The Effect Of Ga On Corrosion Behavior And Mechanical Properties Of Mg–Sn Alloys

As the"green engineering material of the 21st century",magnesium alloy has the characteristics of high specific strength and specific stiffness,large vibration damping capacity,small density and easy cutting,which makes it have a broad application prospect and development space in the fields of aerospace,rail transit,electronic devices and biomedical.However,there are some weaknesses in the application of magnesium alloy:poor corrosion resistance and low absolute strength.In many commonly used magnesium alloy series,Mg–Sn system alloy has obvious precipitation strengthening effect and low preparation cost,so the research on this alloy system as the matrix has attracted increasingly attention of scholars at home and abroad.The results show that Sn can not only improve the mechanical properties of magnesium alloy,but also significantly improve its corrosion resistance.In recent years,it has been found that the addition of Ga to magnesium alloy can also improve the corrosion resistance of the alloy without loss of mechanical properties.In this paper,as-cast Mg–5Sn–xGa（x=0,0.5,1,2,3 wt.%）alloy was prepared by non vacuum melting of pure metal in resistance furnace.The whole melting process was carried out in CO₂ and SF₆ shielding gas with volume ratio of 99:1.The as-cast alloy was homogenized and extruded to obtain as-extruded alloy.Then,the effects of Ga on the microstructure of Mg–5Sn alloy as cast and as extruded were studied by means of X-ray fluorescence spectrometer（XRF）,optical microscope（OM）,X-ray diffractometer（XRD）,scanning electron microscope（SEM）,electron probe microanalyzer（EPMA）and electron backscatter diffractometer（EBSD）.The corrosion and mechanical properties of the as cast and as extruded alloy system were tested by potentiodynamic polarization curve,immersion weight loss test and universal testing machine.It was found that the addition of Ga in as cast Mg–5Sn alloy did not change the original dendrite morphology,but refined the grain size of the alloy.When 0.5 wt.%Ga element is added,no new phase containing Ga element is formed in the alloy,and all Ga elements are dissolved in the matrix.With the increase of Ga content,the second phase in the alloy changed from（α-Mg+Mg₂Sn）eutectic structure to（α-Mg+Mg₅Ga₂）eutectic structure and Mg₂Sn intermetallic.At the same time,the volume fraction of Mg₂Sn phase decreased,while that of Mg₅Ga₂ phase increased.It is found that there are two ways for Ga to improve the corrosion resistance of as-cast alloy when further investigating the corrosion behavior of the alloy system:（i）a part of Ga element is dissolved into the alloy matrix,because of its high corrosion potential,it can reduce the potential difference between the second phase and the matrix,thus inhibiting the corrosion reaction;（ii）the other part of Ga element precipitates in the form of Mg₅Ga₂ eutectic phase,which has lower corrosion sensitivity than Mg₂Sn phase and can reduce the number of pitting corrosion.Verified by the experiments,the corrosion resistance of as-cast Mg–5Sn–3Ga alloy is the best.It can be seen from the micro-hardness data that the addition of Ga has little effect on the mechanical properties of as-cast Mg–5Sn–xGa alloy.The as-extruded Mg–5Sn–xGa alloy was obtained after hot extrusion deformation treatment of the as-cast alloy,and the microstructure of the as-extruded alloy was studied subsequently.The results show that incomplete dynamic recrystallization occurs in the as-extruded alloy,and the coarse dendrite morphology disappears in the as cast alloy.Instead,the microstructure of the as-extruded alloy consists of recrystallized grains which are obviously refined and elongated grains which are elongated along the extrusion direction.Two intermetallic compounds,Mg₂Sn and Mg₅Ga₂,still exist in the as-extruded alloy,which are precipitated from the matrix in the form of nano particles and distributed in a strip along the extrusion direction.After extrusion deformation,the preferred orientation of the alloy changes,and the base plane of most of the grains changes to the direction parallel to the extrusion,forming the base fiber texture,which plays a role in texture strengthening of the alloy matrix.In the as-extruded alloy,the mechanism of Ga element to improve the corrosion resistance of the alloy is similar to that of the as-cast alloy as a whole.The difference is that after extrusion,the structure of the alloy is more compact and the second phase size is nano particles.Therefore,the corrosion mode transitions from pitting corrosion to filiform corrosion,and the thickness of the corrosion layer is also significantly reduced.The corrosion rate of as-extruded Mg–5Sn–2Ga alloy is the lowest.In addition,the strength and plasticity of the as-extruded Mg–5Sn alloy can be improved by adding Ga.The strengthening mechanism mainly includes texture strengthening and precipitation strengthening.The tensile strength of as-extruded Mg–5Sn–3Ga alloy is the highest,which can reach 313 MPa;the texture deflection angle of Mg–5Sn–1Ga alloy is the largest,and the texture weakening effect is the most obvious,so the plasticity of extruded Mg–5Sn–1Ga alloy is the best,and the elongation after fracture can reach 23.4%.