Microstructure Control And Mechanical Properties Of Mg-Sn-Zn Magnesium Alloys

The new type Mg–Sn–Zn alloys have been given attention recently. Mg2Sn in the alloys(FCC structure, point group m3m, a=0.676nm) has high hardness and high meltingtemperature (770.5oC), which has great potential as room and elevated temperature structuralmaterials for industrial application. Generalized stacking fault (GSF) energy of Mg isreduced owing to dopings of Sn and Zn atoms, which is beneficial to improving activity ofbasal slip, activation of nonbasal slips and a number of deformation twins, and thusimproving the ductility. Moreover, Sn and Zn are necessary elements in the human body, andtherefore, the Mg–Sn–Zn system has a great potential for applications serving as biomaterial.However, microstructure of Mg–Sn–Zn alloys is much coarse under normal gravity casting,leading to inferior mechanical properties. Therefore, it is essential to modify themicrostructure of the Mg–Sn–Zn alloys. Addition of element alloying is one of the effectivemethods to modify the coarse microstructure by the mechanisms of adsorption-poisoningand heterogeneous nucleation. The change of solidification conditions (such as centrifugalcasting) can also refine the microstructure. In addition, rolling and extruding methods caneffectively refine coarse grains and second phases further, which can significantly impovemechanical properties. However, much few studies have been carried out on the rolling,extruding and annealing of Mg–Sn–Zn alloys.There, in this thesis, effects of minor elements (Mn, Y, Sb, Zr) on microstructure andmechanical properties of casting Mg–3Sn–1Zn and Mg–5Sn–1Zn alloys are investigated.Mg–5Sn alloy is fabricated by a centrifugal casting method. Moreover, rolling and extrudingwith anneal methods are also used to refine microstructure and improve mechanicalproperties further.The main results are as follows:1) The as-cast Mg–3Sn–1Zn (TZ31) alloy exhibits strong strain hardening ability(hardening capacity Hc=3.62, strain hardening exponent n=0.55) and high elongation(elongation εp=19.1%, tensile strength σb=168MPa) during tensile deformation at room temperature; The generalized stacking fault (GSF) of Mg is reduced owing to dopings ofSn and Zn atoms, which is beneficial to improving activity of basal slip, activation ofnonbasal slips and formation of a number of deformation twins, leading to the highelongation; High density of twin boundaries restricts dislocation motion;Therefore,multiplication and accumulation of dislocations at the twin boundaries lead to the strongstrain hardening ability of the TZ31alloy; Results obtained here can be used as areference to understand deformation mechanism of Mg alloys with Sn at roomtemperature.2) Better content of Zr addition is0.05wt.%.0.05wt.%Zr significantly refinesmicrostructure of the as-cast TZ31alloy; At room temeprature, the compressive strength(σb) and elongation-to-failure (εf) increase from330MPa and31.9%to379MPa and39.0%, respectively; At150oC, σband εfincrease from209MPa and35.7%to224MPaand42.5%, respectively; Y and Sb can also refine the microstructure and improvemechanical properties of the as-cast TZ31alloy. However, Mn has no obvious effect onthe microstructure and mechanical properties of the as-cast TZ31alloy; Undercentrifugal casting, with the increase of centrifugal radius, the grain size decreases andsegregation of Sn intensifies.3) Better rolling and annealing methods were obtained: Fine recrystallisationmicrostructure (~4μm) can be obtained by rolling at350oC and280oC with15passes(every reduction ratio is less than15%), the annealing temperature and time is275oCand1hour, respectively. The yield strength, tensile strength and elongation-to-failure are131MPa,260MPa and19.3%, respevtively. Better rolling method of extruded TZ31alloy was obtained: Rolling was carried out at350oC with6passes (every reductionratio is no less than20%). With the increase of temperature or time, recrystallisationgrains size increases from3μm (250oC/1h) to9μm(300oC/1.5h). Results obtained herecan be used as a reference to optimize rolling and annealing methods of Mg alloy withSn.4) The Mg2Sn shows thermal stability during the tensile deformation at200oC. Themicrostructure evolution of the rolling TZ31alloy (~11μm) was investigated during the elevated temperatures. At150oC, dynamic recovery occurs. At200oC, dynamicrecrystallization appears. With the increase of the strain, dynamic recrystallizationmicrostructure increases. The mechanical properties of the rolled TZ31alloy can obtainbenefit from the thermally stable Mg2Sn, dynamic recovery and recrystallization atelevated temperatures.5) Effects of combined additions of Sr and Sb on microstructure and mechanical propertiesextruded TZ51alloy were investigated.0.1wt.%Sr–Sb has no obvious effect on the grainsize. When the contents of Sr–Sb increase to0.3wt.%and0.5wt.%, the grain sizeincreases. At room temperature, the yield strength, tensile strength andelongation-to-failure increase from129MPa,251MPa and20.6%to152MPa,273MPaand23.2%by the addition of0.1wt.%Sr–Sb, respevtively. However, the additions ofSr–Sb have no obvious effect on the mechanical properties. The extruded TZ51alloyswith and without Sr–Sb show quasi cleavage with dimple fracture at room temperature.The fracture is primarily dimple fracture at elevated temperature.In summary, in this thesis, effects of minor elements (Mn, Y, Sb, Zr) on microstructureand mechanical properties of casting Mg–Sn–Zn alloys are investigated. Types and contentsof elements addition are optimized. The evolution of static and dynamic recrystallizationmicrostructure was observed, which can be beneficial to the control of grain size afterdeformation. Results obtained here can be used as a reference to design and manufacture Mgalloys with high mechanical propeties.