Behaviour at high temperature of two cobalt-based superalloys reinforced by carbides. Part 2: Case of a medium carbon {Co-0.6C-23.5Cr-10Ni-3.5Ta-7W}-based superalloy

摘要

This second part of the work concerns the fine microstructure version of the second alloy out of the two complex carbide-strengthened cast cobaltbased alloys elaborated in laboratory. This second alloy, also synthesized by melting of pure elements and solidification in fast cooling conditions was here too subjected to high temperature oxidation for its surface and subsurface, and high temperature exposure for its bulk. The same temperatures and durations as in the first partwere applied: 1050 and 1150°C, 66 and 146 hours. In contrastwith the first studied alloy the high temperature oxidation behaviour of this second alloy was generally good, characterized by the appearance and sustainability of an outer covering chromia scale protecting the alloy. Catastrophic oxidation only occurred in some rare locations for the most severe conditions (the highest temperature and longest duration). The bulk microstructure evolved during these tests - a general coarsening of all carbides - with as consequences a decrease in room temperature hardness suggesting a weakening of the mechanical resistance at high temperature, similarly to what was observed for the first alloy.