Papers by Author: Roumen H. Petrov

Abstract: The recrystallization texture of highly cold deformed IF steels is addressed. The latter is characterized by the //ND fibre and a certain spread towards the {311} orientation. The //ND fibre is the optimum texture for enhanced deep-drawing properties whereas the presence of any other component, such as {311}, will deteriorate the plastic anisotropy of the material. Previous works concluded that the recrystallized {311} orientation results from an oriented nucleation process related to the plastic instability of {001} deformed grains. In the present work, the microstructural nature of such plastic instability is investigated by high resolution orientation scanning microscopy on an annealed IF sample after cross-rolling. Present data indicate that localized deformation in near {001} grains plays an essential role in the nucleation of {311} orientations.

Abstract: The core loss and magnetic induction of electrical steels are dependent on the microstructure and texture of the material, which are produced by the thermo-mechanical processing. After a conventional rolling process, crystal orientations of the α-(//RD) and γ-(//ND) fibers are strongly present in the final texture. These fibers have a drastically negative effect on the magnetic properties of electrical steels. By applying asymmetric rolling, significant shear strains could be introduced across the thickness of the sheet and thus a deformation texture with more magnetically favorable components is expected. In this study, an electrical steel of 1.23 wt.% Si was subjected to asymmetric warm rolling in a rolling mill with different roll diameters. The evolutions of both deformed and annealed textures were investigated. The texture evolution during asymmetric warm rolling was analyzed by crystal plasticity simulations using the ALAMEL model. A good fit between measured and calculated textures was obtained. The annealing texture could be understood in terms of an oriented nucleation model that selects crystal orientations with a lower than average stored energy of plastic deformation.

Abstract: Ultra low carbon (ULC) steel samples were deformed in near plane-strain compression mode with different strains, strain rates and temperatures. Different aspects of microstructural developments, for deformed γ (ND//) and θ (ND//) fibre grains, were investigated using X-ray line profiles and high resolution electron diffraction. The study clearly showed increase in grain interior strain localizations and in-grain misorientation at the intermediate deformation temperature. This effect was more apparent in γ-fibre and can best be explained through orientation sensitive recovery. γ-fibre also demonstrated higher potential for increase in dislocation density. This was observed experimentally and simulated through discrete dislocation dynamic simulations. Higher textural softening with stronger increase in dislocation density and possible effects of orientation sensitive recovery appears to define the orientation dependent recovery in low carbon steels.

Abstract: The changes in crystallographic texture are investigated during recrystallization in 5182 and 6016 aluminum alloys, which were subjected to various cold rolling reductions. In the current study, the texture varieties in the recrystallized materials are conditioned by both the material’s chemistry and the applied rolling strain. The presence of large constituent particles produces strain heterogeneities during cold rolling which causes a specific texture development. The development of recrystallization textures is discussed based on experimental data and results of crystal plasticity simulations. A recrystallization model based on nucleation and growth selection is presented. The driving force for nucleation was accounted for by applying continuum mechanics crystal plasticity models. For the modeling of the strain heterogeneities around hard undeformable particle a FE-code was used.

Abstract: Cast iron components in combustion engines, such as cylinder blocks and heads, are exposed for long periods of time to elevated temperatures and subjected to large numbers of heating and cooling cycles. In complex components, these cycles can lead to localized cracking due to stresses that develop as a result of thermal gradients and thermal mismatch. This phenomenon is known as Thermo-Mechanical Fatigue (TMF). Compacted Graphite Iron (CGI) provides a suitable combination of thermal and mechanical properties to satisfy the performance of engine components. However, TMF conditions cause microstructural changes, accompanied by the formation of oxides at and close to the surface, which together lead to a growth in size of the cast iron. These microstructural changes affect the mechanical properties and accordingly the thermo-mechanical fatigue properties. The aim of this research is to provide insight into the microstructure evolution of CGI, with its complex morphology, under TMF conditions. For this, optical and scanning electron microscopy observations are made after cyclic exposure to air at high temperature, both without and with mechanical loading. It was found that the oxide layers, which develop at elevated temperatures, crack during the cooling cycle of TMF. The cracking results from tensile stresses developing during the cooling cycle. Therefore, paths for easy access of oxygen into the material are formed. Fatigue cracks that develop also show oxidation at their flanks. In order to quantify the oxide layers surrounding the graphite particles, Energy Dispersive X-Ray Analysis (SEM-EDX) and Electron Probe Micro Analysis (EPMA) are used.

