Microstructural analysis of the 〈111〉 and 〈110〉 nickel single crystals subjected to severe plastic deformation by hydroextrusion

Abstract

The effect of hydrostatic extrusion (HE) on the microstructure and crystalline orientation of the 〈111〉 and 〈110〉 nickel single crystals was examined. The crystals were deformed by two-step hydrostatic extrusion to achieve the true strain εr=2.4. After the extrusion the samples had the form of cylindrical rods. The mechanical properties of the extruded samples (expressed in terms of their microhardness) were compared with the mechanical properties of ultra-fine grained nickel obtained by subjecting polycrystalline nickel to HE. The microhardness of the two deformed crystals appeared to be similar. The microstructure of the samples and the orientation evolution were examined using transmission electron microscopy (TEM), electron back scattered diffraction (EBSD) and X-Ray diffraction (XRD).Both the deformed crystals had an inhomogeneous ultra-fine-grained structure (as observed by TEM). The average grain diameter was 300nm. The majority of the grain boundaries had high angle disorientations (EBSD).In both the deformed samples the predominating orientation was 〈111〉 (XRD). In the 〈111〉 oriented crystal 95% of the initial orientation was preserved whereas in the 〈110〉 oriented crystal the initial orientation was predominantly transformed into 〈111〉 and 〈100〉.