Grain Boundary Self-Diffusion in Polycrystalline Tungsten at Low Temperatures

Grain boundary (GB) self-diffusion was investigated in sintered tungsten material by the radiotracer serial-sectioning technique using the 185W radioisotope. Measurements were carried out in the low-temperature range from 1160 to 1323 K in the Harrison type-C kinetic regime yielding directly the GB diffusion coefficient DGB = 1.41 × 10 - 5 exp ( - 294 kJ mol - 1/RT) m2 s ^{- 1}. The activation enthalpy Q_GB amounts to about 0.55 times the value Q_V of W volume self-diffusion. This ratio is larger than observed for GB diffusion in very pure metals (Q_GB 0.35 to 0.45Q_V). From additional measurements at higher temperatures, 1873 to 2173 K, in type-B kinetic regime, a smaller value of Q_GB 0.45Q_V was estimated. By combining all results it is found that the GB diffusivity in the low-temperature range is considerably reduced with respect to the extrapolated high-temperature results. This observation is discussed in terms of a strong segregation effect of (spurious) impurities like C and P in the tungsten GBs.