The Effect Of Alloying Elements Addition On Microstructure And Mechanical Properties Of The AZ91D Magnesium Alloy

As the lightest structural metallic materials, magnesium alloys have many unique and excellent properties, such as low density, high specific strength and stiffness, good damping capacity, excellent electromagnetic shielding and machining characteristics, good processing properties and casting properties, so they are important on automobile and electric industry. AZ91 alloy is the most broadly used magnesium alloys in industry, however, because of poor high-temperature behavior, AZ91 alloy is not suitable for the use of structural materials over 120℃. Rare earth elements are the most effectual elements in increasing mechanical properties of magnesium alloy at high temperature. The trademark of rare earth magnesium alloys accounts for more than 50% in all magnesium alloys in the world, and the rare earth magnesium alloys take special possession in the heat resistant magnesium alloys. In the present work, the microstructure and mechanical properties of AZ91D alloy with different additions of La, Nd and Ce were studied by many analyse and testing means, such as optical microscope(OM), scanning electron microscope(SEM), energy disperse spectroscopy(EDS), X-ray diffractometer (XRD), tensile testing and hardness testing etal.The experimental results show that, suitable addition of La, Nd and Ce can obviously refine the structure of AZ91D magnesium alloy, meanwhile, the refined microstructure and precipitated phase pattern lead to a improvement in hardness, elongation, tensile strength at room and high temperature, and so on. With the increasing addition of La in AZ91D alloy, the mechanical properties of AZ91D alloy rise at first and then drop. With the 1.0% addition of La, the hardness, the tensile strength at room temperature and 200℃reach their maximum value simultaneously, which are 74.7HB, 217MPa and 159MPa respectively and are 31.6%, 26.2% and 27.2% higher than AZ91D alloy without La addition. Meanwhile, the elongation increase 124% at room temperature and 141% at 200℃, reach to 2.8% and 9.5% respectively.When added Nd to the alloy, new needle-like and block phase of Al3Nd formed, which refined grains and increased the mechanical properties of the alloy by inhibiting the slipping of grain boundary. The mechanism of grain refinement of Nd on AZ91D alloy is that the new phase Al3Nd has the face-centered cubic structure and its lattice parameter is familar with Mg₁₇Al₁₂, then it can act as nucleation center to catalyze the nucleation of Mg₁₇Al₁₂ phase. On the other hand, the Nd enrichment on the solid/liquid interface front during solidification induce constitutional supercooling, then it accelerate the formation of equiaxed crystal. With 0.4% Nd addition, the tensile strength of the alloy at both room temperature and 200℃reach maximum values of 227MPa and 158.52MPa respectively, and the elongation are 3.35% at room temperature and 10.55% at 200℃.The addition of Nd and Ce break the network-like structure of Mg₁₇Al₁₂ and skeletal shape or granular Mg₁₇Al₁₂ phase appear dispersely on the grain boundary or inside the grains. At the same time, new aciculate and granular compounds of Al₄Ce and Al2Nd are formed in the alloy. With the combined effect of fine grained strengthing, precipitated phase strengthing and solution strengthing, the microstructure and mechanical properties on AZ91D alloy obviously improved. The research shows the refining effect on microstructure is the best when the addition is 1.0%Nd and 0.6% Ce, and the tensile strength of the alloy reach the peak value, the values are 228MPa at room temperature and 161.9MPa at 200℃, which is 32.6% and 29.4% higher than AZ91 without RE. The elongation is also best, which reach to 3.2% and 11.5% respectively.