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First evidence for mechanism of inverse ripening from in-situ TEM and phase-field study of δ′ precipitation in an Al-Li alloy

J. Park, R. Darvishi Kamachali, S.-D. Kim, S.-H. Kim, C. S. Oh, C. Schwarze, I. Steinbach.

Scientific Reports, 9, 3981, (2019)

Dissolution of the precipitates after (a) 11 hours (b) 12 hours, and (c) 13 hours of aging is shown. The white arrow indicates a dislocation. The number density of the precipitate rapidly decreased between 12 and 13 hours. Evolution of (d) the precipitate quantity (number) and (e) average precipitate radius is presented and compared to the simulation results.

Abstract
In-situ TEM investigation of aging response in an Al–7.8 at.% Li was performed at 200 °C up to 13 hours. Semi-spherical δ′ precipitates growing up to an average radius of 7.5 nm were observed. The size and number of individual precipitates were recorded over time and compared to large-scale phase-field simulations without and with a chemo-mechanical coupling effect, that is, concentration dependence of the elastic constants of the matrix solid solution phase. This type of coupling was recently reported in theoretical studies leading to an inverse ripening process where smaller precipitates grew at the expense of larger ones. Considering this chemo-mechanical coupling effect, the temporal evolution of number density, average radius, and size distribution of the precipitates observed in the in-situ experiment were explained. The results indicate that the mechanism of inverse ripening can be active in this case. Formation of dislocations and precipitate-free zones are discussed as possible disturbances to the chemo-mechanical coupling effect and consequent inverse ripening process.

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