Vacancy Sinks Research Articles

Effect of δ alumina fibers on the aging characteristics of 2024-based metal-matrix composites

The age-hardening precipitation reaction in aluminum matrix composites reinforced with discontinuous alumina fibers was studied using the differential scanning calorimetry (DSC) technique, microhardness tests, and transmission electron microscopy (TEM) observation. Composites fabricated with the 2024 alloy matrix were infiltrated through a ceramic preform using a squeeze-casting process. The alumina fibers had a considerable effect on the aging response of the matrix alloy in composites. Alumina fibers caused suppression of Guinier—Preston (GP) zone formation in composite that reduced the peak hardening during artificial aging. The suppression of GP zone formation in composites is believed to be due to the fiber-matrix interface, which acts as a sink for vacancies during quenching. Moreover, the presence of reinforcement does not alter the kinetics of the subsequent artificial aging of these Al2O3/2024Al composites.

A new model of the pest-like disintegration of intermetallics due to intergranular oxidation

A new diffusional model of disintegration of intermetallics during oxidation was developed. The model is concerned with intermetallics in which the pest phenomenon occurs as a result of intergranular oxidation. An example of such intermetallics is NbAl 3. It was assumed that, in these cases, oxidation leads to an excess of vacancies at grain boundaries. The migration of the excess vacancies to the surface facilitates the diffusional growth of grain boundary cracks. The steady state vacancy concentration ahead of the crack tip was found from the diffusional equation compromising the vacancy sources and sinks. The rate of crack propagation was found through the vacancy flux to the crack tip. Two different steady state regimes of crack growth (fast and slow) were established, and the temperature of transition from one regime to the other was determined. The fast regime was related to the disintegration of intermetallics containing a substantial number of structural vacancies. The slow regime corresponds to the case when no disintegration occurs. The rate of diffusion crack propagation is only slighly affected by the gradients of the internal stresses and of the equilibrium vacancy concentration in the oxidation zone. The decisive role of the vacancy excess generation in the fast crack diffusion propagation was revealed. Activation energies of the process were estimated.

Formation of the bands of anomalous oxygen precipitation in Czochralski-grown Si crystals

This paper reports some results on anomalous oxygen precipitation in CZ-grown Si crystals with uniform oxygen distribution and proposes a mechanism consistent with these observations. Experiments have been carried out on crystal growth with constant pulling rate, with a step change in pulling rate and temporary cessation of pulling. To modify thermal growth conditions, an additional heater has been used. The thermal history was specified following temperature measurements in the growing crystals. Distribution of microdefects has been investigated by etching and X-ray topography after 1000°C annealing for 24 h and Au diffusion at 1200°C. It was found that a band of anomalous oxygen precipitation emerged near sinks for vacancies and its shape was closely connected with that of the vacancy-depleted region. The band exhibits reduced or enhanced oxygen precipitation depending on thermal history during crystal growth in the ranges 1150–760°C and 760–610°C. After Au diffusion, a higher concentration of dislocation loops and stacking faults is revealed in the band. A mechanism is proposed to account for the results which also makes it possible to predict a configuration of the band and its behaviour upon heat treatments, starting from the thermal history and taking into account the distribution of intrinsic point defects in a growing crystal.

Atomic short-range ordering in AuFe alloys: a resistometric study

Kinetics and equilibrium of atomic short-range order (SRO) in Au 1− x − Fe x ( x=0.05, 0.10, 0.14, 0.20) have been investigated by measurement of residual electrical resistivity for a cold-rolled and a well-annealed state. Activation enthalpies of SRO relaxation correspond well to the sum of formation and migration enthalpies of vacancies, both determined separately In contrast to Mössbauer measurements a strong temperature dependence of SRO-induced electrical resistivity was observed. It was found that, because of the high density of vacancy sinks, SRO kinetics in the cold-rolled state in considerably slower than in the well-annealed state. Surprisingly, the as-rolled state for the alloys with 5, 10 and 14 at.% Fe has a high degree of SRO, whereas for Au-0.2.%Fe the initial state is disordered.No indications for precipitation of α-Fe were found for the samples with 5,10 and 14 at.% Fe, for both the cold-rolled and the recrystallized state. For 20 at.% Fe, however, a resistivity drift in the cold-rolled state was observed, which may be caused by precipitation of α-Fe.

On the influence of interfaces and localised stress fields on irradiation-induced point-defect distributions in silicon

High-flux 1-MeV electron irradiation in a high voltage transmission electron microscope is used to study the influence of interfaces and localised stress fields on {113}-defect generation in silicon. A semi-quantitative model is presented to explain the observations, suggesting that the silicon oxide/silicon interface is a stronger sink for self-interstitials than for vacancies. It is shown that the position and the height of the maximum of the {113}-defect density strongly depends on the strength of the interface as a vacancy sink and that compressive straining of the silicon substrate slows down the diffusion of vacancies towards the interface.

