Microstructure and Properties of Mg-Y-Nd-xGd Magnesium-Rare Earth Alloys

Microstructure and Properties of Mg-Y-Nd-xGd Magnesium-Rare Earth Alloys
　　Magnesium alloys, as the lightest structural material in practical application, has achieved rapid development since coming out, attributing to its low density, high specific strength and specific rigidity, excellent damping capacity and electromagnetic shielding capacity, as well as recuperability. However, magnesium alloys are limited to be used at elevated temperature due to its poor heat-resistant ability. WE system magnesium alloys were reported to neodymium magnets be the best candidate in practical use till now. Thus, it’s of great significance to research and develop a new heat-resistant magnesium alloy, serving for high technology industry.In this dissertation, a new magnesium alloy system Mg-5Y-3Nd-xGd-0.5Zr(x=04) is achieved by adding Gd element on the basis of WE54. Samples are melted in electric resistance furnace and poured into steel mould, then heat treated by solution and aging treatment.It is shown that precipitates in the as-cast alloys are mainly composed of bone-like Mg12Nd and massive Mg5Gd, segregated around the grain boundary as discontinuous eutectic structure. Most of Y dissolved into Mg matrix. Spherioidization of grains occurs during heat treatment, and much finer precipitates distribute homogeneously inside the grains and around the boundaries.
　　Then the magnesium-matrix solid solution reaches saturation, and the mechanical properties of the alloys are increased by both solution strengthening and precipitation strengthening of the second phases.The mechanical properties of alloys such as hardness and tensile strength improved significantly after T6 treatment. The max Brinell hardness increases from 71 to 95, with Vickers-hardness from 95 to 129. The max ultimate tensile strength(UTS) and yield tensile strength(YTS) at room temperature are 290MPa and 190MPa, respectively, with little change when heated to 250℃, and can still reach up to 220MPa and 180MPa at 300℃, indicating that it is an http://www.chinamagnets.biz/ excellent candidate for heat-resistant magnesium alloys.By DSC and TEM analysis, the plate-like rhombus metastable phaseβ1, coherent with matrix, is found in the alloy after heat treatment, justifying the aging process should beа→β"→β’→β1→β. Meanwhile, many Zr-rich cores are identified under SEM observation, indicating that Zr can refine grains as heterogeneous crystal nucleus.The deformation mechanism of magnesium alloys alters as temperature increases. At room temperature, it is controlled by dislocation gliding, while at elevated temperature, the sliding of grain boundaries dominates as the atoms can migrate much easier. As to alloys after T6 treatment, its plasticity decreases, and fracture mechanism changes from mix fracture of intergranular fracture and cleavage fracture to quasi-cleavage fracture with increase of Gd. The little addition of Gd can improve the strength as well as plasticity of based-alloy, having great strengthening effect.