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

Abstract

TaC carbides are among the most efficient particles for reinforcing cast refractory alloys and superalloys against mechanical stresses at temperature as high as 1000°C and over. The most common TaC-strengthened superalloys contain less than 0.5 wt.%C and 5wt.%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 15wt.%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.

As-Cast Microstructures of High Entropy Alloys Designed to Be TaC-Strengthened

“…It is true that other TaC particles -coarse blocky carbides not mixed with matrixare present in the HEA/TaC2 alloy, but one can think that they can be not detrimental for the mechanical properties at high temperatures. These blocky TaC are similar to the ones previously observed in cobalt-chromium alloys with high contents in Ta en C (15 wt.% and 1 wt.% C) earlier studied [25] . In these alloys earlier studied, thermodynamic calculations evidence that low C and Ta contents led to the appearance of eutectic TaC carbides only, the dendritic matrix being thus the first solid phase to crys-tallize from the liquid state.…”