Regular features of intercrystallite diffusion of cobalt in polycrystalline tungsten

Results of investigations into intercrystallite diffusion in the Co-57(poly -W) system, which were obtained using a new method [1, 2], were interpreted in terms of a modified model [3] describing the structure of crystallite c onjugation regions (CCR's) in bi- and polycrystals. This paper discusses Ar rhenius dependencies lnq(1/T) of the population of states q in the intercry stallite diffusion zone of Co-57 atomic probes (AP's) in tungsten polycryst als (poly-W). Regular features of intercrystallite diffusion largely depend on the interaction of diffusing AP's and point defects with CCR's. This in teraction determines appearance and properties of segregation's of diffusin g AP's and residual impurities at CCR's. Even if diffusion annealing temper ature is low, "vacancy-residual interstitial impurity" complexes (VIIC's), which segregate at CCR's, dissociate thermally. The concentration of free v acancies, which are formed in the segregation zone of VIIC's, is many order s of magnitude higher than the volume concentration of equilibrium vacancie s. These "extrinsic" vacancies escaping VIIC's and "intrinsic" vacancies, w hich also segregate at CCR's, ensure high diffusion mobility of AP's-Co-57 in the segregation zone. Diffusion of substitutional Ap's-Co-57 in lattice regions adjacent to the CCR core gives rise to a two-dimensional diffusion flow in the intercrystallite diffusion (ICD) zone and provides conditions n ecessary for saturation of this zone with AP's even at a temperature of 0.1 8T(m)(T-m is matrix melting point). Low-temperature variation of the popula tion of states q in the ICD zone with increasing temperature of isochronous annealing is explained by the temperature dependence of the segregation co efficient of substitutional Ap's-Co-57. Segregation activation enthalpy of Ap's-Co-57 in poly-W is Q(s) = (31.4 +/- 1.1) kJ/mole. The relative (normal ized to the width d of the CCR core) width 1/d = 5+/-1 of a stationary regi on with a high concentration of point defects, in which the pumping coeffic ient of AP's-Co-57 from the CCR core is much larger than the volume diffusi on coefficient, was measured at low temperatures of isochronous annealing. A new phenomenon - an expansion of the region with a high concentration of point defects (ERHCPD) - was discovered. This phenomenon appears and procee ds at relatively high homologous temperatures of isochronous annealing of t he Co(poly-W) system. Expansion of this region is due to the decay of segre gated "vacancy-interstitial impurity" complexes. The vacancy concentration of the ERHCPD and the binding energy of "vacancy-interstitial impurity" com plexes E-VIIC congruent to (42.6+/-7.0) kJ/mole were estimated using a mode l of the new phenomenon. Measured diffusion activation enthalpy of substitu tional Ap's-Co-57 in the ERHCPD of the Co-57(poly-W) system, Q(pump) congru ent to (114.6+/-6.5) kJ/mole was nearly five times lower than the activatio n enthalpy of volume diffusion, Q(vol) = (522.1+/-0.3) kJ/mole in this syst em.