【作者】 李娟；

【导师】 汪冰峰；

【作者基本信息】 中南大学 ， 材料加工工程， 2014， 硕士

【摘要】 摘要：钛及钛合金具有比强度高、耐腐蚀性能好等优异的特性,在航空航天领域得到广泛的应用。晶粒细化能显著提升材料的综合性能。开展对细晶钛材绝热剪切变形的研究,将能够更好挖掘钛材的动态性能,为制备高强高韧的钛合金提供科学依据。本文以层片状组织TC4钛合金和细晶TA2为研究对象,利用分离式霍普金森压杆(SHPB)在帽型样品上进行可控动态加载实验获取绝热剪切带(ASB),运用多向压缩(MAC)的方法细化晶粒获取细晶TA2。结合金相显微镜(OM)、电子背散射衍射(EBSD)和透射电镜(TEM)的方法探索了TC4钛合金和细晶TA2的绝热剪切变形特征(包括ASB中的微观结构特征,微观演化过程和相变规律),并对TA2的晶粒细化过程和细化机制加以阐明。结果表明：层片状组织TC4钛合金绝热剪切变形时间约为60μs,绝热温升达到1500K,超过其再结晶温度和相变温度。产生的ASB中心区域是由低位错密度的细小等轴晶粒组成,α-Ti相和片状α"-Ti相混合共存,剪切带内发生了相变,光学显微镜下的白亮带是剪切带内相变所致；ASB中形成了再结晶织构；剪切带内等轴晶粒晶界是大角度的几何相关晶界(GNBs)；ASB内细小等轴晶粒的形成是动态再结晶作用的结果。经过6道次的多向压缩变形,TA2的原始晶粒尺寸由25μm剧烈细化到0.2μm；孪生在晶粒细化过程中起主导作用,其细化机制为孪生诱导动态再结晶机制。细晶TA2的绝热剪切变形时间约为50μs,ASB内的绝热温升达到867K,超过其再结晶温度。剪切带中心是由晶粒大小约为50nm的细小等轴晶粒和100-150nm的融合晶粒组成,部分纳米晶粒发生了晶粒长大过程；ASB内等轴晶粒的形成和长大是晶界旋转作用的结果,它是以机械力作用为主和以热作用为辅共同驱动完成的。更多还原

【Abstract】 Abstract:Titanium and titanium alloy have excellent properties, such as high strength, good corrosion resistance, etc. Grain refinement can significantly enhance the properties of materials. The study of adiabatic shearing in ultrafine grained titanium can explore the dynamic property and provide scientific evidence for preparing high strength and high toughness titanium alloy.The article took TC4alloy with lamellar microstructure and ultrafine grained TA2as study object. Split Hopkinson Pressure Bar (SHPB) technique was carried out to obtain adiabatic shear band (ASB). The ultrafine grained titanium with uniform grain size was prepared by multi-axial compression (MAC). The deformation characteristic (the microstructure characteristics of ASB, the microstructure evolution and phase transformation) of adiabatic shearing was investigated by means of Optical Microscope (OM), Electron Back-Scattered Diffraction (EBSD), and Transmission Electron Microscopy (TEM). The grain refinement evolution and mechanism was also explained. The result shows that:The shear deformation time in TC4alloy was60μs. The temperature rise in ASB was about1500K, which is high enough to meet the need of recrystallization and phase transformation. The core of ASB in TC4alloy was consisted of very fine equiaxed grains with low density of dislocations, a-phase grain and α"-phase grain with plate structure were coexisted in the shear band. Phase transformation was occurred in ASB. The white band observed in ASB was the result of phase transformation. Recrystallization microtextures were formed within ASB and the grain boundaries within ASB were geometrically necessary boundaries (GNBs) with high-angles. It was suggested that the dynamic recrystallization was responsible for the formation of fine equiaxed grains in ASB.The coarse initial grains with the size about25μm in TA2were rapidly reduced to about0.2μm after six passes. The mechanical twinning played a significant role in grain refinement. It was suggested that the mechanical twinning inducing continuous dynamic recrystallization was responsible for the formation of ultrafine grained microstructure.The shear deformation time in ultrafine grained TA2was50μs. The temperature rise in ASB was about867K, which is high enough to meet the need of recrystallization. The central region of ASB in ultrafine grained TA2was consisted of a number of equiaxed grains0.05-0.08μm in diameters with low dislocations density and scattered grains0.1-0.15μm in diameters. Some little equiaxed grains in ASB have grown up. The formation of the nanosized new grains and their growth up in the ASB of ultrafine grained TA2processed by MAC during the deformation process is the result of the grain boundaries rotation supported by the mechanical force and assisted by heat effect.更多还原