Microstructural instability in coated single crystal superalloys

• Published in: Journal of materials processing technology Vol. 153-154; pp. 660 - 665
• Main Authors: Reid, M., Pomeroy, M.J., Robinson, J.S.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 10.11.2004
• Subjects: Diffusion; Platinum aluminide; Superalloys; Platinum aluminide; Superalloys; Diffusion
• ISSN: 0924-0136
• DOI: 10.1016/j.jmatprotec.2004.04.132

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–(Ni,Pt)Al coating, respectively. Specimens of the coated alloys were exposed in an oxidising environment for 188, 350 and 750h 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 0.2mm. 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.