Nanostructure of the 35 µm diameter carbon fibre.
Nanostructure of the 35 µm diameter carbon fibre.

Metal composite materials incorporating carbon fibers are of growing interest to the aviation industry as a new option to reduce the weight of engine components and improve environmental performance. Exactly how those fibers perform in such composites could now become clearer, thanks to the work of UK and Czech researchers.

One of the most promising metal matrix composites (MMCs) for gas turbine blades in aircraft engines is a titanium alloy (Ti-6Al-4V) embedded with SiC fibers. The fibers give the turbine blades or ‘blisks’ (short for ‘bladed disks’) their strength. Alexander M. Korsunsky of the University of Oxford and colleagues from the Diamond Light Source at Harwell and TESCAN Brno in the Czech Republic wanted to find out how.

Using a highly complex combination of synchrotron imaging and nano-focused X-ray beam scattering, together with focused ion beam (FIB) stress evaluation, the researchers constructed a map of the structure and strain inside the composite [Baimpas, N., et al., Carbon 79 (2014) 85-92, DOI: 10.1016/j.carbon.2014.07.045]. As the composite components contain both crystalline and amorphous regions, a single technique cannot provide the necessary characterization.

Using the Diamond Light Source at Harwell, X-ray tomography of a cross-section of the composite revealed an approximately regular arrangement of fibers. The technique also allows analysis of the interfacial bonding region between the fibers and matrix, and the internal structure within the fiber itself.

“We can readily discern… the presence of a monofilament carbon core inside the fiber,” says Korsunsky. “We drilled down further to discover that this filament has a fine structure at the nanometer scale that is a consequence of its processing history, and [it is this] that determines the properties.”

The team then used a combination scanning electron microscopy and FIB material removal to build up a picture of the internal structure and stresses inside the monofilament core. Their observations reveal that the transition between amorphous and crystalline regions is associated with significant compressive stresses within the carbon fiber core.

The advantages of this combined analysis approach are numerous, Korsunsky told Materials Today. “We get an insight into the complex graded structure of carbon fibers that has not been directly imaged at this resolution, and show that stress analysis within it is possible. The different analytical techniques we use provide good agreement, meaning that disadvantages of one can be overcome by using another and vice versa.”

The approach could work just as well with other high performance composites utilizing carbon fibers, including those based on polymer matrices, says Korsunsky. Understanding the internal stresses of carbon fibers within composites, and how they arise during material growth and structure evolution, will help optimize new designs that drive superior performance, he adds.

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