Microstructural and mechanical evaluation of Nb-silicide-based in situ composite at ultrahigh temperature

This study evaluated the thermal degradation mode of Nb-silicide-based in situ composite (Nb–19Ti–16Si-4Hf-13Cr–2Al–4B at.%) when exposed to temperatures up to 1600 °C in Ar atmosphere. For evaluation, the Rockwell hardness test, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) equipped with an energy-dispersive spectroscopy (EDS) were used. The as-manufactured specimen was composed of Nb solid solution (Nbss), Laves phase, and two types of silicides ((Nb,Ti,Hf)5Si3) that have hexagonal and tetragonal crystal structures, respectively. Exposed to ultrahigh temperatures, the resulting degradation and transformation of silicides led to the precipitation of face centered cubic (FCC)–Ti phases, which occurred predominantly after 40 h exposure. The decrease in volume fraction of silicide seems to be attributable to decomposition of the silicide, by which phase separation between the silicide and Ti phases occurred owing to the high vapor pressure of Si. As a result, the hardness (HRC) decreased from about 60 to 30. The results show that the evaporation of elemental Si needs to be considered for achieving good thermal oxidation resistance in Nb silicide composites. The results further suggest that one solution for improving the fracture toughness of Nb silicide composites is via control of the microstructure.