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

形状记忆合金（sma）是一种可以在加热时恢复到预先设定的形状的金属。这种能力是由晶体结构的可逆变化驱动的，使它们成为变形飞机、可展开的医疗植入物和固态致动器的候选材料。除了独特的机械反应之外，一些sma中的原子在室温附近的移动速度比目前扩散模型预测的要快100亿倍。结果，在室温下不受干扰的情况下，合金会在数周到数年内自然地变得更强，但延展性也会降低。该项目提出了一种新的理论来解释这种无法解释的行为，该理论基于晶体结构的不稳定性引起的原子之间的局部减弱键，这降低了原子沿晶体内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，增强了对晶体固体扩散的基本理解。该项目的更广泛影响包括为本科生，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，通过科学技术展览和创客空间的贡献进行社区外展，以及与设计与建筑学院合作使用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会导致一类新的可编程材料，其特性在几个月到几年的时间内随着时间的推移而可控地变化。本项目旨在确定在某些形状记忆合金（sma）中观察到的超高速低温扩散的起源，这无法用当前的扩散模型解释。所获得的知识将为基于相不稳定性和晶格软化的晶体固体中增强扩散的新模式建立基本理解。本工作提出了一个基于空位扩散的活化能降低的假设，这是由于不稳定奥氏体的取向依赖的晶格软化。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验证。这也将解释在某些合金中，尽管离熔点很远，析出物的室温快速成核和粗化是如何可能的。在基础科学层面，该项目获得了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相变和异常扩散的关系。在应用科学和工程水平上，提出的工作将澄清sma室温老化效应的起源，并确定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形状变化特性的活雕塑项目吸引学生参与科学和艺术的交叉点，并将先进的材料作为他们工具箱中的一套新工具介绍给设计师和建筑师。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 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.