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The influence of chemical composition on the microstructure of the ␥-titanium aluminide alloy Ti-48Al-2W-0.5Si (at. pct) and the accompanying tensile, low-cycle fatigue, and creep properties has been evaluated. The study showed that small variations in chemical composition and casting procedures resulted in considerable variations in the microstructure, yielding vastly different mechanical properties. Low contents of aluminum and tungsten led to a coarse-grained lamellar (␥/␣ 2 ) microstructure with high creep resistance. A composition close to the nominal one produced a duplex (␥ ϩ ␥/␣ 2 ) structure with favorable strength, ductility, and low-cycle fatigue properties. By controlling the solidification and cooling rates at casting, a pseudoduplex (PS-DP) microstructure with a unique combination of high strength and high fatigue and creep resistance can be obtained. These unique properties can be explained by the diffuse boundaries between the relatively small ␥ grains and the neighboring lamellar colonies, combined with semicoherent interfaces between the ␥ and ␣ 2 phases. At tensile and low-cycle fatigue loading, these boundaries act like high-angle boundaries, producing a virtually fine-grained material promoting strength, whereas at creep loading, grain-boundary sliding is hindered in the semicoherent interfaces leading to high creep resistance.

Two different cast gamma titanium aluminide alloys with refined microstructures were studied, Ti-45Al-2Mn-2Nb (at.-%) containing 0•8 vol.-%TiB 2 ( XD45) and Ti-47Al-2Mn-2Nb (at.-%) containing 0•8 vol.-%TiB 2 ( 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 TiB 2 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.MST/4388T he author is with Volvo Aero Corp., SE-461 81 T rollha ¨ttan, Sweden. Manuscript

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