Abstract:
Mechanically alloyed Ni-20Cr (wt%) nanocrystalline powder was cold spray deposited on SA 516 Grade 70 steel.
The microstructure and high temperature oxidation characteristics of Ni-20Cr nanostructured alloy coated steel
have been investigated. The cyclic oxidation was conducted in air at 900 °C for periods up to 50 h. The coated
steel was examined prior to and after cyclic oxidation using SEM, EDX, FIB, TEM and SAED. The TEM analyses
showed that the coating exhibits equiaxed nanoscale microstructure. Interestingly, the high temperature cyclic
exposure significantly influences the grain growth despite the segregation and preferential formation of nanoislands of Cr2O3 within the coating matrix. Furthermore, the enhanced grain boundary diffusion of Cr in the
nanostructured coating promotes the formation of denser Cr-rich protective surface oxide layer, which possesses
a superior oxidation resistance, i.e. prevents the corrosion of substrate steel. The microstructural changes,
crystallographic and chemical aspects of the Ni-20Cr nanostructured coating upon cyclic exposure are discussed
in the context of its high temperature corrosion protection property
