A recent model developed to predict the smallest grain sizes obtainable by severe plastic deformation has worked well for materials with medium to high stacking fault energies but not for those with low stacking fault energies. To probe this issue, experiments were conducted using a Cu–30 wt.% Zn alloy with a very low stacking fault energy of 7mJ/m2 as the model material. High-pressure torsion was used as the grain refinement technique. The results indicate that stacking faults and twin boundaries play a key role in the grain refinement process such that the smallest achievable grain size was determined by the highest stacking fault and twin density that the system was able to produce. An amorphization of grain boundaries was also observed in the final structure. These observations were very different from those reported for materials having medium to high stacking fault energies and they confirmed the operation of a different grain refinement mechanism.

The Role of Stacking Faults and Twin Boundaries in Grain Refinement of a Cu–Zn Alloy Processed by High-Pressure Torsion. Y.B.Wang, X.Z.Liao, Y.H.Zhao, E.J.Lavernia, S.P.Ringer, Z.Horita, T.G.Langdon, Y.T.Zhu: Materials Science and Engineering A, 2010, 527[18-19], 4959-66