Despite a number of recent investigations into the nature and properties of equilibrium atomic defects in the refractory bcc metals, the mechanisms responsible for the high-temperature mass transport in these materials are not clear. The observed strong high-temperature enhancement in self-diffusion appears to be unexplainable in terms of divacancies, but might result from the presence of small equilibrium concentrations of highly-mobile self-interstitials. Recent vacancy formation measurements in Cr appear to support this view. However, recent studies of diffusion in Nb(W) alloys and high-temperature vacancy migration in W have raised new issues to be considered
