Grain size and grain boundary character distribution in ultra-fine grained (ECAP) nickel

Microstructures of the Ultra-fine grained (UFG) nickel produced through equal channel angular processing (ECAP), pressed 12 times through the BC route to impart a strain of ∼12, were investigated using transmission electron microscopy (TEM) and electron back-scattering diffraction (EBSD). The average grain size obtained from orientation imaging microscopy (OIM) was 0.23 μm, for a tolerance angle of 5° and 0.24 μm from TEM investigations. The microstructure of ECAP nickel was not homogeneous. The grain refinement was more extensive after 12 passes as in the present investigation than 8 passes (studied earlier), when processing in both the cases was through ECAP using route BC. The grain boundary character distribution (GBCD) was determined in the 12-pass ECAP Nickel. The details changed depending on whether the cut-off angle was 5° or 2°. In the present investigation we found that a 5° cut-off gave physically more meaningful results. The Vickers micro-hardness depended on the location in the extruded sample and varied between 1.56 GPa and 2.47 GPa, for an applied load of 25 g. When the load was increased to 50 g, it ranged between 1.6 GPa and 2.3 GPa.