Thermal evolution of cluster assembled Ni

Diffusion properties of Ni3Al cluster assembled nanostructured materials are investigated at the atomic scale. Two different model samples are considered, at equilibrium at 300 K. One is obtained by modelling cluster compaction under 2 GPa external pressure and the second by accumulating low energy deposited clusters on a Ni surface. They differ essentially by their density, the latter sample presenting an interconnected network of nanopores, which is not observed in the former. At elevated temperatures, cluster coalescence is observed in both, as well as an intense atomic diffusion at the internal surfaces and nanograin interfaces. A method is presented which allows, in a good approximation, to distinguish between the two phenomena and to estimate diffusion coefficients. At temperatures above 400 K, it is found for both samples, irrespective to their density, that the diffusion activation energy at the internal surfaces and interfaces is as low as in a liquid while the grain cores remain crystalline