Effect Of Ca Content On Microstructure And Mechanical Properties Of Mg-6Al-2Nd-xCa Alloy

As one of the lightest metallic structural materials,Mg-Al series alloys have been rapidly developed for aerospace and automobile industries in recent years.However,Mg-Al series alloys are difficult to be used at high temperatures for a long time,so it is extremely important to improve their high temperature properties.In this paper,Mg-6Al-2Nd-xCa(x=0,0.5,1.0,1.5,2.0,2.5,3.0)alloys were designed by adding Nd and Ca.The microstructure evolution,mechanical properties at room temperature and high temperatures of as-cast alloys were systematically studied,and the solidification paths of the alloys were analyzed.Finally,the microstructure and mechanical properties of as-extruded alloys at room temperature and high temperatures were studied,and the strengthening mechanism was analyzed.The experimental results showed that:As-cast Mg-6Al-2Nd-xCa alloys are mainly composed of?-Mg matrix,granular phase Al₂Nd,acicular phase Al₁₁Nd₃,skeletal phase Al₂Ca and?-Mg₁₇Al₁₂ phase.With increasing Ca content,the Al₁₁Nd₃ and?-Mg₁₇Al₁₂ phases gradually disappeared,and the Al₂Ca phase at the grain boundaries transformed from the discontinuous network to the continuous network.The solidification curves of as-cast alloys showed that with increasing Ca content,the precipitation temperatures of?-Mg matrix,Al₂Nd,Al₁₁Nd₃ and?-Mg₁₇Al₁₂ decreased,the precipitation temperature of Al₂Nd phase decreased from 630°C to 610°C,and the precipitation temperature of?-Mg decreased from 620°C to 606°C.However,the precipitation temperature of Al₂Ca phase increased from 504°C to 529°C with increasing Ca content.The solidification paths of as-cast alloys were different,and the solidification paths of Mg-6Al-2Nd-xCa(x=0,0.5)alloys were L?Al₂Nd;L??-Mg;L?Al₁₁Nd₃;and L??-Mg+?-Mg₁₇Al₁₂.The solidification paths of Mg-6Al-2Nd-xCa(x=1.0,1.5,2.0,2.5,3.0)alloys were L?Al₂Nd;L??-Mg;and L??-Mg+Al₂Ca.Moreover,the results of remelting-isothermal liquid quenching experiment were consistent with those of solidification curve analysis.The tensile strengths of the as-cast Mg-6Al-2Nd-xCa alloys decreased with increasing Ca content at room temperature.And the tensile strengths of as-cast alloys still decreased with increasing Ca content at 175°C.However,when the temperature was 200°C,225°C and 250°C,the tensile strengths of as-cast alloys first increased and then decreased with increasing Ca content,and the strength reached the highest when the Ca content was 1.0%.The tensile strengths of as-cast alloy with 1.0%Ca content at 200°C,225°C and 250°C were 128.8±2.4MPa,95.3±2.6MPa and 82.4±3.0MPa,respectively.The elongation of as-cast alloys decreased with increasing Ca content at room temperature and high temperatures.When the alloy composition was fixed,the elongation increased with increasing temperature.At room temperature and high temperatures,with increasing Ca content,the fracture mode of as-cast alloys changed from transgranular+intergranular mixed fracture to intergranular fracture.Al₁₁Nd₃,Al₂Ca and?-Mg₁₇Al₁₂ phases all fractured due to the hot extrusion.There were some dynamic recrystallization phenomena in the alloys.At room temperature and high temperatures,the tensile strengths of as-extruded alloys increased with increasing Ca content,while the elongation decreased with increasing Ca content.The tensile strengths of as-extruded Mg-6Al-2Nd-3Ca alloy at room temperature,175°C,200°C,225°C and 250°C were 317.7±5.6MPa,161.7±5.8MPa,133.5±5.5MPa,100.9±5.8MPa and 84.2±6.0MPa,respectively.When the Ca content in the alloy was constant,the tensile strength decreased with increasing temperature,while the elongation increased with increasing temperature.Compared with as-cast alloys,the mechanical properties of as-extruded alloys were greatly improved at room temperature and high temperatures.At room temperature and high temperatures,the fracture mode of as-extruded alloys was ductile fracture.