Order-disorder transformation in a NiCoCrAlY bond coat alloy at high temperature

• Published in: Scripta materialia 2000-11, Vol.43 (11), p.963-968
• Main Authors: Rehfeldt, T, Schumacher, G, Vaßen, R, Wahi, R.P
• Format: Article
• Language: English
• Subjects: Applied sciences; Coating; Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; High temperature; Materials science; Metals. Metallurgy; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Phase transformations; Physics; Plasma spray
• ISSN: 1359-6462; 1872-8456
• DOI: 10.1016/S1359-6462(00)00536-4

To further increase the service temperature of nickel superalloy turbines, thermal barrier coatings are applied, usually with MCrAlY bond coat interlayers for stress accommodation of the thermal mismatch of substrate and ceramic thermal barrier. The microstructure of a MCrAlY bondcoat alloy has been determined by means of scanning electron microscope (SEM) and transmission electron microscope (TEM) in the as-produced state and after heat treatment at elevated temperatures followed by subsequent quenching into water. A beta -phase, a gamma '-phase both of them structurally ordered and a chromium rich phase were found in the as-produced state. The gamma '-phase was shown to undergo an order-disorder transformation between 1273-1373K. The lower stability of the gamma '-phase in comparison with Ni sub 3 Al is assigned to the lower content of Al and to the presence of the additional alloying elements Co and Cr. The structural stability of the beta -phase at high temperatures is not known. This does, however, not necessarily mean that the beta -phase is stable at high temperatures. Due to the simple structure of the beta -superlattice, ordering of the high temperature disordered phase is supposed to be so fast that the disordered state cannot be frozen-in during quenching from high temperatures. Measurements at high temperature only can decide whether the beta -phase is thermally stable in the temperature range of interest or not.