Evaporation of Chromium at the Surface of Ni–Cr–Fe Heat-Resistant Alloys During Long Exposure Times at 1100 °C in High-Vacuum

The evaporation of chromium at elevated temperatures in vacuum, commonly employed in the processing and heat treatment of Ni–Cr–Fe heat-resistant alloys, has been investigated. Two variants of these alloys, each containing distinct alloying elements, were exposed at 1100 °C for durations of 3 to 500 hours. These exposures were conducted within a vacuum environment, both with and without applied stress. During the experiments, the samples displayed the development of a carbide-free layer (M₂₃C₆/M₇C₃) with thicknesses ranging from 10 to 100 μm. Employing Energy Dispersive Spectroscopy (EDS), the composition of the alloys was analyzed as a function of depth beneath the surface. Notably, the samples exhibited a pronounced gradient of chromium content. This gradient corresponded to the dimensions of the carbide-free layer, suggesting that the dissolution of carbides is due to the chromium evaporation process. A noticeable difference in the behavior of the two alloys was identified and deduced to be related the effect of aluminum on the development of concentration gradients and the associated surface evaporation of chromium. These observations were interpreted within the framework of a diffusion-based evaporation model which shed light on the intricate interplay between alloy composition, carbide dissolution, and chromium evaporation during high-temperature exposure.