Microstructure And Mechanical Property Of Al–20Mg₂Si Alloys Modified By Y–Sb

Aluminum alloys enhanced by Mg₂ Si particles have been expected as the candidate material for steel and cast iron in automobile industry due to the advantages such as low density, wear resistance and easy casting, The intermetallic compound Mg₂ Si exhibits a high melting temperature, low coefficient of thermal expansion, a reasonably high elastic modulus and low density, which significantly contributes to the improved mechanical properties of Al–Mg₂Si alloys at both room and high temperatures. However, unexpected coarse dendritic primary Mg₂ Si phase and brittle Chinese script eutectic structures formed in Al–Mg₂Si series alloy under common gravity casting seriously damaged its mechanical properties. Therefore, Mg₂ Si particles must be modified to insure adequate mechanical strength and ductility of the alloys.Recently, substantial investigations have been carried out to modify the morphology and refine the size of Mg₂ Si phases. It is generally accepted that high content of Rare Earth（RE） such as La, Ce, Nd and mischmetal can modify primary or eutectic Mg₂ Si, and the modification mechanisms are elucidated as follows: the enrichment of RE elements in the solid–liquid interfacial front change the surface energy of Mg₂ Si or lead to the constitutional undercooling, thus change the morphology of primary Mg₂ Si crystal. In addition, some other chemical elements such as Sb, Sr, P, Li were reported to be effective modification elements.However, limited investigations have been carried out to study the effect of combined addition of modification elements on the morphology transformation of primary Mg₂ Si crystals. Therefore, it is of great importance to further investigate the effect of co-modification on primary Mg₂ Si crystals and reveal the modification mechanisms.This thesis mainly investigated the effect the combined addition of Y and Sb on the primary Mg₂ Si in Al–20Mg₂Si alloy, the modification mechanisms were discussed detailedly,what is more, both room and high temperature tensile properties of the alloys were tested.The conclusions are as follows:1) Y and Sb were used to modify the primary Mg₂ Si particles in Al–20Mg₂Si alloy, it was found that the combined addition of Y and Sb can obtain better modification results than the simply addition of Y or Sb at the content of 0.5 wt.%. The morphology of primary Mg₂ Si transformed from coarse dendrites into polyhedral shapes, with average size reduced from about 200μm to 20μm.2) The co–modification mechanisms of primary Mg₂ Si in Al–20Mg₂Si alloy can be elucidated as two aspects. On the one hand, Mg3Sb2 compounds act as the heterogeneous nuclei of primary Mg₂ Si and result in the refinement of particle size; on the other hand,some Sb and Y atoms adsorb on the surfaces of primary Mg₂ Si crystal,thus changing the velocity speeds of different crystal faces and the surface energy, which finally leads to the morphology transformation of primary Mg₂ Si crystals.3) The modification effects of Sb and Y–Sb on primary Mg₂ Si in Al–20Mg₂Si–4Cu alloy were investigated. The results show that average size of primary Mg₂ Si was refined to20μm and 50μm by Sb and Y–Sb, respectively. Numerous formations of intermetallic compounds containing Y and Cu weaken the co-modification effect of Y–Sb.4) UTS and elongation–to–failure of the Sb-modified extruded Al–20Mg₂Si–4Cu are superior than the unmodified one at both room and high temperatures. UTS of the modified alloy increased from 220 MPa to 283 MPa, 185 MPa to 213 MPa, with an elongation–to–failure increased from 2.6% to 4.1%, 2.6% to 3.9% at room temperature and 150℃, respectively. However, tensile property of extruded Al–20Mg₂Si–4Cu alloy modified by 0.5 wt.%（Y–Sb） did not improved compared with the unmodified one.