Study On Microstructure And Mechanical Properties Of Mg-4Al-4RE-0.5Zn-0.3Mn(RE=Ce,Nd) Magnesium Alloy

Mg alloy is regarded as green material in the 21th century due to its low density, good damping capacity, high specific strength and good electron- magnetic shielding capacity. As the lightest structural alloy, the usage of the Mg is intensively in response to the light weight of automobile required in the automobile industry. So it is meaningful to develop one kind of Mg alloy which can be used in the automobile above 200℃.The Mg-4Al-4RE-0.5Zn-0.3Mn (RE=Ce,Nd) alloys, prepared by copper and preheat steel moulds, have been investigated systematically including microstructure as well as mechanical properties in present work. The mechanism of microstructure effect on the mechanical properties was analyzed systematically. The aim is to provide theoretical and practical data for the further development of Mg-4Al-4RE-0.5Zn-0.3Mn alloy in this work.The results show that: the precipites of the alloy adding 4%Ce and the alloy adding 4%Nd are Al4Ce, Al₁₀Ce₂Mn₇ and Al₃Nd, Mg₃₂(Al,Zn)₄₉, respectively. Comparing the alloy prepared by copper mould with the alloy prepared by preheated steel mould, mechanical properties are improved at room temperature and 150℃. Especially at 150℃, the ultimate strength and the yield strength of the alloy with 4% Ce addition are increased 30%,52%, respectively. The ultimate strength and the yield strength the alloy adding 4% Nd are enhanced by 33%, 21%, respectively. T6 heat treatment was carried on the alloy containing metastable Al3Nd prepared by preheat steel mould, which was desomposed to Al2Nd and Al11Nd3 after T6 treatment. Comparing with the as-cast alloy, The T6 treatment plays ambigous role on the mechanical properties at room temperature. while the ultmate strength and the yield strength is increased by 17%, 21% at 150℃, respectively. The reasons can be attribute to the precipitates distribute along grain boundary after T6 treatment and the different deformation mechanisms of the Mg alloy at room temperature and elevated temperature.The fracture analysis shows that the fracture surface of the alloy adding 4% Ce prepared by preheat steel mould belongs to cleavage fracture at RT, quasi- cleavage at 150℃, while for the material prepared by copper mould, the fracture surface belongs to quasi-cleavage. The fracture surface of the alloy adding 4% Nd belongs to quasi-cleavage at any condition in this work.The investigation of the compression microstructure shows that texture appeares in the matrix of the alloy adding 4% Ce prepared by preheat steel mould and rosette-like microstructure appears in the matrix of the alloy adding 4% Nd prepared by preheated steel mould. The compression properties of all kinds of the alloy prepared by copper mould, compared with the preheat mould, are enhanced. The macrostructural observation for the deformed samples reveals that the angle of micro-crack and the compression direction is about 45o. It can be seen that the micro-crack emerged mainly in the phase boundary of the massive precipitate from the compression fracture surface. Furthermore the tearing ridge will be generated when the micro-crack meets the precipitate which can not be bridged during the expanding. After that the micro-crack will go on expanding in the same direction.