Influence of the Presence of Aluminum and of its Content on the As–Cast Microstructure of Alloys Designed to be Tac–Reinforced

Four alloys based on either cobalt or nickel, containing tantalum and carbon for creep–resistance purpose (based on TaC carbides), as well two aluminum contents for hot oxidation–resistance purpose (alumina–forming behavior), were conventionally cast. Their microstructures in the as–cast conditions were characterized. As expected, all alloys contain a dendritic matrix and TaC as single carbide type present. However, differences were noticed among the four alloys concerning the distribution of the TaC carbides. As expected, in the two alloys containing 5 wt. % Al, Co-based or Ni-base, the tantalum carbides were successfully obtained as an interdendritic network of eutectic script TaC carbides. This was more or less different for the {10 wt. % Al}–containing alloys for which the TaC phase crystallized according to two successive liquid–solid transformations. Obviously, for the two former alloys, TaC appeared first as pre–eutectic particles, and second, after the dendritic development of the matrix, as eutectic script carbides. The cobalt alloy contained a majority of interdendritic eutectic script TaC and a minority of blocky pre–eutectic TaC segregated to the periphery of the ingot. The carbides distribution in the nickel alloy was inversed, with the greatest part of TaC carbides formed prior to the matrix crystallization and massively segregated in the outermost part of the ingot. The high temperature properties of this last alloy were controlled at 1200°C and this showed that, in addition to the good oxidation behavior, the {10 wt.% Al}–containing nickel alloy demonstrated nevertheless a correct creep–resistance.