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

技术：这个变革性的实验/理论研究项目将调查铁-镓(Fe-Ga)和铁-铝(Fe-Al)合金的伸展或负泊松比行为的起源，这两种合金的测量结果分别为-0.78和-0.45，沿[110]晶体取向。系统的实验制备和表征，加上广泛的密度泛函和分子动力学模拟，将使我们能够预测和优化成分、原子排列、制造条件和测试程序，以合理控制这些金属间化合物的泊松比和弹性常数。这项研究的学术价值在于应用了对弹性常数如何相互作用和产生伸长性的理解，以及1)先进的计算和理论工具来模拟成分定制的二元Fe-Ga和Fe-Al合金，以及2)实验表明，由于这两个二元合金系统的延展性和相对简单性，非常适合于模型验证，并提供了可以指导建模工作的实验结果。成功的结果将是了解原子放置和原子间作用力是如何结合在一起导致金属的这种不同寻常的性质的，这一见解将有助于开发定制的合金性能特征。将对不同成分和处理的单晶Fe-Ga和Fe-Al样品进行研究。PIS将进行拉伸测试并测量其他物理性能(如磁化强度、磁各向异性和光学光谱)，以便在受控温度、磁场、载荷和应变率的情况下与理论计算进行比较。通过密度泛函计算研究了不同类金属分布模式的相图和相稳定性，并直接确定了弹性常数和泊松比。通过对这两个系统的比较研究，可以揭示控制金属间合金中泊松比的关键因素。非技术性：该研究项目的成功还将在MEMS、形状记忆和能量转换等材料研究领域产生强大的商业影响。这两种合金都可以溅射和电沉积，通过成分控制负泊松比来定制不寻常的结构响应的能力表明，新型MEMS器件在工业和国防应用中具有巨大的潜力。教育影响将包括培训两名研究生，他们将接触到理论和实验研究，尽管他们位于不同的海岸，但将在整合跨学科研究进展方面密切合作。除了在适当的时候访问彼此的校园外，PI还计划在夏季学期访问Ames Lab，在那里我们的合作者将向学生展示他们正在学习的合金生长和表征的基础知识。学生将接触到现代理论和实验材料研究的几乎所有方面。此外，研究成果将极大地丰富由联合首席调查员开发的课程“物理和化学中的计算方法”的课程，该课程非常适合在PI的实验室工作的明确的本科生研究经验。将进行几个本科生项目来补充研究生的研究。PI参与地区社区大学和高中学生的外展和招生，并通过NSF REU计划和NSF赞助的科学与工程研究实习(RISE)计划(RISE)指导本科生研究项目，具有丰富的经验。

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.