Microstructure of alpha plus beta dual phase formed from isothermal alpha '' phase via novel decomposition pathway in metastable beta-Ti alloy

摘要

Recently, the formation of the isothermal alpha '' phase (alpha ''(iso), orthorhombic) via aging heat treatment has been reported in several beta-Ti alloys. The microstructure of the alpha + beta dual phase that was obtained from the alpha ''(iso) phase via a novel decomposition pathway (beta -> alpha ''(iso) (single phase) -> alpha + beta) in metastable beta-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 alpha + beta phases was observed using transmission electron microscopy. The microstructure of the alpha + beta dual phase inherited the characteristics of the microstructure of the alpha ''(iso) phase. The microstructural features included irregular-shaped variants, dense and fine twinning (width similar to 50-100 nm). The Burgers orientation relationship was satisfied between the initial beta, alpha ''(iso), decomposed beta, and decomposed alpha phases. Furthermore, only a specific variant combination of the decomposed alpha and beta phases was observed. The crystallographic orientation (i.e., variant) of the decomposed beta phase was similar to that of the initial beta phase. These results indicated the existence of a strict variant selection rule between the alpha and beta phases. The origin of this rule was the alpha ''(iso) variant that was formed before the decomposition. When the Burgers orientation relationship was established, the alpha ''(iso) variant determined the parallel plane and parallel direction between the alpha and beta phases. (C) 2021 Elsevier B.V. All rights reserved.