Microstructural instability in coated single crystal superalloys

Specimens of CMSX-4 and CMSX-10 alloys have been platinum aluminised to give a single phase (Ni,Pt)Al coating and a two phase PtAl₂-CNi,Pt)Al coating, respectively. Specimens of the coated alloys were exposed in an oxidising environment for 188, 350 and 750 h at a temperature of 1100°C. Specimens were then sectioned, polished and examined using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS) and X-ray diffraction (XRD). Decreases in nickel activity and increases in aluminium activity occurred within the substrate close to the coating/alloy interface. These effects resulted in precipitation of additional γ′in the alloy, which caused rafting of the γ′and encapsulation of needle-like sigma phase in γ′. The development of these needle-like sigma phases beneath the diffusion layer in the CMSX-10 alloy at 1100°C is significant, such that they extend to a depth of 0.2 mm. In addition, they cause failure at the coating/substrate interface in the case of the CMSX-10 alloy. For the CMSX-4 alloy, equilibration effects remote from the coating cause further precipitation of sigma phases in isolated cells in γ′ regions. The implications of these instabilities on superalloy properties are discussed.