Deformation modes of the superplastic Al-10wt%Ti alloy at high strain-rates

Gas-atomized powders of Al-10wt%Ti alloy were consolidated by a powder metallurgy process involving hot extrusion. A microstructure of micron-size aluminum grains and fine flake-shaped Al3Ti particles was obtained. The fine-grained Al-10wt%Ti alloy showed unusually high ductility and high strain-rate sensitivity at strain rate of 10(-1) sec(-1) and at temperatures above 600 degrees C. This high strain-rate superplasticity(HSRS) is correlated with those of PM processed aluminum alloys and whisker-reinforced aluminum composites based on the microstructural similarities. The observed flow stress vs. strain rate was analysed by an empirical equation incorporating threshold stress with the stress exponent of 2 and the activation energy of lattice diffusion of Al(i.e., 140kJ/mole). Grain boundary sliding accomodated by dislocation slip is considered as the governing mechanism for the HSRS, and it is interpreted as an extension of fine grain superplasticity. Because liquid phase does not appear in the binary Al-Ti system at the tested temperatures, requirement for incipient melting for HSRS is questioned.