铸造Mg-Al-Zn合金的成分、相组成与凝固路径的关系

RELATIONSHIP OF COMPOSITIONS,PHASE CONSTITUENTS AND SOLIDIFICATION PATHS OF CASTING Mg-Al-Zn ALLOYS

采用SEM,EDS和XRD研究了几种Mg-Al Zn合金的成分、相组成与凝固路径的关系.结果表明,Mg-Al-Zn合金的相组成与Zn/Al质量比有关;随着Zn/Al比值的增加,第二相γ-Mg_(17)Al_(12)的数量减少,逐渐被φ-Mg_(21)(Zn,Al)_(17)取代,直至完全消失.通过搜集合金中可能存在的各相的热力学模型,运用Pandat热力学计算软件平台,计算并比较了平衡和非平衡(Scheil模型及金属型铸造)条件下合金的成分、相组成与凝固路径的关系.结果表明,金属型铸造的凝固过程偏离平衡凝固过程,Scheil模型很好地解释了大多数合金铸造组织的相组成;但是由于Scheil模型不考虑有固相作为反应物参与的包晶反应,对ZA65合金凝固路径和相组成的预测结果与实际铸造组织的相组成不符.

Casting Mg-Al-Zn alloys are promising for the automotive components loaded under both high strength and high ductility, as well as under high temperatures. The relationship of compositions, phase constituents and solidification paths of casting Mg-Al-Zn alloys were investigated by SEM/EDS, XRD and thermodynamic calculations. It is shown that the phase constituents of Mg-Al- Zn alloys are related to Zn/Al ratio; with the Zn/Al ratio increasing, the secondary phase γ-Mg17Al12 is gradually replaced by φ-Mg21（Zn, Al）17 and eventually disappears completely. The phase constituents and solidification paths of the alloys under various conditions, including equilibrium, Scheil and permanent mould casting, were examined by thermodynamic calculation software Pandat with the availability of thermodynamic description of Mg-Al Zn ternary system. The practical casting process deviates from the equilibrium; however, the practical phase constituents of the experimental alloys except ZA65 can be predicted by the Scheil model. Because of no considering the periteetic reaction in which solid participates as reactant, the Scheil model can not correctly predict the phase constituents of ZA65 alloy under permanent mould casting condition.