鋳造-HIPプロセスによって作製したTiAl基合金のミクロ組織と機械的性質に及ぼすZr添加の影響

Abstract

The effects of Zr on tensile strength and ductility of cast-HIP’ed γ-TiAl were investigated. A Ti-48 at%Al alloy was chosen as the base alloy and Zr was added to the base alloy up to 4 at% with Al/(Ti+Al) of 0.48 atomic ratio. In the tensile test of the HIP’ed alloys at room temperature, as the Zr content increased, the yield strength rose rapidly and the elongation decreased. However, the alloys containing 1 at% to 2 at%Zr showed a marked strength improvement with little decreasing elongation, which indicated the excellent strength-ductility balance that was not observed in other ternary alloys we investigated previously. All microstructures of the HIP’ed alloys studied consisted of grains in lamellar form and equiaxed γ grains. The volume fraction of grains in lamellar form increased as the Zr content increased. The yield strength of all the HIP’ed alloys showed a linear relationship with the volume fraction of grains in lamellar form. Thus the yield strength of the HIP’ed alloys could be analyzed on the basis of the rule of mixture in which these alloys were assumed to be composed of grains in lamellar form and equiaxed γ grains. The equiaxed γ region had the Zr segregated zone that enclosed lamellar colonies forming like a network. It was considered that superior strength-ductility balance of HIP’ed Zr-containing alloys was caused by strength improvement resulted from increasing volume fraction of grains in lamellar form and by ductility improvement of equiaxed γ grains due to the Zr segregated zone and of its network-like morphology. The annealing of the HIP’ed alloys at 1603 K resulted in a disappearance of the superior strength-ductility balance and the Zr segregated zone. The relationship between microstructure and tensile property of Zr-containing γ-TiAl was studied in detail.