Advanced Characterization of Martensite - 3D & High Resolution: Session 3
Program Organizers: David Rowenhorst, U.S. Naval Research Laboratory; Michael Mills, The Ohio State University

Tuesday 2:00 PM
July 11, 2017
Room: Comiskey
Location: Hyatt Regency Chicago

Session Chair: Marc De Graef, Carnegie Mellon University


2:00 PM  Invited
Materials Characterization with Atomic Electron Tomography: Mary Scott1; 1UC Berkeley
    Materials design on the atomic scale requires a 3D characterization method that can measure individual atomic coordinates in a sample. Extending atomic resolution electron microscopy to three dimensions via tilt axis tomography enables one to determine the 3D atomic structure of a material without averaging or using a priori structural information. This method, termed atomic electron tomography (AET), has been used to isolate crystalline grains in 3D, to visualize the atomic arrangement of atoms in defects, and to localize individual atoms of a sample with 20 pm precision. From the measured atomic coordinates 3D strain fields, chemical ordering, and the distribution of defects can be determined. These results will be discussed in the context of applying AET to more increasingly systems, such as NiTiHf shape memory alloys.

2:40 PM  
Revealing Transformation and Deformation Mechanisms in High Temperature Shape Memory Alloys Using Advanced Electron Characterization Techniques: Lee Casalena1; Fan Yang1; Daniel Coughlin2; Yipeng Gao1; Glen Bigelow3; Darrell Gaydosh3; Santo Padula3; Ronald Noebe3; Peter Anderson3; Yunzhi Wang1; Michael Mills1; 1The Ohio State University; 2Los Alamos National Laboratory; 3NASA Glenn Research Center
    The development of viable high-temperature shape memory alloys demands a coordinated effort connecting nanoscale crystal structure to macroscale mechanical properties. In this work, several high performance NiTiHf and NiTiAu alloys are comprehensively explored with the goal of enhancing properties conducive to actuator applications in demanding automotive and aerospace environments. NiTiHf has a favorable balance of properties, including high strength, stability and work output at temperatures approaching 300C, yet at reduced cost compared with Pt/Pd/Au containing counterparts. NiTiAu demonstrates work output at significantly higher temperatures – where the benefits may offset cost – but requires further research. Advanced electron characterization techniques are used to explore effects of precipitate structure, coherency, chemistry, and matrix plasticity on alloy behavior to develop an understanding of the critical microstructure-property relationships. These insights are incorporated into microstructural modeling frameworks to further reveal how phase transformations and time-dependent plasticity interact under isothermal deformation and load-biased thermal-cycling conditions.

3:00 PM  
Short Range Order in Low Temperature Aged and Cycled Ni-Ti: Saeid Pourbabak1; Xiebin Wang2; Jan Van Humbeeck2; Bert Verlinden2; Dominique Schryvers1; 1University of Antwerp; 2Catholic University of Leuven
    In this study the structural effects of room temperature aging and/or low temperature cycling on the martensitic transformation in Ni-Ti is investigated. Despite the low temperatures, DSC cycling shows that Ms decreases with time. HRTEM and HAADF STEM imaging in atomic resolution do not show any change in the matrix structure or appearance of a second phase. However, strong localized and periodic diffuse intensity is seen in electron diffraction patterns. The Cluster Model which assigns the shape of the diffuse intensity to that of short range ordered microdomains is applied to analyze the results. The geometry of the diffuse intensity can be explained by columns of pure Ni atoms along <111>B2 directions and which start to form upon cycling at low temperature. Quantification and simulation of diffraction intensities of samples with different thermomechanical history shows that thermal cycling increases the size of the Ni clusters and enhances short-range ordering.

3:30 PM Break