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Abstract : |
A study was carried out to determine the role of Y during high temperature oxidation or an M-Cr-Al-Y-type coating on an Ni-base superalloy using various techniques of electron microscopy. Prior to oxidation exposures, Y was found to segregate within an external surface layer of the coating about 5 Irm in thickness as well as within the interdiffusion zone. After exposure at 1000 or 1100 degrees C in still air, Y exhibited a preferential tendency for segregation at grain boundaries of alpha-Al2O3 scale developed by the coating, however, a finite concentration of Y was also present in solid solution. In addition, some of the Y was found to oxidize into a Y-rich oxide ''pegs'' within the alpha-Al2O3 scale and extending into the coating. Experimental results showed that segregation of Y to grain boundaries could maintain a fine-grained scale after exposure for up to 1000 h at 1000 or 1100 degrees C. It was concluded that, for the coating-substrate system studied, Y could improve the protective nature of alpha-Al2O3 scale by various mechanisms operating simultaneously., |
