Pressure induced irreversible phase transformation in an ordered high-entropy alloy

Abstract

We report on a pressure induced irreversible phase transformation in an ordered eutectic high-entropy alloy (EHEA) AlCoCrFeNi2.1. The suction cast sample was synthesized by arc melting of raw elements resulting in an ordered B2 and FCC phase mixture for eutectic AlCoCrFeNi2.1 alloy. This phase mixture was confirmed by transmission electron microscopy (TEM) where superlattice reflections corresponding to ordered B2 phase were observed while the FCC phase remains disordered. The B2 phase was observed to be enriched in Nickel and Aluminum while the surrounding FCC phase is enriched in Fe, Cr and Co. The suction-cast EHEA sample was studied by X-ray diffraction under high pressure in a diamond anvil cell with platinum pressure marker to 73 GPa at ambient temperature. The B2 to FCC phase transformation is completed by 3 GPa in sharp contrast to BCC to FCC phase transition of 21 GPa in additively manufactured EHEA samples. The X-ray diffraction and TEM studies of pressure recovered samples show presence of only FCC grains without any specific orientation with respect to starting sample. These results are consistent with molecular dynamics simulations that the presence of B2 phase aids in the formation of the B2 to FCC phase transition under high pressures.