ELTE

ELTE TTK

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Cím: Budapest, 1117, Pázmány Péter sétány 1/a.
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E-mail: titkar@metal.elte.hu

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Előadás kivonatok:

Prof. Igor Alexandrov (Ufa State Aviation Technical University, Russia)
Experimental study and computer modeling of severe plastic deformation

High pressure torsion (HPT) is one of most wide-spread schemes for severe plastic deformation. This scheme is often applied for analysis of scientific aspects, connected with formation of bulk ultrafine-grained (UFG) and nanostructured states. At the same time the investigation of the HPT process is a topical issue. This is connected with its multiple-factor character. In this work the results of experimental investigations and computer modeling of processes, accompanying HPT of pure copper are presented. Analysis of the material flow features was made in frames of the developed analytical model. The process of the preferred orientations formation was studied in frames of the viscous-plastic self-consistent model. The interrelation between the components of the stress-strain state and the features of structure- and texture- formation in bulk billets, subjected to HPT, was revealed.

The upgraded Estrin-Tóth dislocation model has been suggested to describe deformation behavior of materials, subjected to severe plastic deformation (SPD), which in its turn is realized under high imposed pressure of several GPa. The model was applied to study high pressure torsion of Cu samples. The dislocation density evolution has been studied in the two phases: in the fragment/cell block boundaries and interiors. The evolution of concentration of vacancies, obtained during the deformation, and annihilation of dislocations during their non-conservative motion, generation of dislocations during multiple cross slip have been considered. Accounting of hydrostatic pressure was made in an explicit form on the basis of the presumption about limitation of the lattice diffusion, controlled by pressure. As a result the causes of achieving high values of flow stress, yield stress, dislocation density, vacancy concentration have been explained.

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Prof. Zuzanka Trojanová (Charles University, Czech Republic)
Magnesium based metal matrix composites

Mg-based composites were manufactured by various processing techniques. The reinforcement was in the form of short fibres or particles. The microstructure developed influences properties of composites. Physical and mechanical properties of Mg based composites were investigated in the temperature range from 293 to 673 K. Internal friction as well as acoustic emission were measured. Strength and deformation behaviour were studied. The volume fraction, shape and composition of the reinforcement significantly influence the properties of a composite. The interface between the matrix and the reinforcement has a great deal of importance in determining the properties of the composite. Usually, at least one component (typically the matrix) is exposed to higher homologous temperatures during such applications. An understanding of elevated temperature deformation is therefore a crucial to the design and development of metal matrix composites.

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Prof. Zuzanka Trojanová - Prof. Pavel Lukac (Charles University, Czech Republic)
Elastic and plastic behaviour of UFG magnesium

UFG Mg samples were prepared by milling procedure in an inert atmosphere and subsequent compacted and hot extruded. The linear grain size of specimens used was estimated by X-ray line profile analysis to be about 100-150 nm. Internal friction spectrum was measured in the temperature interval RT-480°C. Two developed peaks in the internal friction spectrum were obtained at temperatures 103 °C and 358°C (1Hz). The influence of prestraining and subsequent annealing was observed on the low temperature peak. The activation energy of the both peaks was estimated to be 1.16 eV and 1.77 eV respectively. The activation volume has been estimated for low temperature peak. The low temperature peak has dislocation origin, while the high temperature peak is conditioned by the grain boundary sliding.

Compression testing was performed at temperatures from room temperature up to 300°C. Rapid decrease of the yield stress as well as the maximum stress with temperature was observed. This decrease and the flat character of the stress strain curves at elevated temperatures indicate possible contribution of (a) diffusion process/es and grain boundary sliding. Stress relaxation tests were conducted in order to analyse thermally activated processes occurring during plastic deformation.