Optimizing the Surface of Multiphase Al Alloys for Successful EBSD Analysis

Instruments Used

EDAX Pegasus Analysis System with Octane Elect, Velocity™ Super and Gatan PECS™ II with APEX™ 2.0 Software

Background

The precise structure of a specimen must be continuous to the surface and undistorted to collect accurate EBSD orientation measurements. Any surface damage and contamination originating from sample preparation must be reduced.

On materials like aluminum that are soft, it is highly challenging to prepare the specimen using mechanical polishing without causing a thin distorted surface layer to form that covers the true microstructure.

It is even more difficult to simultaneously achieve a suitable specimen surface for all phases when secondary phases with various polishing responses are present. Broad beam ion milling is a method to remove the distorted layer from the full surface without increasing the risk of further damage by mechanical contact.

Materials and Methods

EBSD mapping was used to analyze a multiphase Al alloy sample with Si, Cu, Fe and Sn containing phases after standard mechanical polishing. Orientation data could be captured from the majority of regions.

It was found that particular intermetallic phases did not create indexable patterns at all and other grains demonstrated preparation artifacts (Figure 1, top row).

The sample was positioned in the PECS II after analysis and was milled utilizing 4 kV Ar ions at a low incident angle of 3˚ for 20 minutes. The low angle was chosen to take away the surface topography that had been created around the secondary phase particles.

EBSD patterns could be gathered from all phases for total surface analysis due to the smoothness of the resulted surface (Figure 1, bottom row).

(top row) (left) Secondary electron image of polyphase Al alloy after mechanical polishing (tilted sample). Some phases did not polish well and stand out from the surface. (center) EBSD image quality (IQ) map illustrating that no diffraction patterns could be collected from the protruding grains (black areas). Two grains at the top show an intricate substructure, which is an artifact caused by mechanical polishing. This substructure is also apparent in the inverse pole figure (IPF) map (right). (bottom row) (left) Secondary electron image after ion milling (tilted sample). Most topography has been removed, and all grains now produce indexable EBSD patterns. The artificial substructures have been removed and the true grain structure is displayed in the IQ and IPF maps (center, right).

Figure 1. (top row) (left) Secondary electron image of polyphase Al alloy after mechanical polishing (tilted sample). Some phases did not polish well and stand out from the surface. (center) EBSD image quality (IQ) map illustrating that no diffraction patterns could be collected from the protruding grains (black areas). Two grains at the top show an intricate substructure, which is an artifact caused by mechanical polishing. This substructure is also apparent in the inverse pole figure (IPF) map (right). (bottom row) (left) Secondary electron image after ion milling (tilted sample). Most topography has been removed, and all grains now produce indexable EBSD patterns. The artificial substructures have been removed and the true grain structure is displayed in the IQ and IPF maps (center, right). Image Credit: EDAX

Summary

Al samples consisting of several phases with differing polishing responses can be effectively prepared for EBSD analysis through the use of broad beam ion milling. The ion milling produces a damage-free and clean sample surface and enhances the electron diffraction signal in all phases, irrespective of their polishing resistance.

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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