Relationship between the structure and mechanical properties in ß III titanium alloy

Abstract

The ageing reactions that take place during the heat-treatment of solution-treatedβ III titanium (11.5 wt% Mo, 6 wt% Zr, 4.5 wt% Sn, balance Ti) were followed by detailed structure characterization using electron microscopy. The variations in mechanical properties with heat treatment were also followed systematically. The electron microscopy investigations indicated that the omega phase formed on quenching. The size and volume fraction of the omega phase increased on subsequent ageing,β,ω andα phases were found to co-exist at ageing temperatures between 800 and 900° F (427 and 482° C) for short ageing times. From the observations of interfacial dislocations at theβ/ω interface and the precipitation of fine alpha near the omega particles, with a morphology that is characteristic of the priorω morphology, it is suggested that theα-phase forms directly from the omega phase. The observed increase in yield strength over the solution-treated condition, due to the precipitation ofω phase, was found to agree well with that predicted by the Orowan hardening mechanism. Since the precipitation of fine ellipsoidalα-phase was found to increase the yield strength of the alloy with reasonable ductility, it is suggested that the optimum heat treatment to produce high strength with good ductility inβ III titanium is to age at 900° F (482° C) for 10 to 25 h.