Volume and grain boundary diffusion of chromium in Ni-base Ni-Cr-Fe alloys

Volume and grain boundary diffusion of chromium in Ni-16 mass%Cr-7 mass%Fe alloys containing 0.004, 0.015 and 0.07 mass% of carbon have been measured using radioactive tracer Cr-51 over the temperature range of 858-1424K by serial radio-frequency sputter-microsectioning technique. While the volume diffusion coefficients are largely unaffected by the presence of carbon, the increase in carbon content markedly reduces the mobility of chromium atoms along the grain boundaries. As a result. the difference between the activation energy for lattice and grain boundary diffusion decreases with increase in carbon content. In fact, for 0.07 at% carbon, the activation energies for volume and the grain boundary diffusion are nearly same. Among all three alloys, the difference between the volume diffusion coefficient, D-delta, as well as deltaD(gb) (delta = grain boundary width; D-gb = grain boundary diffusion coefficient) decreases gradually in a regular manner with the rise of temperature. Grain bounadry energy is reduced by addition of carbon and enhanced by temperature, At higher temperatures, there is no difference between the grain boundary energies of the three alloys.