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Super Aluminium

02 March 2019 - Metallurgy

cyclic_strengthening_Sun_2019_TEM.PNG As reported by Sun et al. in Science, ( DOI) a novel way to strengthen aluminium. The article explains that ever since the original 1906 patent on the invention of solid-state precipitation as a way to strengthen aluminium alloys nothing much has happened. The alloy is heated and the more precipitates form, the stronger the material. Although nucleation is favored with decreasing temperature, the nucleation sites require a specific chemical composition helped by the creation of atomic vacancies and the subsequent migration of atoms which is in turn facilitated by higher temperatures. A typical run is 12 hours at 190°C, 8 hours at 175°C and 24 hours at 120°C, as if the manufacturing process is not already energy consuming enough. The aluminium itself is produced in electrolysis.

In the novel processing method as described in the article, strengthening takes place at room temperature. The vacancies are created not by elevated temperatures but by so-called cyclic strengthening where the metal is subjected to cycles of elongation and compression. Many vacancies are created this way for the non-Al atoms (in this study copper and magnesium) to jump into. The differences with thermally treated samples are striking. The cluster size is reduced from 10-50 nm to 1-2 nm with each cluster containing up to 10 atoms. In TEM images the Cu/Mg enriched sites and the vacancy sites are clearly visible.