Although the structure of liquids and glasses may appear highly disordered, the atomic motions in... more Although the structure of liquids and glasses may appear highly disordered, the atomic motions in these phases are rather strongly correlated. Such a correlation, however, cannot be readily described by a pair density function, and higher order correlation functions are necessary. We suggest that the microscopic stresses defined at each atom are quite useful in describing such high order correlations. We discuss how the concept of the microscopic stress fluctuation helps to describe and understand the glass transition and glass formation phenomena. In a molecular dynamics study the local shear stresses were found to become orientationally correlated below a transition temperature, Ts, which is close to the melting temperature. Below Ts the viscosity shows a rapid increase and transverse phonons become more stable. Thus Ts could be viewed as a dynamical glass transition temperature. The conventional glass transition can be described in terms of the elastic percolation of the orientat...
ABSTRACTWe investigate the structures of dislocation cores in a model DO22 type intermetallic all... more ABSTRACTWe investigate the structures of dislocation cores in a model DO22 type intermetallic alloy in the absence of external stresses. The tetragonal distortion produces sessile configurations of superpartial dislocations even when the energies of planar faults are reasonably low. The influence of the core configurations on the mechanical behavior at low temperatures is discussed.
Ordered intermetallic compounds have been investigated very intensively in the recent years since... more Ordered intermetallic compounds have been investigated very intensively in the recent years since a variety of their properties make them superior in high temperature applications over the commonly used disordered alloys. Their elastic moduli are usually high and they have very high yield or fracture stresses that are maintained up to high temperatures. Moreover, compounds based on light elements, such as Ti and Al, can have very low densities and thus high strength-to-weight ratio. Furthermore, these alloys often have high melting temperatures and much lower self-diffusion coefficients than do disordered alloys and, therefore, higher creep strength and corrosion resistance. However, a very significant limitation is their much lower ductility and/or toughness at low temperatures than in disordered alloys (for recent symposia and reviews see Liu et al. 1995; Horton et al. 1995; Westbrook, and Fleischer 1995).
ABSTRACTWe present two tight-binding linear muffin-tin orbitals (TB-LMTO) techniques for electron... more ABSTRACTWe present two tight-binding linear muffin-tin orbitals (TB-LMTO) techniques for electronic structure calculations of extended defects (such as grain boundaries, interphase interfaces, surface layers etc.) in metals. The first is based on the first-principles self-consistent surface Green's function approach within the atomic-sphere approximation (ASA) utilizing two-dimensional periodicity in the layers parallel to the interface. In the second approach the Hamiltonian is constructed within the TB-LMTO-ASA as well, but semiempirical terms are employed to characterize the repulsive part of the interaction and the effect of electrons in interstitial space. While the adjustable parameters have only been fitted to the properties of ideal ground state structure, the semiempirical approach describes correctly the structural energy differences, phonon frequencies etc. Two examples are presented: the electronic structure of the Σ = 5(210)/[001] tilt grain boundary in tungsten is ...
The atomic structure of Σ = 3 tilt boundaries with variously inclined boundary planes has been st... more The atomic structure of Σ = 3 tilt boundaries with variously inclined boundary planes has been studied using the many body potentials for gold. A chain unit model, analogous to the structural unit model, describes the relationship between different boundaries. The basic units in this model are units of the (111) and (112) twin boundaries and the model corresponds to atomic level faceting into these boundaries.
... vor dem Sprung und einer Gleichgewichtsaufspaltung danach, (iv) der Natur der Kernstruktur au... more ... vor dem Sprung und einer Gleichgewichtsaufspaltung danach, (iv) der Natur der Kernstruktur auf {lll}-Ebenen vor und nach dem Quergleitproze , dem sogenannten Escaig ... Au erdem sagt das Modell voraus, da fr Orientierungen nahe [Oil] die Zug/Druck-Asymmetrie am gr ten ist. ...
