'Vacancies' crystal defects key to improved design of lightweight aluminium alloys
Monash University researchers in Australia have used a combination of atomic-scale imaging and simulations to improve the understanding of the theta-prime strengthening phase in the aluminium copper alloy system.
In a study published in Nature Communications, the authors showed that improvement of the phase was enabled by introducing a large influx of specific crystal defects, or 'vacancies.'
They investigated the theta-prime transformation in the binary alloy Al-1.7at.%Cu, an alloy that forms the basis of many commercial alloys used widely in the aerospace industry. They reported direct and rapid nucleation of the theta-prime phase, as well as of an unexpected precipitate phase.
Researchers describe this nucleation pathway as template-directed, as it involves a precursor phase that serves as a structural template for the nucleated phases.
Whereas nucleation is slow and sparse when the bulk alloy is subjected to a conventional heat treatment, the study showed that nucleation is rapid and abundant when the heat treatment is applied to a sample with one of its dimensions at the nanoscale. The study also revealed the critical role of vacancies in enabling template directed nucleation.
The findings have important implications for precipitation mechanisms in nanoscale or nanostructured materials, as well as in conditions associated with large numbers of lattice defects such as materials far-from-equilibrium or subjected to extreme levels of deformation or intense ion irradiation.
Lead author, Associate Professor Laure Bourgeois of the Department of Materials Science and Engineering and the Monash Centre for Electron Microscopy, said: "we showed that transformation becomes much easier after the introduction of vacancies into the theta-prime phase in several situations: in nanomaterials, under irradiation by an electron beam in an electron microscope, and by deforming and heating bulk materials."
"By offering an improved understanding of how the strengthening phase can be promoted, we aim to contribute to the design of better high-strength lightweight alloys that offer superior performance."