Oxidation at 1250°C of {Cr, Ta}‐rich carbides‐strengthened {Ni, Co}‐based alloys

Abstract

Thermogravimetry tests were performed for 70 h at 1250°C in the air for six TaC-strengthened alloys with different values of the Ni/Co ratio. The oxidation and chromia volatilization rates were specified, and the formed oxides were characterized. Metallographic observations show that the alloys rich in Ni (from ≈70 to ≈25 wt.% Ni) stayed chromia-forming until the end of the 70 h. In contrast, catastrophic oxidation locally started in the case of the Co-based alloys, which are too poor in Ni. The numerical exploitation of the mass gain files was done taking into account the mass loss induced by the chromia volatilization phenomenon. Results clearly showed a regular increase in oxidation parabolic constant when Ni is more and more replaced by Co. K E Y W O R D S high-temperature oxidation, mass gain kinetics, post-mortem characterization, superalloys | INTRODUCTIONCarbides are hard particles on which the hightemperature mechanical resistance of some superalloys is based. This principle of reinforcement by a second phase was the main way used for the earliest superalloys. These ones appeared near the middle of the last century. In their case, most of the strengthening particles were chromium carbides. [1,2] More recent cast polycrystalline equiaxed superalloys are still reinforced by carbides. In many cases, these carbides are of various types in the same alloy: chromium carbides, carbides involving other elements, such as Ta, Ti, Nb, and so forth. Some of these alloys contain MC carbides among their carbide populations. These ones can be based on Ni [3][4][5] or on Co. [6][7][8][9] MC carbides can be also present in the oriented microstructures of directionally solidified (DS) Ni-based [10] or Co-based [11] superalloys. MC can be obtained as the single carbide phase present in the microstructure of Co-based alloys [12,13] if the C and M weight contents are well rated. By respecting this condition, alloys containing exclusively eutectic TaC, [14] TiC, [15] or NbC, [16] and even HfC, [17] may be successfully produced. These MC carbides are mainly localized in the grain boundaries and between dendrites. By basing the mechanical properties on these eutectic refractory carbides, one may obtain particularly creep-resistant polycrystalline superalloys for possible uses in a wide range of elevated temperatures, extending to 1200 and maybe 1250°C. [18] Indeed, these high temperatures are still about 100°C below the start of the melting of these alloys but MC carbides are still present in their microstructures. This is a decisive advantage by comparison with γ/γ' nickel-based single crystals. Indeed, these later alloys lose their reinforcing particles above 1100°C. MC-reinforced alloys are also superior to superalloys strengthened by carbides of more classical type: for example, Cr 7 C 3 or (Cr,W) 23 C 6 ) also disappear at such high temperatures. In addition, one may also point out that the conventionally cast MCstrengthened superalloys can be used as structural materials for geometrically complex components for working at ...