The range of ufg metals is wide, ranging from below 1 µm down to 100 nm.
And there is a good range of grain sizes where you can model the material behavior using rather conventional material models based on the early work of Conrad.
This can be easily extended to ultra fine grained fcc metals, and we even showed recently that ufg bcc metals can be well described. Not only in terms of strength, but also concerning rate-sensitivity and activation volume.
If you are interested, we showed this recently in an attached open access article in Acta Materialia (August 2017):
Article Dominating Deformation Mechanisms in Ultrafine-grained Chrom...