Influence of short-range order on precipitate orientation relationships in aluminum containing FCC high entropy alloys

Abstract

High entropy alloy (HEA) phase evolution is governed by the competing roles of high configurational entropy and enthalpy of mixing, including severe lattice distortion, and local, or short range, atomic order. While HEAs have seen unprecedented interest over the last decade, many promising applications have not been realized due to limitations in secondary phase, or precipitate, control. Through high resolution microscopy and spectroscopy coupled with molecular dynamics simulations, we examine the role of chemical complexity on the evolution of precipitates, and specifically on their orientation relationships with their host matrices. Microstructural, chemical, and local order measurements are coupled with atomistic simulations of the structure and energy of the Face Center Cubic (FCC)/Body Center Cubic (BCC)B2 interface, in various possible orientations, using model interatomic potentials. Our local order measurements at the nanometer scale revealed that Cr-(Co/Ni/Fe) bonding becomes less favorable after aging. This finding aligns with our microstructural observations, which show lower Cr and Al content in the FCC phase post-aging. We experimentally observed a non-typical orientation relationship between B2-BCC and FCC matrices was stabilized, which we attribute to this chemical complexity. Our atomistic simulations reveal the significant effect of chemical complexity and local ordering on interface energies. Critically, we connect the local chemical order with the formation of high energy interfaces that lead to unusual orientation relationships. The relationship between local order and the orientation relationships landscape of precipitates within a microstructure presents an opportunity for tuning alloy properties at the level of atomic bonding.