Generation of vacancies in high-speed plastic deformation

In a recent experiment, crystalline metals were subjected to high-speed plastic deformation, and subsequently a number of vacancy clusters were observed without any trace of dislocations. In an effort to explain this result, in the present study fluid-like behavior of solid in ultra-high-speed deformation is considered, and the possibility of spontaneous generation of vacancies analogous to cavitation in high-speed fluid flow is discussed. Similar to a large velocity gradient that induces turbulence in a high-speed fluid flow, large shear stress induced in a solid material during the course of high-speed deformation may generate vacancies instead of dislocations, if the dislocations cannot follow the deformation speed. In this paper, similarities between dislocation in solid and vortex in a fluid discussed, along with similarities between vacancy in a solid and cavitation in fluid, and a mechanism of vacancy production under high-speed plastic deformation of crystalline materials is proposed.     

Modelling of Equilibrium Grain Boundary Solute Segregation under Irradiation

Both radiation-induced excess vacancies and solute-interstitials may enhance solute diffusion. The radiation-enhanced solute diffusion promotes the kinetic process of equilibrium segregation. This effect is especially considerable in the low temperature range. As a complement to modelling of radiation-induced non-equilibrium segregation, the radiation-created vacancy and solute-interstitial-accelerated equilibrium grain boundary solute segregation were theoretically treated. The models were applied to phosphorus segregation in α-Fe subjected to neutron irradiation.     

Quantitative nanoscale tracking of oxygen vacancy diffusion inside single ceria grains by in situ transmission electron microscopy

Oxygen vacancy formation and migration in ceria is critical to its electrochemical and catalytic properties in systems for chemical and energy transformation, but its quantification is rather challenging especially at atomic-scale because of disordered distribution. Here we report a rational approach to track oxygen vacancy diffusion in single grains of pure and Sm-doped ceria at −20 °C to 160 °C using in situ (scanning) transmission electron microscopy ((S)TEM). To create a gradient in oxygen vacancy concentration, a small region (∼30 nm in diameter) inside a ceria grain is reduced to the C-type CeO_1.68 phase by the ionization or radiolysis effect of a high-energy electron beam. The evolution in oxygen vacancy concentration is then mapped through lattice expansion measurement using scanning nano-beam diffraction or 4D STEM at a spatial resolution better than 2 nm; this allows direct determination of local oxygen vacancy diffusion coefficients in a very small domain inside pure and Sm-doped ceria at different temperatures. Further, the activation energies for oxygen transport are determined to be 0.59, 0.66, 1.12, and 1.27 eV for pure CeO₂, Ce_0.94Sm_0.06O_1.97, Ce_0.89Sm_0.11O_1.945, and Ce_0.8Sm_0.2O_1.9, respectively, implying that activation energy increases due to impurity scattering. The results are qualitatively supported by density functional theory (DFT) calculations. In addition, our in situ TEM investigation reveals that dislocations impede oxygen vacancy diffusion by absorbing oxygen vacancies from the surrounding areas and pinning them locally. With more oxygen vacancies absorbed, dislocations show extended strain fields with local tensile zone sandwiched between the compressed ones. Therefore, dislocation density should be reduced in order to minimize the resistance to oxygen vacancy diffusion at low temperatures.     

Inhibition of Electromigration in Eutectic SnBi Solder Interconnect by Plastic Prestraining

Plastic prestraining was applied to a solder interconnect to introduce internal defects such as dislocations in order to investigate the interaction of dislocations with electromigration damage. Above a critical prestrain, Bi interfacial segregation to the anode, a clear indication of electromigration damage in SnBi solder interconnect, was effectively prevented. Such an inhibiting effect is apparently contrary to the common notion that dislocations often act as fast diffusion paths. It is suggested that the dislocations introduced by plastic prestraining acted as sinks for vacancies in the early stage of the electromigration process, but as the vacancies accumulated at the dislocations, climb of those dislocations prompted recovery of the deformed samples under current stressing, greatly decreasing the density of dislocation and vacancy in the solder, leading to slower diffusion of Bi atoms.     

Dislocation structures introduced by high-speed deformation in bcc metals

Systematic experiments were carried out over a wide range of strain rate, 10⁰–10⁶ s⁻¹, so as to reveal the deformation mode in bcc crystals, especially at high strain rate. Dislocation structure showed heterogeneous distribution at low strain rates in all three bcc metals examined. At higher strain rates exceeding 10³ s⁻¹, distribution of dislocations was random, and the formation of small dislocation loops was observed in V and Nb. In Mo, small dislocation loops were not formed by deformation, even at high strain rates. However, post-deformation annealing of an Mo specimen that had been deformed by 20% at 5×10⁵ s⁻¹ produced dislocation loops. The inside–outside contrast method identified these loops to be of vacancy type. These results reveal that in Mo vacancy clusters are not formed directly from the interaction of dislocations, but by the aggregation of vacancies. In V and Nb, the same formation process is believed to occur at high strain rates. These results suggest that the different mode of plastic deformation at high strain rates accompanied by production of vacancies also occurred in bcc metals.     

