Lattice instability and room temperature hyper-diffusion in metastable phase-transforming alloys

非技术概述形状记忆合金（SMA）是在加热时可以恢复到预先编程的形状的金属。 这种由晶体结构的可逆变化驱动的能力使它们成为变形飞机，可部署医疗植入物和固态致动器的候选人。 除了独特的机械响应之外，一些SMA中的原子在室温附近的移动速度比当前扩散模型预测的要快100亿倍。 结果，当在室温下不受干扰时，合金在数周至数年的时间内自发地变得更强，但延展性也更差。 该项目提出了一种新的理论，基于晶体结构的不稳定性引起的原子之间的键合局部减弱，从而降低了晶体内原子沿沿着某些方向运动所需的能量。 该奖项支持理论驱动的实验方法，增强了对晶体固体中扩散的基本理解。 该项目的更广泛的影响包括为本科生，K-12学生，特别是那些来自代表性不足的少数民族和背景的学生提供研究经验，通过对科技展览和创客空间的贡献进行社区宣传，以及与设计和建筑学院合作使用SMA技术的“活雕塑”艺术项目。 从该项目中获得的基本理解可能导致一类新的可编程材料，其性能随时间变化可控，在几个月到几年的过程中。技术摘要本项目旨在确定超快速低温扩散的起源观察到的一些形状记忆合金（SMA），这是不能用目前的扩散模型来解释。 所获得的知识将建立一个新的模式，在结晶固体相不稳定性和晶格软化的基础上增强扩散的基本理解。 这项工作提出了一个假设的基础上减少的激活能的空位为基础的扩散由于取向相关的晶格软化的不稳定的奥氏体。 该假设将通过非弹性X射线散射和电子显微镜在单晶模型中进行实验测试。 它也将解释如何快速的室温成核和粗化的沉淀物可以在一些合金，尽管远离他们的熔点。 在基础科学层面，该项目提供了关于β-钛形状记忆合金中声子色散关系的信息，并确定了软声子模式及其与室温下相变和异常扩散的关系。 在应用科学和工程水平上，拟议的工作将澄清SMA中室温老化效应的起源，并确定可以消除这种不稳定性的策略。从教育和推广的角度来看，该项目将通过一个利用SMA形状变化特征的活雕塑项目，吸引科学和艺术交叉点的学生，该奖项反映了NSF的法定使命，并通过使用基金会的智力价值进行评估，被认为值得支持和更广泛的影响审查标准。

Non-technical SummaryShape memory alloys (SMAs) are metals that can return to a pre-programmed shape upon heating. This ability, which is driven by a reversible change in the crystal structure, makes them capable candidates for morphing aircrafts, deployable medical implants, and solid-state actuators. Beyond the unique mechanical response, atoms in some SMAs inexplicably move up 10 billion times faster near room temperature than predicted by current diffusion models. As a result, the alloy spontaneously grows stronger but also less ductile over a period of weeks to years when left undisturbed at room temperature. This project proposes a new theory for this unexplained behavior based on locally weakened bonding between atoms caused by instabilities of the crystal structure, which lowers the energy required for atomic motion along certain directions within the crystal. The award supports a theory-driven experimental approach that augments the fundamental understanding of diffusion in crystalline solids. Broader impacts of the project include research experience for undergraduate students, K-12 students, and especially those from underrepresented minorities and backgrounds, community outreach through contribution to science and technology exhibitions and makerspaces, and a “living sculpture” artistic project using SMA technology in collaboration with the school of design and architecture. Fundamental understanding gained from the project could lead to a new class of programmable materials whose properties change controllably with time over the course of months to years.Technical SummaryThis project seeks to identify the origins of ultra-fast low-temperature diffusion observed in some shape memory alloys (SMAs), which cannot be explained with current diffusion models. The knowledge gained will establish the basic understanding for a new mode of enhanced diffusion in crystalline solids based on phase instability and lattice softening. This work proposes a hypothesis based on reduction of activation energy of vacancy-based diffusion due to orientation-dependent lattice softening of the unstable austenite. The hypothesis will be tested experimentally in a single crystal model through inelastic x-ray scattering and electron microscopy. It will also explain how rapid room-temperature nucleation and coarsening of precipitates can be possible in some alloys despite being far from their melting points. At the basic science level, the project yields information on the phonon dispersion relations in beta-titanium shape memory alloys, and identifies soft phonon modes and their relationships with both phase transformation and abnormal diffusion at room temperature. At an applied science and engineering level, the proposed work will clarify the origins of the room temperature aging effect in SMAs and identify strategies by which such instability can be eliminated. From an education and outreach perspective, the project will engage students at the intersection of science and the arts through a living sculpture project that utilizes the shape-changing characteristics of SMAs, and introduce advanced materials to designers and architects as a new set of tools in their toolbox.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.