Determination of the structure and orientation of nanometer-sized precipitates in matrix materials via transmission diffraction signals emitted by bulk samples in the Scanning Electron Microscope

Abstract

The characterization of precipitates in bulk materials by Electron Backscatter Diffraction becomes challenging when the precipitate size falls below 200 nm. This is due to the drastic decrease of diffracted intensity with decreasing precipitate size and the rapidly growing signal generated by the surrounding matrix. A new technique is presented and allows determination of the structure and orientation of precipitates far below 100 nm directly in bulk samples in the Scanning Electron Microscope, normally requiring thin lamella extraction and Transmission Electron Microscopy analysis. The technique relies on the selective chemical etching of the matrix material of a sample, combined with the large tilt characteristic of the EBSD technique. As a result of the selective etching, precipitates protrude from the surface and emit what turns out to be a transmission diffraction signal instead of a backscatter diffraction signal, even though the sample remains massive. With this technique successful analysis of precipitates down to 30 nm was performed in a bulk Cu/Cr sample produced by Laser Powder Bed Fusion Additive Manufacturing with standard EBSD hardware. A Monte Carlo simulation confirms that the intensity of the transmission signal collected from a nanometer-sized precipitate protruding from the surface is much higher than the intensity of the backscatter signal collected from the same precipitate embedded right underneath the surface of a flat EBSD sample. Transmission signals emitted by bulk samples are a great opportunity to study features whose size are below the resolution limit of EBSD, like nanometer-sized precipitates, while keeping the possibility of exploring large areas and without the need to produce a thin lamella.