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

Abstract

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 CrAl7,Ni2Al9,FeNiAl9,NiAl3,Ni2Al3 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 NiAl with different Al (Al-rich) and Ni (Ni-rich) saturations.At the interfacial boundary of NiAl phases with different composition precipitates are presented as a randomly distributed individual inclusions of solid solutions based on Cr and Cr3Si. At the ”coating - substrate” interface they are presented as a continuous secondary reaction zone, consisting of a large number of precipitates Cr(Ni) and Cr(Ni,Al) (Ni80Cr20 alloy) and a smaller amount of Cr(Fe,Ni) and Cr(Fe,Al,Ni) (Ni60Cr15 alloy). The formation of these heterogeneity elements is due to the limited solubility of Cr in NiAl.A protective oxide α−Al2O3 is formed on the coating surface as a result of heating. An increase in the exposure time leads to the formation of the NiAl2O4 spinel type oxide along with Al2O3 on the surface of the coating on the Ni80Cr20 alloy.