114 resultados para Severe Plastic-Deformation


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Titanium and its alloys are excellent candidates for biomedical implant. However, they exhibit relatively poor tribological properties. In this study, a two-step treatment including surface mechanical attrition treatment (SMAT) combined with thermal oxidation process has been developed to improve the tribological properties and biocompatibility of Ti. Ti after two-step treatment shows excellent wear-resistance and biocompatibility among all Ti samples, which can be ascribed to the highest surface energy, well crystallinity of rutile layer on its surface. Overall, the two-step treatment is a prospective method to produce excellent biomedical Ti materials.

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Severe plastic deformation via equal-channel angular pressing was shown to induce characteristic ultra-fast diffusion paths in Ni (Divinski et al., 2011). The effect of heat treatment on these paths, which were found to be represented by deformation-modified general high-angle grain boundaries (GBs), is investigated by accurate radiotracer self-diffusion measurements applying the 63Ni isotope. Redistribution of free volume and segregation of residual impurities caused by the heat treatment triggers relaxation of the diffusion paths. A correlation between the GB diffusion kinetics, internal friction, microstructure evolution and microhardness changes is established and analyzed in detail. A phenomenological model of diffusion enhancement in deformation-modified GBs is proposed.

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An equiaxed ultrafine-grained (UFG) microstructure was successfully produced in a Ti-6Al-4V alloy with an average grain size of 110-230. nm through symmetric and asymmetric warm rolling of a martensitic starting microstructure. The UFG material displayed a combination of ultrahigh strength and ductility at room temperature. Compared with the conventional symmetric rolling, the asymmetric rolling process led to a more pronounced effect of microstructure refinement and a higher tensile ductility. The optimum mechanical response was obtained though the asymmetric rolling at 70% reduction, offering an ultimate tensile strength of 1365. MPa and a total elongation of ~23%. Apart from the magnitude of grain refinement, the inclination of basal texture component from the normal towards the rolling direction during asymmetric rolling and possible strain induced β to martensite transformation may concurrently contribute to a remarkable tensile strength-ductility balance.

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Application of damage model in combination with finite element analysis to design and optimization of equal channel angular pressing - conform of commercially pure titanium against ductile failure is demonstrated. The properties required for precise simulation of the process and prediction of damage accumulation (equivalent stress as function of equivalent strain and temperature and low bound ductility function) are obtained in the temperature interval 20-400 °C and described in details.

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Machining of titanium is quite difficult and expensive. Heat generated in the process of cutting does not dissipate quickly, which affects tool life. In the last decade ultra fine grained (UFG) titanium has emerged as an option for substitution for more expensive titanium alloys. Extreme grain refinement can be readily performed by severe plastic deformation techniques. Grain refinement of a material achieved in this way was shown to change its mechanical and physical properties. In the present study, the microstructure evolution and the shear band formation in chips of coarse grained and UFG titanium machined to three different depths and three different feeding rates was investigated. A change in thermal characteristics of commercial purity Ti with grain refinement was studied by comparing heating/cooling measurements with an analytical solution of the heat transfer boundary problem. It was demonstrated that an improvement in the machinability can be expected for UFG titanium. © 2012 Springer Science+Business Media, LLC.

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In this work, compaction by warm equal-channel angular pressing (ECAP) with back pressure was used to produce Ti-6Al-4V billets from both commercially pure (CP) titanium and titanium hydride (TiH 2) powders, which were mixed with pulverised binary Al-V master alloys of two distinct Al/V ratios and with elemental aluminium powder to arrive at the nominal alloy composition. It was demonstrated that the right combination of temperature, high hydrostatic pressure and plastic shear deformation permits consolidation of the powder mixture to maximum green densities of 99.26%. Moreover, after direct compaction of blended elemental powders by equal-channel angular pressing (ECAP) with back pressure, the sintering temperature required for chemical and microstructural homogenisation of the compacts could be reduced by 150-250°C. This was possible due to high green density, increased contact area between powder particles and the formation of fast diffusion paths associated with grain refinement by severe plastic deformation. The sintered Ti-6Al-4V billets exhibited a maximum density of 99.88%, Vickers hardness of 409-445 HV1 and ultimate tensile strength in the range of 1000-1080MPa. In contrast to findings of other authors, the use of TiH 2 powders in conjunction with ECAP processing did not bring any benefits with regard to the production of the Ti-6Al-4V alloy.

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Recent development of characterisation techniques and computer simulation has extended our ability to access atomic scale information regarding materials microstructure evolution. New results from such techniques have significantly progressed our knowledge about solute behaviour during the earliest stages of decomposition of the solid solution. This chapter updates current understanding about solute clustering and discusses the effect of solute clustering and micro-alloying on precipitate microstructure evolution in aluminium alloys. In addition, a brief review is given on the effect of severe plastic deformation on precipitate evolution in Al alloys.

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In ultrafine-grained (UFG) materials produced by severe plastic deformation (SPD) techniques such as ECAP (equal channel angular pressing), bimodal grain size distributions have been observed under different circumstances, for example shortly after ECAP, after rest or anneal and/or after mild cyclic deformation at rather low homologous temperature. It has been shown that the mechanical monotonic and fatigue properties of some UFG materials can be modified (sometimes enhanced) by introducing a bimodal grain size distribution by a mild annealing treatment which leads, in some cases, to a good combination of strength and ductility. Here, the conditions under which bimodal grain size distributions evolve by (adiabatic) heating during ECAP and during subsequent annealing or cyclic deformation will be explored, and the effects on the mechanical properties, as studied by the authors and as reported so far in the literature, will be reviewed and discussed. In particular, the role of temperature rise during ECAP will be considered in some detail.

