Twin Nucleation and Migration - Modeling and Experiments

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This project is aimed at developing a hierarchical methodology for advanced materials design utilizing the most advanced first principles/mesomechanics tools combined with experiments. We plan to focus on bcc alloys such as Fe-Cr, Fe-Cr-Co and austenitic B2 shape memory alloys to develop our sequential multi-scale design approach. These alloys are of significant interest, but there has been no attempt to develop models to predict their deformation response from first principles. The deformation behavior of these alloys is characterized by significant twinning activity. A continuum twin nucleation model based on first-principle calculations will be developed. The research will determine how the energy barriers evolve and establish twin nucleation and twin migration stress levels, through consideration of the entire energy barrier transients, by carefully defining nucleation and migration phenomena. It is envisaged that confirmation of the model validity at the corresponding length scale by precisely measuring the local deformations associated with twinning including the twin shear strain. Observations of deformation are made at both the macro- and micro- levels using digital imaging techniques at multiple magnifications.The broad impact of the work is that our methodology will accelerate the design of advanced structural materials by avoiding the large test matrix approach and optimization trials. The mechanical response of bcc alloys have been predicted based on crystal plasticity models, which rely on a large number of experimentally determined constants. Overall, the emphasis on first principles/mesoscale mechanics combined with experiments is unique. In addition, there will be several outreach activities ranging from short courses to industry and advanced researchers, to learning modules with activity kits for K-12 students and their teachers, which will broaden the educational impact of the approach.