The relationship between microstructure and age hardening response in the metastable beta titanium alloy Ti- 11.5 Mo-6 Zr-4.5 Sn (beta III)

The influence of heat treatment and oxygen content on the aging response and micro-structure of the metastable Β-phase titanium alloy Ti-11.5Mo-6Zr-4.5Sn (Beta III) has been studied using light and electron metallography and hardness measurements. Increasing the oxygen from 0.17 to 0.28 wt pet was shown to suppress Ω-phase formation and accelerate a-phase formation. Changing the solution treatment from above to below the Β-transus was shown to significantly increase the rate of α-phase formation at residual dislocations and subboundaries present in the warm worked material. Direct aging has been shown to greatly retard the rate of Ω-phase formation by comparison to quenching and aging. Duplex aging first at a temperature in the Ω-phase formation range and then at a temperature in the a-phase formation range results in a very fine dispersion of α-phase particles and a very high hardness. It is suggested that this α-phase forms by aninsitu Ω → α transformation. In quenched samples small amounts (10 pct) of cold work were shown to accelerate the formation of Ω-phase during subsequent aging as shown by both electron microscopy and by hardness measurements. Finally, some remarks are included to indicate the application of our observations to the commercial heat treatment of Β-III.