本文關鍵詞： 純鎢 多向壓縮變形 微觀組織 熱學性能　出處：《合肥工業(yè)大學》2015年碩士論文　論文類型：學位論文

【摘要】：鎢因具有獨特的物理性能、良好的化學穩(wěn)定性能及優(yōu)異的高溫強熱性,被廣泛地應用于航空航天和核能領域。粉末燒結制備的材料普遍存在晶粒粗大、組織疏松以及孔隙多等缺陷。大塑性變形工藝具有強烈致密組織和細化晶粒的能力,能夠達到有效降低組織缺陷的目的。其中多向壓縮工藝(MDC)由于操作過程簡單、成本低,是目前有望直接應用于大批量制備塊體細晶材料的一種典型大塑性變形方法。通過大塑性變形工藝致密和細化組織,實現(xiàn)高性能鎢材的制備過程,對今后鎢的工業(yè)化應用具有重要意義。為此,本文基于變形強化機制理論,采用高溫條件下MDC工藝對燒結態(tài)純鎢材料進行不同變形參數(shù)(單次壓下量、變形道次)的實驗研究。通過光學金相顯微觀察、XRD分析和EBSD分析手段對變形過程中晶粒細化機制和微觀結構演變規(guī)律進行深入地探討,結合顯微硬度測試對變形組織的強韌化效果進行驗證。結果表明：單次變形量為50%的一道次MDC變形對純鎢具有明顯的致密和細化作用,顯微硬度值提高明顯。變形過程中鎢晶體以(110)面的滑移變形為主,同時進行(211)面的孿生變形。變形組織內部位錯密度、微觀應變及大、小角度晶界數(shù)量提高明顯,并且晶界分布有從小角度晶界向大角度晶界轉變的趨勢。分析認為MDC變形過程中晶粒細化過程與晶界的轉化現(xiàn)象有關,隨著MDC變形的進行,初始大尺寸晶粒受變形應變的誘發(fā),晶粒內部位錯發(fā)生增殖、密度增大,而小角度晶界的本質是位錯,晶體滑移的發(fā)生使得小角度晶界(位錯)會相遇纏結,堆積畸變到一定程度時便形成取向差較大的新晶界,完成晶界取向差的轉化。新晶界的產(chǎn)生表明細小晶粒的生成,組織得到了細化,達到了變形強化純鎢組織的目的。對變形前后材料的熱膨脹性能和熱擴散性能進行不同溫度區(qū)間的測試,結合材料熱學性能的物理基礎知識,分析組織變化對純鎢材料熱學性能的影響。最后利用差示掃描量熱技術對變形組織的再結晶行為進行初步的測定分析。分析發(fā)現(xiàn),變形后材料在高溫環(huán)境中表現(xiàn)更好的熱擴散性能,而材料的熱膨脹性能有所增加。相比初始樣,變形后的材料在高溫條件下氧化現(xiàn)象極大減少,表現(xiàn)出更加優(yōu)越的高溫穩(wěn)定性。純鎢材料的再結晶溫度不受微觀組織結構變化的影響,使得MDC變形前后純鎢材料的可加工溫度區(qū)間保持不變,這對于MDC變形純鎢材料的后續(xù)成形加工過程具有十分重要的意義。
[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.
【學位授予單位】：合肥工業(yè)大學
【學位級別】：碩士
【學位授予年份】：2015
【分類號】：TF124;TG146.411

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