The Research On Microstructure, Properties And Creep Behaviors Of Mg-Zn-Al-X Alloys

Developing magnesium alloys with low cost, the excellent combination of mechanical properties and good corrosion resistance on the Mg-Zn system for the creep-resistant structural components (served at not higher than 200℃) in the fields of automobile and airspace industries can overcome the limitation of high cost associated with the alloys containing RE and is of great significance to expand the applications of magnesium alloys. Effects of Sn and the combined Sn and Ca addition on microstructural stability, mechanical properties, creep resistance, corrosion resistance and etc. of the as-cast Mg-6Zn-2Al alloy are systematically investigated on the basis of the optimization of the Al content in the Mg-6Zn system. The solutionizing and aging characteristics of the Mg-6Zn-2Al based alloys as well as mechanical properties of the as-aged alloys are examined. The hot-working aibility, the distribution of precipitates and mechanical properties of the alloy containing Sn and Ca are explored. In addition, the creep behaviors of the as-cast Mg-6Zn-2Al based alloys and the applying mechanism of Sn are also studied. The aim is to explore a kind of novel creep-resistant magnesium alloy containing Sn and Ca and thus to offer the detailed experimental data and theoretical guidance for the development of low-cost high-performance magnesium alloys. The research results are given below:(1)The Mg-6Zn-2Al alloy exhibits good combination of mechanical properties, better corrosion resistance than AZ91 and ZA85 and good microstructural stability during long-term thermal exposure to 150℃and 200℃among the as-cast Mg-6Zn-xAl(x=2,4,6) alloys, which can be selected for further alloying.(2)The as-cast Mg-6Zn-2Al-ySn(y=0.5,1,2,3) alloys are mainly composed of three phases i.e.α-Mg, MgZn and Mg2Sn. Coarse Mg2Sn particles from the solidification of the Sn-rich liquid and short-rod like precipitates from solid decompostion are observed. The Sn addition plays a role in refining the MgZn divorced eutectics and suppressing its formation. These alloys exhibit excellent microstructural stability during the long-term thermal exposure to 150℃, while they show a decrease in the overall hardness with a prolonged time during thermal exposure to 200℃, resulting from the aggregation and coarsening of Mg2Sn.The Sn addition reduces the corrosion potential, decreases the cathodic corrosion process and enhances the anodic process, but has no obvious influence on the corrosion resistance.The Sn addition contributes to an obvious improvement on the ambient and elevated-temperatures strength and has a beneficial influence on the compressive creep resistance. The yield strength of these alloys increases with the Sn content at ambient temperature and the alloy with 0.5mass%Sn has the highest tensile strength and elongation, while the alloy with 0.5mass%Sn has the highest tensile strength and yield strength at 150℃. Quasi-cleavage fracture is the dominant fracture mode of these alloys tested at ambient temperature and 150℃. Cleavage and quasi-cleavage fracture are preferential at ambient temperature and 150℃respectively with the increment of Sn. The steady compressive creep rates (175℃/50MPa) of the alloys containing Sn are lower than that of the base alloy from 10% to 30%, while that (190℃/50MPa) of the alloy with 3mass%Sn is 56% lower.(3)Trace Ca addition (0.2mass%Ca) can enhance microstructual stability of the Mg-6Zn-2Al-3Sn alloy during the long-term thermal exposure to 200℃. It also results in a great improvement on corrosion resistance, the ambient and elevated-temperature strength and compressive creep resistance. The alloy with Ca has the steady compressive creep rates (160-190℃/50-70MPa) lower than that free of Ca from 7% to 18% and exhibits better corrosion resistance than AZ91, which can be selected as the composition design to prepare a kind of low-cost creep-resistant magnesium alloy.(4)The Sn and the Sn/Ca addition lead to the incomplete dissolution of the MgZn eutectics in the Mg-6Zn-2Al alloy even after holding at 345℃for 28h. The formation and the dissolution of GP zones are surpressed by the Sn addition, while they are enhanced by trace Ca. The formation ofβ1' is also enhanced by the latter. Aging contributes to higher yield strength, but leads to higher creep stress sensitivity. In addition, it has no similar influence on creep resistance. The alloy with a higher Sn content exhibits higher ambient and elevated-temperature yield strength and creep resistance.The long-rod likeβ1' and the short-rod like Mg2Sn precipitates are dominant in the as-aged Mg-6Zn-2Al-3Sn alloy, while the additional blockyβ1' precipitates are involved in the alloy with trace Ca. Trace Ca plays the role in modifying the morphology ofβ1', reducing the aspect ratio of the long-rod likeβ' and surpressing Mg2Sn aggregation, contributing to the improvement in the ambient and elevated-temperature yield strength and compressive creep resistance of the as-aged Mg-6Zn-2Al-3Sn alloy and has no obvious effect on the plasticity. (6)The as-extruded Mg-6Zn-2Al-3Sn-0.2Ca alloy (extruded at 330℃and with the extrusion ratio of 25) has fine equiaxed grains, lower anisotropy and good combination of mechanical properties, with the ultimate strength and the elongation of 387MPa and 9% respectively. Mg2Sn particles act as the nucleation sites of dynamic recrystallization during hot extrusion. A massive nano-scale precipitates (β1' and Mg2Sn) are formed in the T5 state, while a massive nano-scale precipitates are also observed in the T6 state. The alloy in the T5 state has higher strength and elogation than the T6 state, ascrbie to its finerβ1' precipitates with higher density.The wrought alloys exhibit inferior creep resistance to the as-cast. However, the as-extruded alloy has comparative ambient tensile properties to commercial wrought magnesium alloys and higher creep resistance, whose steady creep rates (175℃/50-70MPa) are at the range of 10-7s-1 and lower than those of the as-extruded Mg-2at.%Sn-1at.%Zn-0.1at.%Mn and Mg-4Al-2Sr-0.3Mn creep-resistant alloys reported in the literatures, showing the possibility for the development of creep-resistant wrought magnesium alloys without RE.(7)The addition of 3mass%Sn has an obvious improvement on the tensile creep resistance of the as-cast Mg-6Zn-2Al alloy and this effect is more evident at higher temperatures and under bigger stresses, resulting from the lowered diffusion coefficient of magnesium lattice, the refinement of the MgZn eutectics and the formation of Mg2Sn particles. The Sn addition contributes to the higher creep activation energy and the lower creep stress exponent. Dislocation climb is dominant in the creep processes of both alloys, while cracking of the MgZn eutectics is the main cause for the reduced creep resistance at higher temperatures and under bigger stresses.