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

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 beta-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 alpha '' phase are observed in X-ray diffraction (XRD) patterns, thus the material is quite lean in beta-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 degrees C to the cold-rolled material precipitated the omega phase causing embrittlement. Increasing the ageing temperature from 80 to 300 degrees C increased the stiffness, made the elastic regime more linear, and further embrittled the alloy. The low temperature heat-treatments precipitate both omega and alpha '' phases. A higher temperature ageing treatment at 450 degrees C increased the yield strength to over 1 GPa and caused embrittlement, indicating co-precipitation of alpha and omega phases. (C) 2015 Elsevier B.V. All rights reserved.