中文摘要：

To obtain a higher microstructural refining efficiency,and improve the properties and processing ability of hypereutectic Al-25Si alloy,a new environmentally friendly Al-20.6Mn-12Ti-0.9P-6.1Cu(by wt.%)master alloy was fabricated;and its modification and strengthening mechanisms on the Al-25Si alloy were studied.The mechanical properties of the unmodified,modified and heat treated alloys were investigated.Results show that the optimal addition amount of the Al-20.6Mn-12Ti-0.9P-6.1Cu master alloy is 4wt.%.In this case,primary Si and eutectic Si as well asα-Al phase were clearly refined,and this refining effect shows an excellent long residual action as it can be heat-retained for at least 5 h.After being T6 heat treated,the morphology of primary and eutectic Si in the Al-25Si alloys with the addition of 4wt.%Al-20.6Mn-12Ti-0.9P-6.1Cu alloy changes into particles and short rods.The average grain size of the primary and eutectic Si decreases from 250μm(unmodified)to 13.83μm and 35μm(unmodified)to 7μm;theα-Al becomes obviously finer and the distribution of Si phases tends to be uniform and dispersed.Meanwhile,the tensile properties are improved obviously;the tensile strengths at room temperature and 300 oC reach 241 MPa and127 MPa,increased by 153.7%and 67.1%,respectively.In addition,the tensile fracture mechanism changes from brittle fracture for the alloy without modification to ductile fracture after modification.Modifying the morphology of Si phase and strengthening the matrix can effectively block the initiation and propagation of cracks,thus improving the strength of the hypereutectic Al-25Si alloy.

英文摘要：

To obtain a higher microstructural refining efficiency, and improve the properties and processing ability of hypereutectic AI-25Si alloy, a new environmentally friendly AI-20.6Mn-12Ti-0.9P-6.1Cu （by wt.%） master alloy was fabricated; and its modification and strengthening mechanisms on the AI-25Si alloy were studied. The mechanical properties of the unmodified, modified and heat treated alloys were investigated. Results show that the optimal addition amount of the AI-20.6Mn-12Ti-0.9P-6.1Cu master alloy is 4wt.%. In this case, primary Si and eutectic Si as well as e-AI phase were clearly refined, and this refining effect shows an excellent long residual action as it can be heat-retained for at least 5 h. After being T6 heat treated, the morphology of primary and eutectic Si in the AI-25Si alloys with the addition of 4wt.% AI-20.6Mn-12Ti- 0.9P-6.1Cu alloy changes into particles and short rods. The average grain size of the primary and eutectic Si decreases from 250 IJm （unmodified） to 13.83 pm and 35 IJm （unmodified） to 7 tim; the e-Al becomes obviously finer and the distribution of Si phases tends to be uniform and dispersed. Meanwhile, the tensile properties are improved obviously; the tensile strengths at room temperature and 300 ℃ reach 241 MPa and 127 MPa, increased by 153.7% and 67.1%, respectively. In addition, the tensile fracture mechanism changes from brittle fracture for the alloy without modification to ductile fracture after modification. Modifying the morphology of Si phase and strengthening the matrix can effectively block the initiation and propagation of cracks, thus improving the strength of the hypereutectic AI-25Si alloy.