A Study of the Effects of Ta and Y in the Microstructure and Oxidation of MCrAlY Based Bond Coating Alloys

Ni based superalloys in power plants operating with a wide range of fuels using combined cycle power plant technologies require protection for high temperature oxidation and type I and type II hot corrosion. MCrAlY overlay coatings are environmental protection coatings that can provide an optimised corrosion response over a wide range of turbine operating conditions that are likely to be encountered in utility turbines, working with multi-fuel capability. In this study we have concentrated on Co and Cr rich, Ni-Co-Cr-Al based bond coat type alloys. We shall report on the effects of Ta and Y individually and simultaneously in the as cast and heat treated (1200 ºC/ 100 h, 200h) microstructures and oxidation behaviour of Ni-23Co-20Cr-8.5Al (wt%) based alloys. The paper shall discuss phase formation and stability with particular emphasis on solidification path and composition of phases.  The oxidation behaviour of the alloys at 975 ºC will also be compared.