Characteristic length and time in electromigration

A characteristic length can be defined as the distance a vacancy moves under an electromigration force to do thermal work KT. The times for a vacancy to drift and diffuse this characteristic length are equal and thus define a characteristic time. The diffusion coefficient and the drift velocity are unity when these characteristic values are used as units of length and time. Therefore, when these units are used in the absence of vacancy sources and sinks, the vacancy continuity equation contains no parameters. These considerations are used to estimate the length current density threshold, the electromigration failure times, the edge-displacement velocity, and the incubation time. These estimates are consistent with published experimental results.

Effect of Solutioning Conditions on Aging Response in Ti–15V–3Cr–3Sn–3Al

An effect of the solutioning conditions on the aging response (precipitation, age hardening) in a β type titanium alloy, Ti–15V–3Cr–3Sn–3Al, is examined.The aging response is strongly affected by the solutioning conditions and closely related to the concentration of quenched-in vacancies, which is a function of the quenching temperature and the density of vacancy sinks such as β grain boundaries, dislocations, etc. For example, the ω (≤250°C) and the α (300∼350°C) precipitations and the age hardening are greatly accelerated in the specimens in which a large amount of quenched-in vacancies are contained, i.e. the specimens quenched from high temperatures and/or with a small density of vacancy sinks. Furthermore, the α phase precipitates very homogeneously inside the β grains at the aging temperatures between 300 and 500°C. However, the precipitation delays and the denuded zones are sometimes observed in the area in which vacancy sinks exist, e.g. in the vicinity of the β grain boundaries or the isolatedly formed twins, or in the area in which dislocations are introduced due to the weak plastic deformation.In the specimens quenched from lower temperatures in the β region and having a smaller β grain size or a slightly higher density of dislocations, the delay of the aging response is significant and the α phase precipitation is heterogeneous. These phenomena are considered to be caused by a stronger effect of small amount of excessive vacancies, which is due to the relatively high density of the vacancy sinks and the lower quenching temperature, rather than the effect of the β grain boundaries and dislocations as precipitation sites.

Participation of helium in discontinuous transformations in metallic alloys

AbstractThe influence of helium on the diffusion at a migrating boundary has been investigated during discontinuous precipitation in a Co–14 at.-%Al alloy and during discontinuous dissolution in an Al–15 at.-%Zn alloy. The helium was introduced into the alloys via a plasma treatment using various fluxes. Two opposing processes have been observed depending on the flux. One, below a critical flux, led to an increase in the linear growth rate of discontinuous precipitation and discontinuous dissolution owing to an enhanced concentration of vacancies, which acted as carriers of solute atoms in the diffusion process. The other process, above the critical flux, resulted in the appearance of helium bubbles, which acted as mechanical obstacles to the moving discontinuous precipitation and discontinuous dissolution front and as sinks for the excess vacancies. The effect of helium on the diffusion ahead of the advancing front during discontinuous precipitation and discontinuous dissolution has been well described u...

Characterization of point defect generation at silicon surfaces using gold diffusion

The utility of observations of Au diffusion for the characterization of point defect generation and annihilation rates at surfaces and of vacancy distributions and diffusivity in the substrate is demonstrated. Particularly noted are the effects of ambient gases and surface finish on the diffusion process and on the resultant distributions of electrically active Au. Au was deposited on commercial float‐zone Si in a vacuum system after the Si had reached the diffusion temperature (1233 K) and had been annealed in various ways. Contrary to a previously published report, electrically active Au with a one‐sided profile is found when the Au is deposited and annealed in a vacuum. Previously reported two‐sided profiles for electrically active Au are obtained when millitorr levels of O2 are present in the ambient gas during diffusion or during a preanneal prior to Au deposition. It is concluded that the polished or etched silicon surfaces lack the imperfections needed to make them effective sources or sinks for vacancies or self‐interstitials, but that surface roughening caused by O2‐induced evaporation of SiO is sufficient to create very effective sources or sinks.

