A thermodynamic and kinetic basis for understanding chromium depletion in Ni-Cr-Fe alloys

Abstract

Thermodynamic and kinetic models are constructed to describe the development of the chromium depleted zone in Ni-Cr-Fe alloys heated in the range 773-1173 K. The models are interactive and constitute a computer program called DEPLETE. The thermodynamic model is constructed using the Kohler method for the description of the free energy of a multi-component system. It provides the chromium concentration at the carbide-matrix interface as a function of alloy composition and temperature. The kinetic model tracks the shape of the chromium profile as a function of time at temperature and grain size. Model results show that the interfacial chromium concentration decreases for increasing carbon concentration and decreasing heat treatment temperature. Experimental verification of the model is made using high resolution energy despersive X-ray analysis via STEM. Measured results agree well with model results for the dependence of chromium depletion on various input parameters as well as the magnitude and shape of the chromium depleted zone. Experimental measurements also show that the grain boundary carbides are of the form M7C3 where M is about 96% chromium. Results confirm that carbide precipitation at the grain boundary is controlled by volume diffusion of chromium in the matrix and that in the temperature range 873 to 1073 K the chromium concentration in the grain boundary accurately approximates the carbide-matrix interfacial concentration.