本文選題：純鎢 + 高壓扭轉(zhuǎn)　； 參考：《合肥工業(yè)大學(xué)》2017年碩士論文

【摘要】：難熔金屬鎢由于具有良好的力學(xué)性能、高溫強(qiáng)熱性及耐腐蝕性等特點,在航空航天、核工業(yè)等眾多領(lǐng)域擁有廣泛的應(yīng)用前景。采用粉末冶金法制備的工業(yè)純鎢,因燒結(jié)溫度高及雜質(zhì)元素污染等,常存在孔隙多、組織粗大、晶界弱化等問題,導(dǎo)致鎢的塑性加工性能差、韌-脆轉(zhuǎn)變溫度高且易于氧化,限制了其應(yīng)用與發(fā)展。高壓扭轉(zhuǎn)工藝基于高靜水壓力和劇烈剪切變形,能夠有效閉合孔隙、改善材料組織性能,在較低的溫度下獲得具有非平衡大角度晶界的超細(xì)晶組織,促進(jìn)晶界滑移和位錯演化,有效提高低塑、高強(qiáng)材料的低溫變形能力。通過高壓扭轉(zhuǎn)工藝制備綜合性能優(yōu)異的純鎢材料,對促進(jìn)其工程化應(yīng)用具有積極的推動作用。本文基于變形強(qiáng)化理論,采用半限制型高壓扭轉(zhuǎn)模具,在較低溫度下對燒結(jié)態(tài)純鎢進(jìn)行不同扭轉(zhuǎn)圈數(shù)下的大剪切塑性變形,通過金相組織觀察、X射線衍射分析及EBSD技術(shù),分析了純鎢高壓扭轉(zhuǎn)變形的變形特點,深入研究變形過程中微觀組織和形變亞結(jié)構(gòu)的演變規(guī)律。結(jié)果表明:燒結(jié)體純鎢組織經(jīng)高壓扭轉(zhuǎn)變形后得到顯著的致密和細(xì)化,形成具有明顯方向性的纖維組織;純鎢的高壓扭轉(zhuǎn)變形主要以{110}晶面的滑移變形為主,隨著扭轉(zhuǎn)圈數(shù)增大,為彌補(bǔ)單系滑移的不足,沿{112}、{200}晶面的滑移變形增強(qiáng);變形組織內(nèi)微觀應(yīng)變逐漸增大,而亞晶尺寸與位錯密度隨應(yīng)變量積累先增大后減小;高壓扭轉(zhuǎn)變形主要通過剪切破碎和形變誘導(dǎo)位錯演化的方式細(xì)化晶粒,該過程伴隨著小角度晶界向大角度晶界的演化以及特定晶粒取向的形成,當(dāng)變形量達(dá)到一定程度后,變形組織發(fā)生明顯的動態(tài)再結(jié)晶晶粒長大現(xiàn)象,組織內(nèi)形成再結(jié)晶立方織構(gòu);高壓扭轉(zhuǎn)過程中,由于受材料本身變形機(jī)制、應(yīng)力狀態(tài)及動態(tài)再結(jié)晶行為的影響,純鎢組織內(nèi)部產(chǎn)生“織構(gòu)起伏”效應(yīng)。對不同扭轉(zhuǎn)圈數(shù)下的純鎢進(jìn)行顯微硬度、壓痕形貌及再結(jié)晶行為表征,結(jié)合材料微觀組織的演化特點分析純鎢高壓扭轉(zhuǎn)變形的強(qiáng)韌化機(jī)理和變形組織的熱穩(wěn)定性。結(jié)果表明:純鎢燒結(jié)體經(jīng)高壓扭轉(zhuǎn)變形后顯微硬度明顯提高,室溫韌性得到改善,且強(qiáng)韌化效果和均勻性隨著扭轉(zhuǎn)圈數(shù)增加明顯提高,純鎢高壓扭轉(zhuǎn)變形主要通過晶粒細(xì)化和位錯增殖實現(xiàn)強(qiáng)韌化;高壓扭轉(zhuǎn)變形后純鎢的再結(jié)晶溫度未發(fā)生明顯下降,不同扭轉(zhuǎn)圈數(shù)下的再結(jié)晶組織具有較好的尺寸穩(wěn)定性;變形后材料的形變儲存能增大,再結(jié)晶激活能降低,但由于位錯重排和非平衡晶界轉(zhuǎn)變對儲存能的釋放作用,純鎢變形組織的熱穩(wěn)定性相對較好。
[Abstract]:Refractory tungsten has been widely used in many fields such as aerospace and nuclear industry due to its good mechanical properties, high temperature and high thermal properties and corrosion resistance. Due to high sintering temperature and contamination of impurity elements, the industrial pure tungsten prepared by powder metallurgy has many problems such as large porosity, coarse microstructure and weakening grain boundary, which leads to poor plasticity, high ductile-brittle transition temperature and easy oxidation. It limits its application and development. Based on the high hydrostatic pressure and severe shear deformation, the high-pressure torsion process can effectively close the pores, improve the microstructure and properties of the material, obtain ultrafine grain structure with non-equilibrium and large-angle grain boundaries at low temperature, and promote the grain boundary slip and dislocation evolution. Effectively improve the low-temperature deformation capacity of low-plastic and high-strength materials. The preparation of pure tungsten materials with excellent comprehensive properties by high pressure torsion process has a positive effect on promoting its engineering application. Based on the deformation strengthening theory, the large shear plastic deformation of sintered pure tungsten under different torsion cycles was carried out at low temperature by using a semi-limited high-pressure torsion die. X-ray diffraction analysis and EBSD technique were observed by metallographic structure. The deformation characteristics of high pressure torsional deformation of pure tungsten are analyzed, and the evolution of microstructure and deformation substructure during deformation is studied. The results show that the microstructure of pure tungsten sintered by high pressure torsion deformation is densified and refined, and the fiber structure with obvious directionality is formed, and the high pressure torsional deformation of pure tungsten is mainly caused by slip deformation of {110} crystal plane. The slip deformation along {112} and {200} crystal plane increases with the increase of torsion circle number, and the subcrystal size and dislocation density increase firstly and then decrease with the accumulation of strain variables. The high-pressure torsional deformation mainly refines the grain by the way of shear fragmentation and deformation induced dislocation evolution. This process is accompanied by the evolution of small angle grain boundary to large angle grain boundary and the formation of specific grain orientation. When the deformation amount reaches a certain extent, Dynamic recrystallization grain growth occurs in the deformed microstructure, and recrystallized cubic texture is formed in the microstructure, which is affected by the deformation mechanism, stress state and dynamic recrystallization behavior of the material during high pressure torsion. The effect of texture fluctuation is produced in pure tungsten microstructure. The microhardness, indentation morphology and recrystallization behavior of pure tungsten under different torsion cycles were characterized. The strengthening and toughening mechanism of pure tungsten under high pressure torsion deformation and the thermal stability of deformation microstructure were analyzed in combination with the evolution characteristics of microstructure. The results show that the microhardness and room temperature toughness of pure tungsten sintered by high pressure torsion deformation are obviously improved, and the toughening effect and uniformity are obviously increased with the increase of torsion circle number. The recrystallization temperature of pure tungsten after high pressure torsion deformation is not decreased obviously, and the recrystallization structure with different torsion circles has better dimensional stability. The deformed storage energy increases and the recrystallization activation energy decreases after deformation, but the thermal stability of the deformed microstructure of pure tungsten is relatively good due to the effect of dislocation rearrangement and non-equilibrium grain boundary transition on the release of storage energy.
【學(xué)位授予單位】：合肥工業(yè)大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：TG146.411

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