Collaborative Research: Coupled Explicit Thermodynamics of Plasticity - An Innovative Model for Twinning Crystals

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Most metallic materials are polycrystalline in nature and behave as aggregate composites of single crystals as the basic building block. Metallic materials generally deform elastically by atomic bond distortion or plastically by atomic bond rearrangement. Plastic deformation takes place by the mechanisms of dislocation motion and deformation twinning. Dislocations are atom sized features which move by applied stress and have been relatively well studied and are described by many advanced theories. Deformation twins are single crystal sized features and are not well understood or represented by theories of deformation. This award supports fundamental study of deformation twinning and overcoming limitations to modeling of this plasticity mechanism. The new knowledge and computational capability will have a broad range of applications to advanced manufacturing, structure survivability, and transportation. In addition, the project will support the education of graduate and undergraduate students in the areas of computational micromechanics, materials science, and crystal plasticity. The collaborative nature of the award will also afford the students the opportunity to engage across the two campuses.Plasticity in metallic materials is mediated by both dislocation slip and deformation twinning. The disparity in characteristic length between the two mechanisms is a limitation to crystal plasticity finite element simulation of polycrystals as deformation twins cannot be represented by volume average. The objective of this project is to develop a computational framework for the explicit representation of deformation twins based upon an embedded weak discontinuity technique. The embedded weak discontinuity is driven by single crystal theory which accounts for twin nucleation, propagation, and growth within a consistent thermodynamic and physical basis. Dislocation slip will also be represented with a thermally activated based theory and will allow slip to occur in both twinned and untwinned regions. The interaction of dislocations with the twin boundaries will also be modelled during the twin growth phase. This project will focus upon the model material of high-purity titanium. Validation experiments will be conducted on large grain samples to allow for detailed characterization. These deformation experiments will be conducted at different initial temperatures and sample temperatures will be measured during deformation to evaluate the model prediction of the Taylor-Quinney factor.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.