Deformation mechanisms of hexagonal close-packed-multi-principal element alloys (HCP-MPEAs) with equiaxed structures

The successful fabrication of multi-principal element alloys (MPEAs) with stable single-phase face-centered cubic (FCC) and body-centered cubic (BCC) structures has enabled numerous studies to highlight their excellent mechanical properties and distinct deformation mechanisms. However, the solid-solution strengthening (SSS) and deformation mechanisms of hexagonal close-packed (HCP)-MPEAs remain poorly understood due to the lack of stable single-phase HCP alloys. In this study, equiaxed single-phase HCP structures were successfully developed in Ti₄₅Zr₄₅Al₁₀, Ti₃₄Zr₃₃Hf₃₃, Ti₃₅Zr₃₀Hf₃₀Al₅, and Ti₃₀Zr₃₀Hf₃₀Al₁₀ alloy systems through precise thermomechanical processing and subsequent heat treatment. Ti₄₅Zr₄₅Al₁₀, Ti₃₀Zr₃₀Hf₃₀Al₁₀, and Ti₃₅Zr₃₀Hf₃₀Al₅ exhibited high 0.2% proof strength from 25°C to 600°C. The 0.2 % proof stress increased with both mixing entropy (ΔS_mix) and average atomic radius misfit (δ), aligning with calculations that indicate a stronger SSS effect at higher δ values. Density functional theory calculations further reveal that Al plays a crucial role in enhancing SSS. Deformation was primarily governed by (1010) prismatic slip. The low activation volume and high-stress exponent of these alloys at 600°C suggest that minor obstacles, such as clusters or short-range order, hinder dislocation motion, thereby contributing to significant SSS.