The relationship between atomic structure and elastic properties of grain boundaries is investiga... more The relationship between atomic structure and elastic properties of grain boundaries is investigated from both discrete and continuum points of view. A heterogeneous continuum model of the boundary is introduced where distinct phases are associated with individual atoms and possess their atomic level elastic moduli determined from the atomistic model. The complete fourth-order tensors of both the atomic-level and the effective elastic moduli are determined, where the latter are defined for sub-blocks from an infinite bicrystal and are calculated here for a relatively small number of atom layers above and below the grain boundary. These effective moduli are determined exactly for the discrete atomistic model, while only estimates from upper and lower bounds can be determined for the continuum model. Comparison between the atomistic results and those for the continuum model establishes the validity of this definition of elastic properties for heterogeneous structures at these scales. ...
Flow stress measurements were performed on single crystalline Ni,(Al, Ta) as a function of temper... more Flow stress measurements were performed on single crystalline Ni,(Al, Ta) as a function of temperature, orientation, strain rate and sense of the applied uniaxial stress to check the predictions of the Paidar er al. model [ha merall. 32,435 (1984)]. It was found that the critical resolved shear stress (CRSS) for (11 l)[TOl] slip depends not only on the test temperature and orientation of the samples, as other investigators have previously observed, but also on the sense of the applied stress. The orientation dependence of the tension/compression asymmetry, including the regions where the asymmetry is a maximum (positive), a minimum, and where it disappears, is as predicted by the model. The applied stress changes the activation enthalpy of cross slip primarily through its effect on constricting the Shockley partials during cross slip and only secondarily on directly promoting (111) to (010) cross slip. A maximum attainable CRSS for (111) [TO11 slip, the saturation stress, is also in agreement with the model. It was also found that the CRSS for (11 l)[TOl] slip is strain rate independent, but the CRSS for (OOl)[TlO] slip shows a strong positive strain rate dependence. The temperature at which the peak in the flow stress vs temperature curve occurs increases with increasing strain rate and decreases with increasing ratio of RSS on (OOl)[TlO] divided by that on (lll)[TOl]. When the deformation occurs by (OOl)[TlO] slip the stress-strain curve exhibits clearly defined, continuous yield points. R&urn&-Nous avons effect& des mesures de la contrainte d'ecoulement sur des monocristaux de Ni,(Al, Ta) en fonction de la temperature, de l'orientation, de la vitesse de deformation et du sens de la contrainte uniaxiale appliqute afin de verifier les previsions du modele de Paidar ef al. [Acra metall. 32, 435 (1984)]. Nous avons trouve que la fission critique reduite (CCR) pour le glissement (111) [TO]] dtpendait non seulement de la temperature de l'essai et de l'orientation des echantillons, comme d'autres auteurs l'avaient deja signale, mais egalement du sens de la contrainte appliquee. La variation de l'asymetrie traction/compression en fonction de l'orientation, y compris dans les regions ou l'asymttrie est maximale (positive), minimale ou s'annule, correspondait aux previsions du modtle. La contrainte appliquie modifie l'enthalpie d'activation du glissement divie essentiellement par son effet sur la constriction des partielles de Shockley au tours du glissement d&vie, et seulement secondairement en favorisant le glissement d&e, (010) par rapport au glissement d&vie (111). La valeur maximale de la CCR que l'on peut atteindre pour le glissement (11 l)[TOl], cad la contrainte de saturation, est egalement en accord avec le modele. Nous avons egalement trouve que la CCR pour le glissement (11 l)[TOl] ne dipendait pas de la vitesse de deformation, mais que la CCR pour le glissement (OOl)[TOl] prtsentait une forte variation positive en fonction de la vitesse de deformation. La temperature du pit des courbes de la contrainte d'ecoulement en fonction de la temperature augmentait avec la vitesse de deformation et elle diminuait lorsqu'on on augmentait le rapport de la CCR de (00l)[Tl0] sur la CCR de (lll)[TOl]. Lorsque la deformation se produisait par glissement (OOl)[TlO], la courbe contraintedeformation prisentait des limites elastiques continues, bien dtfinies. Zuaammenfasaung-An Einkristallen Ni,(Al,Ta) wird die Fliegspannung in Abhlngigkeit von Temperatur, Orientierung, Abgleitgeschwindigkeit und Vorzeichen der angelegten einachsigen Spannung gemessen, urn die Aussagen des Modelles von Paidar et al. [Acra metal/. 