Creep resistance of certain alloys of the Ti–AI–Zr–Mo–Si system

Abstract

An investigation has been made of the effects of alloy composition and of heat treatment on the high-temperature creep resistance of some α-alloys of the Ti–Al–Zr–Mo–Si system related to the IMI 685 Ti-alloy composition (Ti–6Al–5Zr–0·5Mo–0·25Si wt-%). At least two mechanisms are rate controlling in the temperature range 660–860 K. At low temperatures in this range the creep exhibits nearly a thermal characteristics, the rate being only slightly dependent on temperature and characterized by very low apparent activation energy values ∆H_ap and by some anomalous values of stress exponent n. The behaviour is attributed to a drag creep mechanism. The high values of ∆H_ap and n observed for creep at higher temperatures are attributed to microstructural changes which occur during the tests. Stress-assisted precipitation occurs on dislocations during creep and it is proposed that the creep rate is limited by the rate at which the dislocations can break away from the precipitates.