Journal of Physics: Condensed Matter  (1988) 3833. 1 'LIIXVLRQFRQWUROOHGJUDLQJURZWKLQDSULPDU\FU\VWDOOL]DWLRQ $YUDPL([SRQHQWV5HYLVLWHG T Pradell 1 , D Crespo 2 , N Clavaguera 3 and M T Clavaguera-Mora 4 1 ESAB, Universitat Politècnica de Catalunya. Urgell 187, 08036-Barcelona, SPAIN. 2 Dept. de Física Aplicada, Universitat Politècnica de Catalunya. Campus Nord UPC, Mòdul B4, 08034-Barcelona, SPAIN. 3 Física de l'Estat Sòlid, Facultat de Física, Universitat de Barcelona. Diagonal 647, 08028- Barcelona, SPAIN. 4 Grup de Física de Materials I, Dept. de Física, Universitat Autònoma de Barcelona. 08193- Bellaterra, SPAIN. 14 November 1997 $EVWUDFW The anomalous behaviour of the Avrami exponents found in the primary crystallisation of amorphous alloys leading to nanostructured materials is considered. A kinetic model able to adequately treat such phase transformation has been formulated by means of the implementation of a soft impingement diffusion mechanism after a transient interface controlled growth. A decrease in the nucleation rate as crystallization proceeds has also been considered. Comparison of the model with experimental data is performed, giving excellent agreement. The soft impingement diffusion mechanism is demonstrated to be responsible for the anomalous behaviour of the Avrami exponent, the decreasing nucleation rate being a second order effect.