Theoretical analysis of annealing behaviour of mixed distributions of dislocation loops, voids, and gas bubbles: comparison with annealing behaviour of irradiated reactor component

A theoretical analysis of the annealing behaviour of irradiation damage in the form of mixed distributions of dislocation loops, voids, and bubbles is presented. Two limiting cases are considered. The first is where no external vacancy sources and sinks exist, such that vacancies are conserved during the annealing process and the second is where such sources and sinks do exist. Numerical examples are given and comparisons are made with the annealing behaviour of an irradiated reactor component. The influence of an applied stress on the annealing behaviour is evaluated.MST/1592

The Effect of Vacancies Grown into Silicon on Gold Diffusion

ABSTRACTWe propose a generalized model for gold diffusion in silicon based on the effect of the high concentrations of vacancies and vacancy complexes in the as-grown silicon. The monovacancy profiles calculated using this model are identical to the substitutional gold profiles calculated using the kick-out model. We deposited Au on commercial float zone Si in a vacuum system after the Si had reached the diffusion temperature (1233 K) and had been annealed in various ways. Contrary to previously published reports, we find the electrically active Au with a nearly one-sided profile when the Au is deposited on samples which were preannealed in vacuum. We conclude that annealed silicon surfaces lack the imperfections needed to make them effective sources or sinks for vacancies or self-interstitials. We propose that this can cause a high degree of supersaturation in the as-grown silicon crystal since the point defects cannot annihilate at the surfaces to maintain equilibrium as the crystal is cooled.

Metal-ceramic interfaces: Sources and sinks for mass transfer

The closure of interface voids during metal-ceramic diffusion bonding studied for a number of metal foils joined to TZP zirconia. In the case of bonds to large grained Cu and Ni foils the final voids to close on the interface are found to rest on grain boundaries in the metal foil where they intersect the metal-ceramic interface. Bonds made to foils of smaller grain sized Fe and Ni do not show this behaviour. When the final interface voids occur at the intersections of metal grain boundaries with the metal-ceramic interface, voids are also seen to nucleate on grain boundaries within the metal foil away from the interface. These observations suggest that the metal-ceramic interface does not act as an efficient vacancy sink during interface formation. Possible mass transfer routes are proposed to allow vacancies to sink on metal grain boundaries within the foil during interface void closure. The nucleation of voids within the metal foil during diffusion bonding is possibly caused by interactions of creep and diffusional mass transfer during bonding.

Ti-15V-3Cr-3Sn-3Alの時効特性におよぼす溶体化処理条件の影響

An effect of the solutioning conditions on the aging response (precipitation, age hardening) in a β type titanium alloy, Ti-15V-3Cr-3Sn-3Al, is examined. The aging response is strongly affected by the solutioning conditions and closely related to the concentration of quenched-in vacancies, which is a function of the quenching temperature and the density of vacancy sinks such as β grain boundaries, dislocations, etc. For example, the ω (≤250 o C) and the α (300-350 o C) precipitations and the age hardening are greatly accelerated in the specimens in which a large amount of quenched-in vacancies are contained, i.e. the specimens quenched from high temperatures and/or with a small density of vacancy sinks

Chemical compatibility of coated systems at high temperatures

Abstract Increasing the gas inlet temperatures in industrial gas turbines and aero engines requires careful selection of coatings which offer sufficient chemical compatibility with low interdiffusivity. Rapid diffusion of aluminium and cobalt from Co(Ni)CrAlY coatings into nickel-based alloys, with subsequent precipitation of the more stable β-(Ni,Co)Al in the interdiffusion zone and diffusion of nickel into the coating, limits the maximum operation temperature of such coatings in industrial gas turbines to about 950 °C. The degradation of aluminide and Ni(Co)CrAlY coatings is characterized by the diffusion of nickel into the coating, the subsequent inward diffusion of aluminium and the precipitation of Cr23C6 carbide in the interdiffusion zone which can be explained by the higher chromium activity in this zone. Therefore, these coatings have a higher temperature capability than Co(Ni)CrAlY coatings. Oxide-dispersion-strengthened (ODS) alloys as well as single-crystal alloys have a high susceptibility to Kirkendall void formation. In ODS alloys porosity formation occurs in the interdiffusion zone which can be explained by lower condensation energies of vacancies at incoherent phase boundaries between dispersoids and the sorrounding matrix. Porosity formation in single-crystal alloys is favoured in the coating or at the coating-substrate interface. Grain boundaries may act as sinks for vacancies with subsequent porosity formation.

Point-defect equilibrium during diffusion of purely substitutional impurities into elemental crystals.

Diffusion of exclusively substitutional solutes (B) into elemental metallic or semiconducting matrices via exchanges with vacancies (V) is treated by starting from a quasichemical reaction which includes impurity-vacancy pairs (BV) as a third species, i.e., BV\ensuremath{\rightleftarrows}B+V. This approach enables us to answer the question whether during foreign-atom penetration the vacancy concentration of the host crystal remains in thermal equilibrium. Within the present concepts a distinction can be made between the BV-controlled and the V-controlled mode of the vacancy mechanism. It is shown that commonly the former case, in which vacancy equilibrium persists, is realized. In the V-controlled case the B incorporation is limited by the removal of excess isolated vacancies originating from dissociation of BV pairs. Conditions for the appearance of this diffusion mode are pointed out. We also discuss the role of dislocations as vacancy sinks, the validity of the mass-action law associated with the above reaction, and a possible influence of self-interstitials.

