Microstructures with martensitic features induced by absorption of a large amount of hydrogen in a B2 single-phase Ti-22Al-27Nb alloy

Abstract

In Ti-22Al -27Nb, the β→ γ hydride transformation which is a bcc→bct transition in crystal structure exhibits some of the characteristics that are considered typical of martensitic transformation. We investigated surface relief and microstructure of a pre-deformed β 0(B2) single-phase specimen hydrogenated to different hydrogen contents. When β hydride is formed, no distinctive changes are found on their electro-polished surfaces. However, surface relief with a thin plate-like morphology is found after the β→ γ hydride transition. The plates contain a high density of aligned striations which are determined to be <111> screw dislocations. Crystallographic features of the β→ γ hydride transformation analyzed by the Bowles–Mackenzie method agree well with the experimental observation. The role of aligned dislocations in improving the hydrogen absorption/desorption properties of Ti-22Al-27Nb is discussed based on a discovery of the similarities between the strain around <111> screw dislocations and that induced by the formation of hydrides. Possible ways to control the hydrogen absorption/desorption properties via defects engineering are proposed.