Phase transformations induced by thermo hydrogen treatment in Ti-15 at.% Fe and Ti-15 at.% Cr alloys

Abstract

Hydrogen charging at 1000 °C to two β-Ti alloys, Ti-15 at.% Fe and Ti-15 at.% Cr, was found to induce the precipitation of B2 TiFe and C15 TiCr₂, respectively. For Ti-15 at.% Fe, the β/TiFe orientation relationship (OR) is Cube-Cube type. At 14 at.% H (nominal), an FCC layer containing {111} twins, having identical composition to the β phase and Baker-Nutting OR with TiFe, formed at β/TiFe interface upon water quenching (WQ). At 28 at.% H, WQ produces α′ martensite and athermal ω instead of the FCC layer at the interface. For TiCr₂ precipitates in Ti-15 at.% Cr, at 16 at.% H, there exist two dominant β/TiCr₂ ORs, leading to three types of twin pairs in TiCr₂. The β/TiCr₂ interface as well as the twin boundaries are decorated by TiCr₂ stacking faults. At 28 at.% H, the ORs evolve to Kurdjumov-Sachs type, and an FCC layer with internal twins was observed at the β/TiCr₂ interface. The diffusionless formation of the FCC layers in the two alloys as well as the occurrence of twinning effectively relieved the large strains at β/TiFe and β/TiCr₂ interface. An analysis based on displacement gradient tensor has been conducted, and the results provide effective means for understanding the twin variant selection and strain relieving mechanism.