High temperature low cycle fatigue properties of two cast gamma titanium aluminide alloys with refined microstructure

Abstract

Two different cast gamma titanium aluminide alloys with refined microstructures were studied, Ti–45Al–2Mn–2Nb (at.-%) containing 0·8 vol.-%TiB₂ (XD45) and Ti–47Al–2Mn–2Nb (at.-%) containing 0·8 vol.-%TiB₂ (XD47). The fine grained, nearly lamellar microstructure of XD45 shows superior low cycle fatigue (LCF) properties compared with the coarser duplex structure of XD47. The lifetime for both alloys can be attributed to the amount of inelastic strain in each cycle and the difference in life between XD45 and XD47 increases as the strain range decreases. Overall, XD45 exhibits better LCF properties due to a combination of higher yield strength, lower elastic modulus, and smaller sized lamellar colonies. In both XD45 and XD47, a majority of the fracture initiations occurred at the surface or subsurface at or near weak spots in the microstructure. Such weak spots can be defects such as pores/cavities or surface damage but also TiB₂ laths, lamellar colonies oriented perpendicular to the loading direction and debonded gamma grains or grain clusters acting as stress raisers. Both alloys exhibit multiple crack initiation and stable crack growth.