Grain boundary (GB) self-diffusion was investigated in sintered tungsten material by the radio-tracer serial-sectioning technique using the W-185 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 D-GB = 1.41 x 10(-5) exp(-294 kJ mol(-1)/RT) m(2) 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) approximate to 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) approximate to 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. [References: 26]
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