Atom probe tomography (APT) yields atomic-scale 3D chemical composition and imaging. This material analysis method is suitable for grain boundary segregation analysis (Figure 1). Samples need to be prepared with a sharper tip for APT analysis. Grain boundary analysis requires locating the boundaries within roughly 200 nm of the sample tip in order to perform characterization effectively.
Figure 1. APT provides 3D chemical composition and imaging at the atomic scale and is uniquely suited to the analysis of grain boundary segregation.
Conventional APT Sample Preparation
Conventional sample preparation for APT analysis using either a focused ion beam (FIB) or electropolishing and subsequent analysis in a transmission electron microscope (TEM) is usually difficult and inefficient (Figure 2).
Figure 2. Traditional APT sample preparation is often problematic and inefficient.
Atom Probe Assist
Atom Probe Assist is an innovative way to monitor the position of grain boundaries between FIB milling steps in order to ensure the detection and exact positioning of a grain boundary for subsequent APT analysis. The Atom Probe Assist allows the use of transmission-EBSD (t-EBSD) for crystallographic orientation measurement, and imaging and determination of grain boundary position rapidly and conveniently within a single instrument.
The Atom Probe Assist is designed for t-EBSD measurements of APT tips. It can identify the position of grain boundaries between annular FIB milling steps and determine the types of various grain boundaries, where random grain boundaries are represented in black color and special twin boundaries are represented in white color to aid in site-specific FIB lift-out sampling (Figure 3).
Figure 3. The preparation is complete when the grain boundary between the two different orientations (colored yellow and blue) is located within 200 nm of the sample tip.
Smart t-EBSD Pattern Collection
Obtaining useable t-EBSD patterns is a challenging task when handling conical APT samples because of the constant variation in the shape and thickness of the specimens. The development of specialized background corrections improves image quality and ensures precise band detection and pattern indexing from thick and thin regions of the APT samples (Figure 4). The detector collection parameters are also optimized specifically for t-EBSD collection to minimize the time and effort necessary to obtain the required information.
Figure 4. Specialized background corrections have been developed to improve image quality and ensure accurate band detection and pattern indexing from both thin (left) and thick (right) areas of the APT specimens.
Specimen Qualification in a Single Instrument
A FIB-SEM is required for Atom Probe Assist to mill the samples annularly to the prescribed dimensions for APT analysis. The sample is placed on the instrument in a way that makes the specimen tip and the FIB beam parallel. This arrangement enables milling, SEM imaging, and the acquisition of t-EBSD data without sample movement, optimizing specimen yield through the elimination of the risk of damage occurring during sample transfer between two instruments (Figure 5).
Figure 5. Atom probe configuration
Enhanced Grain Boundary Contrast
t-EBSD mapping helps to determine the position of grain boundaries. This data acquisition between each FIB milling step enables the detection and monitoring of the grain boundary position. The sample can be considered ready for APT analysis if the grain boundary is an appropriate distance from the sample apex. It is possible to collect maps rapidly (˜3 minutes), enabling the rapid determination of the grain boundary position, and reducing sample contamination (Figure 6).
Figure 6. Enhanced grain boundary contrast
Grain Boundary Qualification
Traditional EBSD data can be collected from a specimen using Atom Probe Assist to determine the character of grain boundaries from a sample under analysis. Grain boundary character data is critical for gaining knowledge about grain boundary segregation problems, as various grain boundary types show different segregation behavior. This can be translated into different material properties behavior and it is possible to target specific boundaries easily for Atom Probe preparation and analysis by determining the grain boundary character (Figure 7).
Figure 7. Orientation map where specific grain boundary types are identified.
Using Atom Probe Assist with Hikari Plus EBSD Camera
The use of the Hikari Plus EBSD Camera (Figure 8) in conjunction with the Atom Probe Assist enables:
- Orientation precision < 0.1°
- Data collection rates up to 1,000 indexed points per second
- Phosphor screen optimized for high speed/high sensitivity operation
- Low noise detector for high-quality EBSD and t- EBSD pattern collection
Figure 8. EDAX Hikari EBSD Camera Series
Atom Probe Assist is used with TEAM™ EBSD Software
The TEAM™ EBSD Software (Figure 9) can be used with Atom Probe Assist:
- Transmission-EBSD mode
- Customized Atom Probe Assist data collection mode
- Custom APT map shape with an automatic or user-defined step size down to 1 nm
- Smart systems for constant optimization of dynamic data collection parameters
Figure 9. TEAM™ EBSD Software
This information has been sourced, reviewed, and adapted from materials provided by EDAX Inc.
For more information on this source, please visit EDAX Inc.