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

Abstract

Many cast chromia-forming nickel-based superalloys are reinforced by carbides. In such alloys the primary chromium carbides or tantalum carbides rapidly lose their strengthening effect in service at high temperature because of decrease in volume fraction and morphology evolution. Other carbides, HfC for example, are more stable at high temperature and they can be candidates for the reinforcement of this type of superalloy. In this work, three nickel-based alloys containing 25 wt.%Cr, 0.25 or 0.50 wt.%C, and Hf with contents high enough (3.7 and 5.6 wt.%) to promote the formation of numerous primary HfC carbides, were elaborated by foundry. They were tested in oxidation by air during 46h at 1200°C, with thermogravimetry measurement and metallographic characterization. All the mass gain curves obtained are parabolic and the oxidation rates of the studied alloys are only slightly faster than for corresponding ternary nickel alloys containing the same chromium and carbon quantities. The obtained values of the parabolic constant k p and of the chromia volatilization constant k v , deduced by applying the {m × dm/dt = k p-k v × m} method, are typical of a chromia-forming behavior. However, small quantities of HfO 2 and NiCrTaO 4 oxides are noticed in addition to the chromia scale. In the bulk, the volume fraction and morphology of the HfC carbides only changed a little.

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.…”

“…This locally promoted the change of slow oxidation into fast catastrophic oxidation, phenomenon which may expand to the whole surface despite chromium contents remained rather high in subsurface (decreased only down to around 20wt.% against 25wt.% initially). This never occurred for the HfC-containing nickel-based alloys [15], favored by the easy chromium diffusion through the nickel matrix. But, in contrast, this occurred sooner for the HfC-containing cobalt-based alloys [14] since these ones oxidized much faster all along the same isothermal stage, due to the globally less easy chromium diffusion through the cobalt-based matrix.…”

Creep and oxidation kinetics at 1100°C of nickel-base alloys reinforced by hafnium carbides

“…applied to the obtained alloys [44] demonstrated that these carbides were much more stable at high temperature than chromium carbides in nickel alloys, and even more than the TaC carbides present in some cobalt-based superalloys. The oxidation behavior at 1200°C of the obtained alloys was rather good [45] but, unfortunately, their creep resistance was catastrophic at this temperature [46]. These previous studies showed that HfC-reinforced nickel-based alloys may be used at 1200°C but in absence of applied stress.…”

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

“…The nickel-based alloys were also previously studied in isothermal oxidation [9]. They were also prepared by casting from pure elements, using the same HF induction furnace under Argon.…”