Irreversible microstructure changes in thin film reactions

One of the most interesting microstructure changes in thin film reaction is the formation of amorphous alloys by slowly heating a bilayer of crystalline thin films. For example, we obtain amorphous alloys in bilayers of Ni/Zr and Rh/Si by slowly heating them around 250°C. There is a critical thickness in the formation; if the amorphous layers grow thicker than a thousand angstroms, they tends to crystallize. On the other hand, if we heat those bilayers above 400°C, crystalline compounds form directly. Clearly there is a competing formation between the amorphous and the crystalline phases. The most likely kinetic reason in favor of forming the amorphous phase at the lower temperature is the faster rate of free energy change. A brief review of the amorphous alloy formation by slow heating and a discussion of the driving force, kinetic process, and prediction of first phase formation will be given in this paper to honor the 70th birthday of Professor James C.M. Li.