32,435 (1984)] zu priifen. Die kritische FlieBspannung im System (11 l)[TOl] hangt nicht nur von der Versuchstemperatur und der Orientierung der Probe ab, wie andere Autoren berichtet haben, sondern such von Vorzeichen der angelegten Spannung. Die Orientierungsabhiingigkeit der Zug/Druck-Asymmetrie-einschliet3lich der Orientienmgen, bei denen die Asymmetrie maximal, minimal oder gerade Null ist-stimmen mit den Voraussagen des Modelles i&rein. Die iiullere Spannung veriindert die Aktivierungsenthalpie der Quergleitung iiberwiegend durch ihren EinfluB auf das Einschniiren der aufgespaltenen Versetzungen wiihrend des Quergleitens und nur in zweiter Linie durch direkte Einwirkung auf die Quergleitung von (111) nach (010). Die maximal erhaltbare kritische FEeDspannung bei (11 l)[TOl]-Gleitung, d.h. die Sittigungsspannung, stimmt ebenfalls mit dem Model1 iiberein. Die kritische FlieDspannung in (111) [TO]] war unabhangig von der Abgleitgeschwindigkeit. Dagegen zeigt diejenige fur Gleitung in (OOl)[~lO] eine starke positive Abhiingigkeit. Die Temperatur, bei dem ein Maximum in der Temperaturabhiingigkeit der Fliegspannungauftritt, nimmt mit ansteigender Abgleitgeschwindigkeit zu und sinkt mit zunehmendem Quotienten aus den FlieDspannungen im (001)[Tl0]-und (11 l)[TOl]-Gleitsystem. Rei Gleitung auf (OOl)[TlO] weist die Verformungskurve wohldefinierte kontinuierliche FlieBeinsHtze auf.
Modelling and Simulation in Materials Science and Engineering, 1999
... It is seen from figure 2 of this paper that the best agreement with ab initio results is atta... more ... It is seen from figure 2 of this paper that the best agreement with ab initio results is attained for the potential of Yan et al [7], used in the present study, and for the potential constructed by Ludwig and Gumbsch [47]. In addition Clapp et al [48] studied the tetragonal deformation ...
In this paper we first analyze the effect of covalent bonding upon the structure of stacking faul... more In this paper we first analyze the effect of covalent bonding upon the structure of stacking fault type planar defects on (111) planes and k/k interfaces in the lamellar TiAl. A remarkable finding of ab initio LDA based calculations is that the anti-phase domain boundary (APB) is not a metastable fault in TiAl. The main reason why this has not been found in studies employing central-force potentials is that the covalent bonding naturally leads to a rather high APB energy and, consequently, to an instability. A combination of atomistic calculations and HREM observations revealed that the covalent character of bonding, in particular the d-bond between Ti atoms, also remarkably affects the local atomic structure of the ordered twin with APB. The propensity for preservation of these bonds induces an asymmetry in the structure of this interface. A similar effect has also been found in the complex stacking fault (CSF). Understanding of this phenomenon is crucial when interpreting HREM observations. Furthermore, the Monte Carlo calculations presented in this paper suggest that in Ti-rich alloys a significant segregation takes place to some k/k interfaces leading to the formation of a very narrow region of the DO 19-Ti 3 Al in their cores.
Fully self-consistent ab initio electronic structure calculation of theoretical tensile strength ... more Fully self-consistent ab initio electronic structure calculation of theoretical tensile strength is performed for the first time using full-potential LAPW method. As a specific example, tensile strength of single-crystalline tungsten loaded uniaxially along the (001) and (111) directions is analyzed. Although tungsten is elastically nearly isotropic (C44 ≈ C′), theoretical tensile strength exhibits a marked anisotropy (σ001th = 0.289 Mbar,
ÐEmpirical many-body potentials of a Finnis±Sinclair-type have been constructed for the Pb, Pb± B... more ÐEmpirical many-body potentials of a Finnis±Sinclair-type have been constructed for the Pb, Pb± Bi and Pb±Ni systems using both experimental data and physical quantities derived from ab initio linear mun±tin orbitals calculations. Monte Carlo simulations using these potentials have been employed to compute the solubility limits of Pb±Bi and Pb±Ni alloys, as well as the segregation pro®les at the (111) surface of Pb 95 Bi 05. The pro®les have been found to be oscillatory, with a preferential segregation of Bi occurring at the ®rst surface atom layer, and a slight depletion of Bi in the second layer, in fair agreement with the results of the ab initio linear mun±tin orbitals calculations. The calculations on Pb±Ni show that within the solubility limit of Ni in Pb, Ni does not segregate to the Pb(111) outermost surface layer, although weak segregation to the second layer is found.
