Diffusion of Nb with Cr, Fe, Ni, Mo, and stainless steel

Abstract

Diffusion couples were prepared at about 1100° C from pure Nb and several compositions of stainless steel and also with each of the components of the steel, namely Cr, Fe, Ni, and Mo. Using the electron probe microanalyzer to measure the composition across these diffusion zones, intermediate phases of stoichiometric composition were found for each of the binaries. In Nb-Cr diffusion the phases NbCr 2 and NbCr 7 were always present and after long heating times a NbCr phase with excess Nb in solid solution was observed as well. In Nb-Fe diffusion the intermediate phase was NbFe 2 plus excess Fe in solid solution, but there were isolated Nb-rich compounds far out into the iron depending on the impurity composition of the iron. Nb-Ni couples often melted near 1100° C although the eutectic composition is given as 1175° C in some of the literature. Below 1095° C NbNi with excess Nb in solid solution was observed, and Nb 2Ni precipitated in the NbNi matrix on cooling. Above 1095° C, at near melting, rapid diffusion led to zones as long as 1400 microns with NbNi 3 precipitating in an NbNi matrix on cooling. Nb and Mo were miscible at all compositions and the diffusion coefficient ranged from 3 to 7 × 10 −14 cm 2/sec. Diffusion of Nb with stainless steel showed a main zone of about 40 % Nb, 40 % Fe, 8 % Cr, and 5 % Ni (all as weight percent) which was speculated to contain a mixture of NbFe 2, NbCr 2, and NbNi. A small region near the Nb side of the diffusion zone was speculated to contain a mixture of NbFe 2, NbCr, and Nb 2Ni.