The Investigation Of Microstructure Evolution And Mechanism Of Al-Si Alloys Modified By Rare Earth Yttrium

In order to achieve the purpose of reducing self-weight and improving fuel efficiency,Al-Si alloys have been widely used in aerospace and automotive fields,which is attributed to its good fluidity,low density,low coefficient of thermal expansion and high specific strength.However,the coarse α-Al dendrites or coarse and irregular Si phases exist in conventional cast Al-Si alloys,and the stress concentration is easily formed at the tip of the Si phase,which reduced the mechanical properties of Al-Si alloys.Therefore,the effect of rare earth yttrium(Y)and Y/Al-5Ti-1B on the morphology and size of α-Al,primary Si and eutectic Si in cast Al-Si alloys(hypereutectic Al-20 Si alloy and multi-component near eutectic Al-13Si-3Cu-2Ni-0.5Mg alloy)were investigated in this paper.Meanwhile,the effect of heat-treatment process on the microstructure of Al-13Si-3Cu-2Ni-0.5Mg alloy was researched,and defined the relationship between microstructural evolution and tensile properties in Al-Si alloys.In addition,the modification mechanism of rare earth Y to Si phase was explored,which provides a scientific basis and theoretical guidance for the industrial application of Al-Si alloys.The rare earth Y presented the obviously modification effect on Si phases in hypereutectic Al-20 Si alloy.Compared with the unmodified Al-20 Si alloy,the irregularly star-liked and coarsely block-shaped primary Si was refined to uniformly and finely regular block-like when the addition amount of rare earth Y is 0.8%.The average size of primary Si reduced by 62.9% from 89μm to 33μm and the aspect ration decreased by 38.6% from 2.07 to 1.27,respectively.In addition,the eutectic Si was modified from coarse flake/needle to fine fibrous structure.With the modification of rare earth Y on the microstructure of Al-20 Si alloy,its mechanical properties also have obvious improvement.The elongation(EL)increased by 22.5%from 1.11% to 1.36% when the addition amount of Y is 0.6%,and the ultimate tensile strength(UTS)enhanced by 47.9% from 94 MPa to 139 MPa when the content of Y is 0.8%.And the fracture mode of alloy gradually transformed from a classic brittle trans-granular fracture to the ductile-brittle mixed fracture style.The Y/Al-5Ti-1B displayed a remarkably modification effect on Si phases in Al-20 Si alloy.The morphology of primary Si was refined from the irregularly star-liked and coarse block-shaped to uniformly and finely regular block-like when the 650℃ modifiers mixed melt(0.6% rare earth Y and 1.0% Al-5Ti-1B)were addedinto the Al-20 Si alloy melt.The means size of primary Si decreased to 29μm and the aspect ration reduced to 1.47,respectively.Similarly,the coarsely plate-liked or acicular eutectic Si was modified to multi-branched fine fibrous,and its average roundness decreased by 70.3% from 7.8 to 2.32.The UTS of Al-20 Si alloy enhanced to 154 MPa and the EL improved to 1.79%.Moreover,the fracture feature of alloy gradually shifted from brittle style to ductile-brittle mixed mode.The rare earth Y could effectively improved the microstructure of as-cast Al-13Si-3Cu-2Ni-0.5Mg alloy.The coarse α-Al dendrite was refined into uniformly cell-liked crystal or equiaxed crystal when the addition amount of Y is 0.8%,and the average secondary dendrite arm spacing(SDAS)reduced by 64.6% from 12.7μm to4.5μm.Meanwhile,the eutectic Si was modified from the coarsely flake-shaped or needle-liked to multi-branched fine fibrous,and the average roundness decreased by56.1% from 9.8 to 4.3.Apart from this,the net-shaped or fish-bone-liked inter-metallic compounds evolved into the regularly block-liked rich-aluminum phase(AlCuNiY)containing the rare earth Y element.When the rare earth Y was added to Al-13Si-3Cu-2Ni-0.5Mg alloy,the rare earth element existed in the form of Y-containing compound at the growth front of eutectic Si,which could change the growth direction of Si phase to promote the formation of Si twins and {111}<112>texture,and also inhibit the continuous growth of eutectic Si to achieve the modification of eutectic Si from coarse flake-like to fine fibrous structure.The tensile properties of as-cast Al-13Si-3Cu-2Ni-0.5Mg alloy under the elevated-temperature and room-temperature have been significantly improved with the modification of rare earth Y.Compared with the unmodified alloy,the EL of as-cast alloy increased by 32.1% from 1.12% to 1.48% when the concentration of Y is 0.6%,and the UTS of as-cast alloy enhanced by 45.8% from 166 MPa to 242 MPa when the addition amount of Y is 0.8%,respectively.Meanwhile,the0.8%Y-modified as-cast alloy possess the optimally comprehensive mechanical properties and its Q-value improved by 52.6% from 173 MPa to 264 MPa.And the fracture mode of as-cast alloy changed from brittle fracture to ductile-brittle mixed fracture style with the modification of rare earth Y.In addition,the UTS of as-cast Al-13Si-3Cu-2Ni-0.5Mg alloy under 200℃ enhanced by 33.8% from 160 MPa to214 MPa when the addition level of Y is 0.8%,and the EL of alloy increased by40.5% from 1.16% to 1.63%.In T6 heat-treated(530℃ solution treatment 10 h and 180℃ ageing treatment12 h)Al-13Si-3Cu-2Ni-0.5Mg alloy,the eutectic Si presented a fine particle that sizeis about 5~8μm,and the inter-metallic with complex morphology evolved into Chinese character or small pieces,which were uniformly distributed in the alloy matrix.When the concentration of rare earth Y is 1.0%,the room-temperature UTS enhanced by 32.4% from 238 MPa to 315 MPa,and the EL improved by 38.6% from1.14% to 1.58%,respectively.