Microstructure and positron lifetimes of zirconium modified aluminide coatings

The microstructure of the zirconium modified and non-modified aluminide coatings was examined by the EDS, XRD, TEM and the positron annihilation spectroscopy methods. Both coatings have a double layer structure: β-NiAl phase on the top and γ′-Ni3Al below. Small zirconium nanoparticles were found along grain boundaries in the β-NiAl phase. The positron lifetime in both coatings is the same. The formation of zirconium precipitates neither affects, the number of defects nor the volume diffusion. Zirconium nanoparticles that precipitate along grain boundaries stand against the outward diffusion of Al ions through the coating to the coating/oxygen interface. This “blocking effect” may be responsible for the reduction of the alumina scale growth rate and may delay pore formation on the coating/oxygen interface.