In situ annealing experiments on hydrogenated amorphous-Si thin films coated with a thin layer of Al were performed under a scanning electron microscope equipped with a heating stage and an energy dispersive spectroscope. A sequential change in microstructural features due to interfacial diffusion at the apex of the boundary between Al and amorphous Si has been delineated. Transmission electron microscopy studies affirmed the evolution of polycrystalline Si as a result of the phase transition from amorphous to randomly oriented fine grained Si. A possible mechanism has been postulated to explore the metallurgical aspects of crystallization phenomena that persists during phase transformation. (c) 2006 The Electrochemical Society.