Microstructural properties, thermal stability and superplasticity of a ZK60 Mg alloy processed by high-pressure torsion

An extruded ZK60 magnesium alloy was used to investigate microstructure, hardness and tensile properties after processing by 5 turns of high-pressure torsion (HPT) at room temperature. EBSD results confirmed the successful production of an ultrafine-grained structure with a mean grain size of ~700 nm with reasonable homogeneity and a majority of grains oriented parallel to the shear direction. This material also reached a homogeneous microhardness across the disk with an average hardness value saturated at Hv ?124 from the as-received hardness value of Hv ?74. The obtained high value is due to a high density of dislocations, the very small grain size and texture strengthening. The microhardness retained homogeneity after annealing samples processed by HPT for 40 hours at 448 K. However, the hardness value dropped to Hv ?85 while the mean grain size increased to ~2.1 ?m. These changes may be a result of restoration processes and consequent texture softening. Specimens processed by 5 turns of HPT exhibit excellent superplastic properties with a maximum elongation of 940% at 523 K and an optimum strain rate of 1.0×10-4 s-1. Significant superplasticity was observed at 448 K due to the stability of the bimodal structure at lower temperatures. This can assist the microstructure to accommodate grain boundary sliding and intragranular slip simultaneously and postpone any necking