HIGH-ANGLE GRAIN-BOUNDARY DIFFUSION OF CHROMIUM IN NIOBIUM BICRYSTALS

Grain boundary diffusion of Cr in [100]/[001] symmetrical Nb bicrystal s with varying tilt angles, theta, was investigated in the temperature range of 958-1197 degrees C. For low angle boundaries, the grain boun dary diffusion is dominated by the dislocation pipe mechanism. in whic h the activation energy Q' is independent of theta and the pre-exponen tial factor (sD'delta)(0) varies linearly with sin(theta/2). For high angle boundaries, the grain boundary diffusion coefficients, sD'delta, fall into minima, whereas the activation energies Q' attain maxima at the CSL boundaries, Sigma 13 (22.6, 67.4 degrees), Sigma 17 (28.1? 61 .9 degrees) and Sigma 5 (36.9, 53.1 degrees). The pre-exponential fact or (sD'delta)(0) also varies with theta, but the variation is random. The results of the study agree with those of the f.c.c. system, confir ming that grain boundary diffusion is strongly affected by the boundar y structure and associated energies. Copyright (C) 1996 Acta Metallurg ica Inc.