Influence of tantalum on the rates of high temperature oxidation and chromia volatilization for cast (Fe and/or Ni)-30Cr-0.4C alloys
Résumé
The equiaxed superalloys obtained by classical foundry process are often reinforced by interdendritic carbides appeared during solidification. It is notably the case of cobalt-base alloys, but this principle of strengthening can also be used for nickel and iron-base alloys. Thus, in addition to chromium, the (well known) role of which is to bring a good resistance against high temperature oxidation to the conventionally cast superalloys, tantalum, which is a very oxidable element too, can also play a role in the high temperature oxidation of (Fe and/or Ni) -base alloys. In this work the high temperature oxidation behaviours of two iron-base alloys and two iron-nickel base alloys, containing 0.4 wt.%C and two amounts of tantalum 3 wt.%Ta and 6 wt.%Ta, are studied, with comparison with results obtained previously for ternary M30Cr0.4C alloys and two nickel-base alloys with 0.4%C and 3% to 6%Ta.
When present in Fe-base and (Fe,Ni)-base carbon-containing alloys, tantalum leads to a TaC carbides network which seems to influence the high temperature oxidation behaviour. Their mass gains are higher than for Ta-free alloys, especially for the highest temperatures. Moreover the oxidation of Ta leads to difficulties for determining the parabolic and chromia volatilization constants by using the {m x dm/dt = Kp – Kv x m}- treatment. This is particularly true in the case of iron-base alloys in which the diffusion of Ta towards the oxidation front is the easiest because of its availability in grain boundaries. Thus, an efficient reinforcement by tantalum carbides is probably easier to obtain in Fe-base and (Fe,Ni)-base alloys than in Ni-base alloys, but unfortunately with a significant decrease of the resistance against high temperature oxidation of alloys.
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