Accurate precipitate analysis made easy

Our fully equipped new TEM allows us to detect 20 times more precipitates in less than 20% of the standard operator time.

Hui Shi Maggie

Accurate precipitate analysis made easy

In high-strength low-alloy (HSLA) steel grades, the presence of precipitates, such as Ti-rich precipitates, is key. Their size, distribution, and density are of utmost importance. In the past, OCAS manually determined the size distribution and chemical composition of precipitates, a slow and cumbersome process. To improve precipitate analyses, OCAS moved to Thermo Scientific’s Automated Particle Workflow (APW) in 2020, which made it possible to quantify nano-sized precipitates more accurately in a shorter timeframe. This workflow increased the efficiency and statistical relevance of these analyses, enabling the research centre to boost productivity and the accuracy of results.

GETTING READY: FROM SAMPLE PREPARATION TO STITCHING

OCAS currently uses 3 methods of sample preparation to evaluate steel precipitates, depending on the customer’s request: carbon replica, focused ion beam (FIB) lamella and electropolished thin foil. Replica collecting only precipitates provides the largest sample area, but the precipitates no longer retain their original spatial distribution. The electropolished thin foil keeps the original spatial distribution of the precipitates – but, due to its smaller TEM investigation sample volume and influence of the steel matrix, the analysis (statistics) takes more time. The FIB lamella method is ideal for location-specific sample preparation. A carbon replica sample is first used for statistical size distribution characterisation. After the homogeneity is evaluated for several of the hexagonal grid holes on a carbon replica by transmission electron microscopy (TEM) imaging, one grid hole is randomly selected to represent the distribution across the sample for precipitate analysis, and 64 dark field Scanning Transmission Electron Microscopy (STEM) images are automatically launched at the centre of the selected grid hole. These images are then stitched together to form the overview map of the selected hexagonal hole. The total area for the stitching is 117 x 117 µm, and the process takes approximately 2 min for manual set-up and 10 min for automatic imaging and stitching.

MANUAL SET-UP, AUTOMATED WORK COMPLETION

With an overview map, all precipitates across a defined area, including their morphology, size, and distribution are shown with a resolution of 8 nm per pixel. Since for this example only Ti-rich precipitates were of interest, energy dispersive (X-ray) spectroscopy (EDS) mapping was launched to image only Ti-rich precipitates. A 10 x 10 grid was established,

and 100 EDS maps were automatically collected over an area of 58.1 x 58.1 µm. The process takes 5 min of manual set-up and about 17 h of automated work that can be completed overnight. These 100 Ti maps were automatically recorded as part of the workflow and transformed into particle data, where the precipitates are segmented and labelled by image processing software, and the precipitates data (such as equidiameter, aspect ratio, etc.) were recorded as a list. A size distribution histogram of Ti-rich precipitates can easily differentiate the precipitates status of two samples.

EFFICIENCY BOOSTER

The Ti maps on FIB lamella show the original distribution of Ti-rich precipitates in the steel matrix with a spatial resolution of 0.15 nm. Using APW, which combines (S) TEM images with EDS mapping, OCAS can now quickly obtain the morphology, size distribution, and chemical composition of precipitates. We can also accurately analyse the distribution of both small and large precipitates over a defined area to obtain the comprehensive data needed for product development R&D. This results in 20 times more statistically relevant data for precipitates – and in only 1/3 of the manual working time that was required by OCAS’s previous workflow.

Comparison of size distribution of Ti-rich precipitates on 3376 µm² replica between sample A and B