Microstructure and phase composition of diffusion coating formed in NiCr alloys by hot-dip aluminizing

• A uniform coating without defects is formed on the NiCr alloys by hot-dip aluminizing. • The initial coating consists of an Al matrix with C r A l 7 , N i 2 A l 9 , F e N i A l 9 , N i A l 3 , N i 2 A l 3 intermetallic inclusions. • The addition of Fe into the Ni-Cr-Al system leads to a change in the diffusion interaction processes. • Heating of the coatings leads to a diffusion redistribution of the elements and the formation of precipitates with a high Cr content. • A protective oxide film of α − A l 2 O 3 is formed on the coating surface during heating. In this paper, the results of a study of the structure and phase composition of diffusion coatings formed on the surface of Ni80Cr20 and Ni60Cr15 alloys after hot-dip aluminizing and subsequent high-temperature heating are presented. It is shown that as a result of aluminizing on the surface of NiCr alloys a uniform coating without defects is formed.The coating consists of an Al matrix with C r A l 7 , N i 2 A l 9 , F e N i A l 9 , N i A l 3 , N i 2 A l 3 intermetallic inclusions. It is found that heating of the coatings at 1100 °C leads to a diffusion redistribution of the elements over their thickness and the formation of a layered quasi-two-phase structure of Cr (in case Ni80Cr20 alloy) or Cr and Fe (in case Ni60Cr15 alloy) solid solutions in N i A l with different Al (Al-rich) and Ni (Ni-rich) saturations. At the interfacial boundary of N i A l phases with different composition precipitates are presented as a randomly distributed individual inclusions of solid solutions based on Cr and C r 3 S i . At the ”coating - substrate” interface they are presented as a continuous secondary reaction zone, consisting of a large number of precipitates C r ( N i ) and C r ( N i , A l ) (Ni80Cr20 alloy) and a smaller amount of C r ( F e , N i ) and C r ( F e , A l , N i ) (Ni60Cr15 alloy). The formation of these heterogeneity elements is due to the limited solubility of Cr in N i A l . A protective oxide α − A l 2 O 3 is formed on the coating surface as a result of heating. An increase in the exposure time leads to the formation of the N i A l 2 O 4 spinel type oxide along with A l 2 O 3 on the surface of the coating on the Ni80Cr20 alloy.