梯度微纳米金属极薄带剪/压复合变形下的强韧机理及尺寸效应

Under the tendency of microminiaturization of 3C products, there are strong needs to the metal foil with high quality in the field of electron, computer, medical treatment, energy and traffic. It makes research and development of foil rolling became a new hotspot. It is found that mechanical behavior of metallic materials with dimensions down to the micro- nanometer scale is very different from that of the corresponding bulk materials. The traditional deformation mechanism is not applicable. Based on previous studies, we have found that stronger shear deformation will induce the special evolution of microstructure and texture with shear deformation during asynchronous rolling and enhance strength and plasticity of material. BUT, the evolution mechanism is still unclear. The evolution of microstructure and texture induced by the shear-compressed deformation during asymmetrical rolling titanium will be investigate. Correlation of mechanical response characteristics under the coupling of multiple characteristic dimensions will be study through observation and analysis of microstructure evolution of material deformation. The fine grained titanium with different grain orientation and grain boundary orientation will be prepared by asymmetrical rolling. Microstructure morphology (grain morphology, grain size and distribution), twin boundary surface microstructure (twin, twin density, surface spacing) , grain orientation and grain boundary orientation are characterized by means of analysis technology. The different shear/compressive strain will be obtained by adjusting the key process parameters. The calculation formulas of geometric parameters，such as neutral angles and cross shear ratio in asymmetrical rolling will be found. The effect of different shear/compressive strain on microstructure, the type of texture and texture component are investigated. The relationship between shear-compressed strain microstructure, Different feature size and mechanical properties is obtained from analytical data. The grain size, grain orientation and grain boundary structure and the performance are compared. On the base of above studies, Strengthening and toughening mechanism and size effect of micro/nano structure foil during shear-compressed deformation will be clarified. The optimal process parameters are determined ultimately during asymmetrical rolling titanium. The project has significant theoretical and practical significance It provides theoretical and technical support for the improvement of the plastic deformation theory and preparation technology of ultra-fine crystal foil.

微电子元器件、微机电系统等尖端微制造领域的快速发展，对极薄带尺寸和性能提出更高要求。然而当特征尺寸（晶粒尺寸、孪晶层厚和带厚）减小至微纳米量级时，会呈现与宏观尺度性能相异的尺寸效应，传统的变形机制不再适用。本课题拟采用异步轧制引入强剪切变形制备极薄带并获得梯度微纳米结构，但在该特征尺度下的强韧机理和尺寸效应及其变形机制尚不清楚。通过对材料变形过程微观组织观察和性能测试，分析多个特征尺寸及耦合作用下尺度相关的力学响应特征的相互关系，明确尺寸效应特征及门槛值，统计多晶材料的晶粒、晶界和三叉晶界体积分数，探讨变形过程滑移、孪生、织构及晶界的相互作用，揭示搓轧区比例对梯度结构组织细化及性能的影响规律，明确梯度微纳米结构、特征尺寸及力学性能本质关系，在此基础上阐明剪/压变形下梯度微/纳米结构极薄带强韧机理及尺寸效应变形机制，为完善超细晶极薄带材的塑性变形理论和制备技术提供理论及技术支撑。

高精度金属极薄带材的需求日益增多，为了探寻轧制过程中其力学性能与微观组织的演变规律，本文主要通过机械轧制法制备出极薄钛带材，并通过数值模拟、纳米压痕、金属拉伸、X射线衍射、扫描电子显微镜以及电子背散射衍射等分析测试手段对极薄钛带材的力学性能及微观组织进行表征分析，为金属极薄带材的制备提供理论依据。本项目取得的主要进展如下：(1)利用有限元对极薄钛带材进行轧制模拟，根据弹塑性理论进行剪切应力的等效应变计算，结果表明随着累积变形量的增加，等效应变会逐渐增大,从20%累积变形量的0.582增至80%累积变形量的7.514。轧制过程中合适的润滑条件与辊凸度能够明显的降低粗糙度，改善表面质量，提高极薄钛带材尺寸公差精度；(2)在轧制变形过程中，滑移为异步轧制主要的塑性变形机制，其中｛11 ̅00｝<112 ̅0>柱面滑移为主，易于启动，协调塑性变形。同步轧制塑性变形机制由孪生向滑移转变，其中主要孪晶类型为{112 ̅2}<1 ̅1 ̅23>（压缩孪晶）与{101 ̅2}<101 ̅1>（拉伸孪晶）；(3)在极薄钛带材轧制过程中会出现HCP-Ti向FCC-Ti的相转变现象，并且FCC-Ti的相含量会随着累积变形量的增加而增加，由20%累积变形量下的0.56%增加至80%累积变形量下的5.23%。在宏观与微观织构观察下，随着累积变形量的增加，极薄钛带材主要织构类型为{0001}面平行于ND方向，<10¯10>平行于RD方向的柱面织构；(4)极薄钛带材拉伸性能会呈现出尺寸效应：经650℃/1h退火后，随着钛带厚度减薄，抗拉强度与延伸率都会出现越弱的特性，带材截面处呈现少/单层等轴晶。经轧制后极薄钛带材硬度显著提高，在80%累积变形量下极薄钛带材位移深度达到1000nm时所需载荷比原始带材提高57mN。极薄钛带材断裂行为从塑性断裂向解理断裂转变。