Effect Of Rare Earth Addition On The Microstructure And Properties Of Hypoeutectic Al-Mg₂Si Alloy

In this paper,the effects of rare earth elements Sc and Er on the microstructure and mechanical properties of the hypoeutectic Al-11Mg₂Si alloy was studied by means of optical microscope,X-ray diffractometer,scanning electron microscopy and electronic universal testing machine.The results are summarized as follows:This finding implies that simultaneous primary grainα-Al refinement and eutectic Mg₂Si modification could be achieved by minor Sc alloying in hypoeutectic Al-Mg₂Si alloys.The simultaneous grain refine of primaryα-Al phase and modification of Mg₂Si phase are attributed to the formation of Al₃Sc heterogeneous nucleus,the Sc segregation and the corresponding composition supercooling in front of the growing Mg₂Si phase.Moreover,the morphology of the eutectic Mg₂Si phase is changed from a coarse flake-like shape to fine and short fibrous.The tensile strength,yield strength and plastic strain at fracture are significantly enhanced with the addition of Sc.When Sc increases up to 0.25 wt%,the tensile strength is enhanced by 24.2%from 269MPa to 334MPa,yield strength by 41.0%from 134MPa to189MPa and the plastic strain at fracture by 84.6%from 5.2%to 9.6%.The eutectic Mg₂Si phases and eutectic（α-Al+Mg₂Si）grains modification in hypoeutectic Al-Mg₂Si alloys could be achieved by suitable minor Er alloying.The addition of Er causes a morphological variation of eutectic Mg₂Si from a coarse maze-like structure to a dense point-like structure and some branched morphology.When the Er content increases to0.1wt%,the aspect ratio of Mg₂Si（R_A）and interlamellar spacing（ILS）of eutectic Mg₂Si phase are obviously reduced from 15.4μm and 6.8μm for the base alloy to 1.2μm and 0.4μm for the alloy with 0.1wt%Er addition,respectively,and the eutectic（α-Al+Mg₂Si）grains size are reduced from 41.2μm to 9.6μm.When the content of Er is enhanced to 0.1-0.2wt%,the tensile strength can reach a peak value of 340MPa,which increases by 25.9%compared with the virgin alloy.The elongation increases prominently from 3.87%to 12.56%at 0.1wt%Er content.The tensile fracture morphology of the base alloy is composed mainly by some cleavage platforms,tear ridges and a few dimples.With the addition of Sc and Er,in addition to further reduction of the dimple sizes,a large number of deep and dense dimples appeared on the fracture surface,exhibiting obvious ductile fracture characteristics.The simultaneous grain refine of primaryα-Al phase and modification of Mg₂Si phase are attributed to the formation of Al₃Sc heterogeneous nucleus,the Sc segregation and the corresponding composition supercooling in front of the growing Mg₂Si phase.The Er atoms may be adsorbed on the growth interface of eutectic Mg₂Si during the growth process of eutectic Mg₂Si,which plays a restrictive effect in the growth of eutectic Mg₂Si.The reduction and growth inhibition of eutectic（α-Al+Mg₂Si）grains are caused by the enrichment of Er in the phase growth front of eutecticα-Al and Mg₂Si phase and the corresponding component undercooling in front of the grain boundary.