| Aluminum and its alloys are widely used in aviation,aerospace,automotive and electronic devices because of their low density,good corrosion resistance,high specific strength,and excellent electrical and thermal conductivity.With the development of science and technology,higher requirements are imposed on its subsequent processing performance The uniform and fine equiaxed crystal structure can eliminate the anisotropy of the casting structure,and at the same time suppress the crack growth,which improving the comprehensive mechanical properties of the ingot Therefore,the research on grain refinement of aluminum and its alloys has been widely concerned by materials researchers.In this paper,AlCoCrFeNiTix(x=0.0,0.5,1.0)high-entropy alloys(HEAs)with different contents Ti were prepared and added into pure aluminum.The effects of HEAs on the microstructure and mechanical properties of pure aluminum were investigate.At the same time,the mechanism of refinement of ?-Al grains by HEAs was clarified,and defined the relationship between the evolution of microstructure and mechanical properties in the alloy.In addition,the effect of AlCoCrFeNiTi alloy on the microstructure and mechanical properties of Al-7Si alloy was studied,which refinement mechanism on Si phase was explored(1)When AlCoCrFeNiTi alloy is added into pure aluminum,the coarse?-Al columnar crystal structure in pure aluminum is refined into fine equiaxed crystals.Moreover,a higher HEA concentration results in a finer?-Al grains.The refined samples formed Al3Ti,Al3Ni,and dense nano-AlFeCoNi phases with BCC structure.The formation of these intermetallic compounds can significantly enhance the yield strength and tensile strength of the alloy.With the increase of HEA content,the average length of the nanophase increased from 2568nm to 6907nm,and the average diameter decreased from 102nm to 92nm.Compared with pure aluminum,the ultimate tensile strength is enhanced by 145%from 62MPa to 152MPa,the yield strength is increased by 174%from 42MPa to 115MPa,but the elongation is decreased by 33%from 39%to 26%when the addition of HEA increased to 3.0wt.%(2)The addition of AlCoCrFeNiTi0.5 alloy can refine the coarse ?-Al columnar crystal structure in pure aluminum into fine equiaxed dendrites,and its grain refining effect is lower than that of AlCoCrFeNiTi alloy.The dense nanophase containing Al,Co,Fe,Ni,and a small amount of Cr elements was formed in the refined sample.The Ni content at the end of the nanophase was higher and the middle part was lower.When the amount of AlCoCrFeNiTi0.5 alloy is increased to 3.0wt.%,The ultimate tensile strength is increased to 157MPa,and the elongation is reduced to 24%(3)The addition of AlCoCrFeNi alloy to pure aluminum can refine the?-Al grains into uniformly distributed fine columnar crystal structures,and the growth direction of this ?-Al structure is opposite to the direction of heat flow.When 3.0wt.%HEA is added into pure aluminum melt,the average length of ?-Al columnar crystals is 35?m and the average width is 7?m.The ultimate tensile strength of the sample is increased to 147MPa,and the elongation is reduced to 18%(4)The addition of AlCoCrFeNiTi alloy can effectively refine the eutectic silicon and ?-Al grains of the Al-7Si alloy.When 0.2wt.%AlCoCrFeNiTi alloy is added,the ?-Al grain size of Al-7Si alloy is significantly reduced,and the secondary dendrite arm spacing is reduced by 2.3%from 26?m to 20?m.The morphology of eutectic silicon changed from coarse long needles to fine short rods and granules,and its average length is decreased by 63%from 7.98?m to 2.94?m.In addition,the refined sample formed an Fe-rich intermetallic compound with a size of about 3.5?m,which mainly contained Al,Si,Co,Fe,Ni elements and a small amount of Cr and Ti elements.This intermetallic compound is distributed at the Si/Al interface,which seriously hinders the growth of the Si phase.The ultimate tensile strength of the alloy is increased by 16.07%from 168MPa to 195MPa,and the elongation is increased by 51.24%from 12.1%to 18.3%. |
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