Effect of La addition on microstructure and mechanical properties of hypoeutectic Al-7Si aluminum alloy

The effect of La addition (0,0.1,0.2,0.4,wt.%) on the microstructure,tensile properties and fracture behavior of Al-7Si alloy was investigated systematically.It is found that the La appears in the Al-7Si alloy in the form of AlLa and AlSiLa phases.La addition can refine the secondary dendrite arm spacing (SDAS) and eutectic Si particles,which are decreased by 7.9% and 7%,respectively,with the optimal La content of 0.1wt.%.Because when 0.1wt.% La is added,a relatively higher nucleation undercooling of 37.47 °C is observed.Higher undercooling degree suggests that nucleation is accelerated and subsequent growth is restrained.After T6 heat treatment,compared with the without La,the ultimate tensile strength of the alloy with 0.1wt.% La is enhanced by 5.2% from 333 MPa to 350.2 MPa and the elongation increases by 73% from 7.37% to 12.75%,correspondingly.The fracture mode evolves from the ductile-brittle mixed fracture to ductile fracture mode.However,when La element content reaches a certain value of 0.4wt.%,serious segregation takes place during the solidification process.The formed brittle phases deteriorate the tensile properties of the alloy and the fracture mode of Al-7Si-0.2/0.4 La changes to mixed ductile-brittle fracture mode.