Extraordinary superplastic properties of hot worked Ti–45Al–8Nb–0.2C alloy

Abstract

Superplastic behavior of the Ti–45Al–8Nb–0.2C alloy in ultrafine-grained condition has been investigated. The ultrafine-grained condition was produced via hot extrusion at T ≈ 1250 °C followed by unidirectional isothermal forging at T = 950 °C. The produced material showed extraordinary superplastic elongations (δ = 770–1342%) at T = 850–1050 °C and enhanced values of the strain rate sensitivity coefficient, m > 0.3 at ε′∼10−4–10−3 s−1. The obtained superplastic properties and the results of microstructural examination of the tensile strained specimens suggest that the grain boundary sliding was the predominant deformation mechanism during superplastic flow. In spite of a low content of the α2−Ti3Al phase and absence of the β(B2) phase, the produced ultrafine-grained microstructure showed surprisingly slow dynamic grain growth during superplastic flow at T = 850–1050 °C. The activation energy was defined to be Q = 303 kJ/mol suggesting that the predominant deformation mechanism during superplastic flow was grain boundary sliding controlled by volume diffusion of equally aluminum and titanium in γ-TiAl.