Refinement Of Eutectic Mg₂Si In Mg-Al-Zn-Si Alloy Prepared By Solidification And Deformation

In this paper the microstructures and tensile properties of Mg-2Al-1Zn-1Si alloy modified with or without Sr ? Sb and Sr+Sb were investigated, especially the morphologies and distribution of eutectic Mg₂Si in the alloys were analyzed. The refinement mechanisms of eutectic Mg₂Si were preliminary discussed. Based on the above study, the alloys were undergone isothermal treatment or reciprocating extrusion.The effects, therefore, of the thermal and mechanical working on microstructures and tensile properties of the alloys were investigated. Some results are concluded as following.Alloying with trace Sr element, eutectic Mg₂Si in the studied alloys can be refined due two reasons. One is that Sr is an active element which can inhibit Mg₂Si growth by undercooled the melt ahead the growing Mg₂Si. Another is that Sr can dissolve in Mg₂Si and then change the preferential growth mode of Mg₂Si. Alloying with trace Sb element,eutectic Mg₂Si in the studied alloys can also be refined due two different reasons with Sr. The adding Sb can react with Mg and Sr respectively and generate Mg3Sb2 and SbSr2 phases. The new generated phases are good substrates for the growth of Mg₂Si.Alloying Mg-2Al-1Zn-1Si alloy with 0.7%Sr, the eutectic Mg₂Si can be refined to about 2 ?m from 100 ?m. Moreover, the distribution of Mg₂Si particles on matrix is more uniform in compare with those without alloying. As a result, the tensile strength and elongation of the alloy increase to 143 MPa and 5.5%, respectively. Alloying Mg-2Al-1Zn-1Si alloy with 0.75%Sb, the eutectic Mg₂Si can be refined to 32 ?m from100 ?m, and the tensile strength and elongation at this point increase to 135 MPa and4.5%, respectively. Alloying Mg-2Al-1Zn-1Si alloy with 0.7%Sr and 0.5%Sb, the effect of refinement of Mg₂Si is the best, the morphology transforms from coarse Chinese script into fine particle and the distribution is dispersed. As a result, the tensile strength and elongation of the alloy soar to 162 MPa and 5.5%, respectively.It is interesting that isothermal treatment along is a very effective mean to refine eutectic Mg₂Si. With increasing isothermal treatment time, coarse Chinese script Mg₂Si can gradually degrade into spherical particles and redistribute homogeneously on matrix.As fine as 2.4 ?m of Mg₂Si particles can be obtained, however, which were about 100?m before thermal treatment. It is relatively disappointment that the refinement effect on eutectic Mg₂Si is almost rare of the alloys modified with Sr, Sb or Sr+Sb. This is perhaps partly due to the difficulties of Si in the alloy caused by those modified elements which changed the growth mode of Mg₂Si. The real reason need to be study further.The fragmentation effect of reciprocating extrusion to eutectic Mg₂Si modified with Sr?Sb or Sr+Sb is very obvious. Mg₂Si can break into granules by coefficient of modification and reciprocating extrusion. The degree of diffuse distribution is enhancement, but the uniform of the particle decreases. On the basic of this, the tensile strength of alloying Mg-2Al-1Zn-1Si alloy with 0.7%Sr increases to 234 MPa; the tensile strength of alloying Mg-2Al-1Zn-1Si alloy with 0.75%Sb increases to 223 MPa;when alloying Mg-2Al-1Zn-1Si alloy with 0.7%Sr and 0.5%Sb, the tensile strength increases to 251 MPa. The refinement mechanism of reciprocating extrusion is that eutectic Mg₂Si broken into tiny particles via the coactive of normal stress and shear stress, and particles diffuse enhanced through double flow of horizontal and vertical.The inhomogeneous size of Mg₂Si particles is concerned with Sr or Sb dissolved into Mg₂Si phase.