MICROSTRUCTURE AND FRACTURE-BEHAVIOR OF TI-44AL-XM DERIVATIVES

The microstructures of Ti-44Al alloys containing either 2 at.% Mo or 2 at.% Cr have been studied on as-cast, as HIPped and on heat-treated samples. Room temperature tensile tests have been carried out on the as-HIPped samples. It has been found that the microstructure of the as-cast sample of Ti-44Al-2Mo consists of large lamellar alpha2/gamma Colonies and B2 and gamma blocks at subgrain boundaries, which divide the colonies into a number of small grains. The microstructure of the as-cast Ti-44Al-2Cr is fully lamellar alpha2/gamma with a small amount of segregation. The Ti-44Al-2Mo alloy shows a novel microstructure consisting of gamma grains, (alpha2 + gamma) lamellae and B2 plates across the (alpha2 + gamma) lamellae either after HIPping at 1250-degrees-C or after annealing at 1200-degrees-C, while large lamellar colonies were observed in the Ti-44Al-2Cr sample after the same treatments. Tortuous fracture surfaces with small cleavage facets and diverted crack paths have been observed by SEM in the Ti-44Al-2Mo samples. Delamination along alpha2/gamma interfaces and lamellar colony boundaries is the predominant failure mode of the Ti-44Al-2Cr sample. As HIPped samples of Ti-44Al-2Mo with a duplex microstructure modified by B2 have much higher room temperature tensile strengths and elongation than the as-HIPped samples of Ti-44Al-2Cr.