Thermal Point Defects Research Articles

An atomistic view on Oxygen, antisites and vacancies in the [formula omitted]-TiAl phase

The ordered intermetallic γ-TiAl phase plays an important role in determining mechanical properties of engineering γ-TiAl based alloys. During alloy production, interstitial O is introduced in these alloys and in the γ-phase in particular. It is anticipated that it can have a significant impact on mechanical properties of the alloys due to localized dislocation pinning at O atoms and other point defects. In this paper, we apply a standard thermodynamic approach concerning point-defects in ordered compounds together with density functional theory and finite element method calculations to investigate the impact of O on thermal point defect formation and the associated defect formation eigenstrains in the practically relevant Ti-rich off-stoichiometric γ-TiAl phase. The results show that O is strongly bound to vacancies and Ti antisite defects and may substantially increase their equilibrium concentration. Calculated concentrations and eigenstrain values of antisites and vacancies in the presence of O offer a possibility of their interaction with dislocations, promoting dislocations pinning within the strain aging temperature regime of TiAl alloys.

Ultra-fast disordering by fs-lasers: Lattice superheating prior to the entropy catastrophe

Point defects and lattice heating are two major sources for the catastrophic disordering of a crystal in equilibrium. I demonstrate that the thermal point defects formation time in a femtosecond laser (fs-laser)-excited solid is the longest of all relaxation times, while the ultra-fast contribution to the entropy changes from electrons is minor in comparison to the catastrophe value. Thus non-thermal disordering solely by electron excitation prior to the energy transfer to the lattice is proved to be thermodynamically impossible. The swiftly excited solid can be disordered only if a lattice is superheated over the critical temperature defined by the entropy catastrophe. The presented analysis of experiments on fs excitation of different solids is consistent with theory.

Boundary effects on universal growth of islands driven by ion-beam irradiation

This paper considers the boundary conditions imposed by step edge sinks on an assembly of thermal point defects in which reactions among antidefects may dominate defect lifetimes. A model of sink behavior is described that incorporates the concepts of effective chemical potential ${\ensuremath{\mu}}^{\ensuremath{\ast}}$ and effective defect temperature ${T}^{\ensuremath{\ast}}$ into the action of defect sinks. The law of mass action exactly couples the defect concentrations when ${T}^{\ensuremath{\ast}}=T$. An application of the results to a problem of current interest is described. Specifically, an earlier demonstration of universality in island growth driven by ion-beam irradiation is reanalyzed to determine the extent to which universality is sensitive to the boundary conditions imposed by the step edges on the diffusion field of reacting thermal defects.

Surface islands nucleated by a beam of energetic self-ions

Many processes that control materials synthesis hinge on a nucleation event that initiates a first order phase transition. Symmetries that constrain nucleation of elementary nanostructures are therefore of general interest. Using low-energy electron microscopy, we observe the flux at which adatom and advacancy islands nucleate when clean $\mathrm{Pt}(111)$, in the temperature range 750--$1300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, is bombarded by a beam of ${\mathrm{Pt}}^{\ensuremath{-}}$ ions of various energies. The results reveal a previously unobserved symmetry between the chemical potentials ${\ensuremath{\mu}}^{*}$ required to nucleate the two types of island. Linear response theory is employed to relate ${\ensuremath{\mu}}^{*}$ to ion beam flux; the observations confirm that its use is valid above $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The observed magnitudes of ${\ensuremath{\mu}}^{*}$ agree within a factor 3 with predictions made earlier by Pimpinelli and Villain. In addition, the observations show that adatoms and advacancies on $\mathrm{Pt}(111)$ form a strongly reacting assembly, with the life cycle of thermal point defects determined mainly by formation and annihilation as pairs, rather than by processes at fixed sinks such as surface step edges.

Can vacancies be the main reason of FeAl alloys hardening?

The literature provides quite a lot of facts against the ‘vacancy hardening’ as the main cause of FeAl alloys hardening. The compositional dependency of the vacancy concentrations has been investigated and related to room temperature yield stress of the FeAl alloys. Thermal point defects concentration was measured in a FeAl based alloy and applied to calculate the high temperature strengthening. The studies have shown that vacancies are not the main reason of FeAl alloys hardening.

