Study On Properties And Modification Mechanism Of Heat-resistant Phases In Magnesium-rare Earth Alloys

Various magnesium alloys are widely used.As the lightest metal structure materials,magnesium alloys are mainly used in the aerospace,transportation,electronic 3C and many other areas.However,most magnesium alloys have to work below 120?due to the lower creep resistance.Alloying is one of the effective methods to improve the high temperature performance of magnesium alloys.Si and RE elements are the most widely used alloying elements.The AS and AE alloys obtained by adding Si and RE elements are most widely used heat-resistant magnesium alloys.But the casting property of the AS alloy is poor and the cost of the AE alloy is high.Both of the two alloys have the disadvantage that the morphology of the reinforcing phases is not easily controlled.Mg₂Si and Al₄Ce were tend to form the shape of dendritic and long needle during the process of solidification.This paper mainly studies the Mg-Al-Si-Ce system alloys.The morphology of the heat-resistant phases such as Mg₂Si,Al₂Ce,Al₁₁Ce₃ and Al₄Ce can be controlled by adjusting the composition of the alloy.Furthermore,the microstructures and properties of Mg₂Si,Al₂Ce and Al₁₁Ce₃,Al₄Ce and other high melting point phase were calculated by the method of first-principles calculation.In order to find out the morphological characteristics and modification mechanism of heat resistant phase in Mg-Al-Si-Ce alloy.The mechanism of shape and size control of heat resistant phase based on component control is proposed,which can provide theoretical guidance for developing low cost heat-resistant magnesium alloys.In Mg-Al series alloy,Al₁₁Ce₃,Al₄Ce and Al₂Ce compounds are easily formed by Al and Ce atom.The research shows that Al₁₁Ce₃ and Al₄Ce are tend to be formed when content of Ce is below 1wt.%.The addition of a small amount of Ce to Mg-Al series alloy has little effect on the morphology of eutectic Mg₂Si,but it can improve the morphology of the primary Mg₂Si phase and change it from the shape of petaloid to regular block.When the Al content is about 3wt.%,Mg₂Si is easy to form more polygonal small particles in the alloy,and long needle shaped Al₁₁Ce?Al₄Ce?phase can form interface with Mg₂Si phase.The mismatch degree of Al₄Ce?100?and Mg₂Si?100?is 0.59%<6%,which is very likely to form heterogeneous nucleation interface.The mismatch degree of Al₂Ce?100?and Mg₂Si?100?is 12.02%,since the calculated mismatch degree of Al₂Ce?100?and Mg₂Si?100?is at the critical value,therefore,the Al₂Ce phase is less likely to be the core of the Mg₂Si heterogeneous nucleation,but the formation of the interface is possible.Influence of alloy composition and solidification conditions,needle-like Al₄Ce phase and bulk-like Al₂Ce phase are more likely to form.Al₄Ce and Al₂Ce can form a stable interface with Mg₂Si and effect on crystal growth.The larger states of Al₄Ce and Al₂Ce phase are difficult to become the heterogeneous nucleation cores of Mg₂Si,but Al₄Ce,Al₂Ce can form stable interfaces with Mg₂Si phase,which have some influence on the growth of crystal.Addition of Ce in Mg-Al-Si magnesium alloy can not only reduce the amount of Mg₁₇Al₁₂ and improve its morphology,but also form Al-Ce reinforcing phase,resulting in multiple strengthening effects such as solution strengthening,precipitation strengthening and grain boundary strengthening.The test of hardness and mechanical properties results show that,the hardness of Mg-x Al-2.5Si-Ce alloy increase with the increase of Al content.When the Al content is above 2wt.%,the mechanical property of the alloy is about 130MPa at room temperature and the elongation increase with increase the content of Al.The electronic properties,mechanical properties and thermodynamic properties of Mg₂Si,Al Ce,Al₂Ce,Al₃Ce,Al₁₁Ce₃ and Al₄Ce phases in the alloy were calculated by the first-principles calculation method.The results of binding energy show that the stability of the compounds decreases in the order of Al₁₁Ce₃,Al₂Ce,Al Ce,Al₃Ce and Al₄Ce.The calculation results of enthalpy of formation show that Al₁₁Ce₃ and Al₄Ce are easy to form in the alloying process,followed by Al₃Ce and Al₂Ce,and finally Al Ce is formed.The calculation results of elastic properties show that Al Ce,Al₂Ce,Al₁₁Ce₃ and Al₄Ce satisfy the stability condition of the crystal except for Al₃Ce.Al-Ce compounds belong to brittle materials,the plasticity from weak to strong is Al Ce,Al₄Ce,Al₃Ce,Al₂Ce and Al₁₁Ce₃.The anisotropy of Al-Ce phase was decreased in order of Al₃Ce,Al₁₁Ce₃,Al₂Ce,Al Ce and Al₄Ce.The calculated results of electron density of states show that the Al atom and Ce atom in each Al-Ce phase have obvious hybrid phenomena except Al Ce phase,the emergence of hybridization indicating that Al atoms and Ce atoms have formed a strong bond.The results of metallic property show that the metallicity decrease in order of Al₁₁Ce₃,Al₃Ce,Al₂Ce,Al₄Ce,Al Ce.The temperature of Debye decreases according to Al₁₁Ce₃,Al₂Ce,Al₄Ce,Al₃Ce and Al Ce,which indicates that there is a strong bond between Al₁₁Ce₃.Al₁₁Ce₃ has higher thermal conductivity and the bond in Al Ce compound is weakest.The above studies have accumulated basic data for the further research on the properties of Al-Ce phase.