The microstructure of full dense hot pressed ultra-high-temperature ceramics, namely ZrB 2 , HfB 2 , HfC, and TaC doped with 15 vol% of TaSi 2 , was characterized by x-ray diffraction, scanning and transmission electron microscopy. Concerning the borides, the presence of carbides and oxide of the transition metals was observed well dispersed along the matrix of the composites. The ZrB 2 and HfB 2 grains displayed a core-shell structure: the core was constituted by the original MB 2 grain and the shell by a (M,Ta)B 2 solid solution which grew epitaxially on the core. The compositional misfit and the difference between the coefficient of thermal expansion between the core and the shell was accommodated by low angle grain boundaries and dislocations pile-up, especially pronounced in the ZrB 2 -based composite, where a higher amount of Ta entered the lattice. At deeper investigations, Ta 5 Si 3 , Ta 4.8 Si 3 C 0.3 and Ta 5 SiB 2 , with Zr or Hf impurities were detected at the triple points. Wetting of the grain boundaries was observed. Based on the microstructural features, thermodynamical calculation and the available phase diagrams, it is suggested a densification assisted by a Ta-Si-O-B or -C liquid phase. Also for the carbide-based systems, the formation of solid solution was confirmed. The surrounding (M,Ta)C solid solution was interfaced to the original MC grain by a shark toothed dislocation chain. At the triple point junctions Ta 5 Si 3 C x phases with Hf impurities were detected, but in this case the grain boundaries were clean. The sintering mechanisms of early transition metals borides and carbides with addition of TaSi 2 as sinter-additive is reported in the present study, on the basis of the microstructural evolution observed upon sintering and in the light of phase diagrams and thermodynamical calculations. The microstructure of the composites here presented are finally compared to composites sintered with addition of the same amount of MoSi 2 .
Characterization of Ultra High Temperature Ceramics via Transmission Electron Microscopy - Part II: UHTCs sintered with addition of TaSi 2 -
Laura Silvestroni;Diletta Sciti
2010
Abstract
The microstructure of full dense hot pressed ultra-high-temperature ceramics, namely ZrB 2 , HfB 2 , HfC, and TaC doped with 15 vol% of TaSi 2 , was characterized by x-ray diffraction, scanning and transmission electron microscopy. Concerning the borides, the presence of carbides and oxide of the transition metals was observed well dispersed along the matrix of the composites. The ZrB 2 and HfB 2 grains displayed a core-shell structure: the core was constituted by the original MB 2 grain and the shell by a (M,Ta)B 2 solid solution which grew epitaxially on the core. The compositional misfit and the difference between the coefficient of thermal expansion between the core and the shell was accommodated by low angle grain boundaries and dislocations pile-up, especially pronounced in the ZrB 2 -based composite, where a higher amount of Ta entered the lattice. At deeper investigations, Ta 5 Si 3 , Ta 4.8 Si 3 C 0.3 and Ta 5 SiB 2 , with Zr or Hf impurities were detected at the triple points. Wetting of the grain boundaries was observed. Based on the microstructural features, thermodynamical calculation and the available phase diagrams, it is suggested a densification assisted by a Ta-Si-O-B or -C liquid phase. Also for the carbide-based systems, the formation of solid solution was confirmed. The surrounding (M,Ta)C solid solution was interfaced to the original MC grain by a shark toothed dislocation chain. At the triple point junctions Ta 5 Si 3 C x phases with Hf impurities were detected, but in this case the grain boundaries were clean. The sintering mechanisms of early transition metals borides and carbides with addition of TaSi 2 as sinter-additive is reported in the present study, on the basis of the microstructural evolution observed upon sintering and in the light of phase diagrams and thermodynamical calculations. The microstructure of the composites here presented are finally compared to composites sintered with addition of the same amount of MoSi 2 .I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
