Dislocation density evolution during high pressure torsion of a nanocrystalline Ni–Fe alloy

High-pressure torsion HPT induced dislocation density evolution in a nanocrystalline Ni-20 wt %Fe alloy was investigated using x-ray diffraction and transmission electron microscopy. Results suggest that the dislocation density evolution is fundamentally different from that in coarse-grained materials. The HPT process initially reduces the dislocation density within nanocrystalline grains and produces a large number of dislocations located at small-angle subgrain boundaries that are formed via grain rotation and coalescence. Continuing the deformation process eliminates the subgrain boundaries but significantly increases the dislocation density in grains. This phenomenon provides an explanation of the mechanical behavior of some nanostructured materials. © 2009 American Institute of Physics. DOI: 10.1063/1.3095852 Dislocations play a key role in determining the mechani- cal properties of materials. Manipulating dislocation density and distribution via appropriate processing including anneal- ing and cold working can therefore change the mechanical properties of materials. It has been well known that conven- tional coarse-grained metals and alloys can be softened by annealing and strengthened by cold working.1 Annealing re-