THERE is increasing interest in mechanisms which give plastic deformation at low stresses and moderately high or high temperatures (more than half the melting temperature Tm). Theories of diffusional creep, using either the Nabarro–Herring (NH) mechanism1 based on lattice self-diffusion or Coble's2 variation involving grain-boundary self-diffusion, do not always give strain rates as high as those obtained experimentally. In any case, this approach ignores the participation of grain-boundary sliding, which is often observed under these conditions. Particularly at temperatures below 0.6 Tm, diffusion rates are too slow to make a contribution to creep through NH creep unless grain sizes are very small3, but there is another way in which the diffusion distance may become very small and so give rise to high rates of creep.