【作者】 陈吉华；

【导师】 陈振华；

【作者基本信息】 湖南大学 ， 材料加工工程， 2010， 博士

【摘要】 基于Mg-Zn系开发低成本、综合性能优良的镁合金材料应用于汽车甚至航空航天用耐热结构件中(使用温度＜200℃),可以克服稀土镁合金高成本的局限性,对于拓宽镁合金的应用具有重要意义。本文以Mg-6Zn(mass%)合金为基体,在优化Al含量的基础上,系统地研究了Sn低合金化和Sn/Ca复合合金化对Mg-6Zn-2Al铸态组织与性能的影响规律,考察了铸态合金的固溶时效特性和性能特征,探索了Sn/Ca合金化镁合金的加工成形性能、析出相特征和力学性能特点,并探讨了铸态含Sn合金的蠕变特性和Sn的作用机理。本文的目的在于探索一类Sn和Ca复合强化的耐热镁合金材料,为低成本高性能镁合金的研制提供详细的实验数据和理论依据。主要研究内容和结果如下：(1)研究了Al含量(2,4和6mass%)对Mg-6Zn铸态合金组织与性能的影响,得出Mg-6Zn-2A1合金综合力学性能优良,耐腐蚀性能优于铸态AZ91和ZA85合金,且在150℃和200℃长时间热暴露过程中表现出较高的组织稳定性,是三元合金中一个较佳的成分配比。(2)系统地研究了Sn含量(0.5,1,2和3mass%)对Mg-6Zn-2Al铸态合金组织与性能的影响。结果表明,铸态含Sn合金主要由α-Mg、MgZn和Mg2Sn相组成,其中Mg2Sn相以从液相直接结晶出的粗大颗粒和固态析出的短棒状颗粒存在。Sn具有细化合金中MgZn离异共晶化合物和抑制其生成的作用。上述合金在150℃长时间热暴露过程中组织稳定性良好,而在200℃热暴露后期因Mg2Sn颗粒的聚集长大而硬度有所下降。加入Sn可以降低合金基体的自腐蚀电位、抑制腐蚀反应阴极过程和促进阳极过程,对耐蚀性影响不显著。加入Sn到Mg-6Zn-2Al合金中可以显著提高室温和高温强度并改善抗压缩蠕变性能。室温下,其屈服强度随着Sn含量的增加而明显提高,0.5mass%Sn时抗拉强度和伸长率最高；150℃下,0.5mass%Sn时抗拉强度和屈服强度最高。铸态含Sn合金室温和高温断裂方式为混合型断裂。随着Sn含量增加,其室温断裂倾向于解理断裂,高温断裂倾向于准解理断裂。含Sn合金的稳态压缩蠕变速率(175℃/50MPa)比基体低10%-30%不等,而3mass%Sn合金的稳态压缩蠕变速率(190℃/50MPa)则低56%。(3)系统地研究了少量Ca(0.2mass%)对Mg-6Zn-2Al-3Sn铸态合金组织与性能的影响。结果表明,少量Ca可以提高其在200℃长时间热暴露过程中的组织稳定性,改善耐蚀性,提高室温、高温强度和抗压缩蠕变性能。加Ca合金的稳态压缩蠕变速率(160-190℃/50-70MPa)比不加Ca的低7%-18%不等且耐蚀性优于铸态AZ91合金,是多元合金成分设计的依据。(4)系统地研究了铸态合金的固溶时效特性与组织性能特征。结果表明,加入Sn和Sn/Ca导致合金中MgZn共晶化合物在345℃保温28h后固溶不完全；Sn可以抑制GP区的形成与溶解,Ca则促进GP区的形成与溶解和p1’相的析出。时效处理可以提高铸态含Sn合金的室温和高温屈服强度,增加稳态蠕变速率的应力敏感性,而对抗蠕变性能的影响不一致。高Sn合金的室温、高温屈服强度和抗压缩蠕变性能优于低Sn合金。时效态含Sn合金中析出相以长棒状βl’相和短棒状Mg2Sn相为主,存在Mg2Sn颗粒聚集和多数Mg2Sn相依附βl’相形核与长大的现象；加Ca合金中还存在若干块状的βl’相。Ca改变了时效态Mg-6Zn-2Al-3Sn合金中βl’相的形貌、降低了长棒状pl’相的纵横比并抑制了Mg2Sn相的聚集,从而提高室温、高温屈服强度并改善抗压缩蠕变性能且不影响塑性。(5)系统地研究了Sn/Ca合金化镁合金的加工成形性能、析出相特征和力学性能特点。研究表明,Mg-6Zn-2Al-3Sn-0.2Ca合金经挤压(330℃挤压、挤压比25：1)加工后具有细小的等轴晶组织,各向异性程度低,综合力学性能优良(抗拉强度387MPa,伸长率9%)。变形过程中,Mg2Sn质点是动态再结晶形核的择优位置；按T5处理时基体中析出大量的纳米相(p1’相和Mg2Sn相)；T6态合金中也存在大量的纳米相。T5态合金中长棒状pl’相析出密度高且细小,导致其强度和伸长率高于T6态。挤压态和热处理态合金的抗蠕变性能低于铸态。然而,挤压合金的室温拉伸性能与商业用变形镁合金相当,抗蠕变性能显著改善,其稳态蠕变速率(175℃/50-70MPa)在10-7s-1数量级,比文献报道的性能优良的Mg-2at.%Sn-1 at.%Zn-0.1 at.%Mn和Mg-4Al-2Sr-0.3Mn挤压合金的数据还低。这一结论为不含稀土的变形镁合金抗蠕变性能的改善提供了可能性。(6)探讨了铸态含Sn合金的蠕变特性和Sn的作用机理。研究表明,加入3mass%Sn到Mg-6Zn-2Al铸态合金中可以显著提高抗拉伸蠕变性能且在高温、高应力下的效果更显著,其作用机制是降低镁的自扩散系数、细化MgZn离异共晶化合物和引入Mg2Sn相质点。添加Sn导致合金基体的蠕变激活能提高,蠕变应力指数下降。前者可归因于Sn具有较高的空位结合能,后者则与MgZn离异共晶化合物的细化有关。两种合金在实验条件下的蠕变过程主要受位错攀移控制；MgZn离异共晶化合物开裂是两种合金在较高温度、较高应力条件下抗蠕变性能下降的主要原因。更多还原

【Abstract】 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.更多还原