Data of flow stresses a for pure Cu with ultrafine grains with grain size d approximate to 0.35 mu m produced by severe plastic deformation and grains of conventional size obtained in the range of homologous temperatures T-hom from 0.22 (room temperature) to 0.33 are compared to data for hardnesses H for Cu of various grain structures from single crystalline to d = 0.01 mu m measured by nanoindentation at room temperature. The two sets of data appear to be consistent when sigma approximate to H/3. At room temperature the Hall-Petch relation holds, i.e., the flow stress increases monotonically with decreasing grain size by Delta sigma alpha d(-0.5). At elevated T-hom the saturated flow stress decreases when the grains become ultrafine. The transition from hardening to softening by grain boundaries in the saturation stage is discussed on the basis of a simple statistical dislocation model considering the influence of grain boundaries on the balance between generation and annihilation of dislocations.
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univ erlangen nurnberg, inst werkstoffwissensch ls1, d-91058 erlangen, germany. chinese acad sci, shenyang natl lab mat sci, inst met res, synl, shenyang, peoples r china. max planck inst met res, stuttgart, germany.;blum, w (reprint author), univ erlangen nurnberg, inst werkstoffwissensch ls1, martensstr 5, d-91058 erlangen, germany;wolfgang.blum@ww.uni-erlangen.de
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