CAREER: Heterogeneous Nucleation in Reversible Martensitic Transformations

非技术综述：一些固体经历了可逆的相变，这涉及到原子的重新排列和键的扭曲，导致材料的形状和性质的变化。这一行为可用于开发紧凑高效的执行器、传感器和各种微电子设备。结构转变始于材料中的缺陷或缺陷附近，这有助于原子的重新排列。然而，当考虑小体积材料的转变时，遇到这种罕见缺陷的可能性很低，导致不可预测和低效率的操作。在极端情况下，这可能会完全阻止转换的发生。这个项目的目的是了解缺陷在启动结构转变中所起的作用，并找到一种将这种缺陷引入材料的方法。这将允许设备在微米和纳米长度尺度上操作。这项提议的更广泛的目标包括开发可分布的交互式材料科学实验(“盒子里的实验室”)，这些实验可以远程进行，以吸引远程学生。此外，这项提案将通过有针对性的UG研究机会和与拟议研究相关的具有社会影响力的工程设计挑战，吸引本科生，特别是社区大学的转学学生。技术摘要：无扩散固态相变通常由晶格中稀疏的、高能原子尺度的缺陷形成。材料科学中的重大挑战之一是了解如何利用材料的异质性和无序性来影响材料的性能。特别是，虽然马氏体相变(MTS)中异质形核的位错缺陷模型已经相当成熟，但对于点缺陷簇的作用却知之甚少，尽管最初的观察表明辐照诱导的缺陷簇可以作为有效的形核中心。这项研究的结果将推动MTS成核科学的发展：1)通过测量作为热力学驱动力的函数的自然成核位的数密度，并确定晶格兼容性在控制依赖于尺寸的成核行为中的作用，以及2)通过实验量化辐照诱导的缺陷团簇和He纳米气泡的效力。这些观察结果将被用来开发和测试基于缺陷簇的MT成核模型。这项提议的教育结果将影响UG工程教育目前正在进行的两个范式转变：1)更广泛地采用主动学习方法，以及2)有效和可扩展的远程教育方法。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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:Some solids undergo reversible phase transformations, which involve the rearrangement of atoms and the distortion of bonds, resulting in changes to the shape and properties of a material. This behavior can be used to develop compact and efficient actuators, sensors, and a variety of microelectronic devices. Structural transformations start near a defect or imperfection in a material, which facilitates the rearrangement of atoms. However, when considering transformations in small volumes of material, the probability of encountering such a rare defect is low, resulting in unpredictable and inefficient operation. In extreme cases, this can stop the transformation from occurring entirely. This project's aims are to understand the role that defects play in initiating structural transformations, and to find an approach to introduce such defects into a material. This would allow the operation of devices at micrometer and nanometer length-scales. The broader objectives of this proposal include developing distributable interactive materials science experiments ('lab in a box') that can be performed remotely to engage distance students. Additionally, this proposal will engage undergraduate (UG) students, and in particular transfer students from community colleges, through targeted UG research opportunities and through societally impactful engineering design challenges related to the proposed research.Technical Summary:Diffusionless solid-state phase transformations generally nucleate from sparse, high energy atomic-scale defects in the crystal lattice. One of the grand challenges in materials science is understanding how material heterogeneity and disorder can be harnessed to influence the properties of materials. In particular, while dislocation defect models of heterogeneous nucleation in martensitic transformations (MTs) are fairly well-developed, the role of clusters of point defects is very poorly understood, despite initial observations that irradiation induced defect clusters can serve as potent nucleation sites. The outcomes of this study will advance the science of nucleation of MTs 1) by measuring the number density of native nucleation sites as a function of thermodynamic driving force, and determining the role of lattice compatibility in governing size-dependent nucleation behavior, and 2) by experimentally quantifying the potency of irradiation-induced defect clusters and He nanobubbles. These observations will be used to develop and test a defect-cluster based MT nucleation model. The educational outcomes of this proposal will impact two paradigm shifts currently underway in UG engineering education: 1) broader adoption of active learning methods, and 2) effective and scalable approaches to distance education.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.