NbC TRANSFORMATION DURING AGING IN HP40-Nb HEAT RESISTANT ALLOY

Abstract

In this work, the evolution of MC-type Nb-rich primary carbides in a 35Ni-25Cr-Nb-type refractory alloy, commonly known as HP40-Nb, and its transformation into Ni-Nb silicide known as G-phase (Ni₁₆Nb₆Si₇), has been studied. For this purpose, the experimental technique of scanning electron microscopy was used together with X-ray microanalysis to detect changes in the chemical composition of niobium carbide over time and at a given aging temperature. The microstructure of the studied alloy, in its as-cast condition, consists of an austenitic matrix strengthened by a network of primary eutectic-like carbides rich in chromium and niobium of the M₂₃C₆ and MC types, respectively. During aging of the alloy at high temperatures, microstructural changes take place such as the secondary precipitation of M₂₃C₆ type carbides and the transformation of the primary Nb-rich carbide towards the Ni-Nb-rich silicide. It has been found that the transformation begins at the interface of the niobium carbides with the matrix, progressing towards their interior with the development of aging.