Microstructure And Mechanical Properties Of Indirectly-extruded Mg-Sn Binary Alloys

Mg-Sn based magnesium alloys have been paid increasing attention because the solubility of Sn in the α-Mg matrix is large and decreases rapidly with the temperature decreasing, and the precipitated Mg2Sn intermetallic compound has a high thermal stability. Therefore, the aging-hardness and creep resistance of Mg-Sn binary alloys and the effects of other alloying elements such as Zn, RE, Na, etc, have been increasingly investigated up to date, together with their forming processes. Fine grains of α-Mg matrix and high compressive/tensile yield point asymmetry ratio (R) can be obtained in these deformed Mg-Sn based alloys. However, there are few reports about the essence of high R value and deformation twinning in Mg-Sn based alloy. More interestingly, when the grain size refines to less than10μm, a very large elongation should be obtained at certain temperature and strain rate according to superplasticity theory, but no reports have been addressed. In this work, therefore, the microstructures and mechanical properties of the indirectly-extruded Mg-xSn(x=3,7,14wt.%) alloys have been investigated in detail by means of optical microscopy, scanning electron microscopy, X-ray diffraction and tensile and compressive tests at room and high temperatures in order to establish the relationship between the microstructure and mechanical properties of the deformed Mg-Sn alloys and thus to provide the basic information for the design of new kind of deformed magnesium alloys.The grain size of the α-Mg matrix is refined from220,160and93μm to28,3and16μm in the Mg-xSn(x=3,7,14wt.%) alloys extruded indirectly at300℃, respectively. It is attributed to complete recrystallization during heat extrusion. In particular, the precipitation of a large number of small Mg2Sn particles is homogeneous and dispersive in the α-Mg matrix of the as-extruded Mg-7Sn alloys, which leads to the most remarkable refining because these fine second phase provides heterogeneous nucleus for a-Mg grains and inhibite the growth of the recrystallization grains. Finally, the maximum mechamical properties are obtained in the as-extruded Mg-7Sn alloys, i.e., tensile yield strength, ultimate tensile strength and R value is176MPa,255MPa and0.97, respectively. Moreover, the theoretical calculation shows that the grain refining strengthening is the main mechanism in as-extrued Mg-7Sn alloy contributed to the improved strength.The grain size of the α-Mg matrix is more remarkably refined, i.e,～2μm, when the Mg-7Sn alloy is held at250℃for0.5h and then extruded, because finer particles of the Mg2Sn phase precipitate and uniformly distribute in the a-Mg matrix. When the holding time increases to2h, the a-Mg matrix grain size grows up to about5.0μm because Mg2Sn phase grows larger and reduces the role of the heterogeneous nuleis and hindering the growth of recrystallization grains. So, tensile yield strength, ultimate tensile strength and the value of R are197and160,259and239and0.96and0.88in two kinds of processed Mg-7Sn alloys above, respectively. Microstructures are also investigated in the Mg-7Sn alloy with an a-Mg matrix grain size of2.0μm after deformed with different compressive strains, and the results show that that deforming twinning happened in the α-Mg matrix grains with larger size. It means that basal plain sliding and non-basal plain sliding are the dominant deformation mechanisms. At the same time, the growth of twins can be inhibited due to the existence of fine grains, which makes the drop of compressive yield strength, and thus leads to the high R value of the as-extruded Mg-7Sn alloy.The results of tensile tests at100to350℃for the as-extruded Mg-7Sn alloy with a grain size of2.0μm show that the deformed a-Mg grain extended along the tensile direction results in a low strain rate sensitive ratio m. Therefore, tensile elongation is not high at elevated temperatures and then the surperplasticity is not achieved in the grain-refined alloy.