STRUCTURAL INVESTIGATION OF FE-SI NANOSTRUCTURED MATERIALS

Starting from a mixture of elementary powders of Fe and Si, the mechan ical alloying (MA) process induces an extension of the A2 solid soluti on phase domain up to 27 Si at. %, when using ball milling conditions corresponding to 1.3 watt/g injected shock power (the solid solution p hase domain limit is 9 Si at. % for the thermodynamically stable state ). In this concentration range, an amorphous phase is also detected. I n order to study the disorder type induced by this process, the local chemical environment of the Fe atoms in mechanically alloyed samples a s well as in postannealed samples has been determined by Mossbauer spe ctroscopy. For a Si content less than 15 at %, the Mossbauer spectra s how that the Fe and Si atoms are randomly distributed in the A2 solid solution, following a binomial law in a b.c.c structure. However, for higher Si contents, although the X-ray diffraction (XRD) patterns show the b.c.c structure of the A2 solid solution, the Mossbauer spectra n o longer attest for a random distribution of the Fe and Si atoms in th e crystal structure of the A2 solid solution. So a new analysis, takin g into account the contributions of the internal magnetic fields of th e three sites of the Fe atoms (Fe atoms in the crystalline structure, Fe atoms in the amorphous phase and Fe atoms in the grain boundary pha se), is needed. The Mossbauer spectra of the ball milled and subsequen tly annealed samples are typical of the stable structures, consistent with the nominal stoichiometries. The latter are in agreement with tho se determined by the energy dispersive X-ray analyses (EDXA).