Microstructure and thermophysical properties of as-cast CoNiCrAl bond coat alloys at different Al contents

In this paper, the microstructure and thermophysical properties of two CoNiCrAl bond coat alloys with differing Al contents were investigated. Two CoNiCrAl alloys with compositions of Co-31.7%Ni-20.8Cr-10Al and Co-31.7%Ni-20.8Cr-12Al (all in wt%) were prepared by vacuum induction melting and casting. The microstructure of the as-cast CoNiCrAl alloys was characterised by scanning electron microscopy (SEM), X-ray diffractometry (XRD), energy dispersive spectroscopy (EDS) and electron backscattered diffraction (EBSD). The phase changes of the CoNiCrAl alloys as a function of temperature were studied by differential scanning calorimetry (DSC). Thermophysical properties such as thermal conductivity, thermal diffusivity and thermal expansion were evaluated. The as-cast CoNiCrAl alloys exhibited a two-phase structure, consisting of the Al-rich BCC β phase and Co and Ni-rich FCC γ phase. It was found that the β-phase fraction increased with the overall Al content in the alloy. A two-stage thermal expansion was noted for both CoNiCrAl alloys. In addition, it was shown that the CoNiCrAl alloy with the higher Al content exhibited a lower thermal expansion coefficient at high temperatures. This is because more β phase was formed in the CoNiCrAl-12 alloy, resulting in a reduced thermal expansion coefficient.