S. Dumpala

Integrated atomistic chemical imaging and reactive force field molecular dynamic simulations on silicon oxidation

In this paper, we quantitatively investigate with atom probe tomography, the effect of temperature on the interfacial transition layer suboxide species due to the thermal oxidation of silicon. The chemistry at the interface was measured with atomic scale resolution, and the changes in chemistry and intermixing at the interface were identified on a nanometer scale. We find an increase of suboxide (SiOx) concentration relative to SiO2 and increased oxygen ingress with elevated temperatures. Our experimental findings are in agreement with reactive force field molecular dynamics simulations. This work demonstrates the direct comparison between atom probe derived chemical profiles and atomistic-scale simulations for transitional interfacial layer of suboxides as a function of temperature.

Large area strain analysis using scanning transmission electron microscopy across multiple images

Here, we apply revolving scanning transmission electron microscopy to measure lattice strain across a sample using a single reference area. To do so, we remove image distortion introduced by sample drift, which usually restricts strain analysis to a single image. Overcoming this challenge, we show that it is possible to use strain reference areas elsewhere in the sample, thereby enabling reliable strain mapping across large areas. As a prototypical example, we determine the strain present within the microstructure of a Ni-based superalloy directly from atom column positions as well as geometric phase analysis. While maintaining atomic resolution, we quantify strain within nanoscale regions and demonstrate that large, unit-cell level strain fluctuations are present within the intermetallic phase.

An integrated high temperature environmental cell for atom probe tomography studies of gas-surface reactions: Instrumentation and results

An integrated environmental cell has been designed and developed for the latest generation of Atom Probe Tomography LEAP™ instruments, allowing controlled exposure of samples to gases at high temperatures. Following treatment, samples can be transferred through the LEAP vacuum system for subsequent APT analysis, which provides detailed information on changes to chemical microstructures following the reactions with near-atomic resolution. A full description of the cell is presented, along with some sample results on the oxidation of aluminum and two platinum-group alloys, demonstrating the capability of combining exposure/characterization functionality in a single instrument.

In-Situ Deuterium Charging for Direct Detection of Hydrogen in Vanadium by Atom Probe Tomography

The study of hydrogen embrittlement is of great interest for several decades, owing to the large reduction in maximum elongation of hydrogen exposed materials [1]. Towards this end, we will be studying the hydrogen interaction with material microstructure. Even though there exist several competing theoretical models, providing a mechanism by which dislocations and cracks are expedited through microstructures [2], there is a lack of experimental evidence to support differing theoretical models, owing to two main limitations. Firstly, hydrogen is weakly interacting with many radiations (e.g. x-ray, electron), and thus is difficult to image. Secondly, the interactions are truly atomistic, and thus require a scale-matched imaging methodology. Thus there exists a need for real-space quantitative analysis for identification of H, at these nano scales. Such information would allow for greater understanding of the dislocation H interaction.

High Energy Plume Impingement on Spacecraft Systems

Plume impingement experiments have been performed using a lab-grade, multi-gridded ion source for differing test conditions at beam energies of 150-250 eV. These experiments have been performed with the flux and fluence levels that would be consistent within a formation flight maneuver for a spacecraft with an electric propulsion system. Preliminary results and observations from our experiments with silicon microtip coupons and aluminum 6061-T4 are discussed.

Development of Quantitative Standards for Atom Probe Reconstruction Parameters for Analysis of Interfacial Chemistry

The present work is aimed at developing a standard for defining reconstruction parameters for optimal voxel and chemical thresholds. We develop quantitative techniques for the detection of sharp chemical interfaces from the APT outputs of 3D point cloud and 3D chemical interfaces of continuous geometry. The difficulty lies in mapping discrete data to a continuous output while minimizing the loss in chemical information. In the present work, we develop a novel approach, based on the principles of computational homology, to map the discrete 3D point cloud atomic data to the topology of a continuous chemical interface. The computational homology framework developed can be applied to APT data to get meaningful results and quantify sensitivity of the output on the data reconstruction parameters. To incorporate evaporation physics with chemistry and structure and to provide physical definition of uncertainty in spatially defining phases,

Correlative Imaging of Stacking Faults using Atom Probe Tomography (APT) and Scanning Transmission Electron Microscopy (STEM)

Doping in Ni3Al alloy systems can have a significant enhancement in various properties including high temperature strength, fatigue and creep. The enhanced high temperature mechanical properties of Ni-based superalloys are attributed to microstructure consisting of γ’ matrix strengthened by ordered γ’ precipitates [1]. Segregation or partitioning of the doping elements in these two phases plays a vital role in mechanical properties. Thus correlative atomistic scale chemical imaging techniques are developed for better understanding of these microstructural changes and their effect on the properties.

Data Intensive Imaging for 3D Atom Probe

Atom Probe Tomography (APT) combines atomic imaging with a time-of-flight mass spectrometer to provide direct space three-dimensional, atomic-scale resolution images of materials with the chemical identities of all the detected atoms. Quantitative analysis of APT data through advanced data mining methods enhances the chemical characterization of alloy systems and accounts for the statistical uncertainty associated with missing data and the physics of preferential evaporation. Most techniques currently used to extract precipitate topology and interface information from APT data are efficient; however, they are based on homogenization of the rich point cloud data which inherently loses some information. Furthermore, these methods require a specified, usually heuristic, concentration-level to draw iso-contours in order to extract characteristics of the precipitate topology. These twin issues of homogenization and heuristics are compelling rationale for the development of a robust, scalable, heuristic-free, graph-based framework [1].