Microstructure And Mechanical Properties Of Nanostructured Mg-Gd Alloy Prepared By Accumulative Roll-bonding(ARB)

The microstructure and mechanical properties of a Mg-3Gd(wt.%) binary alloy were investigated in this study. Nanostructured Mg-Gd alloy with high strength was prepared by accumulative roll-bonding(ARB) processing. The deformed samples were annealed to optimize the microstructure and improve properties. The tensile test, electron backscatter diffraction technique and transmission electron microscopy were employed to reveal the mechanical properties and microstructural features of the deformed and annealed samples. The effects of ARB and annealing parameters on the microstructure and mechanical properties of nanostructured Mg-Gd alloy were discussed, and the fundamental mechanisms in improving the strength and plasticity were unraveled from the viewpoints of solute atom segregation and dislocation structure. The main conclusions are summarized as follows:(1) Nanostructured Mg-Gd alloy sheets prepared through 1~4 cycles ARB processing shows high strength but low plasticity. The deformation microstructure is characterized by ultrafine grains and nanotwin lamella. The grain refinement gradually increases with the increase of ARB cycle from 1 to 3, while dynamical recrystallization tends to occur in local regions in the 4 cycles processed sample. In particular, the 2 cycles processed sample shows the superior strength and plasticity, and is chosen for further analysis.(2) After annealing at 190~360? the nanostructured Mg-Gd alloy show better mechanical properties, among which the 190? annealed sample exhibits the greatest strength. As annealing temperature increases from 290? to 310?, the yield strength gradually decreases, but the plasticity is significantly improved. The mechanical property of the 360? annealed sample is pretty poor but still better than that of the original coarse-grained sample.(3) After annealing at 190? for 1 h, apparent Gd segregation at grain boundaries, twin boundaries and defects occurs in the nanostructured Mg-Gd alloy, which may be the fundamental reason for strength improvement. Partial recrystallization takes place in the 290? annealed sample, resulting in a heterogeneous lamella structure composed of ultrafine grains and recrystallized grains, which leads to better yield strength and work hardening rate. After annealing at 310? and 360?, full recrystallized microstructure then can be observed.(4) In the annealed samples, <a>-type dislocation plays a dominant role in the recrystallized grains. With further tensile deformation both <a>-type and <c+a>-type dislocations can be observed throughout the recrystallized grains. The activity of <c+a>-type dislocation thus was considered to be closely associated with plastic deformation of Mg-Gd alloy. Statistical analysis indicates that <c+a> slip and twinning are apt to occur in the grains beyond a critical size. As the annealing temperature increases, the critical grain size for dislocation activity gradually increases.