基于多向压缩工艺的纯钨组织及性能研究

发布时间：2018-01-23 17:31

本文关键词： 纯钨多向压缩变形微观组织热学性能　出处：《合肥工业大学》2015年硕士论文　论文类型：学位论文

【摘要】：钨因具有独特的物理性能、良好的化学稳定性能及优异的高温强热性,被广泛地应用于航空航天和核能领域。粉末烧结制备的材料普遍存在晶粒粗大、组织疏松以及孔隙多等缺陷。大塑性变形工艺具有强烈致密组织和细化晶粒的能力,能够达到有效降低组织缺陷的目的。其中多向压缩工艺(MDC)由于操作过程简单、成本低,是目前有望直接应用于大批量制备块体细晶材料的一种典型大塑性变形方法。通过大塑性变形工艺致密和细化组织,实现高性能钨材的制备过程,对今后钨的工业化应用具有重要意义。为此,本文基于变形强化机制理论,采用高温条件下MDC工艺对烧结态纯钨材料进行不同变形参数(单次压下量、变形道次)的实验研究。通过光学金相显微观察、XRD分析和EBSD分析手段对变形过程中晶粒细化机制和微观结构演变规律进行深入地探讨,结合显微硬度测试对变形组织的强韧化效果进行验证。结果表明：单次变形量为50%的一道次MDC变形对纯钨具有明显的致密和细化作用,显微硬度值提高明显。变形过程中钨晶体以(110)面的滑移变形为主,同时进行(211)面的孪生变形。变形组织内部位错密度、微观应变及大、小角度晶界数量提高明显,并且晶界分布有从小角度晶界向大角度晶界转变的趋势。分析认为MDC变形过程中晶粒细化过程与晶界的转化现象有关,随着MDC变形的进行,初始大尺寸晶粒受变形应变的诱发,晶粒内部位错发生增殖、密度增大,而小角度晶界的本质是位错,晶体滑移的发生使得小角度晶界(位错)会相遇缠结,堆积畸变到一定程度时便形成取向差较大的新晶界,完成晶界取向差的转化。新晶界的产生表明细小晶粒的生成,组织得到了细化,达到了变形强化纯钨组织的目的。对变形前后材料的热膨胀性能和热扩散性能进行不同温度区间的测试,结合材料热学性能的物理基础知识,分析组织变化对纯钨材料热学性能的影响。最后利用差示扫描量热技术对变形组织的再结晶行为进行初步的测定分析。分析发现,变形后材料在高温环境中表现更好的热扩散性能,而材料的热膨胀性能有所增加。相比初始样,变形后的材料在高温条件下氧化现象极大减少,表现出更加优越的高温稳定性。纯钨材料的再结晶温度不受微观组织结构变化的影响,使得MDC变形前后纯钨材料的可加工温度区间保持不变,这对于MDC变形纯钨材料的后续成形加工过程具有十分重要的意义。
[Abstract]:Tungsten is widely used in the fields of aerospace and nuclear energy because of its unique physical properties, good chemical stability and excellent high temperature and strong thermal properties. The large plastic deformation process has the ability of dense structure and fine grain refinement. It can effectively reduce the structure defect, in which the multi-direction compression process (MDC) is easy to operate, and the cost is low. It is a typical large plastic deformation method which is expected to be directly applied to bulk fine grain materials. The preparation process of high performance tungsten can be realized by densification and refinement of microstructure by large plastic deformation process. It is of great significance for the industrial application of tungsten in the future. Therefore, based on the theory of deformation strengthening mechanism, different deformation parameters (single reduction) of sintered pure tungsten materials are carried out by MDC process under high temperature. By means of optical metallography and EBSD analysis, the mechanism of grain refinement and the law of microstructure evolution in the process of deformation were deeply discussed. The strength and toughness of the deformed microstructure were verified by microhardness test. The results showed that a secondary MDC deformation with a single deformation amount of 50% had obvious densification and refinement effect on pure tungsten. The microhardness value is increased obviously. During the deformation, the tungsten crystal is dominated by slip deformation on the 110th plane, and the twinning deformation is carried out at the same time. The dislocation density, microscopic strain and large deformation are observed in the deformed microstructure. The number of small angle grain boundaries increased obviously, and the grain boundary distribution changed from small angle grain boundary to large angle grain boundary. It is concluded that the grain refinement process in MDC deformation process is related to the grain boundary transformation phenomenon. With the development of MDC deformation, the initial large grain size is induced by deformation strain, the dislocation inside the grain proliferates and the density increases, while the essence of small angle grain boundary is dislocation. The occurrence of crystal slip makes small angle grain boundaries (dislocations) encounter entanglement, and when stacking distortion reaches a certain extent, a new grain boundary with large orientation difference will be formed. The formation of new grain boundaries indicates the formation of fine grains and the refinement of microstructure. The thermal expansion and thermal diffusion properties of the materials before and after deformation were tested in different temperature ranges, and the physical knowledge of the thermal properties of the materials was combined. The effect of microstructure changes on thermal properties of pure tungsten materials was analyzed. Finally, the recrystallization behavior of deformed microstructure was determined by differential scanning calorimetry (DSC). The deformed materials exhibit better thermal diffusion performance in the high temperature environment, but the thermal expansion properties of the materials are increased. Compared with the initial samples, the oxidation phenomenon of the deformed materials at high temperature is greatly reduced. The recrystallization temperature of pure tungsten material is not affected by the change of microstructure, which makes the machinability temperature range of pure tungsten material unchanged before and after MDC deformation. This is of great significance for the subsequent forming process of MDC deformed pure tungsten materials.
【学位授予单位】：合肥工业大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TF124;TG146.411

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