Nitrogen Diffusion in Amorphous Silicon Nitride Isotope Multilayers Probed by Neutron Reflectometry

Amorphous silicon nitride is a model system for a covalently bound amorphous solid with a low atomic mobility where reasonable values of self-diffusivities are still lacking. We used neutron reflectometry on isotope enriched Si3 14N4/Si3 15N4 multilayers to determine nitrogen self-diffusivities ranging from 10(-24) to 10(-21) m2/s between 950 and 1250 degrees C. Time dependent diffusivities observed at 1150 degrees C indicate the presence of structural relaxation. For long annealing times (relaxed state) the diffusivities follow an Arrhenius law with an activation enthalpy of (3.6 +/- 0.4) eV. The results are indicative of a direct diffusion mechanism without the involvement of thermal point defects.

First-principles study of constitutional and thermal point defects in B2 PdIn

We performed first-principles calculations to investigate the point defect structure of B2 PdIn. In agreement with experiments, our calculations show that Pd vacancies and Pd antisites are the constitutional point defects in In-rich and Pd-rich B2 PdIn, respectively. To predict the thermal defect concentrations at finite temperatures, we adopted the statistical-mechanical Wagner–Schottky model in combination with our first-principles calculated point defect formation enthalpies. Our calculations suggest that the predominant thermals defects in B2 PdIn are of triple-defect type and not of Schottky type.

Embedded-atom potential forB2−NiAl

An embedded-atom potential has been constructed for the intermetallic compound $B2\ensuremath{-}\mathrm{NiAl}$ by fitting to both experimental properties and ab initio data. The ab initio data have been generated in the form of energy-volume relations for a number of alternative structures of NiAl and ${\mathrm{Ni}}_{3}\mathrm{Al},$ as well as for Ni and Al. The potential accurately reproduces the basic lattice properties of $B2\ensuremath{-}\mathrm{NiAl},$ planar faults, and point-defect characteristics. It also reproduces the energetics and stability of all other structures included in the fit. The potential is applied to calculate equilibrium point-defect concentrations in $B2\ensuremath{-}\mathrm{NiAl}$ as functions of temperature and composition near the stoichiometry. In contrast to previous calculations, the defect formation entropies arising from atomic vibrations are included in our calculation within the quasiharmonic approximation. Such entropies tend to increase the concentrations of thermal point defects in $B2\ensuremath{-}\mathrm{NiAl}$ at high temperatures, but the atomic disorder mechanism remains triple-defect type up to the melting point.

Thermodynamics of constitutional and thermal point defects in B2 Ni1−xAlx

Thermodynamics of constitutional and thermal point defects in B2 Ni_1−<i>x</i>Al_<i>x</i>

Thermodynamics of constitutional and thermal point defects in B2 Ni 1- x Al x

Abstract In view of the discussion on whether or not constitutional vacancies occur in B2 intermetallic compounds, a recently proposed model for point-defect formation in such compounds has been elaborated. In contrast with the results obtained by Ren and Otsuka (2000, Phil. Mag. A, 80, 467) it is shown that the statistical thermodynamic model applied to the available experimental data for B2 Ni1−x Al x (vacancy concentrations as function of composition and temperature, activity data and enthalpy of formation data, both as functions of composition) does not lead to the conclusion that constitutional vacancies do not exist. In fact, all available data, except for the vacancy concentrations experimentally determined by Kogachi et at., are found to be fully consistent with the triple-defect model.

Density of constitutional and thermal point defects inL12Al3Sc

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Lattice defects in silicon rapidly solidified from the melt

The behaviors of melting and rapid solidification of Si have been studied by in situ ultra-high-voltage electron microscopy (UHVEM). Quenching experiments from a partly molten state have also been carried out. The lattice defects introduced are examined in detail by conventional transmission electron microscopy (C-TEM). Planar defects, dislocations, and defect clusters are found to form in the specimens. The results indicate that thermal point defects are associated with the formation of induced defects. Thermally formed stacking fault tetrahedra exhibit that clustering and the collapse of thermal vacancies actually occur in Si at high temperatures. Some dislocations are helical, and this shows that a strong interaction exists between induced dislocations and thermal point defects during cooling from high temperatures.

