Microstructure of α + β dual phase formed from isothermal α″phase via novel decomposition pathway in metastable β-Ti alloy

Abstract

Abstract Recently, the formation of the isothermal α″ phase (α″iso, orthorhombic) via aging heat treatment has been reported in several β-Ti alloys. The microstructure of the α + β dual phase that was obtained from the α″iso phase via a novel decomposition pathway (β → α″iso (single phase) → α + β) in metastable β-Ti alloys was investigated in the present study. Ex-situ X-ray diffraction measurements and scanning electron microscopy were performed to elucidate the phase constitution and the microstructural evolution, respectively, during the decomposition. The detailed microstructure of the α + β phases was observed using transmission electron microscopy. The microstructure of the α + β dual phase inherited the characteristics of the microstructure of the α″iso phase. The microstructural features included irregular-shaped variants, dense and fine twinning (width ~ 50–100 nm). The Burgers orientation relationship was satisfied between the initial β, α″iso, decomposed β, and decomposed α phases. Furthermore, only a specific variant combination of the decomposed α and β phases was observed. The crystallographic orientation (i.e., variant) of the decomposed β phase was similar to that of the initial β phase. These results indicated the existence of a strict variant selection rule between the α and β phases. The origin of this rule was the α″iso variant that was formed before the decomposition. When the Burgers orientation relationship was established, the α″iso variant determined the parallel plane and parallel direction between the α and β phases.