Determining the Origin of the Highly Auxetic Behavior of Iron-Based Alloys

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TECHNICAL: This transformative experimental/theoretical research project will investigate the origin of the auxetic or negative Poisson's ratio behaviors of iron-gallium (Fe-Ga) and iron-aluminum (Fe-Al) alloys, which have been measured at values of as low as -0.78 and -0.45, respectively along a [110] crystallographic orientation. Systematic experimental fabrication and characterization, along with extensive density functional and molecular dynamics simulations will allow us to predict and optimize compositions, atomistic arrangements, fabrication conditions and test procedures for rational control of Poisson's ratio and the elastic constants of these intermetallic alloys. The intellectual merit of the research lies in applying the understanding of how elastic constants interact and produce auxeticity together with 1) advanced computational and theoretical tools for modeling binary Fe-Ga and Fe-Al alloys of tailored compositions, and 2) experiments that because of the ductility and relative simplicity of the two binary alloy systems are well suited for model validation and also for providing experimental results that can guide the modeling efforts. The outcome when successful will be learning how atomic placement and inter-atomic forces combine to result in this unusual property for a metal, an insight that will aid in developing tailored alloy performance characteristics. Single crystal Fe-Ga and Fe-Al samples with varying compositions and treatments will be studied. PIs will conduct tensile tests and measure other physical properties (such as magnetization, magnetic anisotropy, and optical spectra) for comparison with theoretical calculations under controlled temperature, magnetic field, load and strain rates. Density functional calculations will be performed to investigate the phase diagram and phase stability of different metalloid distribution patterns, and to directly determine elastic constants and Poisson's ratios. Comparative studies for these two systems allow disclosure of key factors that govern the Poisson's ratios in intermetallic alloys. NON-TECHNICAL: The success of this research project will also have a strong commercial impact in areas of materials research such as MEMS, shape memory and energy conversion. Both of these alloys can be sputtered and electrodeposited, and the ability to tailor unusual structural responses through compositional control of the negative Poisson's ratio suggests a huge potential for novel MEMs devices for industrial and defense applications. Educational impact will include training of two graduate students, who will be exposed to both theoretical and experimental research and who, although located on opposite coasts, will work closely on integrating research progress across disciplines. In addition to visiting each other's campus as appropriate, PIs have plans for both to visit Ames Lab during the summer term, where our collaborator will show the students the basics of growing and characterizing the alloys they are studying. Students will touch almost all the aspects of modern theoretical and experimental materials research. Additionally, research results will greatly enrich the curriculum of the course "Computational Approaches in Physics and Chemistry" developed by the co-Principal investigator, and it is ideally suited for well defined undergraduate research experiences working in the PI's lab. Several undergraduate projects will be undertaken to supplement the graduate student's research. The PI participates in outreach and recruiting of students at regional community colleges and high schools, and has extensive experience mentoring undergraduate student research projects through the NSF REU program and an NSF-sponsored Research Internship in Science and Engineering (RISE) Program run through or Women in Engineering Office.