Papers by Author: Pavel Lejček

Abstract: This paper shows part orientation issue in the process of Selective Laser Melting (SLM) at four examples of metallic materials (Fe, stainless steel 316L, TiAl6V4 alloy and AlSi11Mg alloy). Horizontally and vertically oriented samples differ in their mechanical properties, especially in plasticity. The causes of these differences are related to a thermal history, microstructural features and porosity. Depending on a particular material, individual effects are manifested under different extents.

Abstract: Despite nanosegregation of solutes at grain boundaries has been intensively studied in the past decades and numerous theoretical data as well as experimental values on characteristic energies and/or enthalpies and entropies exist some questions remain still unanswered. In this paper some of these questions – Which energetic quantities obtained in different ways, experimentally and theoretically, can be mutually compared What is the segregation site of a solute in the grain boundary core – are discussed in more detail. It will be shown that the entropy of grain boundary segregation plays an important and indisputable role in responding some issues and that understanding of its role will help us to elucidate fundamentals of the grain boundary segregation.

Abstract: Effect of solute interaction on interfacial segregation and grain boundary cohesion is modeled on basis of combined Guttmann and Rice-Wang approaches in binary and ternary systems. It is shown that attractive II binary interaction strengthens interfacial segregation and enhances intergranular embrittlement while repulsion exhibits an opposite effects. In a ternary system the segregation is suppressed by the IJ attraction while increased by the repulsion. The effect of the binary interaction is generally stronger compared to the ternary one.

Abstract: Equal channel angular pressing (ECAP) is a promising severe plastic deformation technique for production of ultrafine-grained bulk metals with face centered cubic (fcc) structure. However, the process is often complicated in hexagonal close packed (hcp) metals such as magnesium due to its low forming capability. In this contribution, magnesium single crystals were processed by ECAP through a single pass in order to reveal processes taking place in hcp lattice during severe plastic deformation. The microstructure and texture were investigated by SEM-EBSD. The deformed microstructure contains shear bands, recrystallized regions and mechanical twins. Activity of twinning systems and texture formation are discussed regarding different initial orientation of the single crystals.

Abstract: Literature data on grain boundary properties show surprisingly in some cases reversed courses of the structural dependences than expected, or directly prove their qualitative changes with changing temperature. This “reversed anisotropy” is demonstrated for example of grain boundary segregation. It is shown that this reversed anisotropy of grain boundary properties can be explained by the enthalpy–entropy compensation effect. Serious consequences of altered structural dependence of grain boundary properties for Grain Boundary Engineering are discussed.

Abstract: The knowledge of the structure/property relationship in polycrystalline materials is the basis for successful application of Grain Boundary Engineering. We demonstrate this relationship in the reverse way: from the selective corrosion attack observed in unique sample – loops from excavated necklace dated to the 10th century and manufactured from a Ag–1%Cu alloy – we can deduce the method of manufacturing the objects. Individual grain boundaries in this object were identified by electron back-scattering diffraction. Crystallographic maps of the grain boundaries are confronted with the level of the long-termed selective corrosion attack in the soil electrolyte under conditions of decomposing human body. It is shown that general grain boundaries, which are highly segregated by copper, are preferably attacked by corrosion. The segregated layers represent a less-noble material comparing to the surrounding bulk in this environment. In contrast, the twin and other special grain boundaries are significantly more resistant against this attack.

Abstract: The compensation effect, i.e. the linear dependence between characteristic enthalpy and entropy, has been frequently described in many fields of science and its close relationship to a phase transition is often assumed. In this paper it is shown for example of grain boundary segregation that the compensation effect principally differs from the phase transition although they can occasionally occur at the same temperature.

Abstract: Well-characterised samples such as single crystals, bi-, tri- or multicrystals are needed for basic studies of properties of solids. Growth of such metallic structures by means of the floatingzone melting technique is demonstrated for iron base alloys, FeAl intermetallics and NiTi shape memory alloy.

Abstract: Silver objects obtained from archaeological excavations often exhibit an extended embrittlement. The causes of this embrittlement were studied on silver artefacts buried for more than one thousand years in the soil using light and electron microscopy techniques and EDX microanalysis. Our investigation revealed presence of chlorine, sulphur and oxygen along the grain boundaries suggesting that most likely, the catastrophic failure of the grave objects made of Ag–Cu alloys is caused by an intercrystalline corrosion attack. The role of possible grain boundary segregation and/or precipitation of copper as a path of easy corrosion is discussed.

Abstract: Chemical and structural changes at the grain boundaries were investigated to quantify their influence on fracture behaviour of austenitic stainless steels and model ferritic Fe-Si-P alloys. The balance between the size and the area density of intergranular particles was found to be one of the most decisive factors influencing sensitivity of the steels to intergranular fracture. The precise dependence of the energy of intergranular fracture on the phosphorus grain boundary concentration was also determined.