Václav Paidar

Solute segregation and classification of [100] tilt grain boundaries in α-iron: consequences for grain boundary engineering

A complete classification of [100] tilt grain boundaries into the categories of special, vicinal and general resulting from measurements of grain boundary segregation in α-iron is presented. In contrast to the coincidence site lattice approach at least one special grain boundary was found to exist for each misorientation of adjoining grains irrespective of the value of the reciprocal density of coincidence lattice sites. This finding raises a new challenge to grain boundary engineering. It is suggested that a configuration of typically brittle general boundaries in a polycrystal can be transformed into another one consisting of non-brittle special boundaries under suitable annealing conditions by boundary inclination via boundary migration without any change of grain orientations.

A study of the applicability of many-body central force potentials in NiAl and TiAl

The applicability and characteristics of the central force many-body potentials of Finnis-Sinclair-type for NiAl and TiAl are investigated by studying the variation of the energy of these compounds with structural transformations that correspond to three distinct paths: tetragonal (B2L10), trigonal (B2L11) and hexagonal (B2B19). The energy was computed using both the central force potentials and the full potential linearized augmented plane waves (FLAPW) ab initio method. Comparison of these two calculations provides a means for the analysis of the efficacy of the potentials. The central force many-body potentials reproduce the results of ab initio calculations very satisfactorily for NiAl. This propounds that they are sufficient in atomistic modelling of lattice defects in this compound. For TiAl the central force potentials mimic the results of ab initio calculations qualitatively but are unable to differentiate adequately between structures with practically the same separations of the first and second neighbours. However, the present study provides a justification for the use of these potentials when investigating extended defects in which separations of the first and second nearest neighbours differ significantly from those in the L10 structure.

Segregation and corrosion behaviour of incommensurate 45°[100] grain boundaries in an FeSi alloy: The role of grain boundary plane orientation

Measurements of the temperature dependencies of the chemical composition of 45{degree}[100] incommensurate tilt grain boundaries in an Fe-3.55at.%Si alloy revealed large differences between the segregation behavior of {l_brace}0kl{r_brace} symmetrical and (001)/(011) asymmetrical grain boundaries. The low negative values of the segregation enthalpies of Si, P and C determined at the latter interface as compared to those of the former one are correlated with the cusps found in the orientation dependencies of the etched groove dimensions at a curved grain boundary and with the effective separation of atomic planes parallel to the boundary. The results disclose the surprising fact that the (001)/(011) asymmetrical grain boundary possesses singular character whereas the {l_brace}0kl{r_brace} symmetrical interface is of the general type. The conclusions highlights the importance of the orientation of the grain boundary plane to characterize a grain boundary and its resulting properties.

Dislocation dissociation in TiAl alloys

Two superdislocation configurations consisting in an asymmetric planar dissociation on one octahedral plane, and symmetric wedge shape dissociation on two inclined octahedral planes are analysed using anisotropic elasticity. The presented results can be used to evaluated the stacking fault energies from the measured widths of dissociation.

TEM investigation of non-crystallographic displacements near antiphase domain boundaries in D03 ordered Fe3Al

Antiphase domain boundaries (APBs) have been studied by means of transmission electron microscopy in a D0{sub 3} long-range ordered Fe{sub 3}Al single crystal. Two types of APB contrasts have been observed. The normal {pi}-type APB contrast was imaged with superlattice reflections. This type of contrast is caused by the basic displacement R{sub 0} = 1/4{l_angle}111{r_angle} or 1/2{l_angle}111{r_angle} across the APB. On the other hand, the anomalous {alpha}-type APB contrast was imaged with fundamental reflections. The latter contrast was weak, asymmetric in dark field and discontinuous along a close APB loop. It has been demonstrated that the anomalous {alpha}-type contrast is caused by a small non-crystallographic displacement r of atoms near the APBs. Detailed examination of the anomalous APB contrast indicated that the fault vector r depends on the basic displacement R{sub 0} as well as on the APB orientation. The main component of r was found to be perpendicular to the APBs with R{sub 0} = 1/4{l_angle}111{r_angle}. The addition displacement is discussed in terms of the mechanical and chemical relaxations of the APBs and with reference to the anomalous plastic behavior of Fe{sub 3}Al.

Polarization of plastic deformation modes in polysynthetically twinned TiAl crystals

Polarization of deformation twinning (its propagation in a certain sense but not in the opposite one) is taken for granted. However, the same phenomenon can occur for a superdislocation glide as well, as is demonstrated in this paper. The consequences for plastic deformation of polysynthetically twinned TiAl crystals with the lamellar interfaces parallel to the loading direction are discussed. It is not the interface itself that is an obstacle for propagating deformation but also the fact that a deformation mode with the parallel Burgers vector cannot be activated in the neighboring lamella due to the directionality of superdislocation motion leading to additional stress increase.

Modelling of Recrystallization and Grain Boundary Migration by Cellular Automata

Within past ten years, the importance of cellular automata (CA) models of both recrystallization and grain boundary migration has been recognized. It can be shown that CA provides a computationally efficient mathematical framework for simulations of physically relevant models of the above processes. Generally, CA works in such models with a vast number of identical copies of several generic processes – as recrystallization, nucleation, grain boundary migration, etc. – which interact locally and results in a complex global response. The attention is focused to some important aspects of recrystallization, grain growth and CA modelling. Finally, possible future development of more physically relevant models are briefly outlined.

Theoretical analysis of dislocation splittings in MoSi2

Due to a lower symmetry of the tetragonal C11b structure when compared with the cubic BCC lattice, the 1/2 BCC dislocations. This has essential impact on their behaviour and, consequently, on mechanical properties. Various types of dislocation dissociations are analyzed in the frame of anisotropic elasticity with the help of the data from ab initio calculations of γ-surfaces for generalized stacking faults.

Planar Defects on (112) in BCC Crystals

The parameters of exponential many-body Finnis-Sinclair potentials corresponding to qualitatively different crystal lattice stability were selected and their behaviour was studied. Furthermore, a model with pairwise Lennard-Jones potential was also considered. The attention was paid to the stability of different crystal structures and the properties of simple interfaces such as stacking faults and twin boundaries were investigated.

Plastic deformation of bicrystals composed of polysynthetically twinned TiAl crystals

Abstract Two types of interfaces can be distinguished in TiAl polycrystals: special rotational interfaces on the {111} planes for which the misorientation angles about the normal to the interface plane are multiples of 60°; and grain boundaries in common sense with arbitrary orientations of the boundary plane and rotation axis. The former are the boundaries in the lamellar structure parallel to one of the close packed atomic planes and the latter separate differently oriented colonies of such lamellae. The effect of both these interfaces on the plastic deformation of TiAl intermetallics is discussed. In particular, the lamellar interfaces as strong obstacles for moving dislocations induce a specific type of combined plastic deformation when the glide of ordinary dislocations operates simultaneously with twinning. On the other hand, the grain boundary can affect the dislocation processes at its vicinity by the arising compatibility stresses.