Effect of heat treatment on coating microstructure applied by high activity diffusion process on IN738LC

Abstract

To study the effect of heat treatment on the microstructure and composition of high activity aluminide diffusion coating, a number of IN738LC specimens were coated at 850 and 1050°C via high activity aluminising process for 4 h and then heat treated at 1120°C for 2 h. Then, the microstructures of the coatings were examined via X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). For the high temperature process, the thickness of the outer layer of the coating increased basically due to the inward diffusion of aluminium, which demonstrated that a chemical potential gradient exists for the inward diffusion of aluminium even after 4 h of inward diffusion at 1050°C. The EDS results provided the evidence that the partition of interdiffusion zone into two separate zones could be due to the outward diffusion of chromium. Surprisingly, the decomposition of the precipitates formed in the first step was observed, indicating that those precipitates were not stable.