Phase-field simulation of spinodal decomposition and its effect on stress-induced martensitic transformation in Ti–Nb–O alloys

Abstract

In this study, the phase-field method was used to simulate the spinodal decomposition of the β phase in Ti–23Nb–XO (atomic percent (at.%)) alloys (X = 1, 2, 3, 4, and 5) at 1073 K and the subsequent stress-induced β → α” martensitic transformation (MT) at 300 K. At 1073 K, the β phase separates into an O-rich β1 phase and an O-lean β2 phase in some Ti–23Nb–XO alloys (X = 2, 3, 4, and 5). The tendency toward phase decomposition increases with an increase in oxygen content. When an external tensile stress of 240–270 MPa is applied to certain Ti–23Nb–XO alloys (X = 1, 2, and 3) along the [0 1 1]β direction at 300 K, a stress-induced β → α” MT occurs. In Ti–23Nb–XO alloys (X = 4 and 5), the β1 → α” MT first occurs at 50–110 MPa and the nanoscale α” phase with a size of around 2 nm is formed, reflecting the (β1 + β2) microstructure. This change is followed by a β2 → α” MT at 660 MPa in Ti–23Nb–4O alloy; however, the β2 → α” MT does not occur even with an external tensile stress of 800 MPa applied along the [0 1 1]β direction in Ti–23Nb–5O alloy. Further, the calculated hysteresis loop of the stress–strain (S–S) curve at 300 K becomes slim as the oxygen content increases. It is therefore concluded that the S–S characteristics of Ti–Nb–O alloys are strongly influenced by the (β1 + β2) microstructure, which gives rise to a nanoscale heterogeneity of the driving force for the β → α” MT.