Study Of Tensile And Creep Properties At High Temperature On Mg-Al-Ca Alloys With High Ca/Al Ratio

Magnesium alloy is desirable for 3C products and motor industry due to its high specific strength, good rigidity, low density and good electromagnetic shielding performance and so on. However, its usage as high temperature component was restricted by its low strength and poor creep properties at high temperature. Researcheres have found that the addition of calcium to Mg-Al alloy was benefitial for magnesium alloys'high temperature properties. Also, calcium is much cheaper for magnesium alloy compared with rare earths.Most of the studies about Mg-Al-Ca alloy were under the condition of Ca/Al< 0.8, and the study about Ca/Al> 0.8 was few. In the present study, the mechanical and creep properties of Mg-2Al-2Ca, Mg-3 Al-3Ca, Mg-4Al-4Ca and Mg-5 Al-5Ca are researched, the results show that:(1) In the alloys of Mg-2Al-2Ca, Mg-3Al-3Ca, Mg-4Al-4Ca and Mg-5Al-5Ca, the ratio of Ca to Al is greater than 0.8. The secondary phases in these alloys consist of large number of and a few number of Mg2Ca after melt and extruded at high temperature. Al2Ca and are stable at high temperature. While the Al2Ca is more stable than Mg2Ca at high temperature, the melting point of Al2Ca is 1079?, and it is benefitial for high temperature properties.(2) After tensile test at ambient temperature, the results show that Mg-3Al-3Ca and Mg-5Al-5Ca alloys have the highest tensile strength at room temperature, and the ?b=333MPa. While the tensile strength of Mg-2Al-2Ca and Mg-4Al-4Ca are 292MPa and 289MPa, respectively.(3) The tensile strength of Mg-4Al-4Ca alloy at the tempetature of 175? is 200MPa, as the tensile temperature increased to 250?, the tensile strength decreased to 95MPa. It is the highest among thses alloys. The strain rate sensitivity exponent (m) of Mg-2Al-2Ca is 0.18, it is the highest among thses alloys. The m-value of Mg-5Al-5Ca is 0.11, it is the lowest among thses four alloys. The calculated n-value of Mg-2Al-2Ca is 0.67, Q-value is 60.5kJ/mol.(4) Mg-5Al-5Ca alloy showed the most excellent creep resistance, and the steady-state creep rate (?) is 2.08×10-8 s-1. After crept for 100h, the elongation (8) of Mg-5Al-5Ca is 0.97%. The calculated stress exponent n for the extruded Mg-5Al-5Ca at 175? is 6.67. The creep activation energies for the extruded Mg-5Al-5Ca at the stress of 60 MPa and 70 MPa are 90 kJ/mol and 109 kJ/mol, respectively. The results indicate that the main creep deformation mechanism of Mg-5Al-5Ca alloy is dislocation climbing mechanism. Also, the creep performance of Mg-5Al-5Ca is affected by grain boundary sliding mechanism.