Observation of phase transitions at the surface of Al-3 at. % Ag

Özet

In the bulk binary alloy Al-3 at.% AOAg, $Ag_2Al$ precipitates are formed below 690 K which are reversibly dissolved at elevated temperatures. Here we report on this phase transition observed at two different surfaces. At the (111) surface, Ag-rich clusters are found which possess hexagonal stucture, characteristic of $Ag_2Al$. At the (100) surface the cluster size was followed as a function of temperature by monitoring the bandwidth of the Ag 4d states in x-ray photoelectron spectroscopy. Since the bandwidth measures the coordination number of the emitting atoms, it directly reveals the short-range order of the Ag atoms at the surface. The measurements show that the dissolution of the Ag-rich clusters already starts at temperatures at least 100 K below the bulk transition, and the observed hysteresis behavior is indicative of a first-order phase transition at the surface.