Study On The Microstructure And Properties Of Mg-Al-Ca Based Heat-resistant Magnesium Alloy

In this paper, AX51(Mg-4.6Al_-0.8Ca-0.24Mn) alloy and AX52(Mg-5.0Al_-2.0Ca) alloy were tested respectively with the addition of the different content of Ce-rich and La-rich mischmetal(MM) to analyze their microstructure and mechanical properties. By ways of optical microscope, scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD), the microstructure and properties of experimental alloys were discussed at room and high temperature respectively. Fracture surface features and the heat-resistant mechanism of the experimental alloys were also studied. Results demonstrated that the microstructure and grain size of the experimental alloys were refined after the addition of Ce and La. With the appearance of the heat-resistant phase Al₄Ce and Al₄La, the high temperature properties were improved largely. Existence of the phase of Al₂Ca found in the alloy of AX52 but not in the AX51 alloy could be attributed to the ratio of Ca and Al_. When Ca/Al<0.2, there was no Al₂Ca in the alloy; when 0.22Ca would exist in the microstructure of the alloy. Moreover, with the addition of Ca, while the addition of Ca was superfluous, the microstructure would be thick, which was harmful to the mechanical properties of the alloy.The mechanical properties of experimental alloys were also studied at room temperature and high temperature respectively. It revealed that the mechanical properties of AX51 alloy were better than AX52 alloy at room temperature, but it was reverse when the temperature was at 150℃. In addition, the comprehensive properties of experimental alloy was the best when the content of MM was controlled at 1.5%. Apart from this, the fracture feature of the experimental alloy was quasi-cleavage fracture and ductile fracture respectively before and after the addition of MM to AX51 alloy. Accordingly, the fracture feature of the AX52 alloy was brittle fracture and ductile fracture respectively before and after the addition of MM to AX52 alloy, which was attributed to the superfluous quantity of Ca.The microstructure and mechanical properties of the AX51 alloy and AX52 alloy were improved obviously by the heat treatment technology in this paper. For the solution treatment, solution time and temperature were the key factors for the properties of alloys. When the solution condition was 420℃×48h, the phase ofβ-Mg₁₇Al₁₂ dissolved in the Mg matrix completely, except for the small quantity of the phase of Al₄Ce and Al₄La. Besides the effect of the solution, as well as the aging technology. When aging condition was 180℃×24h, the mechanical properties of the experimental alloy were the most excellent among the four experimental alloys, not only the phase ofβ-Mg₁₇Al₁₂ was precipitated, but also a large amount of the phase of Al4Ce and Al4La.The comprehensive properties of alloys were affected by the different heat treatment technologies. Under the solution treatment, Theσb,σ0.2 andδof the experimental alloy was enhanced greatly because of the solution of the phase ofβ-Mg17Al12, Al4Ce and Al4La both at room temperature and high temperature. However, exceptδ, the alloy'sσb andσ0.2 were improved more greatly by aging treatment than solution treatment, which was attributed to the precipitation of the phase ofβ-Mg₁₇Al₁₂, Al₂Ca, Al₄Ce and Al₄La. These heat-resistant phases could nail in the grain boundary, hampering the sliding and climbing of the grain-boundary.