Effect of Nb contents on the surface tellurization and intergranular cracking behaviors of Ni-Nb-C alloys

Abstract

The effect of Nb addition on the surface tellurization and intergranular cracking behaviors of the Ni-xNb-C alloys in tellurium (Te) vapor have been investigated in this study. It was found that the main tellurization products on the surface are Ni3Te2 layer and Ni2Nb4C lamellar carbides, and the latter only form in the high-Nb alloys. A small number of NbO2 particles can be also observed on the surface of Ni3Te2 layer in the high-Nb alloys which result from the residual oxidizing media. The characterizations on the tellurization products proved that the Nb addition cannot introduce the Nb-related protective layer to suppress the tellurization reaction. The cracked tellurization product layer would allow Te to reach the substrate and attack the grain boundaries. The distribution depth of Te at the grain boundaries was found to decrease with the Nb concentration increasing, especially in the range from 0 to 1 wt%. Because of the abundant supply of Te to the substrate, it can be deduced that the diffusion of Te along grain boundaries becomes slow due to the addition of Nb. Accordingly, the intergranular cracking is significantly suppressed by adding 1 wt% Nb, but the improving effect become not evident with more additions of Nb. In this work, the improving mechanism was ascribed to the slower diffusion of Te along the grain boundaries in Nb-containing alloys. There still need the further experimental investigations to uncover the improving mechanism at the atomic scale in the future.