Microstructure and chemistry of aluminide coating of Ni–Cr and Ni–Co–Cr alloys

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Abstract
Alloys of composition Ni–20Cr, Ni–35Cr, Ni–50Cr, Co–35Cr, Ni–5Co–20Cr, and Ni–20Co–20Cr have been aluminided by pack cementation. Morphology of the coatings varies considerably among the different alloys. The most striking differences are the avoidance of formation of Kirkendall‐type voids in the Ni–Co–Cr alloys, and the degree of continuity of the α‐Cr layer formed. Both features need to be controlled to avoid coating loss by cracking and spallation. From the appropriate phase diagrams, the sequence of morphologies formed by aluminization can be rationalized. The relation of total aluminum deposition to alloy composition results from phase equilibria and phase solubilities, and phase diagrams can be used to rationalize this relation as well.

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