In Situ Observation of Microstructure and Precipitate Phase Transformation during the Solidification of Mg-Containing GH3625 Alloy at Different Cooling Rates

Abstract

In practical applications, intermetallic compounds like Laves phase and metal carbides adversely affect the performance of nickel-based superalloys. Using a high-temperature confocal laser scanning microscope, the solidification process of as-cast GH3625 alloy containing Mg at different cooling rates (−20, −35, and −50 °C min⁻¹) is studied. Fitting curves of the volume fraction of the solid phase with solidification temperature before and after Mg treatment are obtained. Trends of solid phase transformation rates with solidification temperature are determined. Differential scanning calorimetry is employed to analyze and statistically evaluate the melting temperature range and enthalpy of each phase during the melting process. Experimental results demonstrate that Mg treatment significantly accelerates the alloy solidification at the cooling rates of −20 and −35 °C min⁻¹, while reducing the area of residual liquid phase at the same solidification temperature, disrupting the Laves/NbC eutectic relationship, and regularizing NbC morphology, transitioning its distribution from aggregation to dispersion. After Mg treatment, the precipitation of the Laves phase is significantly reduced. As a result, the influence mechanism of Mg treatment on the phase transformation and microstructure of GH3625 is clarified based on homogeneous nucleation theory.