Diffusion in metallic glasses

Rutherfordα-particles backscattering technique was employed for measurements of diffusion rates in metallic glasses. Effects of relaxation, crystallization and plastic deformation on diffusion rates were also investigated. It has been observed that the diffusion rates of a metallic solute are of the same orders of magnitude in both metal-metal and metal-metalloid glasses. A higher diffusivity is likely if there is a large difference between melting points of the solute and matrix. Relaxation has no effect on diffusion, however, diffusivity increases on crystallization. An increase in diffusivity is also observed on plastic deformation of metallic glass.     

Some comments about fatigue crack initiation in relation to cyclic slip irreversibility

In the present work, an overview of recent measurements of plastic strain irreversibility obtained with atomic force microscopy (AFM) is used to discuss the opportunity to define fatigue crack initiation in terms of a critical value of plastic strain irreversibility. We illustrate the fact that the critical value of height emergency associated with deformation band, necessary to crack initiation, is a decreasing function of the plastic strain localization in band. This phenomenon can be understood in terms of an excess of elastic energy associated with mobile dislocations and an interaction between vacancy mobility and oxygen diffusion.     

Modelling accelerated solid-state diffusion under the action of intensive plastic deformation

A mathematical model of accelerated solid-state diffusion during mechanical alloying (MA) in a binary substitutional system A-B is developed. An individual lamellar particle formed due to fracturing/cold welding during a preliminary stage of MA is considered. Interdiffusion occurs via the vacancy mechanism. During the plastic deformation, jog dragging by moving screw dislocation generates non-equilibrium point defects (vacancies and interstitial atoms), which can diffuse, interact with edge dislocations and recombinate. To evaluate the point defect generation rate, a simple Hirsch-Mott theory is employed. Numerical simulation has been performed for a repeated “deformation-rest” cycle at 100°C using realistic parameter values. The influence of non-equilibrium vacancies on the atomic diffusion is evaluated. It reveals itself via the increase of partial diffusivities of substitutional atoms and through the cross-link terms in the matrix of interdiffusion coefficients. The incoherent phase boundary between parent phases (pure elements) is considered as a sink for non-equilibrium vacancies. Due to interplay of these factors, substantial alloying by solid-state diffusion is observed after a reasonable time of MA (4000 s).     

Learning About 3d Melting With Solid Helium

Theories of bulk melting rely on a sudden proliferation of crystalline defects in order to destabilize the lattice. However, these scenarios were never confirmed experimentally, as it is impossible to approach the transition sufficiently to see critical behaviour. In experiments investigating plastic flow of solid ⁴He, we observed such critical behaviour involving a dramatic decrease of the resistance to shear near the first order bcc-hcp transition at 1.772K on the melting curve. By measuring the plastic flow over a wide range of shear stress, we were able to differentiate between the contribution of vacancies and of dislocations to this process. We argue that the mechanism by which the crystal loses its shear resistance involves coupling of vacancy diffusion with a transverse phonon which softens as the transition is approached. Dislocations seem to play no fundamental role in this process. These results suggest a refinement of the Lindemann picture, as the phonon mode which plays a dominant role in this process is near the edge of the Brillouin zone.     

Dependence of structure and properties of vacuum condensates of some metals and alloys on conditions of deposition

We present data on the relationship between the strength and hardness of vacuum condensates of aluminium, copper and Cu-Al alloys and the structure formed as a result of the combined effect of the main parameters of the deposition process (the rate of condensation, pressure, temperature, crucible material and substrate). The mechanical strength of the condensates is discussed from the viewpoint of their contribution to strengthening the compact grain structure, the initial dislocation density and the rate of accumulation during plastic deformation.We propose a formula which is convenient for numerical calculation and which relates the concentration of the element in the condensate to the composition of the evaporated binary alloy. We also present values for the thermodynamics activities of copper and aluminium in the melt, which were obtained by analysing the composition of the condensate at different stages of formation.New data are presented on the phase state of Cu-Al and Cu-Sn films which indicate a reduction of the solubility of aluminium in copper to 3–5 wt.% at 20–400°C, and we give data on the mutual diffusion coefficients D? in Cu-Al condensates, which exceed by 3–6 orders of magnitude the D? values of the massive analogues.     