Grain boundaries are the most common interfaces always present in materials if they are not in a ... more Grain boundaries are the most common interfaces always present in materials if they are not in a single crystal form. Their presence affects a large variety of material properties since many important physical processes, such as diffusion, decohesion, segregation, cavitation, corrosion etc., occur preferentially at grain boundaries (for recent reviews see [1-3]). The property most strongly affected by grain boundary phenomena in metallic materials is their mechanical strength. It has been well established that segregation of various alloying elements and impurities to grain boundaries frequently makes the boundaries particularly suitable paths for brittle cracking at low temperatures ([4-12], for a number of reviews see ref. 13). Similarly, segregation of alloying elements has a pronounced influence, either detremental or beneficial, on fracture occurring during creep by cavitation at grain boundaries [14-17]. Both the effect of segregated impurities on cohesion and their fast diffusion in grain boundaries play the major role in the recently discovered, important phenomenon of brittle fracture at high temperatures [18-21].Grain boundary cracking is also the main problem encountered in intermetallic compounds which would be otherwise very attractive structural materials for high-temperature applications (for reviews see [22, 23]). In crystallographically complex compounds the brittleness may be attributed to the insufficient number of slip systems so that an extensive plastic deformation cannot develop. However, in f.c.c. based L12 compounds, such as Ni3Al, the available deformation modes are adequate and yet the intergranular fracture occurs readily.
The ideal tensile test in transition metal disilicides MoSi2 and WSi2 with a C11b structure is si... more The ideal tensile test in transition metal disilicides MoSi2 and WSi2 with a C11b structure is simulated by ab initio electronic structure calculations using the full-potential linearized augmented plane wave method. The theoretical tensile strength for [001] loading is determined for both disilicides and compared with that of other materials. A full relaxation of all external and one internal structural parameter is performed, and the influence of each relaxation process on energetics and strength of materials studied is investigated. Differences in the behavior of various interatomic bonds including tension-compression asymmetry are analyzed and their origin in connection with the changes of the internal structural parameter is traced. For comparison, the response of bonds in MoSi and CoSi with B2 structure to the [001] loading is also studied
We demonstrate that the straightforward application of the Nudged Elastic Band (NEB) method does ... more We demonstrate that the straightforward application of the Nudged Elastic Band (NEB) method does not determine the correct Peierls barrier of 1/2 111 screw dislocations in BCC metals. Although this method guarantees that the states (images) of the system are distributed uniformly along the minimum energy path, it does not imply that the dislocation positions are distributed uniformly along this path. In fact, clustering of dislocation positions near potential minima occurs which leads to an overestimate of both the slope of the Peierls barrier and the Peierls stress. We propose a modification in which the NEB method is applied only to a small number of degrees of freedom that determine the position of the dislocation, while all other coordinates of atoms are relaxed by molecular statics as in any atomistic study. This modified NEB method with relaxations gives the Peierls barrier that increases smoothly with the dislocation position and the corresponding Peierls stress agrees well with that evaluated by the direct application of stress in the atomistic modeling of the dislocation glide.
Phonon spectra of bicrystals with relaxed grain boundary structure display a variety of localized... more Phonon spectra of bicrystals with relaxed grain boundary structure display a variety of localized modes including long wavelength acoustic modes. Continuum solutions for localized waves that incorporate elastic properties of the interface via jump relations agree well with the latter modes. •In contrast, classical solutions that depend only on bulk elastic properties do not. This demonstrates that the distinct atomic structure of the interface is a controlling factor, and it is shown how local properties can be incorporated into continuum analyses of interface phenomena.
