Study on the anomalous thermal expansion behavior of cold-rolled Ti-34 Nb alloy and its novel tailoring strategy

Abstract

In this study, we have investigated the anisotropic in-plane thermal expansion behavior of cold-rolled (CR) Ti-34 Nb alloy and its relationship with temperature by means of continuous heating and cyclic heating-cooling thermal expansion measurement methods. A new microscopic model including the morphology of β and martensite (α″) phases, cold rolled textures and α″ content is proposed, which successfully explains the mechanism of anomalous positive thermal expansion of transverse direction (TD) and negative thermal expansion of rolling direction (RD). It is found that the coefficient of thermal expansion (CTE) of CR-Ti-34 Nb alloy is controlled by α″ content. The instantaneous CTEs of TD and RD (absolute value) both increase first and then decrease (nonlinearity) within the temperature range of anomalous thermal expansion, which is attributed to the increase of α″→β transformation rate (temperature increases by 1 °C) with increasing temperature and the significant decomposition of α″ phase above 260 °C. Moreover, we have excluded the influence of the anisotropy of α″ and internal stress on CTE. According to the abovementioned findings, α″ content is adjusted by pre-aging treatment for the first time, and a wide range of CTEs (−33.0 × 10−6 k−1 ~ 0.4 × 10−6 k−1) are obtained in CR-Ti-34 Nb alloy, including a variety of low (−6.3 × 10−6 k−1, −3.3 × 10−6 k−1) and zero (−1.5 × 10−6 k−1, −0.9 × 10−6 k−1, 0.4 × 10−6 k−1) CTEs with a wide temperature window of 275 °C (25 °C ~ 300 °C).