Phase Stabilities of Γ/Γ′ Feconialti High-Entropy Alloys Driven by Enthalpy-Entropy Competition

Abstract

Entropy engineering is commonly identified as an effective approach for phase stabilization of high-entropy alloys (HEAs). However, this high entropy philosophy in γ/γ′ HEAs seems incompatible with their intermetallic nature. Taking γ/γ′ (FeCoNi)86Ti7Al7 HEAs as object, we theoretically viewed the individual/synergetic contributions of entropy and enthalpy to the stability of γ/γ′ HEAs. Our results show that high entropy accelerates the stabilization of high-entropy γ phase but it shows limited positive effects even negative effects on the stability of high-entropy γ’ intermetallics, especially high vibrational entropy. The competitive relationship between enthalpy and entropy plays the critical role in the precipitation of γ’ intermetallics. Tailoring a large enthalpy would significantly increase the order-disorder transformation temperature (Tod) and thus promote the stability of high-entropy γ’ phase at elevated temperatures. In addition, we bridge the complex enthalpy-entropy competition to the empirical parameter (δ) of atomic size difference for further rational design of novel γ/γ′ HEAs. Based on the enthalpy-entropy relationship, we check the influence of principal-element variation and alloying-element doping on the crucial Tod value of γ’ phase. It is found that increasing Fe contents would deteriorate the stability of γ’ while doping Nb, Mo, Ta, V, W elements effectively enhance it.

Keywords: High-entropy alloys, γ/γ′ dual phase, Phase stability, Enthalpy-entropy competition, First principles