Superplasticity of metastable ultrafine-grained Ti[sbnd]6242S alloy: Mechanical flow behavior and microstructural evolution

Abstract

Herein we quantitatively clarify the effects of grain size, β fraction, and morphology on the high-temperature deformation behavior of the Ti6Al2Sn4Zr2Mo–0.1Si alloy. For this purpose, five materials were subjected to high-temperature tensile deformation: the UFG1 specimen (having an equiaxed morphology with dα = 0.78 μm and Vβ = 2.8%), the UFG2 specimen (having an equiaxed morphology with dα = 0.99 μm and Vβ = 24.2%), the FG1 specimen (having an equiaxed morphology with dα = 2.65 μm and Vβ = 11.2%), the FG2 specimen (having an equiaxed morphology with dα = 4.12 μm and Vβ = 11.0%), and the STQ specimen (with an acicular α′ martensite morphology). The UFG1 specimen is produced by hot-rolling of the STQ specimen having an acicular α′ martensite microstructure at 750 °C. The UFG2 specimen is prepared by heat treatment of the UFG1 specimen at 400 °C. The FG1 specimen is as-received Ti6242S alloy plate, and the FG2 specimen was prepared by heat treatment of the FG1 specimen at 900 °C. The UFG specimens exhibited higher ductility associated with frequent activation of superplasticity than the FG specimens owing to the effect of decreasing grain size. The STQ specimen exhibited higher ductility at 700 °C than the FG specimens. Quantitative analysis of the deformation mode according to internal-variable theory revealed much more grain boundary sliding in the UFG specimens. A comparison of the deformation behavior of the UFG1 and UFG2 specimens revealed excellent superplastic ductility in the UFG2 specimen at higher strain rates (10−3 and 10−2 s−1) and in the UFG1 specimen at lower strain rates (5 × 10−4 and 10−4 s−1). This behavior is ascribed mainly to different accommodation mechanisms during deformation of these specimens; dynamic β precipitation from supersaturated α microstructure occurred in the UFG1 specimen, whereas a decomposition process in which supersaturated β precipitates dissolve into the α phase was enhanced in the UFG2 specimen. In addition, the excess β precipitation observed in the UFG2 specimen led to enhanced α/β grain boundary sliding, resulting in further enhancement of the superplasticity.