Martensite 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.
The 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.
