Kinetics of high temperature oxidation of chromium rich HfC reinforced cobalt based alloys

Abstract

Mostly known to improve the high temperature oxidation resistance of superalloys, hafnium may also form carbides. Several per cents of Hf allow developing a dense carbide network to mechanically strengthen alloys. Here, the high temperature oxidation behaviour of three HfC containing cobalt alloys was characterised at all steps of a thermogravimetry test: heating, isothermal stage and cooling, compared with two Co-Cr-C model alloys. The five alloys were heated in synthetic air, maintained at 1200°C during 50 h and then cooled. The mass gains were plotted versus time or according to {mxdmldt^Kp-mx Kv) to specify the isothermal kinetic constants, or versus temperature to determine how oxidation acts during heating and oxide spallation occurs during cooling. Compared to the ternary alloys, the oxidation of the HfC reinforced alloys starts earlier but leads to lower mass gains during heating, the isothermal oxidation is faster and oxide spallation occurs later.

High Temperature Oxidation of HfC-Containing Chromium-Rich Iron-Based Alloys

“…Thus, in chromium-rich iron-based alloys too, Hf additions in sufficiently great quantities allowed the development of numerous HfC carbides in the interdendritic spaces, mainly with a eutectic nature and a script-like shape but also with a blocky shape for some of them. This is a common point with the similar Hf-containing cobaltbased alloys [14] and Hf-containing nickel-based alloys [15] which both presented also these repartition and shape of eutectic HfC carbides. However the presence of obviously blocky pre-eutectic HfC was met essentially in the cobalt alloys.…”

“…As for these Hf-rich cobalt-based and nickel-based alloys previously elaborated by foundry and tested in oxidation at the same temperature the presence of so high quantities of hafnium influences the high temperature oxidation behavior of the corresponding Fe-25Cr-C base alloys. The kinetics were accelerated by the presence of hafnium, less than observed for the cobalt-based alloys [14] but more for the nickel-based alloys [15]. By comparison with the nickel-based alloys for which the scale thicknesses were rather homogeneous, this is due to the formation of thick multi-constituted oxide islands.…”

Kinetics of High Temperature Oxidation and Chromia Volatilization for HfC-Containing Nickel-Based Alloys

“…Indeed they are globally oxidation resistant but probably also corrosion resistant thanks to their high chromium contents, and creep resistant thanks to the good stability of the eutectic HfC carbides at high temperature, a creep-resistance which may be better than for TaC-reinforced cobalt-based alloys for which the fragmentation of the TaC was more severe for similar duration at the same temperature [20]. In contrast with similar cobalt-based alloys containing also high fractions of HfC carbides but not so resistant against high temperature oxidation [21], these alloys are thus ready to be tested at high temperature, without protective coating, to verify the later point: notably specifying their resistance against creep deformation. Values of the linear constant k l , of the parabolic constant k p and of the constant chromia volatilization constant k v , the two later ones being derived from the m × dm/dt = f(m) plots.…”

Oxide Spallation During Post-isothermal High Temperature Oxidation Cooling of Cr-rich Cast Alloys Highly Alloyed with Hf

“…The cobalt-based alloys, previously studied in isothermal oxidation [8], were prepared by casting. The pure elements (Alfa Aesar, purity > 99.9%) were melted together in a High Frequency induction furnace (CELES) under an inert atmosphere of 300millibars of pure Argon.…”