Probing Microstructure-Martensitic Transformation Couplings in Metamagnetic Shape Memory Alloys

非技术概述：形状记忆合金（sma）由于施加力和/或施加温度的变化，其原子结构的重新排列介导了可逆的形状变化。某些类型的sma也对磁场有反应，在某些情况下，可逆的形状变化伴随着材料磁性质的急剧变化。这些sma被称为“超磁性sma”——在这种情况下，“超”表示“超越”传统的磁性行为——机械和磁场之间非常强的耦合使它们成为传感器、执行器甚至固态制冷的理想选择。除了它们的实际应用之外，超磁性sma是一种迷人的材料，因为它们的形状变化行为对其结构的微小变化非常敏感，而这些变化又可以通过不同的加工技术来改变。虽然许多研究小组提出了热处理诱导超磁性sma行为变化的不同机制，但大多数解释依赖于合金整体性能的变化，例如原子构型的总体有序/无序程度。pi和其他人最近的工作表明，更多的局部效应（即微观结构）可能起着意想不到的作用。该奖项支持实验/理论相结合的努力，旨在阐明微观结构演变对这些系统响应的作用。所获得的知识可用于提高对基于sma的超磁性器件行为的控制程度。该奖项还支持本科生（UG）和研究生在最先进的实验和计算材料技术的培训。PI和Co-PI目前领导一个多功能材料REU项目，该项目招收的学生将由参与该项目的研究生或与元磁性sma相关的本科生研究项目指导。此外，参与该项目的博士生将通过注册由PI指导的跨学科研究生培训计划（D3EM）接受材料科学，信息学和设计方面的培训。在这个项目中产生的研究元素也将作为案例研究在德克萨斯A&amp；M计算材料科学暑期学校，由PI共同组织超过八年。技术概述：PIs提出了一个实验/理论方案来阐明基于nimni的超磁性形状记忆合金中马氏体相变（MT）的潜在微观结构机制和相应的长度尺度效应。超磁性sma表现出广泛的复杂现象，这是由于微观结构、构型无序和多种相变（有序、铁磁、铁弹性/马氏体）之间的相互作用。pi最近的工作表明，NiCoMnIn合金显示出复杂的，非线性的，非单调的依赖于MTs，以及它们的抑制和重新出现，对小的成分变化和热处理。然而，这种行为不能用全局（热力学）自由度的变化来解释，例如空位浓度或总体有序/无序度。本项目的基本假设是中尺度微观结构特征与超磁性sma相变行为之间存在非常强的联系。该项目的主要目标是通过实验和模拟的紧密结合，建立对基于nimni的超磁性sma中多重相变的复杂化学和热处理依赖性的潜在微观结构机制的基本理解。先进的合成和表征将结合最先进的多尺度计算材料科学框架，以了解微观结构特征与马氏体相变之间的联系。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-Technical Summary: Shape Memory Alloys (SMAs) undergo reversible shape changes, mediated by re-arrangements in their atomic structure, as a result of the application of forces and/or by imposing changes in temperature. Some types of SMAs also respond to magnetic fields and in some cases the reversible shape change is accompanied by dramatic changes in the magnetic properties of the material. These SMAs are known as meta-magnetic SMAs- 'meta' in this case indicates 'beyond' conventional magnetic behavior- and the very strong coupling between mechanical and magnetic fields make them ideal for sensors, actuators, and even solid-state refrigeration. Beyond their practical applications, meta-magnetic SMAs are fascinating materials because their shape-change behavior is extremely sensitive to minute changes in their configuration, which can in turn be changed by different processing techniques. While many groups have proposed different mechanisms by which heat treatments induce changes in the behavior of meta-magnetic SMAs, most explanations rely on changes to global properties of the alloys, such as the overall degree of order/disorder in their atomic configuration. Recent work by the PIs and others suggest that more local effects (i.e., microstructure) may play an unexpected role. This award supports a combined experimental/theoretical effort that seeks to elucidate the role of microstructure evolution on the response of these systems. The knowledge gained can be used to increase the degree of control over the behavior of meta-magnetic SMA-based devices. This award also supports the training of undergraduate (UG) and graduate students in state-of-the-art experimental and computational materials techniques. The PI and Co-PI currently lead an REU program on Multi-functional Materials and students recruited into this program will be mentored by graduate students involved in the project or undergraduate research projects related to meta-magnetic SMAs. Moreover, the PhD students involved in the project will receive training in materials science, informatics and design through enrollment in an interdisciplinary graduate training program (D3EM) directed by the PI. Elements of the research produced in this project will also be used as case studies at the Texas A&M Computational Materials Science Summer School, co-organized by the PI for over eight years.Technical Summary: The PIs propose an experimental/theoretical program to elucidate the underlying microstructural mechanisms and the corresponding length scale effects on the martensitic transformation (MT), in NiMnIn-based meta-magnetic Shape Memory Alloys. Meta-magnetic SMAs exhibit a wide range of complex phenomena resulting from the interplay between microstructure, configurational disorder and several phase transitions (ordering, ferromagnetic, ferroelastic/martensitic). Recent work by the PIs has demonstrated that NiCoMnIn alloys show complex, non-linear, non-monotonic dependence of the MTs, as well as their suppression and re-appearance, on small compositional changes and heat treatments. This behavior, however, cannot be explained by changes in global (thermodynamic) degrees of freedom, such as, vacancy concentration or overall degree of order/disorder. The underlying hypothesis of this project is the existence of a very strong connection between mesoscale microstructural features and the phase transformation behavior of meta-magnetic SMAs. The main goal of this project is to establish a fundamental understanding of the underlying microstructural mechanisms responsible for the observed complex chemistry and heat treatment dependence of multiple phase transitions in NiMnIn-based meta-magnetic SMAs through the close coupling of experiments and simulations. Advanced synthesis and characterization will be combined with state-of-the-art multi-scale computational materials science frameworks to understand the connection between microstructural features and the martensitic transformation.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.