Nickel Enriched Diffusion Layers on a NiAl Alloy

Abstract

The intermetallic phase NiAl is a perspective material for high-temperature and shape memory effect applications. Formation of Ni5Al3, Ni2Al, Ni3Al phases which influence the extent of martensitic transformation in NiAl have been studied up to now with controversial results. We have investigated (using SEM and local elemental analyses) the microstructure of nickel enriched surface layers on a Al-79 wt.% Ni alloy. The layers were prepared by diffusion annealing and subsequently given two different heat treatments: at 930°C outside the Ni5Al3 region and at 500°C within the Ni5Al3 region of the phase diagram. In the specimen which was only diffusion annealed separate islands of Ni5Al3 phase elongated in the direction of the concentration gradient could be recognized within the nickel enriched surface layer. In the samples additionally annealed at 500°C, a well defined continuous layer of the Ni5Al3 phase situated 0.4 mm below the specimen surface was found. In the samples annealed at 930°C, isolated Ni3Al precipitates were observed. Their number and size gradually increased with increasing nickel content.