Microstructure Evolution And Strengthening Study Of Mg-Sn Based Magnesium Alloys

As the lightest structure materials, magnesium alloys have attracted great attention with regard to their application in field such as automotive, aerospace, electronic communications and defense, named as the 21 st century green engineering materials.Mg-Sn based alloys have shown great potential in developing Mg products with high strength and high ductility for their good creep resistance and precipitation hardening.A study regarding the effect of Ba, Li, Al, Zn and Gd addition respectively and pre-deformation on the microstructure evolution and mechanical properties of Mg-5Sn base alloys is performed, with the aid of X-ray fluorescence(XRF), scanning electron microscopy(SEM), transmission electron microscopy(TEM), X-ray diffractometer(XRD), X-ray energy dispersive spectroscopy(EDS) and mechanical testing machine.The main results are summarized as follows:When elements such as Ba and Li are added into the Mg-5Sn alloys which comprise ?-Mg phase and Mg2 Sn phase, it is found from the microstructure that new phases such as Ba Mg2Sn2 or Li2 Mg Sn respectively are formed. Ba Mg2Sn2 phase in the form of feather shape mainly distribute along the grain boundaries, and it is found under the TEM observation that Ba Mg2Sn2 phase is composed of numerous nanocrystals with a radius less than 50 nm rather than a monocrystal, in contrast to a large extent with Mg2 Sn phase, a phase to be only a monocrystal. This could be explained via the oriented attachment mediated growth mechanism of Ba Mg2Sn2 phase.With regard to the Li2 Mg Sn phase, it possesses the similar crystal structure and lattice parameter with Mg2 Sn phase, and exists in the form of lamellar structure in the cast condition. With appropriate trace addition of Ba and Li, they could greatly refine the grain structure, which is tied to the strong barriers to grain growth exerted by the solute segregation and intermetallic compound. Care should be taken that the influence of the Li addition on is attributed to the solute segregation, while Ba addition intermetallic compound.With the addition of Ba or Li, it is found that Ba addition is detrimental to the mechanical properties, while Li addition benefits. With the increment in the amount of Ba, the strength and ductility are reduced during both tension tests and compression tests. On the one hand, this could be explained by the fact that with the addition of Ba,a large number of intermetallic compound(Ba Mg2Sn2 phase) that can not be resolved into the ?-Mg phase are formed. This type of phase is larger than Mg2 Sn phase in size,broadening the effective spacing between second phases, and active as weaker barriers for prismatic slip and {10-1 2} twinning, thus lower tension yield strength and compression yield strength is the result. On the other hand, Ba Mg2Sn2 phase is formed at the sacrifice of Sn, and this would reduce the amount of Mg2 Sn phase. However,trace addition of Li benefits to overall mechanical properties of the Mg-Sn alloys, an effect resulting from the refining the grain size and the second phases serving as strong barriers for slip and twinning. In addition, trace addition of Li could retard the grain growth during dynamic recrystallization. This could refine the grain size, greatly strengthening alloys and reduce the tension-compression yield asymmetry.The influence of 1 wt. % Al,Zn and Gd addition on Mg-5Sn-0.3Li alloy that consists of mainly ?-Mg phase, Mg2 Sn phase and Li2 Mg Sn phase is investigated. Our results show that with trace addition of 1 wt.% Al or Zn, no new second phase is formed, whilst 1 wt. % Gd addition results in the formation of granular Gd Mg Sn phase distributed mainly in the interior of grains. Addition of Al and Zn greatly refine the microstructure of Mg-Sn-Li alloy due to the strong barriers for grain growth by solute segregation and intermetallic compound and the solute segregation taking a major part.On the contrary, Gd addition exerts little effect on the grain size, attributing to the consuming of Sn via the formation of Gd Mg Sn phase. Since Sn possess a higher growth restrict factor than Gd, thus reducing the value of growth restrict factor.With regard to the tension-compression yield asymmetry(CYS/TYS) of Mg-5Sn-0.3Li alloy, the Mg-5Sn-0.3Li alloy possesses a low CYS/TYS with a value of 1. Addition of Al or Zn increases the value of CYS/TYS whilst Gd addition decreases. This is because the solute strenghthening effect resulting from Al or Zn addation, which highly enhances the tensile yield strength and ultimate strength. As for Gd addition, a large number of Gd Mg Sn phase at the expense of Sn containing intermetallic compounds. Since Gd Mg Sn phase mainly distributes in the interior of grains and the Gd Mg Sn phase enhance twinning to a larger extent than slip. This renders the compressive yield stress higher than tensile one.A study about the role of pre-strain on the microstructure and mechanical properties of random textured Mg-5Sn-0.3Li alloy is performed. Our results show that{10-1 1} twin and {10-1 2} twin are formed during pre-strain of random textured Mg-5Sn-0.3Li alloy, which serves as effective nucleation sites for grains during dynamic recrystallization, thus refining the grain size. This greatly enhances the yield stress and ductility during tension and compression when compared with the unpre-strained Mg-5Sn-0.3Li alloy.