【作者】 张建新；

【导师】 郭学锋；

【作者基本信息】 河南理工大学 ， 矿业工程， 2014， 博士

【摘要】 Mg-Sn-Si系合金是一种潜在的新型耐热镁合金。到目前为止,该合金的组织结构、制备工艺和强韧化机理处于探索阶段,使其应用和发展受到限制。本文首先采用常规凝固法制备Mg-3~8Sn-1.5Al-1Zn-1Si合金,用OM、SEM和TEM观察和分析合金组织,XRD、EDS和DSC等分析合金相结构及组成。通过组织观察和Thermo-Calc相图计算研究了合金的结晶和相变过程;进而在系统研究合金普通凝固组织特征和结构的基础上,探索了合金的组织细化及细化机理。集中研究了铸态合金经Sb变质和变形加工的组织演化过程,通过分析变形合金的动态再结晶机理,揭示了晶粒细化的原因;最后在初步测试不同条件下合金的力学性能后,分析了Mg-Sn-Si系合金的强韧化机制。研究表明,Mg-5Sn-1Si合金的平衡凝固顺序为:L→α-Mg,L+α-Mg→α-Mg+Mg2(Si,Sn)和L→(α-Mg+Mg2(Si,Sn)+Mg2(Sn,Si)),之后一直到室温发生α-Mg→α-Mg+Mg2(Sn,Si)。而非平衡凝固过程包括:L→α-Mg,L+α-Mg→α-Mg+Mg2(Si,Sn),L→α-Mg+Mg2Si,Sn+Mg2Si→Mg2(Si,Sn),Sn+Mg2(Si,Sn)→Mg2(Sn,Si)+Si和Si+Mg2Sn→Mg2(Sn,Si),之后到室温发生α-Mg→Mg2Sn+Mg2(Si,Sn)。其中Mg2(Si,Sn)和Mg2(Sn,Si)相的结构与Mg2Si(或Mg2Sn)相同,二者的晶格常数、纳米硬度和弹性模量等性能介于Mg2Si与Mg2Sn之间。通过计算Mg8Sn3Si、Mg8Si3Sn的结合能发现,Si(Sn)取代Mg2Sn(Mg2Si)面心和顶点位置上的原子是等价的。Mg2(Si,Sn)和Mg2(Sn,Si)三元相通过Sn+Mg2Si→Mg2(Si,Sn),Sn+Mg2(Si,Sn)→Mg2(Sn,Si)+Si和Si+Mg2Sn→Mg2(Sn,Si)发生原子置换,其中Mg2(Si,Sn)相的生成量由Si含量决定,Mg2(Sn,Si)的生成量与被取代的Si原子数量及Sn含量关系密切。在Mg-xSn-Si系铸态合金中,Sb通过Mg2(Si,Sn)和Mg2(Sn,Si)的桥梁作用,对汉字状Mg2Si进行变质处理,细化效果较为显著。正挤压后,Mg2Si相呈现片状聚集,Mg2Sn相偏聚在晶界处。Mg2(Si,Sn)相和Mg2(Sn,Si)相被细化,颗粒尺寸由铸态10μm细化到~3μm。挤压过程中Mg2(Si,Sn)相经机械破碎而细化,Mg2(Sn,Si)相经机械破碎和相变而细化。往复挤压后,组织通过连续或非连续动态再结晶而细化,晶粒尺寸由铸态的30μm细化到~8μm,经过7道次往复挤压,合金组织细小均匀,晶粒细化到7.7μm。经过变质和变形细化后,合金的强度和塑性得到显著提高,一方面是基体晶粒的细化,另一方面是强化相的均匀分布,更为关键的是与“Mg2Si/基体”界面结合情况相比,“Mg2(Si,Sn)或Mg2(Sn,Si)/基体”界面结合力更强,从而提高了合金强度和塑性。往复挤压Mg-5Sn-1.5Al-1Zn-1Si合金经固溶+时效热处理后,其强韧性更佳,200℃的强度保持率为59.7%,Sb变质后强度保持率升高到62.6%;高温变形以晶界滑移为主,变形机制与合金的复合强化、热处理工艺、活性元素作用及动态再结晶有一定关系。更多还原

