Effect of prealuminizing diffusion treatment on microstructural evolution of high-activity Pt-aluminide coatings

The effect of prealuminizing (or prior) diffusion treatment on the evolutio n of Pt-aluminide coatings on the Ni-based superalloy CM-247 has been studi ed by using a single-step, high-activity aluminizing process, Coatings gene rated without any prior diffusion treatment, as well as those formed by ado pting two extreme prior-diffusion schedules (at 850 degrees C for 0.5 hours and at 1034 degrees C for 5 hours), were investigated by analyzing the coa ting structures at various stages of aluminizing. When the dilution of the Pt layer, caused by its interdiffusion with the substrate during the prior diffusion treatment, is only marginal (as in the case of no prior diffusion and diffusion at 850 degrees C for 0.5 hours), the equilibrium Pt-aluminid e coating structure evolves through the formation of two transient layers d uring the initial stages of aluminizing, In contrast, for diffusion at 1034 degrees C for 5 hours, which results in extensive dilution of the Pt layer , the two-phase equilibrium structure (PtAl2 in a matrix of NiAl) in the ou ter layer of the coating is found to develop during very early stages of al uminizing and remains unchanged, even over extended periods of aluminizing. Further, in the case of prior diffusion at 1034 degrees C for 5 hours, Pt is found to remain distributed to a greater extent over the entire thicknes s of the coating than in the cases of limited prior diffusion treatment. Th e present findings underline the significance of the nature of the prior-di ffusion schedule on the microstructural evolution of Pt-aluminide coatings. It has also been found that a prealuminizing diffusion treatment is partic ularly important for Pt-aluminide coatings from the point of view of coatin g adhesion to the substrate.