Deformation Mechanisms of Gradient Steels with High Strength and Ductility

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Gradient metals are designed such that the physical properties change steadily with position. This has many benefits but none more than the ability to achieve enhanced strength and ductility at the same time. There are competing views regarding the mechanisms responsible for this phenomenon, but many of these views have not been validated and reconciled. This award will use a combination of novel in situ mechanical testing and modeling at various length scales to distinguish the various deformation mechanisms in gradient steel microstructures. Structurally gradient steels with high strength and ductility have broad applications in nuclear, petrochemical and automobile industries. The fundamental knowledge derived from this study may be generally applicable for the design of other strong and ductile structural metallic materials. The award will also support an extensive education and outreach plan, including the opportunity for graduate students to work with collaborators at a national laboratory, recruitment of minority undergraduate students as interns, and mentoring of middle and high school students in science fairs.The objective of this grant is to investigate, at a fundamental level, the influence of microstructure gradient on mechanical behavior of steels by integrating severe plastic deformation, in situ micromechanical testing, and crystal plasticity simulations. The goal is to design heterogeneous materials with significantly improved mechanical strength and ductility. The hypothesis is that significant grain coarsening of the elongated grain morphology arises from dynamic recrystallization and/or stress-driven grain boundary migration, and the switching between the two mechanisms should be temperature dependent. A further hypothesis is that the suppression of localized shear softening under the combined influence of deformation mechanisms and gradient microstructure evolution leads to increased strength and ductility. To test these hypotheses, a dislocation plasticity based theoretical framework combining plastic deformation, dynamic recrystallization, and stress-driven grain boundary migration will be developed to study gradient microstructures. The modeling will complement the investigation of the fundamental deformation mechanisms by in situ tension microscopy studies at various temperatures and length scales.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.