UC Berkeley

Hexagonal Close-Packed (HCP) alloys are used in structural materials applications for extreme environments such as those involving cryogenic conditions, or for nuclear cladding. However, the alloy design space explored for HCP compositionally complex alloys, namely High-Entropy or Medium-Entropy Alloys (hereafter referred to as HEAs) has been limited. Here, we propose the Ti-Zr-Hf ternary system as an ideal model for developing single-phase HCP HEAs and systematically investigate its composition-microstructure-property relationships. CALPHAD thermodynamic calculations predicted, and experimental results confirmed, that a stable single-phase HCP solid solution forms over a wide compositional range. The as-cast microstructure exhibited a systematic evolution dictated primarily by Hf content: high-Hf content alloys formed coarse dendritic structures via diffusion-dominant transformations, whereas low-Hf alloys formed fine acicular structures through diffusionless martensitic transformations. This change with Hf content is rationalized by a compositional dependence of the Continuous Cooling Transformation (CCT) kinetics. Nanoindentation revealed a strong solid-solution strengthening effect, with hardness generally increasing from 4.3 GPa in Ti-rich compositions to 5.5 GPa in high Hf-content alloys. Notably, the equiatomic TiZrHf alloy demonstrated an exceptional synergy of high yield strength (1140 MPa) and superior ductility (elongation >16%). Post-deformation analysis revealed that this is governed by the Twinning-Induced Plasticity (TWIP) effect, via the activation of {10-12} extension twins, confirmed by a characteristic misorientation angle of ∼86° across twin boundaries. Comparing alloys of contrasting c/a ratio at fixed Ti content further revealed a transition from twinning- and stacking-fault-mediated plasticity to confined planar slip, consistent with the reduced {10-12} twinning shear and enhanced slip accessibility that accompany a higher c/a ratio; the as-cast morphology, by contrast, modulates the magnitude of the work-hardening response rather than the operative deformation mode. This work establishes the Ti-Zr-Hf system as a cornerstone for developing high-performance HCP HEAs.