Microstructure evolution of as-cast CoNiCrAl bond coat alloys after isothermal heat treatments

Abstract

In this paper, the microstructure evolution of two as-cast CoNiCrAl bond coat alloys after various isothermal heat treatments were investigated. The CoNiCrAl bond coat alloys were prepared by vacuum induction melting and casting process. The as-cast alloys were subjected to heat treatments from 550 °C to 1300 °C for periods up to 100 h to study the effects of temperature on the microstructure and phase distribution. The microstructure of heat treated CoNiCrAl bond coat alloys was characterised by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). The CoNiCrAl alloys exhibited a two phase γ + β structure, with the β phase acting as the primary phase. It was shown that the precipitation of needle-like secondary γ phase was slow at temperatures below 600 °C but became rapid above 900 °C. It was found that the morphology of the secondary γ phase changed from needle-like to rod-like at 1100 °C. A homogeneous γ + β two phase structure was obtained at 1300 °C even after 2 h heat treatment. It was revealed that the heat treatment temperature had a significant influence on the formation and distribution of the secondary γ phase. In addition, the precipitation of sub-micron Cr-rich particles occurred at 1300 °C in both CoNiCrAl alloys, which was due to the low solubility of Cr in the β phase and rapid coalescence of γ phase at elevated temperatures.