Papers by Author: Ruslan Valiev

Abstract: The article represents results of influence of different severe plastic deformation (SPD) techniques on TiNi alloys. It is demonstrated that strength and shape memory effect (SME) of TiNi can be significantly enhanced due to formation of ultrafine-grained (UFG) and nanocrystalline (NC) structures by SPD. Influence of equal channel angular pressing (ECAP), high pressure torsion (HPT), multi-step SPD deformations (ECAP plus cold rolling) on structure, mechanical and functional properties of TiNi alloys is considered. There are represented first results of influence of equal channel angular pressure-Conform (ECAP-C) on TiNi alloys, which is a perspective technology for industrial fabrication of UFG metals and alloys.

Abstract: Severe plastic deformation (SPD) processes, are successfully employed to produce ultra fine grain (UFG) and nanocrystalline (NC) microstructures in Ni50.7Ti49.3 shape memory alloy. The effect of grain size on phase transformations during annealing is investigated by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The results of comparative studies of phase transformations in coarse-grained, UFG and NC alloys after SPD and subsequent long-term (up to 100 hours) annealing at 400С is presented. The functional properties and the innovation potential of UFG NiTi alloys is considered and discussed.

Abstract: This work is devoted to enhancement of strength and ductility of the Ti-6Al-7Nb ELI alloy, which is less harmful from medical point of view for human body in comparison to Ti-6Al-4V. It has been demonstrated that formation of an ultrafine-grained structure in the alloy with the help of equal-channel angular pressing in combination with heat and deformation treatments allows reaching high strength (UTS = 1400 MPa) and sufficient ductility (elongation 10 %).

Abstract: Copper based materials are still the most attractive low resistivity materials for microelectronics and electrotechnics applications, though, all variants developed to combine strength and conductivity, such as solid solutions and composites, suffer from decay in electric conductivity while strength is increased . In a addition, linear decay was also conjectured for pure copper when grain size is refined below the UFG and nanostructured domains (except when grain boundaries are pure twins). Copper alloys with low content of silver and chromium were prepared by high pressure torsion (HPT) with various annealing conditions. Vickers hardness and electric resistivity in the temperature range of 4K-340K, were measured as well as microstructural characterizations were performed using quantitative X-ray diffraction. Depending on the annealing conditions the alloys exhibit from 25% to 75% of IACS electric conductivity at room temperature and hardness in the range of 200 Hv. Origins of both high strength and high electric conductivity were investigated from microstructures analysis, using transmission electron microscopy and mechanical testing.

Abstract: Most ultrafine-grained (UFG) materials produced by severe plastic deformation (SPD) exibit only limited ductility which is correlated with the low strain rate sensitivity (SRS) of these materials. Recently, it was demonstrated that SPD is capable of increasing the room temperature ductility of aluminum-based alloys attaining elongations up to 150%, together with relatively high strain rate sensitivity. In the present work, additional results and discussions are presented on the effect of grain boundary sliding (GBS) and SRS on the ductility of some UFG metals and alloys. The characteristics of constitutive equations describing the steady-state deformation process are quantitatively analyzed for a better understanding of the effects of grain boundaries and strain rate sensitivity.

Abstract: Two Al alloys (AA1570 and AA6061) in the solutionized state have been processed by HPT at room temperature to achieve a homogeneous UFG structure. After HPT, the grain size was found to have a mean value about 100 nm for both alloys. Measured yield stress values of HPT-produced UFG alloys being plotted in terms of the Hall-Petch relationship were found to exceed the plot predictions for the range of ultrafine grain size. For both alloys, Atom Probe Tomography measurements allowed to reveal segregation of solute elements along grain boundaries. The origin of the extremely high strength of the alloys nanostructured by HPT is discussed with a special attention to the influence of such segregations on the emission and the mobility of dislocations.

Abstract: Commercial purity copper was subjected to ECAP and subsequent cold rolling. Structure and mechanical properties were studied using EBSD analysis, TEM and tensile tests. Effect of ECAP number passes on grain size and fraction of high angle boundaries after cold rolling was investigated. Rolling results in grain refinement and HABs fraction increase the more ECAP number passes. UTS increases significantly after rolling. Increase of strength is accompanied by loss of plasticity. Evolution of microstructure and mechanical properties is discussed.

Abstract: Some nanocrystaline Ni was prepared by repeated cold rolling with intermediate folding (F&R). The material was then processed by High Pressure Torsion (HPT) to study the grain evolution under additional Severe Plastic Deformation (SPD). Microstructures were characterized by Transmission Electron Microscopy (TEM) and the impurity distribution was analyzed by Atom Probe Tomography (APT). In this paper, we discuss about the influence of impurities on the grain growth during HPT and on the grain size reduction mechanism during SPD.

Abstract: A hypothesis for two-stage character of deformation under load via the simple shear scheme is suggested. At the first stage in the shear strain range , where - the strain parameter, the metal microstructure changes in the way similar to that during elongation. At the second stage at accidental multi-scale rotative motions, similar to turbulent motions in liquids, take place in the metal. This stage of deformation is the proper simple shear. The results of experiments are presented, which testify in favor of the suggested hypothesis.

Abstract: Nanjing University is hosting the 5-th in a row of NanoSPD conferences, which continues a series of the preceding conferences: in Moscow (1999), Vienna (2002), Fukuoka (2005) and Goslar (2008). This paper highlights the major landmarks of NanoSPD activity for the past years from the standpoint of the International NanoSPD Steering Committee. Special attention is given to topics relating to nanostructuring of metals by SPD processing for advanced properties as well as new trends in developing SPD techniques for practical application. Another concern of the committee is in introduction and discussion of the terminology applied in this new field of science and engineering.