The effect of silicon on the structure and mechanical properties of an α+Β titanium alloy

Abstract

Titanium 4 wt% Al-4 wt% Mo-2 wt% Sn containing 0, 0.25 and 0.5 wt% Si has been solution-treated in the α+Β phase field at 900‡ C. The microstructures obtained at room temperature after cooling from 900‡ C at various rates have been determined using transmission electron microscopy and the partitioning of the elements between the phases has been established using X-ray energy dispersive analysis on the thin foils. The degree of partitioning increases with decreasing cooling rate: aluminium partitions to the α-phase, molybdenum and silicon to the Β-phase and tin remains uniformly distributed. Silicon is found to inhibit the partitioning of molybdenum: this has a profound effect on the stability of the Β-phase and the resultant microstructure. In quenched material containing transformed Β, substantial age hardening can be obtained in the range 350 to 600‡ C and is associated with precipitation within the orthorhombic martensite phase, possibly occurring via a spinodal mechanism. Silicon has little effect on the microstructure of air-cooled samples but contributes to high-temperature strength via dynamic strain ageing.