The Semi-solid Microstructure And Properties Of Mg-6Zn-xCu Magnesium Alloys

As a new type of heat-resistant Mg-Zn alloys, the researches about Mg-Zn-Cualloys which have become commercialized successfully are still in the primary stage.Considerable attentions were concentrated lately on the study of the mechanicalproperties of the sand casting alloys and the wrought alloys, including thedeformation microstructure of the alloy, the alloy phase, and the creep properties andcorrosion resistance properties, while there have been few relevant reports aboutsemi-solid Mg-Zn-Cu alloys. Mg-6Zn-xCu (1,3and5wt.%) alloys were preparedby metal mould casting method. The microstructures of the cast and semi-solidalloys and the effect of Cu content on microstructure and mechanical properties ofalloys were investigated by OM, SEM, XRD and mechanical apparatus, in order toprovide theoretical and practical basis for the further development and research ofthe Mg-Zn-Cu alloys.The results show that the CuMgZn phases were formed when added Cu elementsin alloys, and the amount of CuMgZn compounds increased with increasing of Cucontent. Due to the CuMgZn compounds enriched on the surface of α-Mg duringsolidification, the growth of α-Mg was difficultly and then grains were refinedeffectively. But an excess of CuMgZn compounds precipitates at grain boundariesresulted in local stress concentration, thus it was detrimental to the mechanicalproperties of alloys. Consequently, with addition of1.0%Cu, the values of ultimatetensile strength and elongation are208MPa and13.5%, respectively. The tensilefracture surfaces changed from quasi-cleavage to cleavage and intergranular fracturewith increase of Cu content.And then, the alloys were carried through heat treatments and the results showthat, for Mg-6Zn-1Cu magnesium alloys, the best scheme to gain plastic toughness issolution treated at a temperature of440℃for16h, and the maximum value isδ=15.75%. Then after ageing treated at a temperature of160℃for20h, comprehen-sive mechanical properties of the alloys can be gained due to the significant enhanc-ement effect of the dispersed precipitated phase on alloy matrix. The maximumvalues of the tensile strength, elongation rate and Vickers hardness were all up totheir maximum values which are σ_b=281.67MPa、δ=15.23%、78.46HV. Before andafter solution treatment, the alloy fracture mode correspondingly changes through quasi-cleavage fracture�'quasi-cleavage accompanied by a large number of dimpl-es mixed fracture�'cleavage fracture. After solution treated at a temperature of450℃for8h firstly and then ageing treated at a temperature of160℃for20h, thetensile strength, elongation rate and Vickers hardness of Mg-6Zn-5Cu alloys were allup to their maximum values which are σ_b=246.67MPa、δ=18.75%、62.40HV30, whilethe cast alloys only attain σ_b=198.00Mpa，δ=8.31%，52.83HV.A further study found that the alloys have changed into spherical particles formdendritic structures when Mg-6Zn-xCu alloys were carried through semi-solidisothermal heat treatment. Its evolution as follows: the secondary dendrite arm offand disappeared�'grain growth�'separated refinement and spheroidization�'grain coarsening. After the heat treatment under the relative optimum holdingtemperatures and times were chosen：620℃and15min, fine and globular orspherical grains of Mg-6Zn-1Cu alloys were attained，as well as liquid fraction in thesemi-solid microstructure is about60%．And the Mg-6Zn-5Cu alloys can get smallersemi-solid spherical structure, with an average diameter is only about35μm treatedat a temperature of595℃for15min.