Transmission electron microscopy on Hf- and Ta-carbides sintered with TaSi2

The microstructure of hot pressed Hf- and Ta-carbides with 15 vol% of TaSi2 was characterized by X-ray diffraction, scanning and transmission electron microscopy in order to investigate the densification mechanisms. The microstructure of the carbides was constituted by squared grains and subgrains were recognizable only by transmission electron microscopy: the inner part was constituted by the original MC grain and the outer area by a (M,Ta)C solid solution which grew epitaxially on it. The compositional misfit and the difference of the coefficients of thermal expansion between the two regions were accommodated by 45° grain boundaries and dislocations. At the triple junctions, Ta5Si3 and Ta4.8Si3C0.3, with Hf impurities were detected. The grain boundaries were observed to be clean. The microstructure of the composites containing TaSi2 was subsequently compared to composites sintered with addition of the same amount of MoSi2