LATTICE AND GRAIN-BOUNDARY DIFFUSION OF CHROMIUM IN SUPERALLOY INCOLOGY-800

The diffusion of chromium in Incoloy-800 has been studied by the serial sectioning technique using the radioactive tracer Cr-51 in the temperature range 1060-1510 K for lattice diffusion and 775-1170 K for grain boundary diffusion. The lattice diffusion coefficient could be represented by the relation: D(Cr/Incoloy-800) = 3.24 x 10(-4) exp[(- 287.4 kJ/mol)/(RT)] m2/s. The grain boundary diffusion coefficients were evaluated for most of the experiments by Whipple's method. For a few specimens, 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 equation: D(gb,Cr/Incoloy-800) = 5.80 x 10(-5) exp[(- 184.2 kJ/mol)/(RT)]m2/s. Segregation and mass transport along the grain boundaries have also been extensively studied by an autoradiographic technique. It has been observed that for small-grained specimens at low temperatures (< 980 K) long-distance transport of the tracer atoms is mainly through the grain boundaries.