A Study Of Microstructure And Crystallography Of Magnesium Alloys Containing Ca And Si

Based on an overview on the development history and current researches of magnesium alloys, a detailed discussion is made regarding the research and development of heat resistant magnesium alloys, particularly of those containing Ca and Si as alloying elements. In order to investigate the mechanism by which Ca and Si affect the heat resistance of magnesium alloys, the morphology, crystallography and formation mechanism of the microstructures, as well as the mechanical properties of Mg-Ca and Mg-Ca-Si alloys were intensively investigated by means of optical microscopy, X-ray diffraction, scanning electron microscopy with energy dispersive spectrometer (SEM/EDS), transmission electron microscopy (TEM) and mechanical testing, and the following results were obtained. The as-cast microstructure of Mg-Ca alloys containing low Ca was composed of Mg-rich matrix and small amount of divorced eutectic component Mg2Ca distributed around the grain boundaries; while that of Mg-Ca alloys containing higher Ca composed of typical divorced eutectic (Mg+Mg2Ca) formed along the grain boundary areas. Being a nonequilibrium structure, the divorced eutectic (Mg+Mg2Ca) is unstable, and it is dissolved into the Mg-matrix when heated for solution-ageing treatment, with un-dissolved, discrete and granular Mg2Ca left over at the grain boundaries.Meanwhile, in the Mg-Ca-Si alloys containing low Si, an as-cast microstructure composed of Mg-matrix and CaMgSi compound was formed, with the CaMgSi phase formed at the grain boundaries and within the grains and displayed in three morphologies, namely discrete lump-like, dispersive dot-like, and needle-like. With increasing the Si content in the Mg-Ca-Si alloys,"Chinese Script"Mg2Si was produced in the as-cast microstructure in addition to the aforementioned CaMgSi phase in three morphologies. The microstructure of the as-cast Mg-Ca-Si alloys was not altered very much by solution-ageing treatment, merely with the dot-like and needle-like CaMgSi particles coarsened slightly. The morphologically different CaMgSi and Mg2Si phases were shown to be the products formed at different stages of a ternary eutectic reactions. Specifically, the discrete lump-like CaMgSi was a proeutectic product, the dispersive dot-like one was a product of the three-phase athermal eutectic reaction, and the needle-like CaMgSi and Mg2Si were all the products of the four-phase equilibrium eutectic reaction.The results of tensile tests showed that the room-temperature tensile performance of the as-cast Mg-Ca alloy was not that good, and its tensile strength and elongation decreased with increasing Ca content. After solution-ageing treatments, the room-temperature tensile performance of the alloy was markedly increased, though the falling tendency of the alloy's tensile strength and elongation with increasing Ca content kept unchanged. The solution-aged Mg-Ca alloy possessed a good transient tensile property at an elevated temperature of 200°C, and both its transient elevated temperature tensile strength and yield limit increased with increasing Ca content, though again the falling tendency of its elongation with increasing Ca content remained unchanged.The Mg-Ca-Si alloy in as-cast condition was found superior to the Mg-Ca one in room-temperature tensile properties when its Si content was low, and close to it when its Si content was increased. After solution-ageing treatments, the tensile properties of the low-Si Mg-Ca-Si alloy did not change much, whereas that of the high-Si one was improved markedly, approaching that of its low-Si counterpart.Fractography of the tensile specimens showed that a striking intergranular fracture occurred in the as-cast Mg-Ca alloys, a mixed cleavage and intergranular fracture took place in their solution-aged low-Ca counterparts, and a pure intergranular fracture again existed in the solution-aged high-Ca counterparts. For all the Mg-Ca-Si alloys, as-cast or solution-aged, low Si or high-Si, however, their fracture surfaces were essentially characteristic of intragranular fracture, consisting of large number of cleavage facets.A matrix analysis was made of the possible orientation relationship between the Mg-matrix and Mg2Si phase based on intensive TEM observations. It was shown that no unique orientation relationship was kept between the two phases, although the basal plane (0001) in the Mg-matrix tended to be parallel to some low-index planes in Mg2Si. A brief discussion was made of the formation mechanism of the crystallographic features between the Mg-matrix and Mg2Si precipitate. Two types of twins were observed in the lump-like CaMgSi phase formed in the Mg-Ca-Si alloy, which were shown to be respectively the (102) twin with the (102) plane serving as the twinning plane and the (011) twin with the (011) plane to be the twinning plane; besides these, one type of domain which is formed by a rotation of a certain angle about [100] axis was also found in the lump-like CaMgSi phase. The twinning plane and twinning direction of the (102) twin are (102) and [ 2 01] respectively; and the second undistorted plane for the twin is either (30 2 ) or (201). The twinning elementη2 is [203] for the undistorted plane of (30 2 ), and [102] for the undistorted plane of (201). Meanwhile, the twin plane and twinning direction of the (011) twin are (011) and [011] respectively. In addition to the frequently observed two-fold twin, three-fold twins were also observed in the (102) twinned CaMgSi particles. The crystallography of the three-fold twin was explained in details.