Enhanced Work-Hardening and Toughening Via Contiguous Stress-Induced Martensitic Transformation and Mechanical Twinning in Ti-Zr-Hf-Nb-Al High Entropy Alloys

Abstract

Refractory high entropy alloys (HEAs) have been considered as candidate alloys for next-generation high-temperature applications, but encounter the strength-ductility trade-off dilemma. In this article, we proposed a strategy of strengthening and toughening for Ti-rich refractory HEAs by the initiation of martensitic transformation and mechanical twinning successively. Three Ti-Zr-Hf-Al-Nb HEAs with the tensile strength and elongation up to 1167 MPa and 34%, respectively, which are superb among most of the reported BCC HEAs, have been successfully prepared. By using the SEM, TEM, HE-XRD and EBSD setups, the as-cast and fully recrystallized microstructures for these HEAs were characterized. And the mechanisms for the martensitic transformation and mechanical twinning in these HEAs are deeply explored. It has been clarified that under the as-cast and fully recrystallized state, these HEAs are mainly composed of β phase and a little of α′′ martensite phase. Stress-induced martensitic transformation took place in favor of special variants with the [110]β//LD, which has the maximum transformation strain of 7.8%. At the later stage, mechanical twinning α′′ phase have formed between the α′′ laths and the β matrix with the preferential orientation relationship of [111]β//[101]α′′M//[-10-1]α′′T.