Effect of microstructure evolution on the mechanical properties of a Mg–Y–Nd–Zr alloy with a gradient nanostructure produced via ultrasonic surface rolling processing

Abstract

Ultrasonic surface rolling processing (USRP) was performed on a Mg–Y–Nd–Zr alloy after equal-channel angular pressing. The result shown that the surface roughness was significantly improved, residual stress was introduced to the topmost surface, and ~450 µm gradient nanostructure (GNS) of Mg alloys was obtained after USRP treatment. The mechanism of microstructure evolution and its effect on the mechanical properties of the Mg alloy were studied in detail. There are multiple deformation mechanisms in grain refinement. In the initial stage, the coarse-grain (CG) matrix was subdivided by {101̅2}< 101̅1 > extension twins. Subsequently, the deformed grains were refined into substructures using interaction between twins, interaction between twins and dislocations, and dislocation entanglement. Moreover, the substructure contains a high density of multiple stacking faults and dislocation arrays, which can effectively block dislocation movement and promote dislocation packing. Finally, equiaxed nanograins (NGs) with high-angle grain boundaries were formed via subcrystal rotation recrystallization. With the formation of a GNS, the strength of the Mg–Y–Nd–Zr alloy considerably increased at the expense of a small amount of plasticity. The tensile test results demonstrated that the yield strength (YS) and ultimate tensile strength (UTS) of the Mg–Y–Nd–Zr alloy increased from 171.96 and 262.47 MPa to 216.06 and 330.61 MPa, respectively, and the elongation rate decreased from 20.52% to 14.65% after the USRP treatment. The YS improvement of the Mg–Y–Nd–Zr alloy after the USRP treatment was attributed to grain boundary strengthening, synergistic strengthening, dislocation strengthening, and twin strengthening. Furthermore, the microhardness of Mg–Y–Nd–Zr considerably improved after USRP treatment, and grain refinement was the primary surface strengthening mechanism of the Mg–Y – Nd – Zr alloy after the USRP treatment. The work-hardening capacity of the Mg alloy improved after the USRP treatment, and the synergistic deformation between GNS and CG matrix was the primary reason for the excellent work-hardening capacity of the Mg–Y–Nd–Zr alloy.