Metastable phases in interface controlled materials
Metastable phases may be stabilized at interfaces due to lower surface or interface energies in comparison with the equilibrium phase. An example for this kind of stabilization effect is the phenomenon of surface melting, where a liquid-like film forms at the surface below the actual melting point. The excess energy necessary for the formation of the metastable, undercooled liquid phase is overcompensated by the gain in surface energy. Similar effects have been found in interface controlled materials such as thin films or nanocrystalline materials. In thin films, incoherent interfaces between the film and the underlying substrate contribute significantly to the total energy. The lower interface energy of a metastable film can stabilize the non-equilibrium phase as in the case of surface melting. A study is reviewed of thin Fe(Zr) alloy films which serves as a model system for the investigation of the interface effects. If deposited on suitable substrates, these films grow in the amorphous phase whereas in bulk materials the crystalline solid solution is more stable. Crystallization of the initially amorphous films sets in at a critical thickness where the driving force for crystallization can overcome the stabilizing effect of the interface. Under the experimental conditions, diffusional transport is negligible so that no phase separation can occur. Upon increasing the solute concentration, a polymorphic crystal-to-glass transformation (CGT) is encountered in the volume at a critical solute concentration. At concentrations close to the CGT, the relative stabilities of the competing phases, i.e. the crystalline solid solution and the amorphous alloy, approach each other. Therefore, the chemical composition of the films plays a role equivalent to the temperature in the case of surface melting. In the vicinity of the CGT, the interface energy contributions may dominate phase formation. As a result, a dramatic increase of the critical thickness for crystallization is observed with increasing solute concentration. A thermodynamic model based on the competing energetic contributions can explain the results. A generalized phase diagram is used for the description of interface effects in glass forming systems. Precursor effects like wetting at interfaces close to the conditions for phase transitions in bulk materials are also expected to be significant in systems with a high density of incoherent interfaces. The interfaces may serve as nucleation centres which play an important role for phase transformations in these materials.