Microstructure And Mechanical Properties Of Heat-resisting Mg-Al-Sn Alloys

As the lightest metal structure material, magnesium alloys offer high specific strength,superior damping capacity, excellent machinabilty and castability. Demands from all kinds of industry for increasing lightweight components have draw a great interest in magnesium alloys.The most commonly used alloys are based on the Mg-Al system. However, most commercial alloys(such as AZ series ? AM series) have poor mechanical property under high temperature, this phenomenon is mainly caused by soften of the Mg17Al12 phase when the temperature is beyond 120?. Concomitantly, other alloys systems that have good mechanical property are being investigated to be the substitute. The family of Mg-Sn alloys meets those demands well: the precipitate in Mg-Sn alloys has a high melting temperature (771.5?), which is higher than that of M17Al12 (462?) and MgZn (347?); and the solubility of Sn in alpha-Mg solid solution were very lagre,this provides a fundamental basis for improving the mechanical properties of those alloys through solid-solution treatment.This thesis presents the study on transformation of microstructure for the die-casting Mg-6wt%Al-4wt%Sn-2wt%Zn and Mg-6wt%Al-8wt%Sn-2wt%Zn alloys under different aging temperatures, using Rietveld whole-pattern fitting method to calculate the content of the secondary phase. Influences of the distribution of the second phase and the changing trend of these phases content on mechanical property of the alloys are investigated preliminarily, to provide the theoretical and experimental evidence for the research and application of the new alloys.By the researching, the conclusions follows: the enhancement of tested magnesium alloy's mechanical property manly is the result of second-phase enhancement of the Mg2Sn phase,dispersion strengthening of the Mg17Al12 phase as well as the solution strength of alloy element such as Sn,Al. for the two kinds of alloys, after the operation of heat treatment at 400?about 50h, the Mg17Al12 reticular phase (which becomes soften under high temperature) will resolve and dissolve into matrix, and the content of Mg2Sn phase increases obviously; after the operation of heat treatment at 200? about 50h, the A1 element which is dissolved in the matrix shows obvious desolation transformation, and generating massive Mg17Al12 phase, but the hardening Mg2Sn phase keeps stable; during the operation of 400?-200? multiplex heat treatment , the Mg17Al12 reticular phase will resolve, and generat massive Mg17Al12 dispersed phase inside the matrix grain,and the content of Mg2Sn phase increases obviously meanwhile.The content of the two second phase (Mg17Al12and Mg2Sn) at different heat treating stage were calculated by Rietveld whole-pattern fitting method, the change law of the two second phase content is analyzed.In this paper,we found that the superposition of the dispersion strengthening of Mg17Al12 phase (whin the grain) and the second phase strengthening effect of the Mg2Sn phase has made the Mg-6wt%Al-8wt%Sn-2wt%Zn alloys show the best strengthening effect than the experimental alloys in other stages.