KINETIC ANALYSIS OF SECONDARY PRECIPITATION IN A HP40-Nb ALLOY

Abstract

The HP40-Nb heat resistant alloy (35Ni-25Cr-Nb) was analysed by means of optical microscopy after aging treatments at 1073 and 1173 K for different times, in order to apply the classic Johnson – Mehl - Avrami – Kolmogorov kinetic model (JMAK), and thus calculate the activation energy of secondary M₂₃C₆ precipitation, which occurs during thermal aging. The relevance of this theoretical analysis is to infer the mechanism that controls the nucleation and growth of M₂₃C₆ secondary carbides, since the amount and morphology of these phase influences the mechanical properties as well as the corrosion resistance in service. After performing the kinetic analysis using the JMAK model, the activation energy was found to be 208 kJ/mol, which would indicate that the secondary precipitation in this alloy is controlled by the Cr-diffusion phenomenon along the austenitic matrix.