Effect of long-term thermal exposures on microstructures and mechanical properties of directionally solidified CM247LC alloy

Abstract

A directionally solidified CM247LC alloy was exposed at 871 °C and 982 °C for 1000 h, 5000 h, and 10000 h under free stress in order to study the effect of microstructural degradation on the creep properties. None of the specimens exposed at temperatures up to 10000 h produced any kind of topologically close-packed phases because of the excellent phase stability of CM247LC alloy. The plate-like M₆C carbide was formed only at exposure of 982 °C for 10000 h through a decomposition reaction between γ and MC. Moreover, an M₂₃C₆ carbide layer was observed between the M₆C and the matrix. The exposure at 982°C for 5000 h and 10000 h had a spontaneous rafting of γ′ under free stress, while the exposure at 871 °C for 1000 h, 5000 h, and 10000 h had a non-rafted structure. The spontaneous rafted structure resulted in a drastic decrease in creep life. A 3-dimensional morphology of γ′ in the as-crept specimens, which were pre-exposed at 982 °C for 5000 h and 10000 h, had a non-rafted structure. This microstructural feature proves that the significant decrease in creep life of the specimen resulted from a loss of coherency between γ and γ′.