All the three alloys show a single phase FCC structure, which is retained even after thermal exposure at 1073, 1173 and 1373 K for 96 h. Electron microscopy investigations confirm single phase structure, coarse grain size and equiatomic composition for all the as-processed and heat treated alloys. Binary Gibbs energy-composition (G-x) curves, obtained using a CALPHAD database, have been used to assess the thermal stability of CoFeNi, CoCrFeNi and CoCrFeMnNi. Driving force (D) has been calculated to measure the tendency for BCC and G phase formation in each of the alloy. These calculations indicate that the FCC phase is the most stable in all the three alloys. Phases in heat treated CoCrFeNiV alloy reported in literature, are compared with the present alloys. Our results indicate that increase in configurational entropy need not always correlate with enhanced stabilization of solid solution phases. G-x diagrams present a powerful approach to elucidate the phase formation and stability of multicomponent alloys. (C) 2018 Elsevier B.V. All rights reserved.