Low-temperature superplasticity and coarsening behavior of Ti–6Al–2Sn–4Zr–2Mo–0.1Si

The low-temperature superplasticity and dynamic coarsening behavior of Ti–6Al–2Sn–4Zr–2Mo–0.1Si were established and interpreted in the context of inelastic-deformation theory. The starting microstructure with an equiaxed-alpha particle size of 13 μm was refined to 2.2 μm by a treatment comprising beta annealing/water quenching followed by warm rolling at 775 °C. A series of tension and load-relaxation tests were carried out for the coarse and fine microstructures at strain rates of 10−4 to 10−2 s−1 in the temperature range of 650–750 °C. The fine microstructure exhibited enhanced superplasticity (382–826% elongation) compared to that of the coarse microstructure (189–286% elongation); this trend was attributed to a larger fraction of boundary sliding and lower friction stress for the finer material. With respect to microstructure evolution, the coarsening rate of the alpha particles during deformation was ∼12 times faster than that during static coarsening. Furthermore, both the static and dynamic coarsening rates for the Ti–6Al–2Sn–4Zr–2Mo–0.1Si were 2.7–4.7 times lower than those for the Ti–6Al–4V, a trend attributable to the lower diffusivity of Mo compared to that of V at a given test temperature. The plastic flow behavior of the coarse and fine microstructures was rationalized in terms of microstructure evolution during deformation.