Although the structure of liquids and glasses may appear highly disordered, the atomic motions in... more Although the structure of liquids and glasses may appear highly disordered, the atomic motions in these phases are rather strongly correlated. Such a correlation, however, cannot be readily described by a pair density function, and higher order correlation functions are necessary. We suggest that the microscopic stresses defined at each atom are quite useful in describing such high order correlations. We discuss how the concept of the microscopic stress fluctuation helps to describe and understand the glass transition and glass formation phenomena. In a molecular dynamics study the local shear stresses were found to become orientationally correlated below a transition temperature, Ts, which is close to the melting temperature. Below Ts the viscosity shows a rapid increase and transverse phonons become more stable. Thus Ts could be viewed as a dynamical glass transition temperature. The conventional glass transition can be described in terms of the elastic percolation of the orientat...
ABSTRACTWe investigate the structures of dislocation cores in a model DO22 type intermetallic all... more ABSTRACTWe investigate the structures of dislocation cores in a model DO22 type intermetallic alloy in the absence of external stresses. The tetragonal distortion produces sessile configurations of superpartial dislocations even when the energies of planar faults are reasonably low. The influence of the core configurations on the mechanical behavior at low temperatures is discussed.
Ordered intermetallic compounds have been investigated very intensively in the recent years since... more Ordered intermetallic compounds have been investigated very intensively in the recent years since a variety of their properties make them superior in high temperature applications over the commonly used disordered alloys. Their elastic moduli are usually high and they have very high yield or fracture stresses that are maintained up to high temperatures. Moreover, compounds based on light elements, such as Ti and Al, can have very low densities and thus high strength-to-weight ratio. Furthermore, these alloys often have high melting temperatures and much lower self-diffusion coefficients than do disordered alloys and, therefore, higher creep strength and corrosion resistance. However, a very significant limitation is their much lower ductility and/or toughness at low temperatures than in disordered alloys (for recent symposia and reviews see Liu et al. 1995; Horton et al. 1995; Westbrook, and Fleischer 1995).
ABSTRACTWe present two tight-binding linear muffin-tin orbitals (TB-LMTO) techniques for electron... more ABSTRACTWe present two tight-binding linear muffin-tin orbitals (TB-LMTO) techniques for electronic structure calculations of extended defects (such as grain boundaries, interphase interfaces, surface layers etc.) in metals. The first is based on the first-principles self-consistent surface Green's function approach within the atomic-sphere approximation (ASA) utilizing two-dimensional periodicity in the layers parallel to the interface. In the second approach the Hamiltonian is constructed within the TB-LMTO-ASA as well, but semiempirical terms are employed to characterize the repulsive part of the interaction and the effect of electrons in interstitial space. While the adjustable parameters have only been fitted to the properties of ideal ground state structure, the semiempirical approach describes correctly the structural energy differences, phonon frequencies etc. Two examples are presented: the electronic structure of the Σ = 5(210)/[001] tilt grain boundary in tungsten is ...
The atomic structure of Σ = 3 tilt boundaries with variously inclined boundary planes has been st... more The atomic structure of Σ = 3 tilt boundaries with variously inclined boundary planes has been studied using the many body potentials for gold. A chain unit model, analogous to the structural unit model, describes the relationship between different boundaries. The basic units in this model are units of the (111) and (112) twin boundaries and the model corresponds to atomic level faceting into these boundaries.
... vor dem Sprung und einer Gleichgewichtsaufspaltung danach, (iv) der Natur der Kernstruktur au... more ... vor dem Sprung und einer Gleichgewichtsaufspaltung danach, (iv) der Natur der Kernstruktur auf {lll}-Ebenen vor und nach dem Quergleitproze , dem sogenannten Escaig ... Au erdem sagt das Modell voraus, da fr Orientierungen nahe [Oil] die Zug/Druck-Asymmetrie am gr ten ist. ...
