High-temperature in-situ TEM study of carbides precipitation in sintered or melted molybdenum

An in-situ TEM study of the structural evolution with temperature of two grades of molybdenum partly recrystallized following hardening is performed. In spite of the low total carbon contents, carbide compound precipitation is noted in certain grains. Two distinct phases of metastable carbide α-MoC1–x and stable carbide α-Mo2C types are identified through the analysis of diffraction patterns. The presence of the precipitates might be preceded by an intermediate state prior to precipitation. The precipitation process seems to be more pronounced with the sintered grade than with the melted grade, even though its total carbon content is less. This difference might be explained by the higher chemical heterogeneity of the sintered material. Nous avons etudie par MET in situ l'evolution structurale en fonction de la temperature de deux nuances de molybdene partiellement recristallisees apres ecrouissage. Malgre les faibles teneurs globales en carbone, on observe la precipitation de composes carbures dans certains grains. L'analyse des diagrammes de diffraction permet d'identifier deux phases distinctes du type carbure metastable α-MoC1–x et carbure stable α-Mo2C. L'apparition des precipites serait precedee par la formation d'une structure modulee qui peut etre consideree comme un etat intermediaire precurseur de la precipitation. Ce phenomene de precipitation apparaǐt plus marque pour la nuance frittee que pour la nuance fondue, malgre une teneur globale en carbone plus faible. Cette difference s'explique par la plus forte heterogeneite chimique du materiau fritte.