GRAIN-BOUNDARY SELF-DIFFUSION IN POLYCRYSTALLINE TUNGSTEN AT LOW-TEMPERATURES

Grain boundary (GB) self-diffusion was investigated in sintered tungst en material by the radio-tracer serial-sectioning technique using the W-185 radioisotope. Measurements were carried out in the low-temperatu re range from 1160 to 1323 K in the Harrison type-C kinetic regime yie lding 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 t o about 0.55 times the value Q(V) of W volume self-diffusion. This rat io 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 hig her 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 ra nge is considerably reduced with respect to the extrapolated high-temp erature results. This observation is discussed in terms of a strong se gregation effect of (spurious) impurities like C and P in the tungsten GBs.