Microstructure of aluminized coating on a Ni–Cr alloy after annealing treatment

The effects of annealing on the microstructure of first stage (high-Al activity pack) aluminized coating on Ni–15Cr alloy prepared by pack cementation method were analyzed by transmission electron microscope. The coating consists of a thin layer of γ′-Ni3Al, an interfacial zone of mixed β-NiAl and α-Cr, and a thick outer zone of β-NiAl (A layer) and mixed β-NiAl and α-Cr (B layer). Martensitic transformation was observed in the β-NiAl grains in the interfacial zone. Parallel crystallographic relationship was found at the γ/γ′ interface in the substrate and the α/β interface in the interfacial zone. Cr2Al was found to precipitate in the β-NiAl and α-Cr grains in the B layer of the outer zone. The formation mechanisms of the coating layers, the precipitates, and the observed crystallographic relationships are discussed.