A phase-field study of the aluminizing of nickel

Abstract

A quantitative phase-field approach for multiphase systems that is based upon CALPHAD free energies is used to model the aluminization of nickel wires, wherein vapour-phase alloying is used to deposit Al on the surface of the Ni wire and then the wire is annealed so that to remove all Al gradients and achieve a homogenous Ni-Al alloy. Both processes are modelled and numerical results are compared with experiments. It is found that the kinetics of both processes is controlled by bulk diffusion. During aluminization at 1273 K, formation and growth of intermetallics, Ni₂Al₃ NiAl and Ni₃Al, are strongly dependent on the Al content in the vapour phase. Ni₂Al₃ growth is very fast compared with NiAl and Ni₃Al. It is also found that an intermediate Al content in the vapour phase is preferable for aluminization, since the Ni₂Al₃ coating thickness is difficult to control. Ni₂Al₃ is found to disappear in a few minutes during homogenization at 1373 K. Thereafter, the NiAl phase, in which the composition is highly non-uniform after aluminization, continues growing until the supersaturation in this phase vanishes. Then, NiAl coating disappears concomitantly with the growth of Ni₃Al, which disappears thereafter. Finally, the Al concentration profile in Ni(Al) homogenizes.

Keywords:

• Phase-field method
• coatings
• diffusion
• phase transformations

Acknowledgements

The authors thank Mr Edward Lee Pang (Northwestern University) for his help with metallography and microscopy and Prof. Kevin Hemker (Johns Hopkins University) for helpful discussions.

Additional information

Funding

This research was supported by DARPA under contract W91CRB1010004 (Dr Judah Goldwasser, monitor).