Thermodynamic properties and phase stability of nanocrystalline metals and hydrides

From a theoretical point of view and from experimental evidence, it is shown that thermodynamic phase equilibria are drastically modified when the grain size reaches nanometer scales. Here, the thermal stability of elemental nanostructures with respect to grain growth and melting is discussed, based on a simple vacancy model for grain boundaries. For binary nanostructural alloys, such as niobium-hydrides, exhibiting an order/disorder transition, large deviations from thermodynamic equilibrium conditions are observed corresponding to an increase of the effective temperature ( > 150K) above the critical point, in addition to a stabilization of the nanostructure with an inhibition of considerable grain growth at least up to temperatures around 820K. The model is compared with experimental results on the stability of niobium hydrides.