Investigation On RE Elements Alloying And Composite Of Magnesium Alloys

As the lightest metal structural materials magnesium alloy has low density and high performance in damping capacity, specific strength and stiffness. It is widely used in automobile, aerospace and home appliance area. But its low property as increasing the work temperature and low corrosion resistance limits its application area. Recently, some researches focus on increasing the strength of magnesium alloy and analyzing its mechanism in home and abroad. And this article gives some analysis around these two areas.The work chooses AZ91 alloy and Mg-Y-Nd alloy as matrix alloy. To increase the high temperature strength and wear resistance of the alloys, the investigation force on three aspects. First, analysis the microstructure of heat resistance Mg-Y-Nd alloy under the condition of cast and solution and aging, investigate the strengthening mechanism and test the high temperature properties and friction and wear property of T6 Mg-Y-Nd alloy. Second, fabricate graphite particle and Al2O3 short fibers hybrid Mg-Y-Nd matrix composite, along with the analysis of microstructure, mechanical and wear properties of the composite. Third, fabricate AZ91-xSi alloy and Sb metamorphic AZ91-xSi alloy, compare the microstructure of the alloy between and after metamorphism, then test its mechanical and wear properties.For heat resistance Mg-Y-Nd alloy choose the best solution and aging method, after the solution treatment at 525℃for 8h and aging treatment at 250℃for 16h, compare the microstructure, mechanical and wear properties of cast and T6 treatment alloy. The results show that the microstructure of cast Mg-Y-Nd alloy is composed ofα-Mg solid solution, Mg24Y5 and Mg12Nd eutectic phases which have reticular shape and distribute at the boundary of the grains, furthermore, there are some Y and Nd element solution in the matrix alloy. After aging treatment, the precipitated phases change to Mg24Y5,Mg12Nd and Mg14Nd2Y phases, the shape of precipitated phase inside of grains changes to block and short rod shape, inside grains the precipitated phase have dispersive distribution form; at the boundary of grains the precipitated phase connect as the shape of triangle. The distribution form of the precipitated phase enhance the strength of the alloy, at the same time some Y and Nd element inα-Mg solid solution makes theα-Mg solid solution change to supersaturated solid solution, and lead to lattice distortion of Mg, it baffles the dislocation movement and finally increases the strength of the alloy. So the precipitated phase enhances the strength of alloy by precipitation strengthening and dislocation strengthening. The precipitated phase in aging treatment has orientation relationship with the matrix, at the boundary of Mg14Nd2Y and matrix the dislocation regularly mismatch and show half coherent interface. The tensile test shows that at 200℃and 250℃, the tensile strength of T6 Mg-Y-Nd alloy is higher than AZ91 alloy at high temperature, at 200℃the tensile strength is 210MPa, and its elongation is the same as room temperature, but at 250℃the elongation goes up to 22%. The fracture surface at room temperature shows the character of brittle fracture but the fracture surface at high temperature shows the ductile fracture and has dimple in it. At dry sliding condition, the friction coefficient of T6 Mg-Y-Nd decreases as increasing the test load, at low load it changes fast, but at high load it become stable and when the test load achieve certain value the frction coefficient keeps invariant. The mass loss of the specimen increases as increasing the load, and the wear mechanism changes from oxidation wear to adhesive wear and delamination wear. At 380N, the temperature of the worn surface achieves melting temperature of the alloy and the wear mechanism changes to melting wear, the assistance of melting layer makes the specimen has lower friction coefficient. Analysis the worn surface at 2mm by XRD it shows onlyα-Mg solid solution, but under TEM analysis some square are found, but the square phase can't be confirmed by XRD analysis because of low content. The formation of square phase is the alloy has solution process at high temperature during the friction and wear process, so it forms at non-equilibrium cooling. The salt spray test indicates that the corrosion property of T6 Mg-Y-Nd alloy is better than AZ91 alloy, and adding Y and Nd element can increase the corrosion resistance of Mg alloy.Fabricating composite can increase the heat and wear resistance of the Mg alloy. The test using Al2O3 and graphite as reinforcement fabricates Mg-Y-Nd matrix composite. The content of graphite 5%,10%,15% and 20% in volume fraction, respectively and the Al2O3 is 8 vol.%. The results show that the reinforcement combine closely with the matrix and disperse uniformly. Because of the assistance of Al2O3 short fibers, the microhardness and the tensile strength of the composite is higher than the single alloy, but the elongation decreases. The results of dry sliding wear test show that the friction coefficient of the composite decreases as increasing the test load, at the same load, the friction coefficient of the composite decreases as decreasing the graphite content. The mass loss increases as increasing the load, the composites show different wear mechanism at different test load, at the beginning of the wear process, the mechanism is mainly slight abrasive wear, as the temperature increasing the worn surface shows the character of oxidation wear, then the mechanism changes to delamination wear, but the melting wear can't be found on the worn surface which indicates that the composite has better heat and wear resistance. When the graphite content is above 10%, the graphite on the worn surface and in the subsurface is extruded, then smeared on the worn surface as sliding, it forms lubricating film which can decrease the friction coefficient and friction force.Adding Si can increase high temperature strength of Mg alloy, and the price of which is cheap. Si can form Mg2Si with Mg, Mg2Si which has the melting point of 1085℃can works as strengthening phase especially when the content of Al is low. Mg2Si phase becomes coarsening easily and when it has the shape of chinese script type can lead to stress concentration and make the material become brittle. This test fabricates AZ91-xSi alloy, the Si content is 0.5%,1.0% and 1.5%, then adding 0.5% Sb in the series AZ91-xSi alloy, analysis the microstructure and properties of these two series alloy. The results shows that adding Si in the AZ91 alloy can refine the grains, and some Mg2Si phase of chinese script type are found in the alloy, the tensile strength of the alloy increasing but the toughness decreases and the fracture shows the character of brittle fracture. After adding Sb, the grains become smaller, Sb forms Mg3Sb2 with Mg, Mg3Sb2 becomes the core of heterogeneous nucleation of Mg2Si and it changes the shape of Mg2Si, so the tensile strength and toughness of the alloy increases and the fracture shows the character of ductile fracture. The mechanism of series alloy is similar as T6 Mg-Y-Nd alloy. At the same Si content, the wear resistance of the alloy increases after Sb metamorphic.