Collaborative Research: In-Situ Three-Dimensional Diffraction and High-Resolution Electron Microscopy Study of Modulated Martensites

马氏体是发生在许多技术上重要的金属和陶瓷中的结构变化的产物。调制马氏体是一种由一系列原子层组成的马氏体，它通常与用于医疗植入物和消费产品的形状记忆合金的不寻常特性有关。尽管调制马氏体在科学和工程上具有重要意义，但关于其结构仍有许多未解之谜。与大多数金属合金不同，调制马氏体的原子结构在单晶或晶粒内变化。原子结构也很容易随着施加的应力、温度变化或磁场的变化而变化。本项目采用两种互补的实验技术来阐明模型调制马氏体的结构。研究小组正在利用三维x射线散射来确定从几十纳米到微米的长度尺度上的平均原子结构，并利用三维透射电子显微镜来揭示几纳米范围内的原子结构。这两种技术的结合提供了关于原子层在多个长度尺度上的周期性的信息——这些数据是单独使用任何一种技术都无法获得的。该方法适用于许多具有准周期调制的材料，并且减少了对高分辨率电子显微镜的需要（目前是原子尺度表征的首选技术，但昂贵且耗时）。参与该项目的学生将学习用于开发新材料的重要实验、计算和理论工具。技术描述调制马氏体表现出长周期层状结构和非周期堆积序列的结构性质是一个充满争议的基本问题。这项合作研究揭示了调制马氏体与互补的实验技术：三维（3D）同步加速器x射线衍射和高分辨率电子显微镜。它阐明了原子在空间尺度上的堆叠，从几个原子平面到晶格调制的多个周期。使用Ni-Mn-Ga作为调制马氏体的模型系统，该项目解决了一些基本问题：它们是如何形成的，为什么它们是稳定的，它们的衍射模式是由在扩展体积上的平均产生还是反映局部原子顺序，以及它们如何随温度，应力，磁场演变并影响形状记忆合金的伪弹性性能。研究的目标和范围是：(1)在Argonne国家实验室先进光子源上对Ni-Mn-Ga单晶进行三维衍射测量；(2)利用原子尺度反蒙特卡罗和纳米孪晶细化方法分析三维散射强度数据，确定原子尺度晶格调制；(3)在一系列方向上对小晶体体积进行高分辨率电子显微镜、洛伦兹显微镜和纳米束电子衍射，以关联晶格调制、磁畴和互反空间图；(4)结合已识别的相变机制，建立了描述马氏体和马氏体间相变行为的landau型模型。

Nontechnical DescriptionMartensite is the product of a structural change that occurs in many technologically important metals and ceramics. Modulated martensite is a type of martensite composed of a sequence of atomic layers, and it is often associated with the unusual properties of shape memory alloys used in medical implants and consumer products. Despite the scientific and engineering importance of modulated martensite, there are unanswered questions about its structure. Unlike in most metallic alloys, the atomic structure of modulated martensite varies within a single crystal or grain. The atomic structure also changes rather easily in response to applied stress, temperature change, or magnetic field. This project employs two complementary experimental techniques to clarify the structure of a model modulated martensite. The research team is using three-dimensional X-ray scattering to determine the average atomic structure over length-scales ranging from tens of nanometers to micrometers, and three-dimensional transmission electron microscopy to reveal the atomic structure within regions a few nanometers across. The combination of the two techniques provides information on the periodicity of atomic layers across multiple length scales - data that cannot be obtained by either technique in isolation. The approach is applicable to many kinds of materials with quasi-periodic modulations, and it reduces the need for painstaking high-resolution electron microscopy (currently the technique of choice for atomic-scale characterization, but costly and time-consuming). Students involved in the project are learning important experimental, computational, and theoretical tools used to develop new materials.Technical DescriptionThe structural nature of modulated martensites that exhibit long-period layered structures and aperiodic stacking sequences is a fundamental issue surrounded by controversy. This collaborative research sheds light on modulated martensites with complementary experimental techniques: three-dimensional (3D) synchrotron X-ray diffraction and high-resolution electron microscopy. It clarifies atomic stacking on spatial scales ranging from a few atomic planes to multiple periods of the lattice modulations. Using Ni-Mn-Ga as a model system for modulated martensite, this project addresses some fundamental questions: how they form, why they are stable, whether their diffraction patterns arise from averaging over an extended volume or reflect local atomic order, and how they evolve with temperature, stress, magnetic field and affect pseudoelastic properties of shape memory alloys. The goals and scope of the research are to: (1) perform 3D diffraction measurements on Ni-Mn-Ga single crystals at the Advanced Photon Source of Argonne National Lab; (2) analyze the 3D scattering intensity data to determine atomic-scale lattice modulations using atomic-scale reverse Monte Carlo and nanotwin microstructure refinement methods; (3) perform high-resolution electron microscopy, Lorentz microscopy and nanobeam electron diffraction from small crystal volumes at a series of orientations to correlate lattice modulations, magnetic domains and reciprocal space maps; and (4) develop a Landau-type model to describe the martensitic and inter-martensitic transformation behaviors by incorporating the identified transformation mechanisms.