【Abstract】 Mg-Sn-Si is a potential and new type of heat resistant magnesium alloy system. However, the research on preparation technology, structure as well as strengthening and toughening mechanism of the system are still in the exploratory stage, therefore the application and development of this system are restricted. In this study, Mg-3~8Sn-1.5Al-1Zn-1Si alloys were prepared by using conventional solidification technology. In order to reveal solidification and phase transformation processes of the system, microstructural observation and phase diagram calculation were adapted. For microstructural observation and analyzing, optical microscope, SEM and TEM were used. For microstructural constitutes and phase transformation characteristics evaluating, XRD, EDS and DSC were utilized. For detailed evaluation of solidification of the system, Thermo-Calc phase diagram calculation was used. Based on the analyzing results of solidification, phase transformation and phase dispersion, microstructural refinement processes and mechanism are explored for the alloys with Sb modification or/and mechanical deformation. Mechanical properties of the processed alloys were tested, and strengthening and toughening mechanism were analyzed. Based on the alloy preparation, solidification and phase transformation analyzing, microstructural modification and refinement processing, strengthening and toughening mechanism study, the following results were achieved.The solidification process of Mg-5Sn-1Si alloy under equilibrium consists of L→α-Mg, L + α-Mg→α-Mg + Mg2(Si,Sn) and L→(α-Mg + Mg2(Si,Sn) + Mg2(Sn,Si)). After solidification, α-Mg→α-Mg + Mg2(Sn,Si) occurs from solidus to room temperature. However, under nonequilibrium condition solidification process of Mg-5Sn-1Si alloy consists of L→α-Mg, L + α-Mg→α-Mg + Mg2(Si,Sn), L→α-Mg + Mg2 Si, Sn + Mg2Si→Mg2(Si,Sn), Sn + Mg2(Si,Sn)→Mg2(Sn,Si) + Si and Si + Mg2Sn→Mg2(Sn,Si). After solidification, α-Mg→Mg2Sn + Mg2(Si,Sn) occurs from solidus to room temperature. Mg2(Si,Sn) and Mg2(Sn,Si) have the same space structure as that of Mg2Si(or Mg2Sn), lattice constant are just in between of Mg2 Si and Mg2 Sn. The bonding energy calculating to Mg8Sn3 Si and Mg8Si3 Sn reveals Si(Sn) atoms can replace central Sn(Si) atoms on faces or corner atoms to form Mg2(Si,Sn) and Mg2(Sn,Si). The detailed replacement reactions are the following: Sn + Mg2 Si →Mg2(Si,Sn), Sn + Mg2(Si,Sn)→Mg2(Sn,Si) + Si and Si + Mg2Sn→Mg2(Sn,Si). The quantity of Mg2(Si,Sn) is determined by the Si content, meanwhile, the production quantity of Mg2(Sn,Si) has close relationship with the replaced Si and Sn contents. Measurement analyzing revealed that some peoperties of Mg2(Si,Sn) and Mg2(Sn,Si) phase, such as nanohardness and elastic modulus are also in between those of Mg2 Si and Mg2 Sn phases.For as-cast Mg-xSn-Si alloys, Sb modification effect on Chinese script type Mg2 Si is remarkable due to the formation of Mg2(Si,Sn) and Mg2(Sn,Si) phases which effect as a bridging role.Mechanical working can effectively refine the microstructures of the studied alloys and can also uniform the distribution of strengthening particles on matrix. For those alloys underwent forward extrusion, broken Mg2 Si particles are in a kind of flake aggregation, while Mg2 Sn phase is dispersed at the grain boundaries. Mg2(Si,Sn) and Mg2(Sn,Si) phases are refined and resulted particle size is about 3μm which was about 10μm in as-cast condition. The refinement processes are different to Mg2(Si,Sn) and Mg2(Sn,Si) phases. For Mg2(Si,Sn) particles, it was broken from as cast coarser ones. For Mg2(Sn,Si), however, although it was broken from as cast coarser ones, but the refinement process consisted of phase transformation. For those alloys underwent reciprocating extrusion(RE), matrix were refined through the continuous or discontinuous dynamic recrystallization, the mean grain size ia about 8μm which was 30μm under as-cast state. After 7 RE passes, the matrix grain was about 7.7μm which was much fine and particle distribution was much uniform.After Sb modification or/and mechanical working, strength and plasticity of the alloy can be significantly improved as a consequence of matrix refinement, particle uniform distribution and most crucially strengthened interfacial bonding between Mg2(Si,Sn) or Mg2(Sn,Si)/matrix in comparing with that of Mg2Si/matrix. For RE processed Mg-5Sn-1.5Al-1Zn-1Si alloy, the tensile strength retention rate at 200℃ is 59.7%, after Sb modification and RE processed, the strength retention rate of Mg-5Sn-1.5Al-1Zn-1Si alloy is 62.6%. The deformation under high temperature due priority to grain boundary sliding, therefore, the strengthening mechanism is due to several reasons, such as grain refinement by Sb modification and dynamic recrystallization, particle redistribution, etc. mainly related to compound strengthening at grain boundary as well as within grains.更多还原