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 mu m was refined to 2.2 mu m by a treatment comprising beta annealing/water quenching followed by warm rolling at 775 degrees 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 degrees 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 similar to 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. (C) 2010 Elsevier B.V. All rights reserved.