Deformation behaviour of ultrafine-grained materials

Specimens of pure copper were produced via different processing routes: nanocrystalline samples were prepared by the inert-gas condensation method and submicrometer-grained samples were obtained by means of severe deformation techniques. Mechanical properties were studied using various deformation schemes, viz. tensile, compression, microhardness and miniaturized disk-bend tests. Unusual mechanical behaviour of the submicrometer-grained samples was observed: a combination of remarkable strength with high ductility and a long stage of stable plastic flow at a very low strain hardening rate. The results are discussed taking into account non-equilibrium structure of grain boundaries and triple junctions. Ultrafine-grained samples exhibit much higher strength than conventional polycrystalline copper and the yield stress increases with decreasing grain size. However, both the nanocrystalline and submicrometer-grained samples are weaker than the Hall-Fetch relationship, extrapolated into the ultrafine-grain region, would predict.