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A new model is proposed that aims to capture within a single modelling frame all the main microstructural features of a severe plastic deformation process. These are: evolution of the grain size distribution, misorientation distribution, crystallographic texture and the strain-hardening of the material. The model is based on the lattice curvature that develops in all deformed grains. The basic assumption is that lattice rotation within an individual grain is impeded near the grain boundaries by the constraining effects of the neighbouring grains, which gives rise to lattice curvature. On that basis, a fragmentation scheme is developed which is integrated in the Taylor viscoplastic polycrystal model. Dislocation density evolution is traced for each grain, which includes the contribution of geometrically necessary dislocations associated with lattice curvature. The model is applied to equal-channel angular pressing. The role of texture development is shown to be an important element in the grain fragmentation process. Results of this modelling give fairly precise predictions of grain size and grain misorientation distribution. The crystallographic textures are well reproduced and the strength of the material is also reliably predicted based on the modelling of dislocation density evolution coupled with texture development.

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A study of possibilities given by the developed Cellular Automata–Finite Element (CAFE) multi-scale model for prediction of the initiation and propagation of micro-shear bands and shear bands in metallic materials subjected to plastic deformation is described in the paper. Particular emphasis in defining the criterion for initiation of micro-shear and shear bands, as well as in defining the transition rules for the cellular automata, is put on accounting for the physical aspects of those phenomena occurring in two different scales in the material. The proposed approach led to the creation of the real multi-scale model of strain localization. This model predicts material behavior in various thermo-mechanical processes. Selected examples of applications of the developed model to simulations of metal forming processes, which involve strain localization, are presented in the paper. An approach based on the Smoothed Particle Hydrodynamic, which allows to overcome difficulties with remeshing in the traditional CAFE method, is presented in the paper as well. In this approach remeshing becomes possible and mesh distortion, which limits application of the CAFE method to simple deformation processes, is eliminated.

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The objective of the present work is searching for the correlation between the carbon content in steels and the parameters of the rheological models, which are used to describe the materials behavior during hot plastic deformation. This correlation can be expected in the internal variable models, which are based on physical phenomena occurring in the material. Such a model, based on the dislocation density as the internal variable, is investigated in this work. The experiments including hot torsion tests are used for the analysis.
The procedure is composed of three parts. Plastometric tests were performed for steels with various carbon content. Optimization techniques were applied next to determine the coefficients in the internal variable rheological model for these steels. Two versions of the model are considered. One is based on the average dislocation density and the second accounts for the distribution of dislocation densities. Evaluation of correlation between carbon content and such coefficients in the models as activation energy for self diffusion, activation energy for recrystallization, grain boundary mobility, recovery coefficient etc. was the main objective of the work. In consequence, the model which may be used for simulation of hot forming processes for steels with various chemical compositions, is proposed.

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This work investigates the relationship between the strain rate and the ductility and the underlying deformation mechanisms in an ultrafine-grained Al6082 alloy. At room temperature the uniform elongation of the material exhibits a marked increase with decreasing strain rate. This effect is related to the activation of micro shear banding, which is controlled by grain boundary sliding. The contribution of these mechanisms to uniform elongation is estimated. It is proposed that the grain boundary sliding suppresses the transformation of micro shear bands into macro shear bands. The activity of other deformation mechanisms during plastic deformation of the ultrafine-grained material is also discussed.

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This work focuses on the effect of strain rate on the deformation behaviour of an ultrafine grained Al alloy 6082 produced by equal channel angular  pressing. The uniform tensile elongation was found to increase with  decreasing strain rate very substantially. This effect is discussed in terms of the mechanisms that control plastic deformation of the alloy.

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Shear bands formed during both cold and hot plastic deformation have been linked with several proposed mechanisms for the formation of ultrafine grains. The aim of the present work was to undertake a detailed investigation of the microstructural and crystallographic characteristics of the shear bands formed during hot deformation of a 22Cr-19Ni-3Mo (mass%) austenitic stainless steel and a Fe-30 mass%Ni based austenitic model alloy. These alloys were subjected to deformation in torsion and plane strain compression (PSC), respectively, at temperatures of 900°C and 950°C and strain rates of 0.7s-1 and 10s-1, respectively. Transmission electron microscopy and electron backscatter diffraction in conjunction with scanning electron microscopy were employed in the investigation. It has been observed that shear bands already started to form at moderate strains in a matrix of pre-existing microbands and were composed of fine, slightly elongated subgrains (fragments). These bands propagated along a similar macroscopic path and the subgrains, present within their substructure, were rotated relative to the surrounding matrix about axes approximately parallel to the sample radial and transverse directions for deformation in torsion and PSC, respectively. The subgrain boundaries were largely observed to be non-crystallographic, suggesting that the subgrains generally formed via multiple slip processes. Shear bands appeared to form through a co-operative nucleation of originally isolated subgrains that gradually interconnected with the others to form long, thin bands that subsequently thickened via the formation of new subgrains. The observed small dimensions of the subgrains present within shear bands and their large misorientations clearly indicate that these subgrains can serve as potent nucleation sites for the formation of ultrafine grain structures during both subsequent recrystallisation, as observed during the present PSC experiments, and phase transformation.

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An ultra-fine-grained Al alloy was subjected to compression tests at different strain rates. The strain rate sensitivity of the flow stress was estimated. A strong effect of the baseline strain rate on the mechanisms of plastic deformation was found. It is suggested that a decrease of strain rate results in activation of micro shear banding due to grain boundary sliding. A connection between the strain rate, strain rate sensitivity, and the prevalent deformation mechanism was established.