Mechanical properties of ZK60 magnesium alloy processed by high-pressure torsion

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.</jats:p

An investigation of the limits of grain refinement after processing by a combination of severe plastic deformation techniques: a comparison of Al and Mg alloys

An Al-7075 aluminum alloy and a ZK60 magnesium alloy were processed by acombination of equal-channel angular pressing (ECAP) for 4 passes and high-pressure torsion (HPT) through total numbers of up to 20 turns. Processing by ECAP and HPT were performed at 473 K and room temperature, respectively. Mechanical testing showed an increase in the hardness value of the Al-7075 alloy after a combination of ECAP and HPT whereas in the ZK60 alloy the hardness was reduced. Microstructural images of the Al-7075 alloy revealed verysignificant grain refinement after a combination of ECAP + HPT compared to the individual processing techniques. By contrast, and consistent with the hardness measurements, the average grain size of the ZK60 alloy was larger after processing by the two-step SPD technique. These results are examined and an explanation is presented based on the available microstructural evidence

Orientation imaging microscopy and microhardness in a ZK60 magnesium alloy processed by high-pressure torsion

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 withfractions 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 {1010} 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 15o 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

Microstructure and hardness evolution in magnesium processed by HPT

High pressure torsion provides an opportunity to process materials with low formability such as magnesium at room temperature. The present work shows the microstructure evolution in commercially pure magnesium processed using a pressure of 6.0GPa up to 10 turns of rotation. The microstructure evolution is evaluated using electron microscopy and the hardness is determined using dynamic hardness testing. The results show that the grain refinement mechanism in this material differs from materials with b.c.c. and f.c.c. structures. The mechanism of grain refinement observed at high temperatures also applies at room temperature. The hardness distribution is heterogeneous along the longitudinal section of the discs and is not affected by the amount of deformation imposed to the material