Successful EBSD Data Collection by Removing Oxide Layer from Magnesium Alloys

Instruments Used

EDAX Velocity™ Super EBSD Analysis System and Gatan PECS™ II with APEX™ 2.0 Software.

Background

The accurate structure of a specimen must be continuous to the surface and undistorted in order to acquire precise electron backscatter diffraction (EBSD) orientation measurements. Traditionally, electro- or mechanical polishing is performed to prepare the surface of a specimen for investigation.

On particular materials such as zinc or magnesium alloys that are highly prone to oxidation, an oxide layer can be quickly created if the sample comes into contact with water.

This may occur throughout polishing or afterward by the sample being exposed to water vapor in the air during storage or transport. This type of oxide layer effectively impedes the observation of EBSD patterns and must be excluded prior to analysis.

Materials and Methods

A mechanically polished Mg sample had been stored for several years and a thick oxide layer had formed where no EBSD patterns could be acquired. The sample was positioned in the PECS II and milled with 5 kV Ar ions at a 4˚ incident angle for 30 minutes to remove this oxide while ensuring the sample did not come into contact with water.

The duration was chosen to ensure that sufficient material was taken from the surface to fully uncover the native material. The oxide layer was removed across a multiple-mm2 sample area after milling and was then investigated utilizing EBSD.

(top row) (left) Backscatter electron image of the initial sample surface with small particles and oxide layer. (center) Secondary electron image after ion milling (tilted sample). Dark material at the edges of the image is remaining oxides. (right) Detail of milled area with good quality EBSD patterns. The visible surface topography is caused by a scratch and original small surface irregularities and is accentuated by ion milling at a low incidence angle. (bottom row) (left) Secondary electron image and (center) EBSD IPF on IQ map of a crack exposed by the ion milling. (right) Detail of Mg grain with twin lamellae.

(top row) (left) Backscatter electron image of the initial sample surface with small particles and oxide layer. (center) Secondary electron image after ion milling (tilted sample). Dark material at the edges of the image is remaining oxides. (right) Detail of milled area with good quality EBSD patterns. The visible surface topography is caused by a scratch and original small surface irregularities and is accentuated by ion milling at a low incidence angle. (bottom row) (left) Secondary electron image and (center) EBSD IPF on IQ map of a crack exposed by the ion milling. (right) Detail of Mg grain with twin lamellae.

Figure 1. (top row) (left) Backscatter electron image of the initial sample surface with small particles and oxide layer. (center) Secondary electron image after ion milling (tilted sample). Dark material at the edges of the image is remaining oxides. (right) Detail of milled area with good quality EBSD patterns. The visible surface topography is caused by a scratch and original small surface irregularities and is accentuated by ion milling at a low incidence angle. (bottom row) (left) Secondary electron image and (center) EBSD IPF on IQ map of a crack exposed by the ion milling. (right) Detail of Mg grain with twin lamellae. Image Credit: EDAX

Summary

The oxide layer that efficiently forms on Mg alloys can be removed through the application of broad beam ion milling, without the introduction of further mechanical strain. The resulting sample offers high quality EBSD patterns where the microstructure can be effectively analyzed.

This information has been sourced, reviewed and adapted from materials provided by EDAX.

For more information on this source, please visit EDAX.

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