Elucidating the Impact of Nanoscale Strain and Concentration Fields on Martensitic Transformations in NiTiHf-based Shape Memory Alloys

摘要形状记忆合金（sma）是一类独特的能够恢复其原始形状的合金。这一特性使sma成为许多未来商业和工业技术（如医疗设备、传感器、执行器和航空航天飞行器组件）的主要竞争者。这些金属的形状恢复依赖于原子排列的可逆变化。设计和改造它们的原子结构可以从根本上改变它们的形状记忆性能。在sma中产生被称为沉淀的纳米级颗粒，为改变其局部原子结构和化学性质提供了直接途径，从而实现了目标形状记忆特性。然而，由于缺乏高精度、实时的测量，准确地将纳米析出物的特征和性质与形状记忆性能相匹配仍然是一个挑战，这为建立各种应用的可靠合金设计创造了障碍。pi通过开发原子分辨率，原位电子显微镜技术来定量测量沉淀对sma形状记忆行为的影响，从而解决了这一挑战。获得的知识可以为需要高温操作和改进机械性能的沉淀工程sma应用生成强大的合金设计规则。此外，pi之间的合作研究活动将与教育项目和推广活动相结合，包括为来自当地公立学校的少数族裔高中生提供暑期实习，针对在校学生和远程学习学生（如工业和军事）的课程开发，以及对研究生和本科生的研究培训，重点是合金设计和材料特性。这些教育计划提高了学生对新技术所需关键材料的认识，并鼓励不同背景的学生从事STEM领域的职业。技术概述：在形状记忆合金（SMAs）中，析出相的力学和化学效应在控制马氏体相变（MT）动力学中起着至关重要的作用。然而，明显缺乏系统和定量的实验证据，可以支持或测试由微观结构内非转化共格沉淀修饰的MT物理机制的理论模型。本实验计划旨在量化和阐明由相干析出相诱导的纳米应变场和浓度场对nitihf基sma相变和力学性能的影响。具体来说，pi试图揭示和分离Heusler相和Han相沉淀（当它们共存时）的作用，它们在nitihf基sma中具有明显不同的晶体结构和化学性质。为了实现这一目标，pi在原子尺度材料表征方面利用了一系列独特的专业知识，这些专业知识基于像差校正电子显微镜、电子衍射和原位加热应用，以及合金设计/合成和热机械性能表征。该计划的重点是：(i)开发合金合成路线，允许在控制微观结构和化学性质的情况下共析出Heusler相和Han相；（ii）利用原子分辨电子显微镜、光谱学和扫描电子衍射对析出物周围的应变场和浓度场进行量化，以揭示它们对基体微观形貌的影响；（iii）利用原位温控实验表征沉淀强化sma中的MT机制和相变途径，以确定MT对设计的析出物特性（即应变场和浓度场）的依赖。这项工作为未来的SMA设计和计算建模提供了坚实的基础，提供了对SMA结构-组成-性能关系的定量、全面的理解，以及它们对实际服役环境的动态响应。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。

AbstractNON-TECHNICAL SUMMARYShape memory alloys (SMAs) are a unique class of alloys that can recover their original shape. This feature makes SMAs a leading contender for many future commercial and industrial technologies such as medical devices, sensors, actuators, and components for aerospace vehicles. Shape recovery in these metals relies on a reversible change in atomic arrangements. Designing and engineering their atomic structures can radically change their shape memory performance. Producing nano-sized particles, called precipitates, in SMAs offers a direct route to modifying their local atomic structure and chemistry, thereby achieving targeted shape memory properties. However, accurately pairing the characteristics and properties of nano-sized precipitates to shape memory performance remains a challenge due to a lack of high-precision, real-time measurements, which creates a roadblock to establishing a reliable alloy design for various applications. The PIs address this challenge by developing atomic-resolution, in situ electron microscopy techniques to quantitatively measure the impacts of precipitates on shape memory behavior in SMAs. The gained knowledge can generate powerful alloy design rules for precipitation-engineered SMAs for applications requiring high-temperature operations and improved mechanical properties. Moreover, the collaborative research activities between the PIs will be intertwined with educational programs and outreach activities through summer internships for underrepresented minority high-school students from local public schools, curriculum development targeting both on-campus students and distance-learning students (e.g., industry and military), and research training of graduate and undergraduate students with a strong emphasis on alloy design and materials characterization. These education plans promote student awareness about critical materials needs for new technologies and encourages diverse students to pursue careers in STEM. TECHNICAL SUMMARYMechanical and chemical effects of precipitates play a crucial role in controlling martensitic transformation (MT) dynamics in shape memory alloys (SMAs). However, there is a distinct lack of systematic and quantitative experimental evidence that can support, or test, theoretical models of the physical mechanisms of a MT modified by non-transforming coherent precipitates within the microstructure. This experimental program aims to quantify and elucidate the effects of nanoscale strain and concentration fields induced by coherent precipitates on the phase transformation and mechanical properties of NiTiHf-based SMAs. Specifically, the PIs seek to uncover and isolate the role of Heusler and Han phase precipitates (when they coexist), which have a distinctly different crystal structure and chemistry in NiTiHf-based SMAs. To achieve the goal, the PIs utilize a unique set of expertise in atomic-scale materials characterization based on aberration-corrected electron microscopy, electron diffraction, and in situ heating applications, as well as alloy design/synthesis and thermo-mechanical properties characterization. This program focuses on: (i) the development of alloy synthesis routes that allow for co-precipitation of Heusler and Han phases with controlled microstructure and chemistry; (ii) the quantification of strain and concentration fields around precipitates using atomic-resolution electron microscopy, spectroscopy, and scanning electron diffraction to uncover their effects on the microstructure morphology of the matrix; (iii) the characterization of the MT mechanisms in precipitation-strengthened SMAs and the phase transformation pathways using in situ temperature-controlled experiments for determining the dependence of a MT on the designed properties of precipitates (i.e., strain and concentration fields). This work provides a strong foundation for future SMA designs as well as in computational modeling by offering a quantitative, holistic understanding of the structure-composition-property relationships of SMAs and their dynamic response to realistic in-service environments.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.