Mass transport of cobalt and copper in Monel-400

Abstract

The diffusion of cobalt and copper in Monel-400 has been studied by serial sectioning technique using the radioactive tracers 60Co and 64Cu in the temperature ranges 1070 to 1570 K for volume diffusion and 700 to 1100 K for grain-boundary diffusion, respectively. The volume diffusion coefficients can be expressed as: for Co in Monel-400 (1070–1570 K) D Co = 3.09 × 10 −4 exp( −260.1 kJ/mol RT m 2/s , for Cu in Monel-400 (1070–1570 K) D Cu = 2.88 × 10 −4 exp( −270.8 kJ/mol RT m 2/s . The grain-boundary diffusion coefficients were evaluated for most of the experiments by Whipple's method. For a few experiments Suzuoka's method was also applied. The two methods yielded values of grain-boundary diffusion coefficients that were in good agreement with each other. The grain-boundary diffusion coefficients could be represented by the expressions: for Co in Monel-400 (700–1100 K) D gb,Co = 3.24 × 10 −5 exp( −151.0kJ/mol RT ) m 2/s , for Cu in Monel-400 (700–1100 K) D gb,Cu = 1.13 × 10 −4 exp( 165.8kJ/mol RT ) m 2/s , Autoradiographic technique was also used extensively to study the segregation of the diffusing tracers. It was observed that for small-grained specimens (average grain dia ~ 200 μm) at temperatures above ~ 1000 K both volume and grain-boundary diffusion processes contribute significantly to the penetration of tracer atoms in the bulk of the solid, but at lower temperatures (< 960 K) the migration is mainly through the grain boundaries.