Influence of the Base Element on the Thermal Properties of Non- Ferrous Chromium-Rich TaC-Containing Alloys Elaborated by Conventional Casting: Part 2: Thermogravimetric and Metallographic Study of the Oxidation Start and of the Oxide Scale Spallation

Abstract

After a first part devoted to the study of several thermal chemical and mechanical characteristics of the six (xCo-yNi, bal.)-25Cr-0.4C-6Ta alloys, this second part of their study deals with the oxidation behaviour of these alloys in presence of heating or of cooling. The heating and cooling parts of thermogravimetry files of oxidation tests of which the isothermal parts were earlier studied, were analysed by plotting mass variation versus temperature. During heating the oxidation starts and a first mass gain is achieved prior to the isothermal stage. They are slightly influenced by the Co/Ni ratio. During the cooling oxide scale spallation happens for all alloys. For the nickel-richest alloys this phenomenon takes place for a temperature less decreased in comparison to the cobalt-based alloys. The fall in mass due to the loss of oxides by spallation seems a little higher for the nickel-richest alloys than that for the cobalt-richest ones. More than to differences in thermal expansion coefficient, this difference may be linked to the oxidation-induced mass gain achieved prior to spallation start, which is higher for the cobalt-richest alloys than for the nickel-richest ones.