难熔金属钨高压扭转微观组织演化及力-热性能研究

发布时间：2018-05-20 07:36

本文选题：纯钨 + 高压扭转　；参考：《合肥工业大学》2017年硕士论文

【摘要】：难熔金属钨由于具有良好的力学性能、高温强热性及耐腐蚀性等特点,在航空航天、核工业等众多领域拥有广泛的应用前景。采用粉末冶金法制备的工业纯钨,因烧结温度高及杂质元素污染等,常存在孔隙多、组织粗大、晶界弱化等问题,导致钨的塑性加工性能差、韧-脆转变温度高且易于氧化,限制了其应用与发展。高压扭转工艺基于高静水压力和剧烈剪切变形,能够有效闭合孔隙、改善材料组织性能,在较低的温度下获得具有非平衡大角度晶界的超细晶组织,促进晶界滑移和位错演化,有效提高低塑、高强材料的低温变形能力。通过高压扭转工艺制备综合性能优异的纯钨材料,对促进其工程化应用具有积极的推动作用。本文基于变形强化理论,采用半限制型高压扭转模具,在较低温度下对烧结态纯钨进行不同扭转圈数下的大剪切塑性变形,通过金相组织观察、X射线衍射分析及EBSD技术,分析了纯钨高压扭转变形的变形特点,深入研究变形过程中微观组织和形变亚结构的演变规律。结果表明:烧结体纯钨组织经高压扭转变形后得到显著的致密和细化,形成具有明显方向性的纤维组织;纯钨的高压扭转变形主要以{110}晶面的滑移变形为主,随着扭转圈数增大,为弥补单系滑移的不足,沿{112}、{200}晶面的滑移变形增强;变形组织内微观应变逐渐增大,而亚晶尺寸与位错密度随应变量积累先增大后减小;高压扭转变形主要通过剪切破碎和形变诱导位错演化的方式细化晶粒,该过程伴随着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,当变形量达到一定程度后,变形组织发生明显的动态再结晶晶粒长大现象,组织内形成再结晶立方织构;高压扭转过程中,由于受材料本身变形机制、应力状态及动态再结晶行为的影响,纯钨组织内部产生“织构起伏”效应。对不同扭转圈数下的纯钨进行显微硬度、压痕形貌及再结晶行为表征,结合材料微观组织的演化特点分析纯钨高压扭转变形的强韧化机理和变形组织的热稳定性。结果表明:纯钨烧结体经高压扭转变形后显微硬度明显提高,室温韧性得到改善,且强韧化效果和均匀性随着扭转圈数增加明显提高,纯钨高压扭转变形主要通过晶粒细化和位错增殖实现强韧化;高压扭转变形后纯钨的再结晶温度未发生明显下降,不同扭转圈数下的再结晶组织具有较好的尺寸稳定性;变形后材料的形变储存能增大,再结晶激活能降低,但由于位错重排和非平衡晶界转变对储存能的释放作用,纯钨变形组织的热稳定性相对较好。
[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.
【学位授予单位】：合肥工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG146.411

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