First-principles investigation of thermal point defects in B2 NiAl

The equilibrium concentrations of thermal defects are calculated in the mean-field approximation for the configurational entropy. The stable configurations of point defects in NiAl are discussed.

Overview No. 104 The physical and mechanical properties of NiAl

A critical review of the physical and mechanical properties of NiAl is presented. The physical properties examined include electronic structure and bonding, crystal structure and phase stability, thermodynamic properties, elastic properties, and electrical, magnetic, and thermal properties. Discussion of crystal defects in NiAl include both constitutional and thermal point defects, the core structure and energy of line defects, and planar defects (shear faults, grain boundaries, and free surfaces). The mechanical properties, substructure, and mechanisms of ductility of NiAl single crystals and polycrystals are reviewed in detail, while alloying effects and the deformation of NiAl martensite are briefly described. The fracture toughness, modes of fracture, and cyclic properties reported in the literature are assessed. A critical analysis of diffusion data for NiAl is followed by a discussion of the activation energy and mechanisms of diffusion. This information is related to the creep properties of NiAl, and additional critical comments concerning the substructure and creep mechanisms of NiAl are provided. A review of the environmental resistance of NiAl is followed by a brief discussion of several current and potential applications of NiAl. Concluding remarks include suggestions for future research on NiAl.

Definition, resolution, and contrast in photothermal imaging

Photothermal imaging can be described quantitatively by means of a point spread function (PSF), which represents the image of a buried thermal point defect. From this PSF conclusions can be drawn about amplitude and phase contrast, half-width, and resolution of photothermal imaging. The theoretical findings are stated by measurements from an appropriately designed model sample.

The influence of thermal point defects on the precipitation of oxygen in dislocation-free silicon crystals

The precipitation of oxygen in regularly grown and in situ-quenched dislocation-free Czochralski silicon crystals has been investigated by means of infrared absorption spectroscopy, preferential etching, and x-ray transmission topography. In addition, the effect of doping with acceptor or donor impurities on oxygen precipitation has been studied. The experimental results indicate that at oxygen supersaturation ratios smaller than 20 the formation of nuclei for precipitation predominantly occurs via a heterogeneous in which thermal point defects, in particular silicon interstitials, play an important role.

The effect of doping on the formation of swirl defects in dislocation-free czochralski-grown silicon crystals

During the growth of macroscopically dislocation-free Czochralski silicon crystals three types of swirl defects (types A, B and C) can form. The effect of doping with electrically active impurities (B, Ga, Sb, P and As) and isoelectronic impurities (Sn) on the formation of these microdefects has been investigated by means of preferential etching, copper and lithium decoration, X-ray transmission topography and EBIC-mode SEM. Doping with donors (concentration > 10 17 cm -3) suppresses the formation of A swirl defects (dislocation loops). Doping with acceptors (concentration > 10 17 cm -3) eliminates the formation of B and C swirl defects, while doping with tin has no effect. The new experimental data strongly suggest that the formation of the different types of swirl defects is due to parallel condensation processes involving silicon interstitials and vacancies. The observed doping effects are explained in terms of complex formation as a result of coulomb attraction between dopants and charged thermal point defects.

Swirl Defects in As-Grown Silicon Crystals

ABSTRACTDuring melt-growth of macroscopically dislocation free bulk silicon crystals (floating-zone and Czochralski technique) microdefects can form due to the condensation of thermal point defects (self-interstitials, vacancies). The formation of these imperfections, generally referred to as “swirl defects”, is strongly affected by the growth conditions (e.g. the crystal pulling rate) and crystal purity. The various reported defect formation models will be discussed. Special attention will be paid to the effect of doping on swirl defect formation.