Microstructures And Tensile Properties Of Mg-10Gd-4.8Y-0.6Zr Alloy During Two-direction Forging

Abstract:In the present work, the two-direction forging has been performed on Mg-10Gd-4.8Y-0.6Zr alloys at773K,743K and703K. By using optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and universal tensile testing machine, the microstructures and mechanical properties after different forging passes were studied systematically and the mechanism of grain refinement was discussed in details. Experiment results are obtained as follows:Grain refinement during forging at500℃is dominated by a complicated combination of discontinuous dynamic recrystallization at low strain regions and continuous dynamic recrystallization at medium-to-high strain regions. Similar with500℃, the mainly grain refinement mechanism at470℃is dynamic recrystallization. When forged at430℃, twinning is the dominant deformation mechanism before4passes, with the forging direction changed, the intersections between high-density fine twins lead to the evolution of ultrafine grained structure. After4passes, the static recrystallization during intermediate annealing plays an important role in microstructure refinement.Dynamic decomposition happens during forging at three forging temperature. A small quantity of large size precipitates at500℃concentrates in local area along the elongation direction, with forging temperature decreasing, the size of the precipitates decreases, the number increases and the distribution is much more uniform. When forged at a certain temperature, the number of precipitates increases with the increasing forging passes.When forged at500℃, the grain size is refined gradually with the increase in number of forging passes, it is significantly refined after6passes, i.e., accumulative strain of1.8, resulting in a uniform fine grain structure with an average grain size of approximately9.3μm. As the number of forging passes increased after6passes, the average grain size increases obviously due to the grain growth during the intermediate anneal process. The excellent mechanical properties is achieved after forged for6passes, with ultimate tensile strength336MPa and elongation to fracture21.0%at room temperature. When forged at lower temperature (470℃,430℃), The UTS and YS increase with increasing forging passes, but the increasing dynamic precipitates worsen the ductility.For the6passes specimens, with the forging temperature decreasing, the strength increases and the elongation deceases. For the forged state, the specimen forged at430℃possess the highest strength, with ultimate tensile strength366MPa and yield strength295MPa. Dynamic precipitates at lower forging temperature reduce the ageing strengthening effect. For the T5treatment state, the specimen forged at500℃possess the best mechanical properties, with ultimate tensile strength479MPa, yield strength425MPa and elongation to fracture4.1%.