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

Abstract

Many cast chromia-scale 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. This is due to decrease of the volume fraction of the these carbides and to their morphology evolution. Other carbides, notably HfC, are more stable at high temperature and they can be candidates for the reinforcement of this type of superalloys. 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 prepared in a foundry. They were tested by oxidation in air for 46 h at 1,200 °C with thermogravimetry measurement and subsequent metallographic characterization. All the mass-gain curves obtained are parabolic and the oxidation rates of the studied alloys are only slightly faster than for the corresponding Hf-free ternary nickel alloys containing the same chromium and carbon contents. 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-scale forming system. However, small quantities of HfO₂ and NiCrTaO₄ oxides are observed in addition to the chromia scale. In the bulk, the volume fraction and morphology of the HfC carbides only changed a little. These alloys appeared thus resistant enough against hot oxidation, this allowing first high temperature mechanical characterization in oxidizing atmosphere, before possibly considering these alloys for real applications.