Research On Microstructure, Properties And Superplasticity Of Mg-Al Based Alloys

Lightweight magnesium alloys, especially Mg-Al system, have been widely used as structural materials for automobiles to reduce weight. However, sharply decreasing mechanical properties above 120℃ restrain the wide application of Mg-Al system alloys. In this paper, grain refinement of AM50 and AZ91 alloys of Mg-Al system have been studied, and the reaseach focused on the effects of solute element Ca on microstructure and mechanical properties of the Mg-Al based alloy. The relationship between the chemical compositon, microstructures and mechanical properties of Mg-Al based alloy were discussed to improve their mechanical properties.The influence of Ca on the microstructure of AM50 and AZ91 alloys have been investigated by using optical microscope (OM), scanning electron microscope (SEM), differential scanning calorimetry (DSC) and X-Ray diffraction. The results demonstrate that Ca element can form a new Al₂Ca phase, reduce the quantity of Mg₁₇Al₁₂ phase and refine the grain in both Mg based alloys. The average grain size of AM50-2Ca alloy reduced from 200um of AM50 to 40μm, and that of AZ91-2Ca also decreased from 120μm of AZ91 to 45μm. The grain refinement of Ca on AM50 can attribute to the formation of primary Al₂Ca phase, which acts as heterogenous nucleus. The grain refinement of Ca on AZ91 alloy is mainly attributed to Ca at the solid/liquid interface front during solidification strongly restricts the grain growth of α-Mg phase.The tensile tests in the investigated range show that Ca can improve tensile strength and yield strength of extruded AM50 at ambient temperature and also increase significantly its yield strength at elevated temperature. With Ca addition to the extruded AZ91 alloy, its tensile and yield strength at elevated temperature increased, however, the corresponding strength at ambient temperature decreased. Ca addition changes the fracture patterns of both alloys, that is, the cleavage fracture gradually becames quasi-cleaveage fracture. The brritle Al₂Ca phase, which may cause cracks occurring, results in the premature fracture of both AM50 and AZ91 alloys.The superplasticity of AZ91-1Ca and AZ91-2Ca alloys have been systematically investigated. The results demonstrate that the maximum elongation of 340% of AZ91-1Ca was obtained at temperature 623K and strain rate 7.29×10^-5s^-1, and its strain rate sensitive index m is 0.5. The maximum elongation of 340% of AZ91-2Ca was obtained at temperature688K and strain rate 7.29xlO'V, and its strain rate sensitive index m is 0.48. The deformation mechanism of AZ91-lCa and AZ91-2Ca is also studied. The dominant deformation mechanism is grain boundary glide (GBS). And the accormadation mechanisum is dislocation creep controlled controlled by atom diffusion through grain boundaries or in grains. The research on cavities in superplasticity shows that cavities mainly occur in the vincity of A^Ca phase and also distribute along grian boundaries.