Microstructures and mechanical properties of Mg-15Gd-1Zn-0.4Zr alloys treated by ultrasonic surface rolling process

In this study, the microstructure evolution and mechanical properties of Mg-15Gd-1Zn-0.4Zr (GZ151K, wt%) alloys treated by ultrasonic surface rolling process (USRP) were investigated. Microscopic analysis methods, including atomic force microscopy, electron backscatter diffraction, scanning electron microscopy, energydispersive X-ray spectroscopy, and X-ray diffraction, as well as mechanical performance tests, including microhardness and uniaxial tensile tests, were employed to analyse the strengthening mechanisms of the GZ151K alloy deformed by the USRP. Results showed that the surface morphology of the alloy was significantly improved compared to that of the untreated sample. In addition, the deformation process of GZ151K alloy introduced a large number of twins and dislocations inner the grains, and formed twin and dislocation gradient structures due to the stress/strain gradient along the normal direction. Furthermore, twinning deformation and dislocation slip during USRP alternately occurred and competed with each other to form a higher volume fraction of sub-grains and twin boundaries. Therefore, the microhardness and tensile strength after USRP were significantly enhanced by twin boundaries and dislocation strengthening, whereas the ductility decreased owing to the initiation and extension of microcracks. However, when the pressure during the USRP was 0.3 MPa, minimal surface roughness and superior mechanical properties could be achieved under the twin and dislocation strengthening.