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

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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.