Research On Morphological Evolution And Corrosion Behavior Of Mg₂Si In Al-Mg₂Si Alloys

Nowadays,countries all around the world pay more attention to the environment and resource problems,energy conservation and emission reduction could be a good way to solve these problems.Lightweight of automobile not only could reduce the fuel consumption,but also could lower the carbon emissions.Aluminum alloy could substitute steel and iron in the automobile industry because of the low density and corrosion resistance.By addition of Mg and Si elements in casting process,in situ Al/Mg₂Si composite has been prepared,which promotes the strength and hardness of the aluminum alloy.Unfortunately,the primary Mg₂Si phases and eutectic Mg₂Si in normal as-cast Mg₂Si/Al composites are usually very coarse or fiber-like and thus lead to poor properties of these composites.Therefore,composites with a predominantly coarse or fiber-like Mg₂Si crystal must be modified to ensure adequate mechanical strength and ductility.The morphology of Mg₂Si could be modified by addition of P in Al-20%Mg₂Si alloy and by heat treatment in Al-10%Mg₂Si alloy,which leads to a better mechanical property of Al/Mg₂Si composite.However,the corrosion property has not been investigated after Mg₂Si has been modified.Thus,in this work,we focus on the change of corrosion behavior after the Mg₂Si has been modified in Al/Mg₂Si composite,and we also study the solidification process and microstructure of corresponding alloys.The results are as follows:?1?After the addition of Al–3%P master alloy,Mg₂Si particles are modified,transforming from enormous dendrite?irregular shape?to fine and uniformly distributing polyhedron.Rod-shaped eutectic Mg₂Si in cast Al-10%Mg₂Si alloy is broken down into small fragments and metastable phase precipitates from the matrix after T6 treatment.?2?At the initial stage of corrosion process in NaCl solution,potential of Mg₂Si electrode is anode,and corrosion starts from Mg₂Si particle surface.As the corrosion proceeds,active Mg element in Mg₂Si phase preferentially dissolves with enrichment of Si element,resulting in the electrochemical polarity exchange between Mg₂Si and ?-Al.Consequently,?-Al electrode becomes anode,leading to the corrosion of Al matrix with the formation of pits.?3?Eutectic Mg₂Si phase compared to primary Mg₂Si phase presents preferential dissolution in Al–20%Mg₂Si.Due to the addition of Al–3%P master alloy,Mg₂Si particles are modified,transforming from enormous dendrite?irregular shape?to fine and uniformly distributing polyhedron,which prevents the corrosion pits from propagating in Al matrix and enhances the corrosion resistance of Al–20%Mg₂Si alloy.?4?Spherical and uniforming distributing Mg₂Si and metastable precipitates in Al matrix are the mean reasons for better corrosion resistance Al-10%Mg₂Si alloy with T6 heat treatment.