Effect Of Rare Earth Gd And Semi Solid Isothermal Treatment On Microstructure And Properties Of Mg-6Al Alloy

Mg-6Al-xGd magnesium alloys were prepared by permanent mold casting. The microstructure of the alloy was observed by scanning electron microscopy, energy dispersive spectroscopy, X ray diffraction, transmission electron microscopy and optical microscopy. The effect of Gd on the microstructure and room temperature mechanical properties of Mg-6A1 alloy was studied. The relationship between compression ratio and the deformation microstructure of Mg-6Al-xGd magnesium alloy was investigated based on compression deformation, thereby calculating the activation energy corresponding to different compression ratios. The effect of isothermal heat treatment on semi-solid microstructure and mechanical properties was used to study by semi-solid isothermal treatment on Mg-Al-xGd magnesium alloy. The main conclusions are demonstrated as follows:1. The addition of Gd in the Mg-6Al alloy refine the grain, the ?-Mg₁₇Al₁₂ phase is small and dispersed,and the morphology of the ?-Mg7Al2 phase changes from continuous network to semi reticular and granular. As the content of Gd element is increased, the number of the?-Mg₁₇Al₁₂ phase reduces, and Al2Gd phase with higher melting point can be formed. When the content of Gd is 0.9wt%, the refinement of the alloy is best.2. Mg-6Al-0.9Gd magnesium alloy is prepared by compression. With increase of the deformation, the plastic deformation occurs and the initial dendrite arms are stretched under the stress, which are finally broken into the fine grains. The eutectic melting activation energy of the alloy decreases with increase of the deformation rate from 3236.91kJ·mol^-1 to 2962.99kJ·mol^-1, while the distortion energy increases OkJ·mol^-1 to 273.92kJ·mol^-1. In the process of compression deformation,there are defects in the interior of the structure.3. Taking the Mg-6Al-0.9Gd alloy for isothermal treatment, the fraction of liquid phase increases and the solid particles decreases, which changes from coarse dendrite to fine equiaxed grain with increase of isothermal temperature. The dendrite of the semi-solid alloy disappeares with the dendrite arms fusing when the time for heat preservation is elongated, and the bulk particles separates which are finally changed into the equiaxed grains. With increase of deformation, liquid phase at grain boundary increases and the roundness of the grains improves.4. As the content of Gd element is increased, the tensile strength, impact toughness and elongation of as cast Mg-6Al-xGd magnesium alloy increase and then decrease. When the content of Gd is 0.9wt%, the tensile strength and the elongation of the alloy are 210.93MPa and 9.42%. The increases are 21.4% and 120% when compared with Mg-6A1 alloy. For the as-cast alloy, the adding of Gd element can enhance the impact toughness, resulting in the best property in Mg-6Al-0.9Gd alloy at room temperature, and the impact toughness is 14.5J/cm2,which is 45% higher than Mg-6A1 alloy. After the semi-solid processing, the tensile, the impact toughness and the elongation are 234.63MPa, 18J/cm2 and 12.5% for Mg-6Al-0.9Gd alloy, and the corresponding increases are 11.2%, 24.1% and 32.7% respectively when compared with as-cast Mg-6Al-0.9Gd alloy.