Superplasticity in a multi-directionally forged Mg-Li-Zn alloy

Superplastic behavior of the Mg-8Li-lZn alloy, processed by extrusion at 573 K followed by multi-directional forging (MDF) at 423 K, was studied using shear punch testing (SPT) of miniature specimens at various temperatures and strain rates. Microstructural analysis indicated moderate grain refinement, reducing the gran size from 8 gm in the initial as-extruded condition to 4 pm after 8 passes of MDF. Processing by MDF not only refined the grain structure but also provided a more homogeneous microstructure and a larger fraction of high-angle grain boundaries. SPT was performed at shear strain rates in the range of 3.3 x 10(-3)-1.3 x 10(-1) s(-1) and temperatures in the range of 498-573 K. The strain rate sensitivity (SRS) index (m-value), obtained from the slope of the central region of the sigmoidal shear stress-shear strain rate curves, was as high as 0.51 for the MDF processed material at 548 K. This is in contrast to the as-extruded material that exhibited a linear behavior with a maximum m-value of only 0.29 at the same temperature. For the MDF processed material, the activation energy of 61 kJ mol(-1), which is close to 67 kJ mol(-1) for the grain boundary diffusion in beta-Li, and the high m-value of 0.51 suggest that the prevailing deformation mechanism is grain boundary sliding (GBS) controlled by grain boundary diffusion.