In this project, the PIs advance understanding of the mechanisms that lead to grain-boundary mobility differences, texture development and abnormal growth of grains with a preferred texture/grain orientation in polycrystalline metals. They will use this understanding to develop low cost yet high-performance polycrystalline auxetic and/or magnetostrictive materials that are single-crystal-like. Their approach for this research will combine multiple-scale computational simulations, theoretical models and synergistically integrated insights gained from quantitative experimental studies of recrystallization, grain growth and texture development in rolled sheets of Fe-Ga (Galfenol) and Fe-Al (Alfenol) based binary and ternary alloys. Experiments will be used to explore relationships between anneal protocols and surface energy, grain mobility and texture development. Their hypothesis is that surface energy differences are the dominant driving force underlying the ability to selectively develop a grain structure and texture that is single-crystal-like. This hypothesis will be investigated by creating thermodynamic models of grain growth and texture development. In parallel, first-principle-based computational simulations and experimental studies of Galfenol and Alfenol will be conducted to aid in model formulation and validation, and to identify binary and ternary iron alloys with properties that should impart high auxeticity and/or magnetostriction in materials for which surface energy can be used to promote anisotropy development. This research is aligned with the SusChEM initiative through developing methods for processing magnetostrictive alloys that allow earth-abundant, inexpensive and benign chemicals, e.g. Al, Co, Ga, Mn and Sn, to be used as a replacement for expensive critical materials, the rare-earth elements such as Tb and Dy that comprise ~33at.% of Terfenol-D. The PIs will introduce a method for determining the surface energy of metal grains with a specific crystallographic orientation by tracking the contact angle of a drop of liquid gallium on grain surfaces of known orientation. This method overcomes shortcomings of existing methods, such as water-drop methods, that work for glass and polymeric surfaces with low surface energy and high-temperature destructive and/or creep-based methods that work for amorphous solids and samples for which isotropy is a reasonable approximation (e.g. highly polygranular samples).
NON-TECHNICAL SUMMARY: This research will lead to the understanding needed to achieve the performance capabilities of costly single-crystal alloys in low-cost polycrystalline alloys. Models of atomic structure and energy-based models of crystal growth processes will be used to gain insights into how to control and target the selective growth of desired crystals at the expense of crystals with less favorable mechanical and/or magnetostrictive properties. The iron-aluminum and iron-gallium alloys that are one focus of this project have been targeted because of preliminary results that suggest they are good candidates for a sustainable alternative to magnetostrictive alloys used in industrial and defense applications that contain rare-earth elements like Terbium and Dysprosium. This research aligns well with the need for advances in the development of sustainable materials, as it focuses on methods for processing magnetostrictive alloys that allow earth-abundant, inexpensive and benign chemicals to be used as a replacement for expensive critical materials, the rare-earth elements that are both significantly more costly and significantly less abundant in the Earth's crust. The iron-aluminum and iron-gallium alloys to be studied are highly-auxetic, a mechanical property that is generally found in polymers but rarely in metals. The potential for high industrial impact of a structural auxetic alloy exists, as studies of non-structural auxetics (i.e. polymers) indicate that auxeticity can be used to enhance resistance to fracture and indentation. This research project will also support the training of postdoctoral, graduate and undergraduate students in modeling and processing anisotropic, rare-earth-free, single-crystal-like materials as well as in developing a new method for the measurement of surface energies of anisotropic solids. Students will disseminate research results in journal publications, conference papers and presentations. The team will mentor underrepresented (minority and women) high-school, undergraduate and graduate students under this project. The PIs will both continue to engage in on-going outreach to K-12 students.
