【作者】 张敏；

【导师】 张文征；

【作者基本信息】 清华大学 ， 材料科学与工程， 2007， 博士

【摘要】 本论文工作主要是针对我国资源丰富的镁合金开展系统的归纳和组织设计。在对现有镁合金中的强化效果,尤其是占主导地位的时效析出强化,以及各种合金元素所起的作用进行充分调研和分析归纳的基础上,应用现有的理论模型,设计了Mg-Sn-Mn-Si四元合金,并通过调整成分和热处理工艺,探索了该合金中性能变化趋势。采用金相分析、SEM、TEM等方法系统研究了该系列合金中析出相的种类、形貌、晶体学特征等。经过固溶和时效热处理之后,该系列合金中最主要的析出相为Mg₂Sn相,与基体之间具有多种有理位向关系和无理位向关系。大部分Mg₂Sn呈片条状或长杆状,平躺在（0 0 0 1）α基面内,不能有效阻碍位错运动。Mn和Si的添加促进了块状Mg₂Sn的析出,这种块状相由几组刻面组成,其中大部分刻面为无理界面,但是每组刻面都与至少一个Δg平行,分析了这些刻面的取向以及界面上的匹配状况,实验测量和理论计算相吻合。这些块状相具有更好的强化效果。这些研究为通过理论分析指导材料设计提供了一定的实验依据。除了Mg₂Sn,基体中还有少量Mg₂Si相和Mn5Si3相,时效初期的样品中还含有一种亚稳Mg-Sn相。这是首次在镁合金中发现Mn5Si3相。这种相具有h.c.p.的晶格结构,由（0 1–1 0）Mn5Si3面组成三组平行的刻面。虽然Mn5Si3相与基体之间没有唯一确定的位向关系,但是有一个约束条件,使得系统始终满足Δg//g,并且同时平行于这组Δg和g的刻面是择优界面。综合运用Δg平行法则和O点阵模型定量解释了Mg-Al、Mg-Y-Nd等系统中位向关系与传统Burgers位向关系之间的小角度偏转,并计算了相关系统中析出相与基体之间的界面结构,与前人的实验相吻合。对具有优异性能的Mg-Zn-Y系列合金设计热处理工艺,并分析了该合金的显微组织,发现铸态组织中晶界和晶内都有大量准晶相,这些准晶相能起到很好的强化效果。时效后由于晶界准晶相溶解,硬度值低于铸态组织。同时发现了一种该合金中未曾报道过的Zn2Zr相。更多还原

【Abstract】 Large-scale development and application of magnesium alloys have been greatly stimulated by the demand for lightweight and environment-friendly materials in the automotive industry in recent years. However, conventional magnesium alloys cannot meet the demand of high strength both at room and elevated temperatures. Therefore, a lot of attention has been paid on how to improve the elevated temperature properties. There is a large number of magnesium resources in China, and it is necessary for us to design new magnesium alloys with high performance both at room and elevated temperatures. After literature survey and systematic analysis on microstructures of Mg alloys, we have chosen Mg-Sn based alloys with additions of Mn and Si to investigate the heat-treatment process, microstructures and crystallography. Optical microscopes, SEM and TEM were used to examine the newly developed alloy. After solid solution and age-hardening for a period of time, a large number of Mg₂Sn precipitated in the matrix and they have more than eight types of orientation relationships （OR） with the matrix, some of them are irrational. We have analyzed these ORs thoroughly. Although a lot of plate and rod shaped Mg₂Sn precipitates lie in the basal plane of （0 0 0 1）α, the additions of Mn and Si have stimulated the precipitate of polygonal Mg₂Sn precipitates. This kind of polygonal precipitates is covered by about 4 pairs of facets. Most of the facets are not parallel with any low-indexed plane, however, they are found to be parallel with certainΔg’s. The orientation and matching status of these facets are analyzed by usingΔg’s and morie fringe theory, and the calculated result is consistent with the measured results. The relative planes match exactly at the facets. This will provide theoretical support in designing new alloys.There are also several Mg₂Si and Mn₅Si₃ particles in the present Mg-Sn-Mn-Si alloy. To our knowledge, no publilcation has reported the presence of Mn₅Si₃ precipitates in any Mg alloy before. The Mn₅Si₃ precipitates grow into plates with about 50 nm in thickness and 200 nm in width. In Mn₅Si₃/Mg system, the OR is not fully constrained in 3-dimensional space, but the following OR is often observed: g（1 –1 0 0）Mn5Si3//Δg1//Δg2, whereΔg1 = g（2–1–1 0）Mn5Si3– g（0 0 0 2）Mg,Δg2 = g（3–2–1 0）Mn5Si3– g（0 0 0 2）Mg. When satisfyingΔg parallelism rule, the rotation angle between g（2–1–1 0）Mn5Si3 and g（0 0 0 2）Mg is calculated to be 7.8°, which is consistent with experimental results. The broad facet of the precipitate plate is normal to the parallelΔg’s, agreeing with theΔg parallelism rule.By combiningΔg parallelism rules and O lattice theory, small deviations of about 0.5°（in Mg-Al alloy） and 0.29°（in Mg-Y-Nd alloy） from exact Burgers OR were calculated and interpreted. The lattice matching status of the habit planes of the precipitates, at the Burgers OR and at the calculated OR, is compared. The calculated results are consistent with experimental work.The microstructures of Mg-Zn-Y alloys and effects of heat-treatment in hardness are analyzed. It is found that the as-cast samples have better hardness since there are a large number of eutectic quasi-crystals in grain boundaries. A kind of Zn₂Zr phase is detected, and an intermetallic compound with Mg and rich Y is also detected in Mg-Zn-Y alloy.更多还原