The Efect Of Cr, Mg, Cu Alloying On Microstructure And Properties Of Al-7%Si Cast Alloy

Al-Si is one kind of good structural material in automotive industry,which can produce high quality complex parts due to its good castability,moldability,weldability and other good properties like abrasive resistance and corrosion resistance.However,because Al-Si binary alloy has low strength and it is not heat-treated alloy,the application area of Al-Si alloy is limited.Alloying method is one of important ways to break through the application constraint of Al-Si alloy.This thesis comprehensively studies the effect of Cr,Mg,Si micro-alloying on the structure and properties of Al-7%Si alloy by using two kinds of cast alloy:Al-7%Si-0.3%Mg-0.3%Cr,Al-7%Si-0.3%Mg-0.3%Cr-1.5%Cu.Advanced Cs-corrected electronic microscopy and HAADF-STEM technology are applied to investigating the morphology characteristics and atomic structures of second phases formed during heat-treatment process.The additive of Cr increases the fracture toughness and greatly enhances the strength of alloy.The tensile strength of as quenched Al-Si-Mg-Cr alloy is 70%higher than that of A356 commercial cast alloy.The elongation of Al-Si-Mg-Cr alloy at room temperature is 5%higher than that of A356 commercial cast alloy.There are three phases including????? and Nano-scale Si forming in the Al-Si-Mg-Cr alloy during the homogenization process.Among these phases,Cr-rich phase?????is the dominant strengthening phase.It is found that????? has some common orientation relationships?ORs?with Al matrix.Under these common ORs,the interfaces between?????and Al matrix are coherent.Mg brings about strong aging hardening to Al-Si alloy.??is the dominant strengthening phase during aging process of Al-Si-Mg-Cr alloy.In the underage stage,the volume fraction of??is the decisive factor of strengthening effect.In the overaged stage,some lath-like?phases precipitate in the alloy.The additive of Cu further enhances the strength of alloy on the basis of Mg alloying.Cu changes the precipitate sequence of Al-Si-Mg alloy.Correspondingly,the precipitate sequence of Al-Si-Mg-Cr-Cu alloy is:Super Saturated Solid Solution?SSSS??GP zone????.Cu atoms are found to segregate in the interface of precipitate during aging process.In the under-aged stage,Cu occupies in the Si3 site of^?unit cell.^?phase contains some Cu atoms and is called???in this thesis.During the under-aged process,the high contrast step-unit characteristic is found in the interface of needle-like GP zone and???.The amount of step-unit increases with the coarsening of precipitate.The formation of step unit results from the translation distribution of Unit T?T??,a sub-structure?atomic coordination geometry?in precipitate.Furthermore,it is found that the coarsening of needle-like precipitate is realized by the formation of Unit T?T??,along<100>???.