Stress corrosion cracking in novel aerospace materials
Aerospace manufacturers are continuously exploring novel materials that could support the next generation of aircraft design. This includes advanced aluminum alloys that incorporate increased zinc content, which results in a tougher material with higher quench sensitivity. Unfortunately, these improvements often come at the cost of decreased cracking resistance. Understanding the precise source of environmentally assisted stress corrosion cracking (EAC) would allow manufacturers to finely tune alloy production and potentially overcome the shortcomings of these novel materials. Multimodal microscopy
Multimodal correlative microscopy
Deconvoluting crack initiation can require you to look deep into the interior of a material. This can be accomplished through a variety of direct and indirect means such as X-ray scans or by cutting through the sample to the point of interest. Multimodal analysis, which combines multiple such approaches, is capable of combining the resulting data into a multiscale representation of the sample. Not only does this give a holistic view of the material, it also serves as a guide for the acquisition of highly detailed localized information; large-scale X-ray scans can reveal regions of interest for further analysis with higher-resolution DualBeam methods such as focused ion beam-scanning electron microscopy (FIB-SEM) or plasma FIB-SEM (PFIB-SEM).
EBSD with femtosecond laser ablation Multimodal microscopy
Thermo Fisher Scientific is continuously seeking to enhance the accessibility and utility of correlative workflows. Our latest instrumentation features “tri-beam” PFIB-SEMs that include a femtosecond laser for additional high-volume sample removal. This can be used to quickly access and evaluate regions of interest (ROI) identified with X-ray computed tomography (CT) scans.
In the case of metal alloys, the SEM column and retractable detector can be used to collect electron backscatter diffraction (EBSD) data of the ROI revealed with laser ablation. EBSD provides quantitative information on the phase and orientation of planes within the alloy. By automated sequential laser milling and EBSD analysis, a 3D representation of the ROI can be generated, revealing the exact structure of microscopic cracks within the alloy.
Case study: AA7050 aluminum alloy
Utilizing a Thermo Scientific correlative microscopy workflow, researchers at the University of Manchester were able to locate microscopic cracks in an aluminum alloy with X-ray CT and subsequently analyze them with 3D EBSD in a Thermo Scientific Helios 5 Laser PFIB System. The result was a high-resolution 3D volume that clearly revealed the complex crack path resulting from EAC. They were also able to determine that recrystallization grains played a significant role in the crack path, and that more rigorous control of these grains could increase the cracking resistance of the material.
Correlative microscopy, combining microCT for site targeting with laser PFIB-SEM for serial sectioning, allowed for precise access to a large volume of material containing EAC cracks. This high–SEM–resolution study has provided new insight into the interactions between microstructure and EAC cracks in aluminum alloys, particularly in relation to the size and morphology of the grains as well as deviations in crack direction. Scientists have also developed a novel, optimized lift–out procedure for large–scale serial sectioning, which can be used for similar scientific and industrial research problems.
Bartolomiej (Bart) Winiarski is a staff scientist at Thermo Fisher Scientific.