Microstructural Formation During Wire Arc Spraying of Alloy 625 on Wrought Alloy 625 Substrate

In the present study, important microstructural features of the wire-arc-sprayed nickel-based superalloy 625 were investigated. A combination of optical microscopy and scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), image analysis, X-ray diffraction (XRD) analysis, and transmission electron microscopy (TEM) was used to evaluate the microstructural characteristics of the as-sprayed wire arc-sprayed coating. The apparent density of the coating was determined by the water displacement method, while its theoretical density was calculated from information obtained from XRD analysis. The porosity of the coatings was less than 7 pct from image analysis. The chromium oxide phase, identified by EDS analysis, was distributed throughout the microstructure. Image analysis showed approximately 15 pct of the oxide in the microstructures of the coatings. XRD analysis and the matrix flushing technique indicated the formation of approximately 12 wt pct oxide in the coating microstructure. The lattice parameter of the main fcc Ni alloy phase was found to be 3.5978 Å. SEM images of etched cross sections of coatings revealed areas within the fcc phase regions which were more deeply etched. EDS analysis indicated that these areas had lower concentrations of Mo and Nb than the more lightly etched areas, suggesting the presence of two fcc alloy phases in these coatings. TEM study indicated that the microstructure consisted of Ni-rich regions with Cr₂O₃. The lattice parameter for the Ni grain was found to be 3.46 Å from the high resolution transmission electron microscopy (HRTEM).