Study On The Influences Of Cooling Rate On Solidification Microstrcture Of Magnesium-Aluminum System Alloys

An understanding of the solidification mechanism of magnesium-aluminum alloys, especially, the forming mechanism of Mg-Mg17Al12 eutectic, is essential in order to improve the high-temperature creep resistance, toughness and plasticity of alloys. It will enable the composition and casting conditions of Mg-Al alloys to be tailored to produce sound castings.In this paper, in order to obtain different microstructures under different cooling rates, the magnesium-aluminum alloys, ranging widely from 3wt.% to 38wt.% of aluminum content, were cast in different mould. The microstructures of alloys were observed by using OM and SEM. The weight fractions of Mg17Al12-phase in two different zones of step-type steel-mould samples'microstructures were calculated through quantitative analysis of XRD. The special structures in near-eutectic alloys were analyzed by using DSC and TEM. The eutectic growth mechanics were analyzed based on the theory of solidification kinetics and thermodynamics. The main results are as follows:The influences of the cooling rate is that, the size of primary dendrite grains becomes smaller with the increasing of cooling rate, and secondary or tertiary dendritic arms well develop. Increasing the cooling rate will lead to more precipitateβ-phase in the solidification microstructure, when the alloys has low aluminum content. Whereas, increasing the cooling rate will lead to less precipitateβphase in the solidification microstructure, when the alloys has high aluminum content. Cooling rate has little influence on amount of precipitateβ-phase in eutectic alloy. Increasing the cooling rate will improve the possibility of reaching a high undercooling before nucleation theβphase occurs, which makes eutecticβphase trend to grow as divorced eutectic. The well developed dendrites will increase the tendency of forming divorced eutectic.The eutectic morphologies of Mg-Al system alloy have several types. With the increasing of aluminum content, the eutectic morphology changes from fully divorced to partially divorced,to granular, to fibrous and lamellar. The transitions are continuous, and granular eutectic can seem as transitional morphology between regular eutectic (coupled growth) and irregular eutectic (separated growth). It is transition region, where the weight fraction of eutectic ranges from 30% to 50%. In this region, the morphology of eutectic will be also effected by cooling rate and morphologies of primary dendrites.Under cooling condition of common casts, the plate-like herringbone dendritic eutectic may form in the solidification processing of near-eutectic Mg-Al alloys. That is, plate-like eutectic precipitates firstly, and fibrous eutectics grow lateral as branches from the plate-like eutectic, which seems as trunk of tree. The eutectic combination characters a special tree-like morphology, which has not been reported in former literatures.