Abstract: The bending properties of high strength precipitation-hardening AA6016-type Al alloy thin sheets in pre-aged T4P temper state were studied in this work. Microstructural features like grain boundary particles distribution and volume fraction of the matrix strengthening phases were considered as factors controlling the mechanical properties and the fracture of this grade. Remarkable decrease in ductility, accompanied by severe deterioration of bendability occurred when coarse precipitates were found into the grain boundaries. The in-situ fracture sequence investigations as well as the post-failure surfaces observations indicated that grain boundary ductile fracture mechanisms were involved in the propagation of the cracks during bending. Heat treatment simulations were carried out and the results showed that the precise control of the technological parameters during production of these sheets is the key factor responsible for obtaining an appropriate combination of strength and bendability. Only by providing both, homogeneous distribution of the matrix strengthening phases and a favourable grain boundary structure, the severe and often contradictory requirements for the functional properties of these alloys can be successfully satisfied.

Abstract: The use of Al-Mg-Si-(Cu) heat treatable 6xxx alloys is steadily increasing in the automotive industry. The possibility of weight reduction of the cars in combination with the good formability and high in-service dent resistance of these alloys, make them a favorable material for body panel applications. One of the most common, environment-friendly and easy to perform processes used to join aluminum sheets, is the hemming joining operation. This operation heavily relies on the bendability of the sheets, because they are bent to an angle of 180° over of a radius equal to their thickness. Tearing or cracking of the outer bent surface are often very common. In this study we attempt to understand the relations between the microstructural features of the sheets and their hemming behavior. The hemming experiments are performed in laboratory conditions and the results are discussed together with the data obtained from crystallographic, microstructural and textural investigations. Relations between the hemming appearance, chemical composition, natural aging time and dispersoid’s density are found and discussed.

Abstract: This paper investigates the surface texture evolution after a short phase transformation annealing in low vacuum on ultra low carbon steel sheets alloyed with high Mn and Al and the cold rolled steel sheets of industrial composition alloyed with silicon. The ultra low carbon steel sheets with high Mn and Al show surface monolayer which has a characteristic surface texture components <100>//ND texture and microstructure with special grain morphology. Contrastingly, the industrial composition alloyed with silicon does not show specific surface texture components inspired by surface energy anisotropy at the surface. The composition depth profiling investigations performed on the all steel sheet surface shows that oxidation characteristics of alloying elements at the metal vapour interface have played a decisive influence on surface texture evolution. Further, transformation annealing in higher vacuum reveals that surface texture can be obtained in an industrial composition alloyed with silicon.

Abstract: . It is well known that surface energy anisotropy is one of the driving forces for the orientation selection at the metal-vapour interface. This affects the microstructure and texture evolution at the surface during phase transformation, which is an inherent feature of low-alloyed low-carbon steels. This paper investigates the nucleation and growth of the surface texture by orientation contrast microscopy. It has been found that the surface texture is dominated by {001} oriented grains, which exhibit a remarkable orientation gradient from the centre of the grain towards the edge. The {001} oriented grain centre gradually rotates around a <110> axis in small incre¬mental steps when nearing the edge of the grain. Towards the edge the accumulated rotation angle has commonly reached a value of 30°. Underneath the surface grains (~30 µm) the bulk texture consists of a strong -fibre.

Abstract: The effect of two different intermediate annealing (IA) treatments on texture banding in a roping prone aluminium alloy was investigated. It was found that texture banding occurred in the final annealed material that underwent an IA treatment consisting of slow heating in which there was significant interaction between the recrystallizing grains and the particles in the material. A more uniform distribution of orientations in the final annealed material was obtained in the case of an IA treatment with fast heating so that there was no significant effect of particles on recrystallization.