Study On Modification Mechanism Of Silicon Phase During Solidification Process Of Al(-Zn)-Si Alloy With Eu And P Additions

Al(-Zn)-Si alloys have widespread applications especially in aerospace and automotive industries owing to their superior castability,excellent wear resistance,excellent thermal stability and integrated mechanical properties.However,the mechanical properties are severely weakened since the coarse primary Si and eutectic Si lead to premature crack initiation and fracture in tension conditions.In order to explore the application potentiality of Eu to the Al(-Zn)-Si alloys and improve the mechanical properties of alloys,the paper proposes the use of Eu element for modification to overcome the limitations of traditional modifiers.This thesis mainly carries out the following research work:the interaction between Eu and P in high purity Al-7Si alloy;effect of Eu addition on the growth of eutectic Si;effect of Eu addition on the microstructures and mechanical properties of commercial Al-7Si-0.3Mg alloys;modification mechanism of primary Si in high purity Al-16Si alloy with Eu addition;modification mechanism of primary Si and mechanical properties research in commercial Al-16Si alloy with combined Eu and P additions;modification law of Eu to the pre-eutectic and eutectic Si phases in commercial Al-40Zn-5Si alloy.The paper has drawn some valuable research conclusions listed as follows:(1)The interaction between Eu and P in hypoeutectic Al-7Si alloy is strong.The more P contents in alloys,the more Eu contents are required to modify the eutectic Si to fibrous.This is due to the formation of(Al,Eu)P by Eu solubilizing in AIP,the coarse Al2Si2Eu phases precipitating from melt wrapping P-rich particles and P element solubilizing in coarse Al2Si2Eu phase.High density multiple twins are often found within modified eutectic Si.The Eu element can be adsorbed along the<112>growth direction of Si to poison the twin plane re-entrant edge and at the intersection of Si twins to induce twins.(2)The addition of Eu improves the tensile strength and elongation of Al-7Si-0.3Mg alloy in as-cast and T6 heat treatment condition.When the content of Eu is 0.1%,the quality index in as-cast and T6 heat treatment condition increases by 11.4%and 10.8%.(3)Eu can modify both the primary Si and eutectic Si in high purity Al-16Si alloy.With the increase of Eu contents,coarse five-branched primary Si is gradually modified to smaller octahedral and plate-like primary Si.Simultaneously,the eutectic Si is gradually modified to fibrous.The modification of primary Si by Eu is owing to the formation of Eu solute enrichment layer near liquid-solid interfaces of primary Si,which creates higher constitutional supercooling and restricts the growth of primary Si.(4)Combined Eu and P additions can modify both the primary Si and eutectic Si in commercial Al-16Si alloy and improve the mechanical properties of alloy.When the content of Eu is 0.15%and the content of P is 0.06%,compared to unmodified alloy,the ultimate tensile and elongation increase by 14.1%and 108%,the wear rate decreases by 35.9%.(5)Eu can modify both the pre-eutectic Si and eutectic Si in commercial Al-40Zn-5Si alloy with trace impurity P element.Pre-eutectic Si is gradually modified to nodular and the eutectic Si is gradually modified to fibrous.The addition of Eu improves the tensile property of alloy.When the content of Eu is 0.2%,the ultimate tensile and elongation increase by 16.9%and 383%.(6)Synchrotron real-time imaging experiment results reveal that the primary phase of unmodified commercial Al-40Zn-5 Si alloy is ?-Al dendrite and pre-eutectic Si particles occur in front of dendrites.The needle-like eutectic Si crystals are leading phase during Al-Si eutectic reaction.Afterward,eutectic a-Al nucleates at the interface of eutectic Si,The addition of 0.3%Eu restricts the nucleation of pre-eutectic Si particles due to the formation of(Al,Eu)P by Eu solubilizing in AlP,which reducing the ability of AlP particles to nucleate pre-eutectic Si.(7)High resolution HAADF-STEM images show that the effect of Eu on the growth of pre-eutectic Si mainly including:The stacking faults and parallel twins are caused by the adsorption of single Eu atom columns;The intersecting twins are caused by the adsorption of single Eu atom columns and triangular cluster made up of more Eu atom columns;The poisoning of twin plane re-entrant edge(TPRE)are caused by the adsorption of liner Eu atom columns at the twins.