Grain boundary diffusion and grain boundary phase transition in tungsten in the temperature range of activated sintering

Grain boundary self- and solute (cobalt) diffusion in tungsten was found [Lee et al., Scr. Metall, 1988; Lee et al., Col. de Physique, 1990] to exhibit discontinuities in the Arrhenius behavior at the homologous temperatures of 0.36 < T/T m < 0.4 that surprisingly match the activation sintering temperature of W (T m is the melting point). In the present work, this unusual grain boundary diffusion phenomenon is discussed in terms of a fundamental grain boundary phase transition in W. The experimental data are analysed with respect to predicted segregation-induced grain boundary phase transformation. Competing co-segregation of impurity elements (carbon and phosphor) might induce a discontinuous grain boundary segregation and invoke a grain boundary phase transition which modifies the grain boundary mobilities of substitutional atoms. The improved understanding of grain boundary phase transitions is expected to provide a breakthrough in interpreting the exact mechanism of W-activated sintering.