Role of Deformation Twinning in Strain Hardening and Texture Evolution in Titanium: Experiments and Numerical Simulations

该项目旨在了解a-Ti及其合金中形变孪晶形成的物理机制，以及形变孪晶在a-Ti及其合金应变硬化响应中的精确作用。一个主要目标是开发稳健的晶体塑性模型和模拟工具，能够预测在一系列工业相关的冷加工过程中，这些金属在大塑性应变下的各向异性应力-应变响应的演变以及晶体织构的演变。实验工作包括实验和模型相结合的研究。实验工作包括各种变形路径下的大应变力学测试和变形路径的变化，并利用光学显微镜、扫描电子显微镜的取向图像映射、X射线织构研究和透射电子显微镜对变形样品进行了系统的表征。模拟工作构成了用于单个晶体中孪晶体积分数演化的适当本构函数的公式，以及用于滑移和孪晶硬化的本构描述，包括它们之间的耦合。从实验中阐明的物理学将指导这些本构函数的公式。建议的工作还包括通过直接比较晶体塑性模型对各向异性应力-应变响应以及晶体织构演化的预测与相应的测量结果来广泛验证模型。在晶体塑性模型中，将同时考虑泰勒模型和有限元多晶模型。材料的微观组织状态在变形过程中的形状和尺寸控制、所生产部件的结构完整性以及其在使用中的性能特征(包括机械、电和磁性能及其各向异性)方面起着主导作用。由于本研究中开发的工具可用于设计变形过程，以产生特定应用的最佳显微组织，因此它们可以显著改善各种金属部件的性能特征。这项研究将培养出两名在对美国金属加工业至关重要的技术领域拥有专业知识的博士，美国目前在这一领域落后于几个欧洲国家。此外，两名本科生将参与这项研究。这项研究有望对金属加工行业产生重大影响，因为它将提供预测工具，可以产生大量关于钛合金变形过程中组织演变的定量信息。钛和钛合金是一类重要的金属，在国防、航空航天、生物医学和体育用品工业中有许多商业应用。

The project is aimed at understanding the physics of the formation of deformation twins in a-Ti and its alloys and the precise role of deformation twinning in strain hardening response of the a-Ti and its alloys. A major goal is to develop robust crystal plasticity models and simulation tools that can predict the evolution of both the anisotropic stress-strain response as well as the crystallographic texture evolution in these metals subjected to large plastic strains in a range of industrially relevant cold-working processes. The experimental work involves a combined experimental and modeling study. The experimental work involves includes a range of large strain mechanical tests in a variety of deformation paths and deformation path changes, and a systematic characterization of the deformed samples using optical microscopy, orientation image mapping in the scanning electron microscope, X-ray texture studies and transmission electron microscopy. The modeling work constitutes formulation of appropriate constitutive functions for evolution of the twin volume fraction in the individual crystals, and constitutive descriptions for slip and twin hardening including the coupling between them. The physics elucidated from the experiments will guide the formulation of these constitutive functions. The proposed work also includes an extensive validation of the models by direct comparison of the crystal plasticity model predictions for both the anisotropic stress-strain response as well as crystallographic texture evolution against corresponding measurements. Both the Taylor-type models and the finite element polycrystal models will be considered in the crystal plasticity model. The microstructural state of the material plays a governing role in shape and size control during deformation processing, structural integrity of the produced component, and its performance characteristics in service (including mechanical, electrical and magnetic properties, and their anisotropy). Since the tools developed in this study can be used for designing the deformation process to yield optimal microstructures for a given application, they can lead to substantial improvements in performance characteristics of various metallic components. This study will produce two Ph.D.s with expertise in a technical field important to the US metal working industry, in which the United States is currently lagging behind several European countries. In addition, two undergraduate students will be participating in this research.The proposed study is expected to have a strong impact on the metal working industry, since it will provide predictive tools that can yield a great deal of quantitative information on microstructure evolution during deformation of Ti alloys. Titanium and titanium alloys constitute an important class of metals with many commercial applications in the defense, aerospace, biomedical, and sporting goods industries.