Mechanical properties and deformation behaviour of ultra-fine grained (UFG) Ni subjected to severe plastic deformation (SPD) are investigated. UFG Ni was characterized the homogeneous structure with the grain size 120 nm and high angle nonequilibrium grain boundaries. This state of nickel possesses significant ultimate tensile stress of 1270 MPa.
Investigation by HRSEM of deformation relief on the polished surface of the UFG Ni samples reveals the grain boundaries (GBs) steps already at small degrees of tension, that testifies to engaging of grain boundaries to deformation process at room temperature. After large degrees of the tensile tests the deformation relief revealed a network of crossed shear bands oriented at an angle in the range 35- 45 to the axis of tensile deformation. Shear bands propagated along GBs that are parallel to the plane of maximal shear stress. Formation of shear bands occurs due to strong shift and rotation of grain groups. This leads to a deformation mechanism involving collective relative displacement of grain groups, with extensive grain boundary sliding at room temperature. On the length scale of a few micrometers the material can be regarded as uniform and therefore the local strain distribution becomes more uniform than in coarse-grained materials. It is plausible that this mode of deformation may contribute to the enhanced ductility.
The features of deformation behaviour of Ni with the different grain size and various states of grain boundaries are also considered. Opportunity of achievement in metals and alloys of combination high strength and good ductility due to control of microstructure of materials opens perspectives of its application in industry, particularly, for mirco-systems and for details with complex geometry, obtained due to superplastic forming.