The investigation of carbides evolution in a cobalt-base superalloy at elevated temperature

The carbides evolution characteristics and mechanisms in a cobalt-base superalloy at elevated temperatures between 1140 and 1265 degrees C have been investigated to provide the basis for potential service and heat treatment of the alloy. The in-situ transformation of M7C3 -> M23C6 is observed directly, which has not been reported in previous studies of cobalt-base superalloys. M23C6 carbide nucleates at the M7C3/ matrix interface due to the coherent relationship between M23C6 carbide and the matrix, and grows towards M7C3 carbide. On the other hand, it is found that primary MC carbide degenerates and releases a large number of Ti and W. Thus, residual MC carbide exhibits increased concentration ratios for Ta and Zr and shows enhanced thermal stability. The results indicate that the degeneration behavior of MC carbide is temperature dependent: MC decomposes into M6C carbide at lower temperature and dissolves partially into the matrix at higher temperature. (C) 2016 Elsevier B.V. All rights reserved.