Constitutive Equations for Process and Product Design of Lightweight HCP Metals

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9610130 Anand Whatever the mode of' transportation, materials used in construction of vehicles need to combine -good structural stiffness. strength, and toughness with high resistance to corrosion, and they must be as lightweight as possible in order to save fuel. Aluminum alloys have long been used for skins and frames of aircraft. and are beginning to be used in the automotive industry. Two other important light metals for the transportation industry ire titanium and magnesium. The recent progress in the development of a mathematical theory of polycrystalline plasticity which predicts not only the anisotropic macroscopic stress-strain response and shape changes, but also the evolution of crystallographic texture during deformation, has occurred primarily for materials with face-centered-cubic (fcc) crystals. Much less progress of this type has occurred for materials with hexagonal-close-packed (hcp) crystal structure. Compared with fcc materials, the hcp materials exhibit more complex modes of plastic deformation due to their lower symmetry. Inelastic deformation resistances of different slip systems can be substantially different in the hcp materials, and unlike the fcc aluminum alloys, the hcp titanium and magnesium alloys exhibit deformation twinning. This causes pronounced anisotropies in their macroscopic deformation characteristics, and these anisotropies need to be properly accounted for in developing a robust computational capability for 1. Process Design - deformation processing, and 2. Product design - improved structural response of components, made from titanium and magnesium1. Of particular importance is the development of a fundamental understanding of the inelastic deformation behavior of these materials by slip and twinning, and the development of accurate elasto- viscoplastic constitutive equations which describe this deformation behavior. We propose to develop anisotropic, elasto-plastic constitutive equations and computational procedures for modeling and simulation of inelastic deformation due to both crystallographic slip ,and twinning in hcp titanium and magnesium. The computational capability will be useful in simulating the development of anisotropy due to the evolution of crystallographic texture. The mathematical models and procedures that we propose to develop should be useful in the design of a variety of deformation-processing operations, and the design of components for structural performance. _______________________________ 1 Although the majority of processing of magnesium allows is done by casting, magnesium is also wrought to form tubes, plates, and sheet.