Effect Of Y And B On Microstructure And Mechanical Properties Of Al-10Mg₂Si Alloy

Al-Mg₂Si alloy,as a new lightweight material in aerospace,automotive and other industries,has attracted extensive attention due to its excellent properties such as specific strength,wear resistance and electrical conductivity.The key to prepare the Al-Mg₂Si alloy with high performance in situ authigenic Mg₂Si reinforced phase is how to control the size of?-Al and the shape,size and distribution of primary and eutectic Mg₂Si phases.For the hypoeutectic Al-Mg₂Si alloy,although the existing eutectic Mg₂Si phase and primary Mg₂Si phase are the same Mg₂Si phase,the nucleation and growth conditions during crystallization is different due to the different crystallization conditions and crystallization sequence in the solidification process.Moever,the modification and refining mechanism of the modifier are inconsistent.Therefore,the effect of element B,rare earth element Y and their composite on the microstructure and mechanical properties of hypoeutectic Al-10Mg₂Si alloy is mainly investigated by metallography,SEM,XRD and mechanical property tests.The research results show that:The coexistenc phenomenon of primary?-Al refinement and eutectic Mg₂Si metamorphism occurs in the Al-10Mg₂Si with the addition of Y,and the mechanical properties of Al-10Mg₂Si increase firstly and then decrease.The fracture morphology of the modified alloy also changes from a large area full cleavage plane and tearing edge brittle fracture to a ductile fracture full of dimples.Y and Al will undergo eutectic reaction to form?-Al and Al₃Y before the liquidus.The?-Al acts as the nucleation center of primary?-Al to promote nucleation and thus refines primary?-Al grains.Y has a low solubility in the melt and is discharged in the crystallization process,and then enriches in the front end of the growing eutectic Mg₂Si phase,leading to an increase in the degree of undercooling,which makes the eutectic Mg₂Si deteriorate and change its morphology into dot or fibrous shape.This structure of eutectic Mg₂Si enhances its mechanical properties by impeding dislocation movement.The obvious refinement of primary?-Al and eutectic Mg₂Si in the subeutectic Al-10Mg₂Si alloy is found after the addition of B element.When amount of B is 0.05wt.%,the primary?-Al changes from coarse dendritic to small spherical or short rod,and the aspect ratio decreases by 56.04%.When the 0.03wt.%B is added,the eutectic Mg₂Si show fine fibrous shape.The Al₂B formed by B and Al can act as the heterogenic nucleation center of primary?-Al.Since the atomic radius of B is smaller than that of Al,the lattice constant of primary?-Al can be reduced accordingly,so that the primary?-Al can be refined.When the content of B is 0.03wt.%,the enhancement of mechanical properties is obvious,and the tensile strength and elongation increase from 220MPa and 3.5%to 280MPa and 5.2%.The hardness is also enhanced from 122HV of original state to 187HV.When Y and B are added to Al-10Mg₂Si alloy simultaneously,a large number of primary?-Al present dendritic distribution,but the size regular change of the eutectic Mg₂Si does not appear.Compared with the original state alloy,the tensile strength of the alloy with the addition of 0.5Y+0.07B reaches 237MPa,which is 9.7%enhancement.The hardness of alloy is 141HV,increasing by 27%.There was no a slight decrease in elongation.The decrease of elongation is due to the interaction between element B and element Y,and it is difficult for Y to adsorb alone on the front edge of the solid liquid interface of eutectic Mg₂Si and form compounds.When the primary?-Al is stressed and deformed,the sharp,hard and brittle intermetallic compounds is found at the grain boundary fracture,tearing the Al matrix at the sharp corners,generating the crack source,and the crack diffuses from the thick lamellar boundary of eutectic Mg₂Si,finally leading to the fracture of the material.