Interfacial dynamics in diffusion-driven phase transformations

The migration of ledges in a semicoherent α/β interface is considered to participate in solid-state transformations driven by diffusion. The advance of the ledge and/or the progress of the transformation can require the climb of misfit dislocations both in the ledge and in its path. The creation or annihilation of vacancies required for the transformation and the legde advance is provided by a combination of three vacancy sources or sinks: (a) the net vacancy flux to/from the interface resulting from the difference in lattice plane shift (Kirkendall effect) within the two contacting phases, (b) the climb of misfit dislocations from the interface into the bulk of the α and β phases, and (c) the climb of misorientation dislocations within the interface. Thus, the dynamic action of the interface during the phase transformation would include: (i) climb of misfit dislocations out of the interface, with ensuing dissociation into glissile dislocations which resupply the interface by return glide, and (ii) climb of misorientation dislocations in the interface necessitating a continuing arrival of such dislocations from sources in the bulk or in the interface.

Interdiffusion in oxides (A,B)O (II): Experimental investigation of point-defect relaxation on concentration profiles for MnO-FeO

Abstract Vacancy relaxation phenomena during interdiffusion of FeO and MnO have been studied on the basis of very accurate concentration-distance profiles of the cations, measured by X-ray microanalysis. The profiles clearly depended on annealing time and form of the samples. This observation directly demonstrates that local thermodynamic equilibrium of defects is not established in the reacting system. In addition, the deviation from local equilibrium is most pronounced at the early stage of the reaction in a thick sample. This can be explained by the long relaxation time the cation vacancies need to reach a free surface surrounded by oxygen gas as a vacancy sink.

Evidence for vacancy injection during the oxidation of iron

Iron discs have been oxidised at 890°C on one side only with the other side protected by an inert gas. The scale-metal adhesion was very good. When the face which was normally inert was covered with a non-growing oxide layer, the scale-metal adhesion deteriorated. The results show that vacancies which were produced by oxidation were annihilated within the metal, that the inert face played a part in this and that the scale-metal interface is not a good sink for vacancies.

An investigation of void formation on a bonded interface of power law creep materials containing a cylindrical particle

Three stress solutions for a cylindrical particle on an interface bonded with two identical materials are obtained by using the pseudo-stress function developed previously. The stress solutions correspond to three different bonding mechanisms of a particle interface: 1) complete bonding of a rigid cylindrical particle, 2) radial bonding of the cylindrical rigid particle, and 3) complete bonding with deformable cylindrical particle in a perfectly viscous material. Using stress analysis results, the steady-state vacancy flux distribution at the interface of two materials (material interface) may be expressed as an in terms of the second derivative of normal stress on the interface with respect to two-dimensional space. The behavior of a vacancy sink or a vacancy source is found to be dependent on particle size, relative magnitude of material vs. particle properties, and bonding mechanisms, and loading conditions. In the case of the third stress solution, decohesion at the interface may be prevented by controlling a dimensionless parameter Ω1, which is a function of $${{\sigma _0 ^p } \mathord{\left/ {\vphantom {{\sigma _0 ^p } {\sigma _0 }}} \right. \kern-\nulldelimiterspace} {\sigma _0 }}$$ and the external stress and strain rate hardening exponent, where $$\sigma _0 ^p$$ and σ0 are associated with the particle and matrix constitutive equation $$\bar \sigma = \sigma _0 \dot \bar \varepsilon ^m$$ respectively.

Theoretical modelling of solute segregation to a free surface in irradiated dilute alloys

Abstract In recent years, segregation to a variety of sinks has been observed in a wide range of alloys during irradiation. There have also been a number of theoretical calculations of the segregation to free surfaces in both dilute and concentrated alloys. In this paper, we show that the boundary conditions used have a strong effect on the calculated solute concentration profiles. In most calculations the free surface is assumed to be an ideal sink for vacancies and interstitials, and the calculated solute concentration profiles are very steep close to the free surface. In this paper, we examine the physically more realistic case of rate-limitation boundary conditions for the absorption of vacancies and interstitials at the free surface and show that this reduces considerably the solute concentration gradients close to the surface. We also discuss the implications of this behaviour for the modelling of experimental results of solute segregation.