Effect of Zn Addition on Phase Selection in AlCrFeCoNiZn High-Entropy Alloy

Abstract

The addition of Zn to AlCrFeCoNi high-entropy alloy (HEA) poses intriguing questions as to how it would affect phase evolution. On one hand, Zn has high valence-electron count that should stabilize a face-centered-cubic phase, while, on the other, it exhibits ordering (strong clustering) tendency with Ni and Co (with Cr and Fe), hinting towards phase separation. Here, the phase evolution in AlCrFeCoNiZn was studied using a combination of experimental techniques (XRD, SEM, EDS and DSC) and computational (density-functional theory (DFT), calculation of phase diagrams (CALPHAD), and machine-learning) methods. Mechanically alloyed (MA) and spark plasma sintered (SPS) AlCrFeCoNiZn assumes a metastable single-phase, body-centered-cubic (BCC) structure that undergoes diffusion-controlled phase separation upon subsequent heat treatment to form separate (Al, Cr)-rich, (Fe, Co)-rich and (Zn, Ni)-rich phases. The formation of (Al, Cr)-rich phase, not reported previously in AlCrFeCoNi-based HEAs, is attributed to strong clustering tendency of (Cr-Zn) and (Cr-Ni) pairs, combined with the strong ordering of (Zn-Ni) pair, driving out Cr that in turn combines with Al to form a (Al, Cr)-rich phase. DFT results show the formation of thermodynamically stable L12 phase wherein Cr-Fe-Zn [Al-Ni-Co] preferably occupy1a (000) [3c (0 ½ ½)] positions. The sluggish diffusional transformation to L12 phase from BCC precursors is attributed to the small stacking-fault energy of AlCrFeCoNiZn. The equilibrated HEA exhibits a high microhardness of 8.24 GPa with an elastic modulus of 184 GPa.

Keywords: High-entropy alloy, Phase evolution, mechanical alloying, Machine Learning, density function theory, Phase separation