Effect of precipitation on mechanical properties in the β-Ti alloy Ti-24Nb-4Zr-8Sn

Coakley, James and Rahman, Khandaker M. and Vorontsov, Vassili A. and Ohnuma, Masato and Dye, David (2016) Effect of precipitation on mechanical properties in the β-Ti alloy Ti-24Nb-4Zr-8Sn. Materials Science and Engineering A, 655. pp. 399-407. ISSN 0921-5093 (https://doi.org/10.1016/j.msea.2015.12.024)

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Tensile testing and cyclic tensile loading measurements were performed on heat-treated samples of annealed Ti-2448 and cold-rolled Ti-2448. Quenching from above the β-transus produces an alloy that is highly superelastic has ultra-low elastic modulus (10-25. GPa) and exhibits hysteresis on loading-unloading cycles. On repeated cycling the strain energy absorbed in each cycle decreases. Annealed Ti-2448 exhibits a stable hysteresis loop. Peaks from the α″ phase are observed in X-ray diffraction (XRD) patterns, thus the material is quite lean in β-stabilising additions. The alloy is shown to be highly unstable when heat-treated. A combination of small angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and X-ray diffraction (XRD) was employed to relate the thermally induced microstructural evolution to the change in mechanical properties. A heat-treatment of 80. °C to the cold-rolled material precipitated the ω phase causing embrittlement. Increasing the ageing temperature from 80 to 300. °C increased the stiffness, made the elastic regime more linear, and further embrittled the alloy. The low temperature heat-treatments precipitate both ω and α″ phases. A higher temperature ageing treatment at 450. °C increased the yield strength to over 1. GPa and caused embrittlement, indicating co-precipitation of α and ω phases.