TRANSFORMATION OF SOME AMORPHOUS FePC ALLOYS DURING ISOTHERMAL AGING

Résumé
Des alliages amorphes Fe₇₅P₁₈C₇ et Fe₈₃P₁₁C₆ ont été obtenus par trempe à partir de l'état liquide (twin roller technique). Les feuilles obtenues avaient une épaisseur d'environ 25 µ et une largeur de 2 à 4 mm. Les spectres obtenus sont constitués de pics très élargis et le champ hyperfin moyen est plus important pour l'alliage le plus riche en fer. Les échantillons furent de nouveau observés après un recuit à 350 °C pendant des temps t_a allant jusqu'à une semaine. Le recuit se traduit d'abord par une disparition des orientations préférentielles initiales. Puis, par la formation de clusters d'atome de fer. Le facteur À décroît d'abord très légèrement puis croît après t_a = 2 000 mn. L'étude du rayon X montre la présence d'une phase cubique centrée après une semaine de recuit, le paramètre étant voisin de celui du fer métallique. Il apparaît également du Fe₃P. L'influence de la pression sur la recristallisation est en cours d'étude.

Abstract
The amorphous alloys Fe₇₅P₁₈C₇ and Fe₈₃P₁₁C₆ have been prepared by the twin roller rapid quenching technique. The foils obtained had a thickness of about 25 µ and a width of 2 to 4 mm. The spectra show broad peaks characteristic for such alloys. The mean magnetic splitting is the largest for the iron richest alloy. The amorphous samples were annealed at a constant temperature of 350°C during a time t_a. After an annealing treatment the Mössbauer spectrum was recorded at room temperature. For Fe₈₃P₁₁C₆ spectra were taken for t_a = 0, 350, 850, 1 350, 1 800, 2 400 mn and t_a = one week. For t_a = 350 mn only the relative intensities of the broad lines have changed. This may be interpreted as a decrease of initial preferred orientation. After t_a = 850 mn the typical six line pattern of metallic iron appears in the spectrum indicating the formation of clusters of iron atoms. The relative intensity of the iron six line pattern increases until t_a = 2 400 mn. After the one week anneal there is no further change of the spectrum, which shows also other (but less intense) six line patterns which we have not tried to assign yet. For Fe₇₅P₁₈C₇ spectra were taken for t_a = 0, 500, 1 300, 1 800 mn and t_a = one week. For t_a = 500 mn the pure iron six line pattern is weakly present, its intensity increasing gradually upon furhter annealing. Compared with Fe₈₃P₁₁C₆ however, the relative intensity of the iron lines is lower. For both the alloys the total absorbed intensity changes only slightly as a function of t_a. It seems that the (mean) f-factor firstly decreases and after about 2 000 mn again increases upon annealing. X-ray diffraction of the samples annealed for one week (at 350 °C) shows the presence of a bcc-phase with a lattice parameter close to that of pure iron (and also some Fe₃P). The diffraction peaks are much sharper for the iron richest alloy, indicating that the crystallites are greater in Fe₈₃P₁₁C₆. A Mössbauer study of the amorphous-crystalline transition as a function of pressure is now in progress.