JOURNAL DE PHYSIQUE IV Colloque C4, supplCment au Journal de Physique 111, Volume 4, avril1994 Laser induced phase transition in iron thin films

High power laser pulse irradiation of material surfaces produces a wide spectrum of phenomena, all connected with emerging new technologies. Topics like: ablation, thin films deposition, laser induced chemical reaction are frequently studied. Few studies have been reported in the domain of laser induced solid state phase transitions at-a relative low laser fluence. In this regime the laser pulse acting like a high power heat pulse, the evolution of the system is far from equilibrium and new structures or new mechanism of transformation is expected. In the case of laser thin films heating, the heating rate is of the order of 101OKs-1. If the laser pulse is absorbed only in the thin film and the film is laser irradiated while on the substrate, the rate of cooling can be equally fast, the cold substrate acting as a thermal sink. The thin film is suddenly heated and rapidly cooled, quenching the structure resulting after the laser pulse heating. In a previous paper1 the microstructural aspects of the b.c.c.-f.c.c. phase transition of iron thin films were studied by high resolution transmission electron microscopy. This transition of iron is one of the best known and studied allotropic transformation, mainly in the case of martensitic transformation2. In the first paper', it was pointed out a new aPpect of the phase tranoition process in the laser heating case, by revealing the appearance of a microscopic mixture of the b.c.c. and f.c.c. phases. This mixture represents an incomplete transformation in the same crystalline block and can be understood as a frozen-in stage of the transition. In this paper we report numerical analysis of the laser Article published online by EDP Sciences and available a