Characterising extreme deformation in a failed Al alloy

Introduction

Unexpected metallurgical failure is extremely costly, either due to the additional costs of replacement parts and increased downtime, or due to more catastrophic consequences. Understanding the causes of failure is therefore a critical process; this will often involve microstructural characterisation in order to reveal the processes that lead to the part failure or to understand the mechanisms of the failure itself.

Although electron backscatter diffraction (EBSD) has been used as a key failure analysis tool for many years, the successful measurement of highly deformed materials is challenging, especially for conventional indexing using the Hough transform approach. In addition, the presence of brittle intermetallic phases is often a cause of failure, so it is critical that these phases are reliably measured during EBSD characterisation.

In this regard, EBSD pattern matching methods can be transformative; not only are they much better at successfully indexing diffraction patterns from highly deformed structures, but they can result in improved angular precision as well as more robust separation of phases, especially those that share similar crystal structures. In this application, the benefits of AZtecCrystal MapSweeper for the successful analysis of a failed Al alloy are demonstrated.

Sample and Experimental Details

A failed pole section made from a 7xxx series Al alloy was sectioned and mounted for analysis in the scanning electron microscope (SEM). The surface was prepared for EBSD using a vibratory polisher and then characterised using a field emission gun SEM, equipped with a Symmetry S3 EBSD detector and operating with a beam current of 24 nA and a 20 kV accelerating voltage. EBSD patterns were collected in Symmetry's "Speed 2" mode (156 x 128 pixel resolution) at rates between 180 and 470 patterns per second, were indexed using conventional Hough-based indexing in the AZtec software and then were stored for subsequent reanalysis. Two areas were analysed: a larger region (~300 x 250 µm) adjacent to the fracture surface at a measurement step size of 200 nm, and a smaller area (~50 x 50 µm) in a highly-deformed zone, with a 100 nm step size.

The data were reprocessed using AZtecCrystal MapSweeper, using dynamical simulations with Gaussian weighting of each pattern (to assign greater importance to the centre of each pattern, where the signal to noise ratio is higher). For the smaller, higher resolution dataset, the initially non-indexed points were indexed using Dynamic Template Matching (DTM) – as can be seen from the example, even very poor EBSD patterns can be effectively indexed using DTM, enabling high quality data from the most deformed regions of the sample.

Example EBSD pattern from a highly deformed region of the sample. This quality of pattern precludes successful indexing using the Hough transform method but is easily indexed using the DTM pattern matching approach, with a normalised cross correlation coefficient (R) significantly over the default threshold value of R = 0.15.

Following DTM indexing, all points were analysed using MapSweeper's Refinement Sweep (to improve the angular precision of each measurement) and Repair Sweep (to correct isolated indexing errors and to improve the overall indexing rate). For the larger dataset, only the faster Refinement and Repair sweeps were carried out, enabling full enhancement of the ~1.8 M point dataset in approximately 90 minutes. In all cases, patterns were matched against simulated templates for 3 separate phases: Al, MgZn₂ and Al₇Cu₂Fe.

EBSD results from the smaller analysis area. Top row – conventional Hough-based indexing results, showing (left to right) the phase map (red – Al, blue – MgZn2 and yellow - Al7Cu2Fe), orientation map (IPF-z direction colouring) and Kernel Average Misorientation map (scale 0 - 5°). Bottom row – results following reprocessing using MapSweeper. Scale bar in each image marks 20 µm.

Results

In the highly-deformed, smaller area, the initial indexing rate using the Hough-transform approach was only 70.5%. Following reprocessing using MapSweeper, the indexing rate increased to 96.9% (note that points with a normalised cross correlation coefficient below 0.15 were assigned as non-indexed). This enabled a more rigorous examination of the highly deformed areas, characterised by significant grain size refinement as well as increased defect densities, as shown by the Kernel Average Misorientation (KAM) maps. The improved data quality also highlights the brittle nature of the intermetallic phases and localised strain in the surrounding Al matrix.

In the larger analysis area, the indexing rate increased from 77% to >93% following the MapSweeper reprocessing (excluding the sample mount). This additional information, right up to the fracture surface itself, highlights once again the benefits of MapSweeper for characterising highly deformed regions. In this area, the prevalence of numerous slip bands in the Al matrix, as well as increased dislocation densities surrounding the intermetallic phases, help with the interpretation of possible failure mechanisms in this sample.

EBSD results of the large area adjacent to the fracture surface (left edge of the mapped area), following reprocessing using MapSweeper. Top left – pattern quality map, top right - phase map (red – Al, blue – MgZn2 and yellow - Al7Cu2Fe), bottom right – orientation map (IPF-z direction colouring). Scale bar in each image marks 100 µm.

Summary

The pattern matching methods implemented in AZtecCrystal MapSweeper are ideal for the effective characterisation of highly deformed materials. In this failed Al alloy sample, MapSweeper has enabled a rigorous examination of the most deformed regions close to the fracture surface itself. Not only does DTM indexing deliver high quality information from areas that have undergone significant grain size reduction, but MapSweeper's Refinement and Repair sweeps enhance both the angular precision and the phase discrimination within these datasets. The results indicate the importance of brittle intermetallic phases, concentrating strain in the surrounding Al matrix and potentially resulting in crack initiation, prior to failure.