| As a green engineering material in the 21st century,magnesium alloys possess superior properties such as high specific stiffness,high specific strength,recyclability,excellent electromagnetic shielding and rich resources.It has been widely used in in the field of aerospace,automotive industry,3C and national defense.However,due to its special chemical properties and physical structure,magnesium alloys have some fatal shortcomings,such as low strength,poor plasticity at room temperature and inferior high temperature mechanical properties,which further limits the large-scale application of magnesium alloys in industrial and commercial fields.Consequently,improving the mechanical properties of magnesium alloys at room and high temperature and processing ability of magnesium alloys have gradually become the goal of materials scholars.According to the research reports at home and abroad,the Mg-Sn-Zn-Al based alloy is the key material to develop low-cost and high-performance heat-resistant magnesium alloy.Therefore,in this paper,the traditional melting method was used to prepare Mg-3Sn-5Zn-xAl alloys successfully.The solidification process,phase identification,microstructure evolution and mechanical properties of Mg-3Sn-5Zn-xAl alloy were characterized by OM,SEM,XRD,EBSD,DSC,TEM and tensile test.The effects of different Zn/Al mass ratio and various heat treatment processes on the microstructure and properties of the alloy were systematically investigated.The strengthening mechanism and plastic deformation mechanism are revealed,which provides theoretical support for the design and development of low-cost and high-performance magnesium alloys.The microstructure of as-cast TZ35-xAl alloy is made up of dendrite.With the addition of Al,the dendrite spacing is obviously refined,and the secondary dendrite spacing of TZA353 alloy was reduced to 20.2μm.Second phase of as-cast alloys transformed from MgZn2/α-Mg eutectics and Mg2Sn to icosahedral(I-)phases and Mg2Sn in the wake of decreasing of Zn/Al mass ratio,and no β-Mg17Al2 was found.At the same time,the precipitation sequence of various kinds of secondary phases during solidification was also discussed in detail.After homogenization treatment,the dendrites of as-cast alloys tend to transform into equiaxed grains.I-phase and Mg2Sn exhibit excellent thermal stability during homogenization treatment.The microstructure of as-extruded TZ35-xAl alloys is composed of almost complete recrystallization grains and secondary phases which were crushed into streamlined fragments in the extrusion process.As-extruded alloys exhibited superior strength and ductility balance.Specifically,Mg-3Sn-5Zn-1Al alloy got the highest ultimate tensile strength and elongation,reaching 340 MPa and 20.1%,respectively.EBSD results showed that the average grain size of as-extruded TZ35-xAl alloys was considerably refined and the angle θ between c-axis and ED decreased with the increasing of Al content,leading to higher SF value for basal<a>slip and weakened basal texture.However,the average SF value for non-basal slip in Al-containing alloys did not change obviously,which were much higher than that for basal<a>slip,indicating non-basal slip was still the main deformation mechanism of as-extruded alloys during deformation.TEM results show that numerous fine Mg2Sn,I phase and orthorhombicφ-phase(I-approximant phase)particles are dynamically precipitated in TZA353 alloy during hot extrusion.In addition,the interface between 5-fold symmetry I-phase andα-Mg matrix maintained a certain orientation relationship,namely {1011}α//5f with a slight misorientation(～6°).This rigid interface retained after severe plastic deformation between I-phase and α-Mg matrix may play an important role in enhancing the strength and formability of the alloy.It was found that the effects of two-stage solution treatment process of 330℃/12h+400℃/3h is better than that of single-stage solution treatment process of 400℃/6h.The temperature for artificial aging process was chosen to be 180℃.The T5-treated alloys exhibited higher improvements in yield strength than that of T6-treated alloys,while the ultimate tensile strength did not change obviously and the plasticity decreased significantly.The result was chiefly ascribed to the precipitation strengthening effect after T6 treatment did not compensate the decline of mechanical properties due to excessive grain growth after solution treatment.TZA351 alloy exhibited the best comprehensive mechanical properties after T5 treatment,achieving yield and ultimate tensile strengths of 240 MPa and 323 MPa,respectively,and an elongation of 11.4%.Compared with the single-stage aging process,the double aging process significantly shortened the time to reach the peak aging hardness and showed better aging hardening response.The peak aging hardness of TZA353 alloy was increased by 3.2HV compared with single-stage aging,and reaching to 95.7HV.TEM results showed that after double aging treatment,the main strengthening phase in TZ35 alloy is the long rod-like β1’ which are uniformly distributed in the matrix,and a small amount of short plate-like β2’ were found attached to β1’,while a few coarse β-MgZn precipitated along the grain boundary,but no Mg2Sn precipitates were found.The addition of Al significantly changed the distribution and morphology of precipitates and had a certain refinement effect on the size of precipitates in alloys.The volume fraction of β1’ in TZA353 alloy was significantly reduced,and the quasi crystal I-phase with irregular morphology dispersed in the matrix.The microhardness of TZA353 is higher than that of TZ35 alloy,which may be attributed to the increase in volume fraction and refinement of precipitates.At the same time,the solution strengthening effect of Al may also improve the mechanical properties of the alloy. |
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