In situ and ex situ characterization of microstructure evolution of a γ-γ’ coating on CMSX-4 Plus superalloy during thermal cycling: Insights into Pt diffusion and phase transformations

The microstructure evolution of a Pt-rich γ-γ’ coating deposited on a third generation CMSX-4 Plus nickel-based superalloy was investigated during relatively short thermal cycles (from 300 °C to 1100 °C with a dwell time of 5 min at 1100 °C) using both ex situ and in situ characterization techniques. Ex situ analyses, including Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray Spectroscopy (EDS) and postmortem X-Ray Diffraction (XRD), demonstrated that Pt and Al interdiffusion induced by thermal cycling leads to significant microstructure changes in the coating. Lattice parameters of γ and γ’ phases were revealed to correlate with Pt content. In a novel approach, in situ XRD using laboratory equipment was employed to monitor microstructural evolution during 250 thermal cycles. This unique in situ analysis highlighted the microstructural differences at low and high temperatures within each cycle. For the first time, it was observed that partial dissolution of γ’ precipitates occurs at high temperatures, altering the local chemical composition of both γ and γ’ phases. • Diffusion of Pt and Al drives the evolution of γ-γ’ coating microstructure during thermal cycling. • A direct correlation between γ and γ’ lattice parameters and their respective Pt content was established by XRD and EDS. • First in situ observation of Pt-rich γ-γ’ coating microstructure during cycling. • X-ray diffraction reveals γ' phase dissolution and reprecipitation during cycling. • X-ray diffraction highlights reversible Pt enrichment in γ phase at high temperatures.