Thermodynamic and experimental study of 30 wt-%Cr-containing {Co, Fe or Ni}-based alloys with very high contents in Ta and C

TaC carbides are among the most efficient particles for reinforcing cast refractory alloys and superalloys against mechanical stresses at temperature as high as 1000 degrees C and over. The most common TaC-strengthened superalloys contain less than 0.5 wt-%C and 5 wt-%Ta. Enriching alloys in C and Ta may lead to higher fractions in TaC and therefore possibly to higher mechanical properties for service in hot conditions. The present work explores the microstructures achievable in a cobalt alloy, a nickel one and an iron one, by the introduction of 1 wt-%C and 15 wt-%Ta, first using thermodynamic calculations and second by real elaboration. High fractions in TaC were effectively obtained, script-like eutectic ones, but also blocky preeutectic ones less efficient for reinforcement. These blocky TaC, the formation of which was explained by thermodynamic calculations, tend to aggregate together in some locations, with an overconsumption of Ta. However, some disagreements were noticed between predictions with thermodynamic calculations and as-cast microstructures.