FRACTURE-TOUGHNESS AND HIGH-TEMPERATURE S TRENGTH OF CAST AND HIPED ZR ADDED TIAL

We reported before that cast and HIP'ed TiAl alloys with adequate Zr-a ddition showed the excellent strength-ductility balance at room temper ature. In this study, the tensile strength at high temperatures and th e fracture toughness at room temperature of the Zr-added alloys were i nvestigated. Ti-48 at%Al alloy was chosen as the base alloy and Zr was added up to 4 at% in the base alloy with Al/(Ti+Al) of 0.48 atomic ra tio. Microstructure of Zr-added alloys consisted of higher volume frac tion of the grains in lamellar form and equiaxed gamma grains. Equiaxe d gamma region had the Zr segregated zone which enclosed the lamellar colonies forming like a network. This microstructural feature resulted in the excellent strength-ductility balance at room temperature as di scussed in detail previously. In the results of tensile test at high t emperatures up to 1073 K, 2 at%Zr-added alloy showed higher strength t han that of the base alloy. Fracture toughness at room temperature inc reased with increasing Zr content. For the binary alloy and the Zr-add ed alloys, the tensile strength at 1073 K and the fracture toughness a t room temperature showed a linear relationship with the volume fracti on of the grains in lamellar form. Therefore the advantages of the lam ellar structure brought about the higher tensile strength at high temp eratures and fracture toughness in Zr-added alloys. 2 at%Zr-added allo y is superior to the base alloy and also well balanced compare with th e other binary alloys in the various properties, strength at RT and hi gh temperature, ductility at RT and fracture toughness.