Papers by Author: Julia Ivanisenko

Abstract: A comprehensive investigation of microstructure evolution upon annealing in the temperature range between 100°C and 450°C in SPD-processed nanocrystalline iron had been conducted using transmission and orientation imaging microscopy, and XRD analysis. The asprocessed microstructure was typical of severely deformed metals consisting of grains with a mean size of 110 nm, each subdivided by a network of subgrain boundaries. Additionally, measurements of microstrains using XRD found a very high microstrain level of 0.003 in this material. After annealing at 200°C a drastic decrease of microstrains was observed; whereas no notable changes in the size of grains and subgrains as well in misorientation spectra have been revealed. Thus we relate this decrease of microstrains with recovery of non-equilibrium grain boundaries. Annealing at higher temperatures hadn’t led to further decrease of microstrains, but resulted in slight subgrain growth from 65 to 78 nm accompanied by increase of mean misorientation angle from 5° to 6.3°at 450°C. This indicates the occurrence of coalescence of subgrains as additional recovery mechanism.

Abstract: An overview of the mechanically driven phase transformations taking place in nanocrystalline pearlitic steels in conditions of the severe plastic deformation (SPD), i.e. combination of high pressure and strong shear strains will be given. Conditions of the discussed experiments (room temperature and moderate strain rates) exclude any thermal origin of the observed transformations. One of them is strain induced cementite decomposition, which is a well-documented phenomenon taking place at cold plastic deformation of pearlitic steels. We explain this process taking into account friction forces at the interface between the hard cementite and ferrite. Under the high pressures and stresses higher than the ferrite matrix yield stress, the later one behaves like a viscoelastic fluid. The friction at the precipitate/matrix interface leads to two effects. One is to induce high strains on the precipitates. This leads to shift of thermodynamic equilibrium towards dissolution of the cementite. The second is wear of the cementite phase due to friction at the ferrite/cementite interface and mechanically induced drag of carbon atoms by the ferrite. This had been recently confirmed in 3D AP experiments, which demonstrated that the process of cementite decomposition starts with depleting of carbides with carbon and formation of non-stoichiometric cementite. The existing theories of atom drag by moving dislocations (ballistic models) can be regarded as one of the many possible mechanism of wear discussed by the wear theory. In that respect the process can be called athermal, as temperature indirectly influences wear processes but is not their main cause. We observed also another strain driven transformation in nanocrystalline pearlitic steel during room temperature high pressure torsion. This is a stress induced α→γ transformation, which has never been observed at conventional deformation of coarse grained iron and carbon steels. This was concluded to have occurred due to a reverse martensitic transformation.

Abstract: The paper presents an overview of a number of unusual phase transformations which take place in pearlitic steels in conditions of the severe deformation, i.e. combination of high pressure and strong shear strain. Strain-induced cementite dissolution is a well-documented phenomenon, which occurs during cold plastic deformation of pearlitic steels. Recently new results which can shed additional light on the mechanisms of this process were obtained thanks to 3DAP and HRTEM investigations of pearlitic steel deformed by high pressure torsion (HPT). It was shown that the process of cementite decomposition starts by carbon depletion from the carbides, which indicates that the deviation of cementite’s chemical composition from the stoichiometric is the main reason for thermodynamic destabilisation of cementite during plastic deformation. Important results were obtained regarding the distribution of released carbon atoms in ferrite. It was experimentally confirmed that carbon segregates to the dislocations and grain boundaries of nanocrystalline ferrite. Another unusual phase transformation taking place in nanocrystalline pearlitic steel during room temperature HPT is a stress induced α→γ transformation, which never occurs during conventional deformation of coarse grained iron and carbon steels. It was concluded that this occurred due to a reverse martensitic transformation. The atomistic mechanism and the thermodynamics of the transformation, as well as issues related to the stability of the reverted austenite will be discussed.

Abstract: The effects of equal channel angular pressing (ECAP), further heat treatment and rolling on the structure and room temperature mechanical properties of the commercial aluminum alloys 6061 (Al-0.9Mg-0.7Si) and 1560 (Al-6.5Mg-0.6Mn) were investigated. It has been shown that the strength of the alloys after ECAP is higher than that achieved after conventional processing. Prior ECAP solution treatment and post-ECAP ageing can additionally increase the strength of the 6061 alloy. Under optimal ageing conditions a yield strength (YS) of 434 MPa and am ultimate tensile strength (UTS) of 470 MPa were obtained for the alloy. Additional cold rolling leads to a YS and UTS of 475 and 500 MPa with 8% elongation. It was found that the post-ECAP isothermal rolling of the 1560 alloy resulted in the formation of a nano-fibred structure and a tensile strength (YS = 540 MPa and UTS = 635 MPa) that has never previously been observed in commercial non-heat treatable alloys.

Abstract: Low temperature dislocation- and point defect-related anelasticity in high pressure torsion (HPT) deformed metallic materials bcc Fe-26Al, fcc Fe-36Ni, hcp Ti is studied by mechanical spectroscopy. Internal friction (IF) peaks, which correspond to these phenomena, have different stability against heating. Up to five IF peaks are recorded, at least some of these peaks can be classify as Hasiguti peaks. Mechanical spectroscopy gives a useful tool to study early stages of severely plastic deformed alloys study.

Abstract: Recent studies of nanocrystalline materials have often found that the deformation mechanisms are radically different to those in coarse-grained materials, resulting in quite different mechanical properties for such materials. The use of pearlitic steels for the study of the deformation mechanisms in bcc materials with ultrafine grain sizes is quite convenient, because it is relatively straightforward to obtain a homogenous nanocrystalline structure with a mean grain size as small as 10 nm using various modes of severe plastic deformation (SPD). In this paper we show that highpressure torsion of an initially pearlitic steel results in a nanostructured steel in which austenite has been formed at or close to room temperature. The orientation relationship between neighboring ferrite and austenite grains is the well-known Kurdjumov-Sachs orientation relationship, i.e. the same observed in temperature-induced martensitic transformation of iron and steels. It is shown that this must have resulted from a reverse martensitic transformation promoted by the high shear strains experienced by the material during severe plastic deformation of the nanocrystalline structure. This transformation represents an alternative deformation mechanism that can be activated when conventional deformation mechanisms such as slip of lattice dislocations become exhausted.