Positron Annihilation Studies of Modified Aluminide Coatings on Nickel and Nickel Superalloy

Aluminide coatings deposited by CVD method on CMSX-4 superalloy, modified by palladium and zirconium or hafnium as well as non-modified and palladium modified ones on pure nickel have been studied using positron annihilation spectroscopy. The positron lifetime values and the positron effective diffusion length in the coatings studied point out trapping of positrons in defects associated with dislocations of high density. Palladium used for modification of the aluminide coatings substitutes nickel in the β-NiAl phase. This gives rise to the increase in the positron lifetime indicating a higher number of open volume defects in the crystal lattice. Additional modification of the aluminide coating on CMSX-4 nickel superalloy with zirconium or hafnium causes only slight changes in the positron lifetime which may be related to the differences in location of precipitates in the coatings.