In-situ Investigation On Deformation Behavior Of Mg-Gd Alloy At Room Temperature

Deformation mechanisms inside and among grains have great influence on the deformational behavior of Mg alloys, while study on how grain size and orientation will affect the deformational behavior of Mg alloys is still limited. So, it is of great value to investigate the deformational behavior in mesoscale by discussing the influence of slipping, twining, and grain boundary sliding and rotation on deformation mechanism. In this study, Mg-Gd alloys with different grain size and texture had been processed, and the microstructure evolution and deformational behavior were investigated by in-situ scanning electron microscopy(SEM) and electron back scanning diffraction(EBSD), both deformation mechanism inside(slipping and twining) and among(grain boundary sliding, initiation and propagation of microcracks) grains were tracked during in-situ tensile test at room temperature. Besides, the influence of average grain size, grain orientation and texture on deformational behavior were discussed.The effect of Gd addition(Mg-0.5Gd, Mg-2Gd and Mg-5Gd) on the microstructure and mechanical properties of as-extruded Mg-Gd alloys were discussed. As Gd addition increased, continuous dynamic recrystallization process were hindered by the solute drag effect, which leaded to inhomogeneous microstructure in Mg-2Gd and Mg-5Gd as-extruded rods. Besides, Mg-0.5Gd alloy showed strong basal texture after hot extrusion, while Mg-2Gd and Mg-5Gd alloys gained weak <2-1-11> texture, and the distinction of texture directly lead to distinct mechanical properties in Mg-x Gd alloys. The as-extruded Mg-0.5Gd alloy exhibited much higher yield strength(255MPa), while Mg-2Gd and Mg-5Gd alloys showed better ductility.Mg-2Gd alloy were selected to undertake the following annealing treatment. We found that the dynamically recrystallized grain grew during annealing treatment, Mg-Gd alloys with average grain size ranging from 5 to 57μm were obtained. Besides, the <10-10> texture component of Mg-2Gd alloy was weakened gradually, while the <2-1-11> and <10-11> texture component strengthened gradually with the annealing time and temperature increasing. What’s more, mechanical asymmetry gets promoted after annealing treatment, and the Hall-Petch relationship still stand in Mg-2Gd alloy with the grain size ranging from 5 to 57μm.As the results of in-situ tensile tests at room temperature revealed, basal slip and grain boundary sliding and rotation had been identified and characterized as the main deformation mechanism in as-extruded Mg-2Gd alloy(average grain size: 13.6μm), while both extension and contraction twins barely activated during deformation process. By comparison, extension twin were activated and widely existed in as-annealed Mg-2Gd alloy(average grain size: 38.5μm) during the in-situ tensile test at room temperature, and basal slip remained the main deformation mechanism, but grain rotation and sliding had been hindered because of the bigger grain size. As both the as-extruded and as-annealed Mg-2Gd alloys had similar weak texture, the distinction of deformation behavior could be ascribed as the difference of average grain size. Smaller grain size would effectively refrain the nucleation of twinning and promote grain rotation and sliding during deformation at room temperature. What’s more, slip-induced twinning behavior was identified in as-annealed alloy, and twining promoted slipping mechanism during deformation.As basal slip played very important role on deformation compatibility in both as-extruded and as-annealed Mg-2Gd alloys, a geometric compatibility parameter m had been introduced to describe the geometric relationship of basal slip system between neighboring grains, so that we could study the influence of grain orientation on deformational behavior and the initiation and propagation of microcracks. We found that microcracks initiated only in grain boundaries of both as-extruded and as-annealed Mg-2Gd alloys, and the angle between tensile direction and grain boundary played an important role on miraocrack initiation. The larger the angle was and the smaller the value of m was, the easier microcrack would initiate from this boundary. But the geometric compatibility parameter m did not have a directly influence on the propagation of microcracks. Instead of growing to grain boundaries with smaller m, those microcracks preferred to propagate from one grain boundary to another grain boundary which was more smoothly connected with initiation grain boundary. Besides, twin nucleation could hinder initiation and propagation of microcracks by releasing local stress concentration.