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 Ti 45 Zr 45 Al 10 , Ti 34 Zr 33 Hf 33 , Ti 35 Zr 30 Hf 30 Al 5 , and Ti 30 Zr 30 Hf 30 Al 10 alloy systems through precise thermomechanical processing and subsequent heat treatment. Ti 45 Zr 45 Al 10 , Ti 30 Zr 30 Hf 30 Al 10 , and Ti 35 Zr 30 Hf 30 Al 5 exhibited high 0.2 % proof strength from 25 °C to 600 °C. The 0.2 % proof stress increased with both mixing entropy ( Δ S m i x ) 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 (10 1 ‾ 0) 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. • The equiaxed hcp phase was successfully formed in TiZrAl and TiZrHfAl. • The 0.2 % proof stress of TiZrHfAl was higher than those of conventional Ti alloys at 600 °C. • High solid solution strengthening was obtained in the alloys with high Δ S and δ . • The electric charge density indicates strong bonding of Ti-Al, Zr-Al, and Hf-Al. • Large atomic size misfits and strong bonding cause high solid solution strengthening.