The grain boundary self-diffusion of Hf was studied in the hexagonal close-packed  phase of the alloy, at temperatures ranging from 910 to 1493K, by using radio-tracer and serial sectioning techniques. It was not possible to stabilize the grain size fully by using pre-diffusion heat treatments, so some of the grain boundaries moved during diffusion annealing due to continuing grain growth. Consequently, the measured penetration profiles were untypical in shape and could not be treated by using standard grain boundary diffusion models which considered only stationary grain boundaries. A procedure for treating such profiles was developed on the basis of a simple model for tracer diffusion in a polycrystal which contained both stationary and moving grain boundaries. By using this method, it was possible to determine the product of the grain-boundary self-diffusion coefficient and grain boundary width for stationary grain boundaries. It was also possible to estimate the grain boundary migration velocities by assuming that the product values for stationary and migrating grain boundaries were the same. The activation enthalpy and pre-exponential factor of the product for self-diffusion along stationary boundaries (table 92) were deduced to be 212kJ/mol and 3.5 x 10-13m3/s. In spite of the presence of 3%Zr, these parameters were expected to describe closely the self-diffusion in pure -Hf. The data indicated the operation of a vacancy mechanism for grain boundary self-diffusion in the latter, and were consistent with previous results for other hexagonal close-packed transition metals. A rough estimate of the activation enthalpy for grain boundary migration (195kJ/mol) was in good agreement with that for grain boundary diffusion, and suggested that the activation barriers to atomic transport across, and along, the grain boundaries were almost identical.

F.Güthoff, J.Mishin, C.Herzig: Zeitschrift für Metallkunde, 1993, 84[8], 584-91

Table 92

Grain Boundary Self-Diffusivity in -Hf