No abstract

An extruded ZK60 magnesium alloy was processed by high-pressure torsion (HPT) at room temperature for up to 5 turns under a constant compressive pressure of 2.0 GPa with a rotation speed of 1 rpm. This processing produced an average grain size of ~700 nm. The grain size distributions and textures were examined by electron backscatter diffraction (EBSD) and this revealed some multi-modality in the microstructure at different stages of straining with fractions of both coarse grains and ultrafine grains. EBSD analysis at the mid-radius positions of unprocessed and HPT-processed materials revealed a gradual evolution from a prismatic {101 ത 0} fiber to an ultimate basal {0001} fiber texture with the c-axis parallel to the normal direction.The majority of grain boundaries had misorientations larger than 15 o throughout the processing.The strain hardening tended towards a reasonable hardness homogeneity with a hardenability exponent, η, of 0.07 up to strains of ~20 and with a subsequent hardness saturation at Hv ≈ 125.

Severe plastic deformation routes such as high-pressure torsion (HPT) are capable of producing ultrafine grain sizes in various alloys, specifically in magnesium alloys which exhibit poor ductility due to their hexagonal close-packed (hcp) crystal structure. HPT was performed on ZK60 magnesium alloy samples at room temperature under a pressure of 2.0 GPa up to 5 turns. The Vickers microhardness values were obtained from the centre to the edge of the disc samples and they show a slight hardness gradient with values which are lower at the centre of the samples and higher towards the edge. By increasing the numbers of turns in HPT, the hardness values increase to a saturation level and the gradient is removed. Tensile tests were performed at high temperatures and the results reveal a significant increase in elongation to failure as the numbers of turns in HPT increases. It is shown that microstructural analysis is in agreement with the results obtained from mechanical testing.

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 µm. 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.