Investigation On Microtsructure And Properties Of Mg-5Al-2Si-xCe Alloys

Magnesium alloy is one of the lightest structural materials in metal and has broad application prospects.Mg-Al-Si series magnesium alloys have advantages such as good fluidity and low cost.Mg-Al-Si series alloys were developed as heat resistant alloys used at elevated temperatures,due to the presence of thermally stable intermetallic Mg₂Si phases.However,the Mg-Al-Si series alloys easily form undesirable,large Chinese script type Mg₂Si phases,which would deteriorate the mechanical properties of the alloys.Therefore,the modification of Mg₂Si phases are critical to improve the mechanical properties of Mg-Al-Si series alloys in recent years.In this study,Mg-5Al-2Si alloys were used as basic alloy.The present work aims to modify the alloy by adding different content of(0,0.4,0.8,1.6,2.4,4.0%)Ce element on Mg-5Al-2Si alloy.The performance of microstructure and mechanical properties,and the modification mechanism of Ce on Mg₂Si phases in the alloy were discussed,which provides theoretical basis for improving mechanical properties of Mg-5Al-2Si alloy and expanding further application.The results reveal that?-Mg phase,Chinese script type Mg₂Si phase,polygonal type Mg₂Si phase,Mg₁₇Al₁₂ phase,newly formed Ce Si₂ and Al₁₁Ce₃ phase in the experimental alloys.The addition of Ce changes the size,morphology and distribution of Mg₂Si phases in the alloys.The size of Mg₂Si phases decreased significantly with the increasing Ce content,from 110?m to 30?m for Chinese script type in length,from23?m to 3.3?m for polygonal type in length,and the morphology of Mg₂Si phases transform from large Chinese script type to dispersive block shape.The modification mechanisms of Mg₂Si phases in the alloys could be divided into two categories.On the one hand,the formation of Ce Si₂phases act as an effective heterogeneous nucleation core,promote the growth of Mg₂Si phases,tiny and dispersive Ce Si₂ phases made Mg₂Si tend to diffuse and cannot form large Chinese script type.This trend is more pronounced as the increasing of Ce addition.This may significantly suppress the growth of Mg₂Si phases.On the other hand,the rare earth element Ce is surface-active element,its solid solubility in magnesium alloys is very low(only 0.52%),which is segregated at the grain boundary.While Si is not dissolved in magnesium,Mg₂Si phases formed by Mg and Si are also distributed at the grain boundary.The addition of Ce will affect the interfacial energy of the Mg₂Si phase and hinder its growth.The tensile strength and elongation of Mg-Al-Si-Ce alloys were improved significantly,especially Mg-5Al-2Si-4.0Ce alloy exhibits the best elongation,and improvement percentage of elongation at 150oC are 569%,at 200oC are 820%.The tensile fracture surfaces exhibit typical feature of quasi-cleavage fractures.