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

摘要非技术综述形状记忆合金（SMA）是一类独特的可以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的领先竞争者，如医疗设备、传感器、执行器和航空航天器组件。 这些金属的形状恢复依赖于原子排列的可逆变化。 设计和工程化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中产生纳米尺寸的颗粒（称为沉淀物）提供了一种直接的途径来改变其局部原子结构和化学性质，从而实现目标形状记忆性能。 然而，由于缺乏高精度的实时测量，将纳米尺寸的沉淀物的特性和性质与形状记忆性能准确地配对仍然是一个挑战，这为建立用于各种应用的可靠合金设计造成了障碍。 PI通过开发原子分辨率的原位电子显微镜技术来定量测量SMA中沉淀物对形状记忆行为的影响，从而解决了这一挑战。 所获得的知识可以为需要高温操作和改善机械性能的应用程序的沉淀工程SMA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计划和外展活动交织在一起，包括为当地公立学校代表性不足的少数民族高中学生提供暑期实习，针对在校学生和远程学习学生的课程开发（例如，工业和军事），研究生和本科生的研究培训，重点是合金设计和材料表征。 这些教育计划促进学生对新技术关键材料需求的认识，并鼓励不同的学生追求STEM职业。沉淀物的机械和化学效应在控制形状记忆合金（SMA）中的马氏体相变（MT）动力学中起着至关重要的作用。 然而，有一个明显缺乏系统的和定量的实验证据，可以支持，或测试，理论模型的物理机制的MT修改的非转化相干沉淀物的微观结构。 该实验计划的目的是量化和阐明纳米级应变和浓度场诱导的相干沉淀物的NiTiHf基SMA的相变和力学性能的影响。 具体而言，PI试图揭示和隔离的作用，赫斯勒和汉相沉淀物（当它们共存时），这有一个明显不同的晶体结构和化学NiTiHf为基础的SMA。 为了实现这一目标，PI利用一套独特的专业知识，在原子尺度材料表征的基础上，像差校正电子显微镜，电子衍射，原位加热应用，以及合金设计/合成和热机械性能表征。 该计划侧重于：（i）开发合金合成路线，使Heusler和Han相共沉淀，并控制微观结构和化学性质;（ii）使用原子分辨率电子显微镜、光谱学和扫描电子衍射对沉淀物周围的应变和浓度场进行定量，以揭示它们对基体微观结构形态的影响;（iii）使用原位温度控制实验表征沉淀强化SMA中的MT机制和相变途径，以确定MT对沉淀物的设计性质的依赖性（即，应变和浓度场）。 这项工作提供了一个强大的基础，为未来的SMA设计，以及在计算建模，提供了一个定量的，全面的了解结构，成分，性能关系的SMA及其动态响应的现实in-service environments.This奖项反映了NSF的法定使命，并已被认为是值得的支持，通过评估使用基金会的知识价值和更广泛的影响审查标准。

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.