Tensile and low cycle fatigue properties of Ti-48Al-2W-0.5Si gamma titanium aluminide

摘要

Nine batches of the #gamma# -TiAl alloy Ti-48al-2W-0.5Si with a slightly varying composition have been examined through tensile and low cycle fatigue testing. The study has shown a large influence of the microstructure on the tensile and fatigue properties. A duplex fine grained microstructure has superior properties in comparison with a material low in aluminium with a coarse grained nearly lamellar structure. In addition, materials with the duplex structure show a much smaller scatter in test results. The brittle to ductile trasition behaviour typical for #gamma# -TiAl occurs between 700 and 800 deg C. The fracture mode of the equiaxed #gamma# -grains is altered from a brittle inter-or transgranular mode to a ductile dimple-like fracture mode in this temperature range. The lamellar colonies sustain their brittle fracture behaviour. The low cycle properties can be described with the well-known Coffin-Manson equation. The strain controlled fatigue properties show a strong sensitivity to the strain range, an effect that can be attributed to the high Young's modulus and the relatively low yield strength of the material which elads to inelastic straining and fatigue damage at small strain ranges. The duplex material exhibits longer lives due to larger isotropic hardening and smaller Bauschinger effect, which lead to smaller inelastic strains and damage in each cycle. As in the tensile testing the scatter in results is smaller. The scatter in tensile and fatigue test results is explained by the anisotropic properties of the lamellar colonies. Fractography has shown that initiation often occurs between the lamellas in large lamellar colonies at the surface of the gauge length. The probability to have a lamellar colony with the lamellas oriented perpendicular to the loading axis producing premature failure is increased as the content of this microconstituent is increased.