The relationship between atomic structure and elastic properties of grain boundaries is investiga... more The relationship between atomic structure and elastic properties of grain boundaries is investigated from both discrete and continuum points of view. A heterogeneous continuum model of the boundary is introduced where distinct phases are associated with individual atoms and possess their atomic level elastic moduli determined from the atomistic model. The complete fourth-order tensors of both the atomic-level and the effective elastic moduli are determined, where the latter are defined for sub-blocks from an infinite bicrystal and are calculated here for a relatively small number of atom layers above and below the grain boundary. These effective moduli are determined exactly for the discrete atomistic model, while only estimates from upper and lower bounds can be determined for the continuum model. Comparison between the atomistic results and those for the continuum model establishes the validity of this definition of elastic properties for heterogeneous structures at these scales. ...
Flow stress measurements were performed on single crystalline Ni,(Al, Ta) as a function of temper... more Flow stress measurements were performed on single crystalline Ni,(Al, Ta) as a function of temperature, orientation, strain rate and sense of the applied uniaxial stress to check the predictions of the Paidar er al. model [ha merall. 32,435 (1984)]. It was found that the critical resolved shear stress (CRSS) for (11 l)[TOl] slip depends not only on the test temperature and orientation of the samples, as other investigators have previously observed, but also on the sense of the applied stress. The orientation dependence of the tension/compression asymmetry, including the regions where the asymmetry is a maximum (positive), a minimum, and where it disappears, is as predicted by the model. The applied stress changes the activation enthalpy of cross slip primarily through its effect on constricting the Shockley partials during cross slip and only secondarily on directly promoting (111) to (010) cross slip. A maximum attainable CRSS for (111) [TO11 slip, the saturation stress, is also in agreement with the model. It was also found that the CRSS for (11 l)[TOl] slip is strain rate independent, but the CRSS for (OOl)[TlO] slip shows a strong positive strain rate dependence. The temperature at which the peak in the flow stress vs temperature curve occurs increases with increasing strain rate and decreases with increasing ratio of RSS on (OOl)[TlO] divided by that on (lll)[TOl]. When the deformation occurs by (OOl)[TlO] slip the stress-strain curve exhibits clearly defined, continuous yield points. R&urn&-Nous avons effect& des mesures de la contrainte d'ecoulement sur des monocristaux de Ni,(Al, Ta) en fonction de la temperature, de l'orientation, de la vitesse de deformation et du sens de la contrainte uniaxiale appliqute afin de verifier les previsions du modele de Paidar ef al. [Acra metall. 32, 435 (1984)]. Nous avons trouve que la fission critique reduite (CCR) pour le glissement (111) [TO]] dtpendait non seulement de la temperature de l'essai et de l'orientation des echantillons, comme d'autres auteurs l'avaient deja signale, mais egalement du sens de la contrainte appliquee. La variation de l'asymetrie traction/compression en fonction de l'orientation, y compris dans les regions ou l'asymttrie est maximale (positive), minimale ou s'annule, correspondait aux previsions du modtle. La contrainte appliquie modifie l'enthalpie d'activation du glissement divie essentiellement par son effet sur la constriction des partielles de Shockley au tours du glissement d&vie, et seulement secondairement en favorisant le glissement d&e, (010) par rapport au glissement d&vie (111). La valeur maximale de la CCR que l'on peut atteindre pour le glissement (11 l)[TOl], cad la contrainte de saturation, est egalement en accord avec le modele. Nous avons egalement trouve que la CCR pour le glissement (11 l)[TOl] ne dipendait pas de la vitesse de deformation, mais que la CCR pour le glissement (OOl)[TOl] prtsentait une forte variation positive en fonction de la vitesse de deformation. La temperature du pit des courbes de la contrainte d'ecoulement en fonction de la temperature augmentait avec la vitesse de deformation et elle diminuait lorsqu'on on augmentait le rapport de la CCR de (00l)[Tl0] sur la CCR de (lll)[TOl]. Lorsque la deformation se produisait par glissement (OOl)[TlO], la courbe contraintedeformation prisentait des limites elastiques continues, bien dtfinies. Zuaammenfasaung-An Einkristallen Ni,(Al,Ta) wird die Fliegspannung in Abhlngigkeit von Temperatur, Orientierung, Abgleitgeschwindigkeit und Vorzeichen der angelegten einachsigen Spannung gemessen, urn die Aussagen des Modelles von Paidar et al. [Acra metal/. 