Alloy Development and Optimisation Informed by an Understanding of Cold Dwell Fatigue Sensitivity

Abstract

Near α titanium alloys have demonstrated a sensitivity to fatigue loading when incorporating a dwell period at peak applied stress under room temperature. Historically, this was first reported for the relatively coarse grained variant IMI685 and later exemplified when characterising TIMETAL® 834 (Ti-834). Various mechanical factors have been identified as key drivers for the phenomenon, including time on load and high R ratios. This indicates that sub-critical damage is accumulated via creep style mechanisms and has led to analogies with “cold creep” behaviour. The propensity of quasi-cleavage faceting at the initiation sites of fatigue cracks formed under dwell conditions eventually led to an understanding of stress redistribution between strong and weak grains. This mechanism can be accentuated by more extensive regions of common crystallographic grain orientation in the form of “macrozones”. Through this fundamental understanding, new alloys can now be developed with the intent to circumvent dwell sensitivity. The present paper will focus on the fatigue performance of TIMETAL® 575 (Ti-575), a recently developed alloy optimised for aero-engine applications. Ti-575 was designed for improved strength and fatigue performance. The alloy’s susceptibility to dwell fatigue has been avoided through control of the microstructural evolution kinetics and associated thermo-mechanical process route to induce fine scaled, bi-modal microstructure containing equiaxed primary grains and secondary α laths with inherent random grain orientations, thus minimising the formation of macrozones. This work will detail the dwell fatigue testing, interpretation of data and associated microstructural characterisation, including EBSD, and compare this analysis with similar results from more conventional alloys such as Ti-6Al-4V and Ti-834.