2000 年 40 巻 10 号 p. 1041-1047
Microstructural evolution only by heat treatment has been studied for a Ti–46at%Al–3.5at%Cr alloy, in order to obtain a microstructure which causes superplasticity at high temperatures. By changing the cooling rate from 1 613 K in the a-Ti single-phase region, three kinds of microstructures were identified. Namely, lamellar microstructure appeared by furnace cooling, feathery microstructure took place by air-cooling and massive microstructure prevailed by oil quenching. During subsequent annealing at 1473 K in the β + γ two-phase region, the feathery microstructure turns to fine microdual structure with the equiaxed γ grains with 13 υm in grain size and the β precipitates formed along the β grain boundaries. In a tensile test at 1473 K with a strain rate of 3.2×10-4s-1, this β/γ microdual structure exhibits remarkable superplastic deformation with an elongation of 450%, which is the same with that obtained by the β/γ microdual structure prepared by a thermomechanical processing. On the other hand, the lamellar microstructure and the massive microstructure are not transformed to the β/γ microdual structure with the equiaxed γ grains, resulting in low elongations of 30% and 110% at 1473 K, respectively.
