Effects Of Ca And Sr On The Microstructures And Properties Of AS21 Alloy

The creep resistance of magnesium alloys often decreases drastically with increasing temperature when it exceeds 120℃, so the development of excellent high temperature creep properties of heat-resistant magnesium alloys is one of the main directions. In the automotive industry, the traditional heat-resistant magnesium alloy AS system (Mg-Al-Si) alloy significantly improves the high temperature performance,owing to the decrease of grain boundary network distribution of (3-Mg17Al12 phase as the decrease of Al content and then obtained with high melting point (1085℃) of Mg2Si strengthening phase as the addition of Si element.However, the dendritic morphology of the Mg2Si intermetallic phase in the matrix will hinder the further improvement of high temperature performance. How to modify the particular morphology of Mg2Si phase to give full play of the usefulness to high temperature performance is especially important.In this paper, Ca and Sr are added into the AS21 alloy, the effects and mechanisms with separate addition and combined addition of Ca and Sr on the microstructures and properties have been investigated using modern analysis and test facilities such as Optical Microscopy (OM), X-ray Diffraction(XRD), Differential Thermal Analyzer(DTA), Scanning Electron Microscopy(SEM) with Energy Dispersive Spectrometer (EDS). It offers some experimental basement and theoretical reference for high performance and low cost heat-resistant magnesium alloys.The results show that Ca can significantly modify the dendritic morphology of Mg2Si phase, it still has significant grain refinement on the AS21 alloy. When the addition of Ca is 0.25%, the morphology of Mg2Si phase completely changed from dendritic into uniform and dispersed granular or blocky, with an average size from the original 42μm reduced to 8μm, and the grain size of AS21 alloy were also refined from 105μm to 75μm. Sr can modify the dendritic morphology of Mg2Si phase as Ca,it also has a certain degree of refinement both on the granular Mg2Si phase which has been modified by Ca and the grain of the alloy.After adding 0.1% Sr into the AS21-0.25Ca alloy, the average particle size of Mg2Si phase was less than 5μm, the average grain size of alloy were further reduced to 67μm.The main phases in the AS21 alloy areα-Mg matrix,Mg2Si andβ-Mg17Al12 phase.with more than 0.25% Ca addition, a new phase CaSi2 generated firstly and led to the heterogeneous nucleation of Mg2Si phase as the nuclei. After adding 0.3% Sr into the AS21-0.25Ca alloy, a small amount of needle-like MgCaSi ternary phase was observed.After the solution treatment of holding 12 to 20 hours at 420℃,the P-Mg17Al12 phase of AS21 alloy integrated into the matrix, the dendritic morphology of Mg2Si phase had a large surface tension caused by its developed dendrites, and Si atoms diffused by the interface between Mg and Mg2Si, the diffsion of Si atoms led to the diffusion of dendrites which can also rely on self-serving tendency to gather to the dissolving granular aggregation Mg2Si, so finally the Mg2Si phase dissolved partly by the phenomenon of fracture and balling.After the solution treatment of holding 16 hours at 420℃, the blocky Mg2Si phase in the alloys containing Ca and Sr did not have a surface tension as the dendritic Mg2Si phase, and the diffsion of Si atoms was less than the dendrites, so its shape changed little which performed of its heat resistance was better than the dendritic Mg2Si phase.Ca can greatly improve the high temperature creep properties of AS21 alloy, the creep deformation of AS21-0.25Ca alloy is only 28.6% of AS21 alloy's. A small amount of Sr based on the joining of Ca can further improve the alloy's high temperature creep properties, the creep deformation of AS21-0.25Ca-0.1Sr alloy is 21.4% of AS21 alloy's. As Ca and Sr modified the dendritic morphology Mg2Si into small particles which can significantly reduce the stress concentration near the Mg2Si phase and can be more effective pinning grain boundary sliding and intragranular dislocation motion in the high temperature creep process, thereby enhanc the high temperature creep properties.The liquidus temperature of AS21 alloy decreased with the addition of Ca by 4℃, and thus the superheat was improved, this is conducive to the mobility of solidification process. On the basis of Ca and Sr, the solidification interval decreased, it can improve the fluidity of the alloy.The corrosion potential of AS21 alloy raised with the addition of Ca and Sr, thereby the corrosion stability improved. The corrosion potentials of AS21-0.25Ca and AS21-0.25Ca-0.1Sr alloy were higher than AS21 alloy's by about 0.014V and 0.002V respectively.The mechanism of modification about Ca to Mg2Si phase is that, in the cooling process of liquid alloy, Ca and Si first formed a high melting point CaSi2 intermetallic compound, thus formed scores of fine and dispersed particles in the melt. As temperature decreased, the dispersed particles acted as the cores of Mg2Si heterogeneous nucleation, thereby formed a lot of small shaped Mg2Si bulks in the solidification process.The mechanism of modification and refining about Sr to Mg2Si phase is that, one hand Sr easily enriched in the front of the growt.h interface of Mg2Si phase mechanically to impede the diffusion of Mg and Si atoms so that the growt.h of Mg2Si phase was limited, on the other hand Sr increased the subcooling of solid-liquid interface so that the nucleation rate of Mg2Si phase was increased in the solidification process.