Low temperature super-plasticity response of AZ31B magnesium alloy with severe plastic deformation

Commonly, super-plasticity phenomenon is encountered at low strain rates applied at high temperatures in materials with fine grain microstructure. Recently, several efforts have been invested in the study of the unusual super-plastic behavior of material affected with Severe Plastic Deformation (SPD), in different conditions chosen for practical and economical considerations. In this context, super-plasticity deformation in such a metallurgical state may be achieved at low temperatures with relatively high strain rates. Among other materials, magnesium alloys show this unique deformation effect, mainly due to their tendency to dynamic recovery and recrystallization processes. Alloying elements such as Zr stabilizes fine grain size of ZK60, which shows super-plastic behavior at wide strain rates and at relatively low temperature ranges. In the current research, SPD has been applied to a cast of AZ31B alloy (150-200 μm grain size) using Equal Channel Angular Extrusion (ECAE) technique. The result was 5-10 μm average grain size. The super-plastic response of this alloy was obtained by means of elevated temperature tensile tests, while using round specimens, at low temperature ranges with various strain rates. The grain refinement microstructure obtained by ECAE was analyzed by means of a modern image analysis method. The results, as reflected by the super-plastic (SP) curves (elongation, stress versus strain rate), were found sensitive to the number of passes, but less sensitive to the successive rotations of the sample between the passes. The comparison between the super-plastic response of AZ31B grain refined by ECAE to the same alloy (grain refined by conventional hot extrusion process), indicated that the maximum elongation of the former specimens was shifted to higher strain rate by a magnitude order of two. This dramatic finding indicates the great influence of ECAE technique in modifying the conditions of super-plastic behavior.