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

非技术概述形状记忆合金(SMA)是一种加热后可以恢复到预编程形状的金属。这种能力是由晶体结构的可逆变化驱动的，使它们能够成为变形飞机、可展开医疗植入物和固态执行器的候选材料。除了独特的机械反应外，一些SMA中的原子在室温附近莫名其妙地以比当前扩散模型预测的100亿倍的速度向上移动。因此，当在室温下不受干扰时，合金会自发变得更坚固，但在几周到几年的时间里延展性也会降低。该项目为这种无法解释的行为提出了一种新的理论，该理论基于晶体结构不稳定引起的原子之间局部减弱的成键，这降低了原子在晶体内沿某些方向运动所需的能量。该奖项支持一种理论驱动的实验方法，该方法增强了对晶体固体中扩散的基本理解。该项目的更广泛影响包括为本科生、K-12学生，特别是那些代表人数较少的少数群体和背景的学生提供研究经验，通过对科学和技术展览和制作空间的贡献进行社区推广，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会导致一种新的可编程材料，其性能在几个月到几年的过程中随着时间的变化是可控的。技术概述该项目试图确定在一些形状记忆合金(SMA)中观察到的超快低温扩散的起源，这是目前的扩散模型无法解释的。所获得的知识将为基于相不稳定和晶格软化的晶体固体中增强扩散的新模式建立基本的理解。基于不稳定奥氏体晶格取向软化导致空位基扩散激活能降低的假设。这一假说将在单晶模型中通过非弹性x射线散射和电子显微镜进行实验验证。它还将解释一些合金在远离熔点的情况下，如何在室温下快速成核和粗化析出物。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的相变和异常扩散的关系。在应用科学和工程水平上，这项拟议的工作将澄清SMA中室温老化效应的根源，并确定消除这种不稳定性的策略。从教育和外展的角度来看，该项目将通过一个活生生的雕塑项目吸引学生进入科学和艺术的交叉点，该项目利用SMA的形状变化特征，并向设计师和建筑师介绍先进材料作为他们工具箱中的一套新工具。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 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.