超细晶金属钛和锆的力学行为及变形机理研究

In the present research project, ultrafine grained (UFG) commercially pure titanium and zirconium fabricated by ECAP are chosen as the research materials. The mechanical behaviors and deformation mechanisms of these materials are systematically characterized in quasi-static and dynamic tension and compression, creep at room temperature, as well as fatigue limit and fatigue fracture mechanism are investigated under cycle loading. Effects of temperature, strain rate and the original microstructures including grain size, proportion of high-angle grain boundaries, dislocation pattern and texture on deformation behavior are investigated by the Hopkinson pressure bar, isothermal simulation, nanoindentation, tension creep and fatigue tests. Advanced characterization and measurement techniques such as high-resolution electron microscopy (HREM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction (XRD), and so on, are used to analyze microstructure evolution law such as dislocation density, dislocation distribution, grain boundary structure, slip bands, twinning and texture, to analyze fracture morphology and damage feature of fatigue and creep and to reveal microscopic mechanism in the deformation process. Based on crystal plasticity theory, physical metallurgy theory and experimental results, the physical model, constitution equation and fatigue life prediction model for the metal deformation are established. It is of important signification in basic theoretical research and practical application of UFG materials.

本项目以室温等径弯曲通道变形（ECAP）技术制备的超细晶金属钛和锆为实验材料，研究在静态和动态拉伸和压缩载荷下，循环载荷下的力学行为及变形机理；疲劳性能及断裂机理；室温蠕变性能及断裂机理。采用霍普金森杆实验技术、热模拟实验技术、微纳米压痕实验技术、疲劳实验技术和拉伸蠕变实验技术等，研究变形条件（加载方式、应变速率、变形温度等）和原始微观组织结构（晶粒尺寸、大角度晶界比例、位错组态和织构等）对实验材料力学行为的影响规律；采用HREM、EBSD、TEM、SEM、XRD等先进分析检测技术,观察分析变形组织结构（位错密度、位错分布、晶界结构、滑移（变形）带、孪晶和织构等）的演化规律，以及疲劳/蠕变断口显微形貌特征，揭示变形过程的微观机理。基于晶体塑性力学和物理冶金学相关理论、数值模拟及实验结果，建立相应变形的物理模型、本构方程及疲劳/蠕变寿命预测模型。项目实施对超细晶材料理论研究和应用有重要意义。

本项目采用复合加工（室温ECAP+旋锻）变形技术制备了超细晶金属钛和锆，研究了超细晶钛、锆的静态和动态压缩以及循环载荷下的力学行为及变形机理；疲劳性能及断裂机理；室温蠕变性能及断裂机理。采用HREM、EBSD、TEM、SEM、XRD等先进分析检测技术，观察分析变形组织结构演化规律，以及压缩、疲劳/蠕变(断口)显微形貌特征，揭示了变形过程的微观机理。研究结果表明：复合加工变形技术可制备出300nm以下、较为均匀的超细晶钛、锆组织。超细晶纯钛/锆准静态压缩真应力-真应变曲线呈明显的稳态流变特征。在变形开始阶段，应力随应变的增大而迅速升高，达到最大值后逐渐下降，最终进入稳态阶段。稳态流变应力随温度的升高而降低，随应变速率的增加而升高。纯钛/锆的应变速率敏感因子随组织的超细化而增大，纯锆的应变速率敏感因子范围为0.028-0.132，且低应变速率和高温有利于提高超细晶纯钛/锆的塑性。超细晶纯钛/锆的低周疲劳寿命较粗晶的明显增加，超细晶纯钛/锆的循环寿命是粗晶纯钛/锆的2～4倍，且总应变幅越小，疲劳寿命越高。粗晶的疲劳延伸指数大于超细晶的，而疲劳延伸系数则相反。超细晶纯钛/锆的条件疲劳极限有所提高，超细晶纯锆的条件疲劳极限是粗晶的1.7倍。超细晶纯钛的缺口实验表明，退火有利于降低超细晶纯钛的缺口敏感性。此外，超细晶纯钛/锆的疲劳变形机理主要为位错滑移，同时还伴随有孪生的发生。经退火处理后的超细晶纯钛/锆的稳态蠕变速率降低，蠕变变形量减小，蠕变寿命增加，抗蠕变性能提高。本项目的实施对超细晶纯钛/锆材料的理论研究和进一步应用具有重要意义。