Effect Of Re Element On Microstructure And Mechanical Properties Of Al-Si Alloy

In this paper,the influence of different content of single rare earth Sm,Yb on the microstructure and mechanical properties of hypereutectic Al-20%Si alloy were studied.At the same time,the microstructure evolution and mechanical properties of Al-20%Si alloy combining addition rare earth Yb and Al-5Ti-1B refiner were also studied in the paper.The single rare earth Sm,Yb have excellent refinement effects on the primary Si and the eutectic Si phases in the Al-20%Si alloy.When the content of Sm is 0.6%,the primary Si phases are modified into small blocks with edge passivation,and evenly distributed on the eutectic matrix.The average size of primary Si decreases from 108 ?m to 35 ?m.Additionally,the eutectic Si is completely transformed the fine fibrous structure,and the interlamellar spacing is greatly reduced.When the content of Yb is 0.5%,the morphology of primary Si is completely transformed into a fine and uniformly distributed block structure,and the average size of primary Si is reduced to about 48 ?m,and the eutectic Si is converted into tiny lake-like fibers.Al-5Ti-1B can reduce the average size of primary Si and eute ctic Si in the Al-20%Si alloy,but it cannot change the morphology.However,the composite modifier consisting of Yb and Al-5Ti-1B has an excellent refinement effect on the ?-Al phase,primary Si and eutectic Si in the Al-20% Si alloy.Compared with the unmodified Al-20%Si alloy,the course ?-Al dendrites are refined into relatively smaller equiaxed dendrites and the secondary dendrite spac e is significantly reduced when the contents of Yb and Al-5Ti-1B are 0.5% and 0.3%,respectively.Primary Si is completely transformed into small regular,uniformly distributed block and the average size reduced to 28 ?m.Additionlly,the eutectic Si is completely transformed into fine granular,short plate-like,and fibrous structure.The ultimate tensile strength(UTS)and elongation(EL)have been greatly improved after Sm or Yb modification treatment of Al-20%Si alloy.Compared with the UTS and EL values of unmodified Al-20%Si alloy,the UTS is enchanted by 48.5% from 1 MPa to 153 MPa and EL is increased by 68.8% from 0.45% to 0.76% when the addition amount of Sm increases up to 0.6%.When the content of Yb is 0.5%,the UTS is enhanced by 64.5% from 93 MPa to 153 MPa,and El is increased by 73.6% from 0.41% to 0.71%.When Al-5Ti-1B modifier was alone added,there was no obvious effect on the mechanical properties of Al-20% Si alloy.However,UTS is enhanced by 83.7% from 92 MPa to 169 MPa,and EL is increase d by 92.1% from 0.38% to 0.73% after combined addition of 0.5%Yb and 0.3%Al-5Ti-1B.The fracture morphology of the unmodified Al-20% Si alloy is mainly composed of many coarse cleavage planes and primary Si sections,which are typical brittle fractures.However,the number of cleaved step surfaces decreases,and some torn edges appear on the fracture surface in the modified Al-20% Si alloy.The fracture mode is a brittle fracture-based ductile-fracture hybrid fracture.During the stretching process of Al-20%Si alloy,the cracks mainly initiate at the interface between primary Si,primary Si and the matrix and the interface between eutectic Si and the matrix.Subsequently,the crack propagates in the structure and collects,causing the Al-20% Si alloy to undergo intergranular fracture and transgranular fracture.