Lattice Distortion and Solid-solution Hardening of Multi-component Alloys with Equiatomic Compositions

Radically different from conventional solid-solution alloys with a well-defined base, multi-component alloy (MCA) systems with equiatomic compositions constitute a novel class of so-called high-entropy alloys (HEAs). At present, considerable work has devoted to screen HEAs for mapping their alloy compositions and properties. However, the scientific effort on understanding their physical metallurgy and mechanical behavior is still limited. Based on the recent studies, most HEAs are brittle and lack of room-temperature ductility in tension; on the other hand, those HEAs with fcc solid-solution structures have sufficient ductility but relative low strengths. In view of these findings, this proposal will focus on three basic aspects for fundamentally understanding the solid-solution effect and hardening in these fcc MCAs: 1) lattice distortions by alloying elements with large size difference, 2) solid solubility of these alloying elements, and 3) scientific principles for design of these novel MCAs with desirable strength and ductility.Firstly, we plan to understand the lattice distortions in these MCAs containing atoms with different sizes. The line broadening in traditional x-ray diffraction and pair distribution functions obtained from neutron scattering will be used to reveal the intrinsic lattice distortion due to the atomic-size effects. The advanced methods based on nano-beam electron diffraction (NBED) and neutron total scattering will be employed to characterize local atomic orders, and the results so obtained will be compared with those calculated from first-principles simulations.Secondly, we need to understand the solid solubility in these MCAs. Our studies will aim at establishing the correlation of the solid solubility with the fundamental properties of atomic-size difference, mixing entropy, mixing enthalpy, electronegativity, and electron concentration of solute atoms. Experimental verification will be conducted in the fcc transition-element alloy systems of (NiCoFe), (NiCoCr) and (NiCoFeCr) with additions of Ti and Nb having large atomic sizes.Finally, we intent to develop scientific principles for strengthening these fcc MCAs. By considering the intrinsic lattice distortion, short-range order and solid solubility, the strengthening behavior of these MCAs will be evaluated and predicted. Scientifically, a successful completion of this proposed research will lead to a fundamental understanding of the solute effects and hardening of these novel MCAs. Technically, our ultimate goal is to design the strong and ductile MCAs for structural applications.