Microstructures, Tensile Properties And Deformation Mechanisms Of Mg-3Al-3Sn Magnesium Alloy

Magnesium alloys are promising candidates for light weight structural applicationsmainly because of their low density, high specific strength and recyclability. However,wrought magnesium alloys present poor formability at ambient temperature due to theirstrong basal texture. Note that a strong strain hardening can be obtained with weakeninginitial texture, and then improves the plastic formability. So texture modifying attracts lots ofattentions and more works are required to understand the deformation behaviors in thoseweak textured magnesium alloys. Moreover, weak texture can be easily achieved in castmagnesium alloys; however, too little attentions have being paid to their deformationbehaviors due to the limited ductility, and more research is also needed.In this thesis, the deformation mechanism was revealed in a novel as-cast Mg–3Al–3Sn(AT33) alloy and influences of alloying element and texture evolution on deformationbehaviors were also analyzed. Simuataneously, the influence of pre-deformation on theprecipitation behaviors are also investigated, expecting to show some guidance in improvingthe strength of magnesium alloy sheets.The main results are as follows:1) It is revealed that as-cast AT33alloy exhibits strong strain hardening ability and lowstrain rate dependence in tensile properties, i.e., tensile strength andelongation-to-failure higher than220MPa and30%are observed at strain rate range of1.0×10–1–1.0×10–4s–1; the yield strength shows strong temperature dependence anddecreases from81MPa at room temperature to73MPa and52MPa at100oC and200oC, respectively.2) It is found that a new texture was evolved in as-cast AT33alloy during tensile andcompressive deformation at room temperature. Based on the calculation of Schmidfactors and dislocation densities, the anisotropy in strain hardening behaviors has beenattributed to the differences in deformation behaviors following the new textureformation. It is revealed that the high ductility of as-cast AT33alloy is related to theeasily activated non-basal a and c+aslip systems,{1011}contraction twinsand the occurrence of dynamic recrystallization (DRX);3) The rolling processing has been optimized in AT33alloys, and therefore, influences of intial grain size and speed ratio on microstructure and mechanical properties have beenexplored, which was found that the sample with a large initial grain size for certainduration shows a much finer microstructure and higher strength after five rollingprocessing; contrary to conventional rolling, the sample rolled by differential speedrolling shows finer grain size, stronger basal texture and more precipitates in annealingmicrostructure, which lead to a increase in yield strength from173MPa to219MPa;4) Influence of pre-deformation on the precipitate behaviors is investigated in extrudedAT33alloy. It is found that the aboundant twinning and dislocation defects activatedduring pre-compressive deformation along extrusion direction and normal direction inextruded AT33alloy promotes the precipitate nucleation of-Mg2Sn phase on acrystallographic orientation relationship (OR) of(220)β∥(0001) αand[001]β∥[1210]αduring following aging treatment, which increases the precipitate density andaccordingly improves the micro-hardness. Twin boundaries are especially suitable forthe precipitate nucleation, where the-Mg2Sn particle exhibits OR with-Mg on thehalf part located within twin area.5) It is revealed that the dispersed nano-sized-Mg2Sn particles hinders the dynamicrecovery but increase the dislocation storage, which contribute to a promising tensilestrength during tensile deformation at elevated temperature in fine grained, rolled AT33alloy; simultaneously, the high dislocation content provides a high driving force for theoccurrence of DRX and lead to a large plastic flow, which contributes a significantimprovement of ductility.6) Based on the precipitation regulation in extruded AT33alloy, the heat treatments forAZ61sheets have been optimized, showings some assists for improving the strength ofmagnesium sheets. Compared with the annealed AZ61sheet (300oC/0.5h), the twoaging treatments, e.g., direat aging (200oC/12h) and solid solution (300oC/0.5h)+aging (180oC/48h), show prominent increase in yield strength of about50MPa andachieve fine comprehensive mechanical properties.In summary, the investigation for the deformation mechanisms in high-ductile as-castAT33alloy enrichs the researches of deformation behaviors and shows some guidance forunderstanding the origins for the high ductility in weak textured magnesium alloys; themodifying of the precipitate behaviors by the twins and dislocations generated duringpre-deformation, also providing a valuable insight for designing high strength magnesiumalloys for industrial applications.