Papers by Author: Nikolay A. Krasilnikov

Abstract: The structure and mechanical properties of Al-based alloy 2024 after high-pressure torsion (HPT) was investigated. Alloy 2024 with homogeneous structure and grain size about 70 nm was obtained using HPT at 6 GPa pressure and 5 turns of the anvils at room temperature. The nanostructured alloy possessed very high UTS (Ultimate tensile stress) above 1100 MPa at room temperature, and superplastic behaviour at temperatures over 300°С. The microhardness of the nanostructured alloy after superplastic deformation (1.5 GPa) was greater than that after the standard treatment of the coarse-grained alloy (1.2 GPa). The influence of HPT parameters and heat treatment on the structure and deformation behaviour of the alloy was studied. The opportunity of achieving a combination high strength and good ductility in metals and alloys opens perspectives for industrial applications, particularly, in micro-systems and for high-strength items with complex geometry which could be obtained by superplastic forming.

Abstract: 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°- 45° 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 mirco-systems and for items of complex geometry to be produced by superplastic forming.

Abstract: An ultra-fine grained microstructure was obtained in high purity nickel by a combination of (a) equal-channel angular pressing (ECAP) and (b) hydrostatic extrusion (HE) with a cumulative true strain of ~11.2. The resulting microstructure was examined by light and TEM microscopy. Mechanical properties have been measured by tensile and hardness tests. It was found that HE of ECAP-ed samples leads to a significant grain size refinement (from 330 to 160nm) and to an increase in microstructural homogeneity. SPD nickel, made by a combination of the ECAP and hydrostatic extrusion methods, has high strength and ductility (i.e.: YS=1120MPa and εf = 11%). The microstructure transformation was accompanied by a strength increase of 78% compared to ECAP alone. The results obtained fit well with the Hall-Petch relationship. A combination of ECAP and HE has achieved much better properties than either single process and show it to be a promising procedure for manufacturing bulk UFG nickel.

Abstract: The influence of the equal channel angular pressing (ECAP) temperature (150-350oC) on microstructure of the AM60 magnesium alloy has been investigated using transmission electron microscopy. It was demonstrated that application of various ECAP regimes leads to significant difference in a grain size and volume fraction of precipitates in investigated material. Thermal stability of precipitates and correlation between microstructure and tensile strength are discussed.