Microstructural Evolution And Strengthening-toughening Mechanism Of As-cast And Deformed Mg-Sn-Si Alloys

Mg-Sn-Si is a potential and new type of heat resistant magnesium alloy system. However, the research on preparation technology, structure as well as strengthening and toughening mechanism of the system are still in the exploratory stage, therefore the application and development of this system are restricted. In this study, Mg-3~8Sn-1.5Al-1Zn-1Si alloys were prepared by using conventional solidification technology. In order to reveal solidification and phase transformation processes of the system, microstructural observation and phase diagram calculation were adapted. For microstructural observation and analyzing, optical microscope, SEM and TEM were used. For microstructural constitutes and phase transformation characteristics evaluating, XRD, EDS and DSC were utilized. For detailed evaluation of solidification of the system, Thermo-Calc phase diagram calculation was used. Based on the analyzing results of solidification, phase transformation and phase dispersion, microstructural refinement processes and mechanism are explored for the alloys with Sb modification or/and mechanical deformation. Mechanical properties of the processed alloys were tested, and strengthening and toughening mechanism were analyzed. Based on the alloy preparation, solidification and phase transformation analyzing, microstructural modification and refinement processing, strengthening and toughening mechanism study, the following results were achieved.The solidification process of Mg-5Sn-1Si alloy under equilibrium consists of Lâ†'α-Mg, L + α-Mgâ†'α-Mg + Mg2(Si,Sn) and Lâ†'(α-Mg + Mg2(Si,Sn) + Mg2(Sn,Si)). After solidification, α-Mgâ†'α-Mg + Mg2(Sn,Si) occurs from solidus to room temperature. However, under nonequilibrium condition solidification process of Mg-5Sn-1Si alloy consists of Lâ†'α-Mg, L + α-Mgâ†'α-Mg + Mg2(Si,Sn), Lâ†'α-Mg + Mg2 Si, Sn + Mg2Siâ†'Mg2(Si,Sn), Sn + Mg2(Si,Sn)â†'Mg2(Sn,Si) + Si and Si + Mg2Snâ†'Mg2(Sn,Si). After solidification, α-Mgâ†'Mg2Sn + Mg2(Si,Sn) occurs from solidus to room temperature. Mg2(Si,Sn) and Mg2(Sn,Si) have the same space structure as that of Mg2Si(or Mg2Sn), lattice constant are just in between of Mg2 Si and Mg2 Sn. The bonding energy calculating to Mg8Sn3 Si and Mg8Si3 Sn reveals Si(Sn) atoms can replace central Sn(Si) atoms on faces or corner atoms to form Mg2(Si,Sn) and Mg2(Sn,Si). The detailed replacement reactions are the following: Sn + Mg2 Si â†'Mg2(Si,Sn), Sn + Mg2(Si,Sn)â†'Mg2(Sn,Si) + Si and Si + Mg2Snâ†'Mg2(Sn,Si). The quantity of Mg2(Si,Sn) is determined by the Si content, meanwhile, the production quantity of Mg2(Sn,Si) has close relationship with the replaced Si and Sn contents. Measurement analyzing revealed that some peoperties of Mg2(Si,Sn) and Mg2(Sn,Si) phase, such as nanohardness and elastic modulus are also in between those of Mg2 Si and Mg2 Sn phases.For as-cast Mg-xSn-Si alloys, Sb modification effect on Chinese script type Mg2 Si is remarkable due to the formation of Mg2(Si,Sn) and Mg2(Sn,Si) phases which effect as a bridging role.Mechanical working can effectively refine the microstructures of the studied alloys and can also uniform the distribution of strengthening particles on matrix. For those alloys underwent forward extrusion, broken Mg2 Si particles are in a kind of flake aggregation, while Mg2 Sn phase is dispersed at the grain boundaries. Mg2(Si,Sn) and Mg2(Sn,Si) phases are refined and resulted particle size is about 3μm which was about 10μm in as-cast condition. The refinement processes are different to Mg2(Si,Sn) and Mg2(Sn,Si) phases. For Mg2(Si,Sn) particles, it was broken from as cast coarser ones. For Mg2(Sn,Si), however, although it was broken from as cast coarser ones, but the refinement process consisted of phase transformation. For those alloys underwent reciprocating extrusion(RE), matrix were refined through the continuous or discontinuous dynamic recrystallization, the mean grain size ia about 8μm which was 30μm under as-cast state. After 7 RE passes, the matrix grain was about 7.7μm which was much fine and particle distribution was much uniform.After Sb modification or/and mechanical working, strength and plasticity of the alloy can be significantly improved as a consequence of matrix refinement, particle uniform distribution and most crucially strengthened interfacial bonding between Mg2(Si,Sn) or Mg2(Sn,Si)/matrix in comparing with that of Mg2Si/matrix. For RE processed Mg-5Sn-1.5Al-1Zn-1Si alloy, the tensile strength retention rate at 200℃ is 59.7%, after Sb modification and RE processed, the strength retention rate of Mg-5Sn-1.5Al-1Zn-1Si alloy is 62.6%. The deformation under high temperature due priority to grain boundary sliding, therefore, the strengthening mechanism is due to several reasons, such as grain refinement by Sb modification and dynamic recrystallization, particle redistribution, etc. mainly related to compound strengthening at grain boundary as well as within grains.