Mechanical properties and deformation behaviour of ultra-fine grained nickel.

The mechanical properties and deformation behaviour of ultra-fine grained (UFG) Ni subjected to severe plastic deformation (SPD) were investigated. The UFG Ni characterized possessed an homogeneous structure with a grain size of 120 nm and high angle nonequilibrium grain boundaries. This nickel possessed an ultimate tensile stress of 1270 MPa. Investigation of the deformation relief on the polished surface of the UFG Ni sample by HRSEM revealed grain boundaries (GBs) steps already at small degrees of tension that testifying to the involvement of grain boundaries in the deformation process at room temperature. After significant tensile strain the deformation relief revealed a network of crossed shear bands oriented at an angle in the range 35 degrees-45 degrees to the axis of tensile deformation. Shear bands propagated along GBs parallel to the plane of maximal shear stress. The formation of shear bands occurred due to a strong shift and rotation of grain groups. This led to a deformation mechanism involving collective relative displacement of grain groups, with extensive grain boundary sliding at room temperature. Over a length 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 deformation behaviour of Ni having different grain sizes and various grain boundary states are also considered. The Opportunity to achieve a combination high strength and good ductility by control of the microstructure of in metals and alloys opens perspectives industrial applications, particularly, for micro-systems and for items of complex geometry to be produced by superplastic forming.