32,435 (1984)] zu priifen. Die kritische FlieBspannung im System (11 l)[TOl] hangt nicht nur von der Versuchstemperatur und der Orientierung der Probe ab, wie andere Autoren berichtet haben, sondern such von Vorzeichen der angelegten Spannung. Die Orientierungsabhiingigkeit der Zug/Druck-Asymmetrie-einschliet3lich der Orientienmgen, bei denen die Asymmetrie maximal, minimal oder gerade Null ist-stimmen mit den Voraussagen des Modelles i&rein. Die iiullere Spannung veriindert die Aktivierungsenthalpie der Quergleitung iiberwiegend durch ihren EinfluB auf das Einschniiren der aufgespaltenen Versetzungen wiihrend des Quergleitens und nur in zweiter Linie durch direkte Einwirkung auf die Quergleitung von (111) nach (010). Die maximal erhaltbare kritische FEeDspannung bei (11 l)[TOl]-Gleitung, d.h. die Sittigungsspannung, stimmt ebenfalls mit dem Model1 iiberein. Die kritische FlieDspannung in (111) [TO]] war unabhangig von der Abgleitgeschwindigkeit. Dagegen zeigt diejenige fur Gleitung in (OOl)[~lO] eine starke positive Abhiingigkeit. Die Temperatur, bei dem ein Maximum in der Temperaturabhiingigkeit der Fliegspannungauftritt, nimmt mit ansteigender Abgleitgeschwindigkeit zu und sinkt mit zunehmendem Quotienten aus den FlieDspannungen im (001)[Tl0]-und (11 l)[TOl]-Gleitsystem. Rei Gleitung auf (OOl)[TlO] weist die Verformungskurve wohldefinierte kontinuierliche FlieBeinsHtze auf.
Modelling and Simulation in Materials Science and Engineering, 1999
... It is seen from figure 2 of this paper that the best agreement with ab initio results is atta... more ... It is seen from figure 2 of this paper that the best agreement with ab initio results is attained for the potential of Yan et al [7], used in the present study, and for the potential constructed by Ludwig and Gumbsch [47]. In addition Clapp et al [48] studied the tetragonal deformation ...
In this paper we first analyze the effect of covalent bonding upon the structure of stacking faul... more In this paper we first analyze the effect of covalent bonding upon the structure of stacking fault type planar defects on (111) planes and k/k interfaces in the lamellar TiAl. A remarkable finding of ab initio LDA based calculations is that the anti-phase domain boundary (APB) is not a metastable fault in TiAl. The main reason why this has not been found in studies employing central-force potentials is that the covalent bonding naturally leads to a rather high APB energy and, consequently, to an instability. A combination of atomistic calculations and HREM observations revealed that the covalent character of bonding, in particular the d-bond between Ti atoms, also remarkably affects the local atomic structure of the ordered twin with APB. The propensity for preservation of these bonds induces an asymmetry in the structure of this interface. A similar effect has also been found in the complex stacking fault (CSF). Understanding of this phenomenon is crucial when interpreting HREM observations. Furthermore, the Monte Carlo calculations presented in this paper suggest that in Ti-rich alloys a significant segregation takes place to some k/k interfaces leading to the formation of a very narrow region of the DO 19-Ti 3 Al in their cores.
Fully self-consistent ab initio electronic structure calculation of theoretical tensile strength ... more Fully self-consistent ab initio electronic structure calculation of theoretical tensile strength is performed for the first time using full-potential LAPW method. As a specific example, tensile strength of single-crystalline tungsten loaded uniaxially along the (001) and (111) directions is analyzed. Although tungsten is elastically nearly isotropic (C44 ≈ C′), theoretical tensile strength exhibits a marked anisotropy (σ001th = 0.289 Mbar,
ÐEmpirical many-body potentials of a Finnis±Sinclair-type have been constructed for the Pb, Pb± B... more ÐEmpirical many-body potentials of a Finnis±Sinclair-type have been constructed for the Pb, Pb± Bi and Pb±Ni systems using both experimental data and physical quantities derived from ab initio linear mun±tin orbitals calculations. Monte Carlo simulations using these potentials have been employed to compute the solubility limits of Pb±Bi and Pb±Ni alloys, as well as the segregation pro®les at the (111) surface of Pb 95 Bi 05. The pro®les have been found to be oscillatory, with a preferential segregation of Bi occurring at the ®rst surface atom layer, and a slight depletion of Bi in the second layer, in fair agreement with the results of the ab initio linear mun±tin orbitals calculations. The calculations on Pb±Ni show that within the solubility limit of Ni in Pb, Ni does not segregate to the Pb(111) outermost surface layer, although weak segregation to the second layer is found.
