Microstructural Characterization of Zn-3Mg(wt.%) Processed by High-Pressure Torsion

High-pressure torsion (HPT) was carried out on a cast-then-homogenized Zn-3Mg(wt%) alloy under 6 GPa pressure for 1, 5, 15, and 30 turns at room temperature. During HPT it is well known that strain increases with increasing the number of turns and increasing radius from the center of the disks. The Vickers microhardness increased with increasing the HPT strain, but it reached a saturation level of approximately 135Hv. The maximum hardness achieved by the HPT processing was approximately 25% higher than the as-homogenized casting prior to HPT processing. After the HPT processing, the microstructure in different regions of the samples was characterized by X-ray diffraction, scanning electron microscopy, electron backscattered diffraction, atom probe tomography, and transmission electron microscopy. Grain size and morphology, and the formation of different intermetallics were analyzed to explain the hardness distribution. The phases present in the as-homogenized alloy were pure Zn and Mg₂Zn₁₁ intermetallic. The application of HPT did not introduce any observable phase transformations. The volume fraction of the pure Zn and Mg₂Zn₁₁ intermetallic phases remained approximately the same at both low and high HPT strains, but the grain size of the pure Zn phase refined from about 3 microns in the center of the disks after 1 turn to nano-level at the higher strains where the radius approached 5mm and the number of turns increased to 15 and 30. Both the pure Zn grains and the Mg₂Zn₁₁ intermetallic regions combined to form a lamellar structure in the as-cast and cast-then-homogenized microstructures, which is expected based on this being close to the eutectic composition. The pure Zn grains became equiaxed due to the dynamic recrystallization caused by HPT. Mg₂Zn₁₁ intermetallic regions became circular-shaped at lower HPT strains, whereas they transformed into an ultrafine lamellar structure at higher HPT strains.