| High Pressure Torsion (HPT) is a simple and effective severe plastic deformation method to produce ultrafine grained or even nanocrystalline microstructures. However, at a certain applied strain no further refinement of the microstructure occurs. HPT deformation of bulk Ni at room temperature leads to a typical saturation microstructure with a mean grain size of approximately 200 nm. In the present work, it will be shown that additional heavy cold rolling of a saturation microstructure does not help to overcome this limit. Furthermore, a strategy to bypass this limitation in the grain refinement using HPT deformation is presented. The main key lies in the reduction of grain boundary migration during the deformation. Thus bulk nanocrystalline Ni samples with an average grain size below 30 nm can be produced. |
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