Microstructural control for superplasticity simply by heat treatment without thermomechanical processing in a Ti-46Al-3.5Cr alloy

Abstract

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 α-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 1 473 K in the β+γ two-phase region, the feathery microstructure turns to fine microdual structure with the equiaxed γ grains with 13 μm in grain size and the β precipitates formed along the γ grain boundaries. In a tensile test at 1 473 K with a strain rate of 3.2×10-4 s-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 1 473 K, respectively.