No abstract

Ti-6Al-4V alloy having a heterogeneous microstructure composed of ultrafine-equiaxed-a-grains and fine-lamellar-a-grains is investigated for microstructural changes during superplastic deformation at temperature of 700 C. The Ti-6Al-4V alloy having an optimum fraction of fine-lamellar-a-grains exhibits an excellent superplastic property and the highest elongation of 583% (tested at 700 C 10 À3 s À1 ). This is mainly due to the optimized activation of grain-boundary-sliding and additional accommodation mechanism associated with frequent occurrences of dynamic recrystallization and b precipitation at boundaries during deformation of the heterogeneous starting microstructure. The present result suggests the possibility that optimizing the starting microstructure so as to have an optimum heterogeneousmicrostructure serves as an additional stress accommodation mechanism and leads to a large superplastic elongation.

Titanium alloys are widely used in aerospace components, with the most widely used alloy being (α+β)-type Ti-6Al-4V (hereafter designated as Ti-64) alloy owing to its high specific strength and high formability associated with superplasticity. This work examines the tensile deformation behavior of the Ti-64 alloy with the acicular α′ martensite microstructure tested at from 700°C to 900°C. Higher tensile-elongation and higher strain-rate-sensitivity value are seen in the Ti-64 alloy with the α′ martensite microstructure as compared to that with the lamellar (α+β) microstructure. During deformation of the α′ martensite microstructure at 700°C or 800°C, acicular microstructure evolves into fine equiaxed (α+β) structure, whereas there is no apparent change in microstructure in the case of the lamellar (α+β) starting microstructure. This result indicates that dynamic globularization during deformation is strongly enhanced in the acicular α′ martensite starting microstructure, thereby leading to higher tensile elongation.