Grain boundaries are the most common interfaces always present in materials if they are not in a ... more Grain boundaries are the most common interfaces always present in materials if they are not in a single crystal form. Their presence affects a large variety of material properties since many important physical processes, such as diffusion, decohesion, segregation, cavitation, corrosion etc., occur preferentially at grain boundaries (for recent reviews see [1-3]). The property most strongly affected by grain boundary phenomena in metallic materials is their mechanical strength. It has been well established that segregation of various alloying elements and impurities to grain boundaries frequently makes the boundaries particularly suitable paths for brittle cracking at low temperatures ([4-12], for a number of reviews see ref. 13). Similarly, segregation of alloying elements has a pronounced influence, either detremental or beneficial, on fracture occurring during creep by cavitation at grain boundaries [14-17]. Both the effect of segregated impurities on cohesion and their fast diffusion in grain boundaries play the major role in the recently discovered, important phenomenon of brittle fracture at high temperatures [18-21].Grain boundary cracking is also the main problem encountered in intermetallic compounds which would be otherwise very attractive structural materials for high-temperature applications (for reviews see [22, 23]). In crystallographically complex compounds the brittleness may be attributed to the insufficient number of slip systems so that an extensive plastic deformation cannot develop. However, in f.c.c. based L12 compounds, such as Ni3Al, the available deformation modes are adequate and yet the intergranular fracture occurs readily.
The ideal tensile test in transition metal disilicides MoSi2 and WSi2 with a C11b structure is si... more The ideal tensile test in transition metal disilicides MoSi2 and WSi2 with a C11b structure is simulated by ab initio electronic structure calculations using the full-potential linearized augmented plane wave method. The theoretical tensile strength for [001] loading is determined for both disilicides and compared with that of other materials. A full relaxation of all external and one internal structural parameter is performed, and the influence of each relaxation process on energetics and strength of materials studied is investigated. Differences in the behavior of various interatomic bonds including tension-compression asymmetry are analyzed and their origin in connection with the changes of the internal structural parameter is traced. For comparison, the response of bonds in MoSi and CoSi with B2 structure to the [001] loading is also studied
We demonstrate that the straightforward application of the Nudged Elastic Band (NEB) method does ... more We demonstrate that the straightforward application of the Nudged Elastic Band (NEB) method does not determine the correct Peierls barrier of 1/2 111 screw dislocations in BCC metals. Although this method guarantees that the states (images) of the system are distributed uniformly along the minimum energy path, it does not imply that the dislocation positions are distributed uniformly along this path. In fact, clustering of dislocation positions near potential minima occurs which leads to an overestimate of both the slope of the Peierls barrier and the Peierls stress. We propose a modification in which the NEB method is applied only to a small number of degrees of freedom that determine the position of the dislocation, while all other coordinates of atoms are relaxed by molecular statics as in any atomistic study. This modified NEB method with relaxations gives the Peierls barrier that increases smoothly with the dislocation position and the corresponding Peierls stress agrees well with that evaluated by the direct application of stress in the atomistic modeling of the dislocation glide.
Phonon spectra of bicrystals with relaxed grain boundary structure display a variety of localized... more Phonon spectra of bicrystals with relaxed grain boundary structure display a variety of localized modes including long wavelength acoustic modes. Continuum solutions for localized waves that incorporate elastic properties of the interface via jump relations agree well with the latter modes. •In contrast, classical solutions that depend only on bulk elastic properties do not. This demonstrates that the distinct atomic structure of the interface is a controlling factor, and it is shown how local properties can be incorporated into continuum analyses of interface phenomena.
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Papers by Vaclav Vitek