Study On The Evolvement Of Solidification Microstructure Of Magnesium-Alnminum System Alloys Under Rare Earth Nd Modification Condition

In order to improve the solidification structure of magnesium(Mg)-aluminum(Al) system alloys, enhance their mechanical properties, and provide some reference to the study of strengthening and toughening Mg-Al alloy, the effect of adding rare earth Nd to the evolution of Mg-Al alloy's solidification structure was studied in this article. The microstructure was analyzed by optical microscope and scanning electron microscopy(SEM), compositions and microzone composition of phases were analyzed by energy dispersive spectrometry(EDS), struture and compositions of every phases in the Mg-Al alloys were analyzed by X-ray diffraction (XRD). Specially studied the type transition and pattern character of precipitated phase in the solidification microstructure of the modified Mg-Al alloys in which the content of Al is approaching the composition of eutectic. The result indicated that:(1) The change law of Mg-Al alloys'organization is approximately same before and after the alloys modified,it was that the amount ofα-Mg increased and the amount of eutectic decreased accordingly as the increasing of aluminum content. But corresponds to each ingredient organization, the rule is different before and after the alloys modified. To the low aluminum content alloy, the eutectic organization's continuity reduced obviously, which in the crystal boundary of organization after modifying, the quantity of it reduced, and the primaryαdendrite's branch became advanced.(2) The modification of Nd made a large number of dendrite appearing in the solidification microstructure of the high Al content(about 33%Al) Mg-Al alloys;In the range of hypoeutectic component, theαdendrite precipitated first. As the Al content gradually increasing and approaching the eutectic point, theβhalo appeared around theαdendrite and become thicken continuously, the amount ofαdendrite decreased, and then theβhalo changed intoβdendrite, the amount of which increased and whose branch became advanced. The result of Nd modification made the solidification microstructure of Mg-Al alloy approaching the eutectic point changed greatly, which changed from typical unmodified symbiotic eutectic toαdendrite,βdendrite and symbiotic eutectic concomitant structure after modification.(3) From the thickest of step permanent mould to the middle of step permanent mould to the thinnest of step permanent mould to graphite mould, as the cooling rate enhanced, crystal grain of Mg-Al alloy became finer gradually, phases in the organization distributed uniform, and the quantity of eutectic increased gradually.(4) Three factors of the aluminum content, the cooling rate and the rare earth Nd modification effect toα-Mg crystal grain size mutually, and their function superposed. Among them, the enhancement of cooling rate and the increase of aluminum content could minish the crystal size, adding of the rare earth Nd element caused the crystal grain branching to be developed, the crystal grain size increased.