Plastic deformation of bcc metal thin foils without dislocation

Heavy plastic deformation of fcc metal thin foils to fracture has been found recently to proceed without involving dislocations, and it results in the formation of high density of vacancy clusters. Thin foil specimens of bcc metals such as V and Mo were plastically deformed to fracture in in situ elongation experiments under an electron microscope. Morphology of thinning and fracture was found to be similar to fcc metals, and no dislocation was observed during heavy deformation. Electron diffraction analysis at the tip of a crack during deformation confirmed a large elastic deformation of up to 5%. Unlike in fcc metal thin foil specimens, point defect clusters were not observed near fractured tips. This difference is attributed to the difference in vacancy reaction, though the deformation in bcc metals without dislocation most likely does produce vacancies.     

Some metallurgical aspects of Dynamic Strain Aging effect on the Low Cycle Fatigue behavior of C–Mn steels

Carbon–Manganese steels and associated welds are commonly used, and sometimes to sustain loads in the Low Cycle Fatigue domain. Nevertheless, the metallurgy of these C–Mn steels is rather complex, due to the interaction of solute atoms (carbon and nitrogen) with dislocations during deformation which leads to metallurgical instabilities: Lüders strain, Static Strain Aging (SSA) and Dynamic Strain Aging (DSA). The DSA phenomenon is an interaction during the test between solute atoms and dislocations which are submitted to an supplementary anchorage if the temperature is sufficient to allow the diffusion of solute atoms leading to a discontinuous plastic deformation localized in bands associated with serrations on the stress–strain curve. In C–Mn, the temperature domain where the phenomenon is present is from 150 °C to 300 °C. If these metallurgical instabilities induce an increase in hardness, unfortunately they produce a decrease of ductility detrimental to components safety. The results of the DSA effect on LCF behavior in C–Mn and Low Alloyed steels reported in the literature are very confused and contradictories. In this study, two C–Mn steels with a different sensitivity to DSA are investigated in the Low Cycle fatigue domain. As reported from some authors, the fatigue life seems enhance or reduce in the temperature domain where the DSA is maximum, but the decrease of the strain rate always decreases the number of cycles to failure.     

Estimation of lattice strain in nanocrystalline silver from X-ray diffraction line broadening

The lattice strain contribution to the X-ray diffraction line broadening in nanocrystalline silver samples with an average crystallite size of about 50 nm is studied using Williamson-Hall analysis assuming uniform deformation, uniform deformation stress and uniform deformation energy density models. It is observed that the anisotropy of the crystallite should be taken into account, while separating the strain and particle size contributions to line broadening. Uniform deformation energy density model is found to model the lattice strain appropriately. The lattice strain estimated from the interplanar spacing data are compared with that estimated using uniform-energy density model. The lattice strain in nanocrystalline silver seems to have contributions from dislocations over and above the contribution from excess volume of grain boundaries associated with vacancies and vacancy clusters.     

Microstructural and compositional modification of In0·53Ga0·47As/In0·52Al0·48As multiquantum wells using rapid thermal annealing process

Abstract

Quantum well intermixing of In_0·53Ga_0·47As/In_0·52Al_0·48As multiquantum wells (MQWs) in an impurity free vacancy diffusion (IFVD) mechanism was investigated to observe the intermixing aspect and the effect of defects in MQWs with regard to the microstructural aspect using transmission electron microscopy. The MQWs were grown on a GaAs (100) substrate using compositionally graded buffer layers via molecular beam epitaxy, and the MQWs were annealed at 750 and 900°C for 30 s via rapid thermal annealing for quantum well intermixing using IFVD method. In the fabricated MQWs, defects, such as stacking faults, twins and dislocations, were not generated at 750°C. The diffusion of Ga in the well layer for the quantum well intermixing started from the top well layer, because the SiO₂ layer that supplied vacancies for the quantum well intermixing was at the top of the sample. Additionally, in the same well layer, the intermixing did not show equality, because these vacancies were not supplied homogeneously. Especially, in the 900°C annealed case, many dislocations were generated from the cladding layer. These dislocations contributed to new vacancy generation sites, thus the quantum well intermixing was accelerated.     

Growth of extrusions in localized cyclic plastic straining

Cyclic strain localization into persistent slip bands produces on the surface of cyclically deformed materials persistent slip markings in the form of extrusions and intrusions. The localized cyclic plastic straining in persistent slip bands leads to the production and annihilation of dislocations as well as point defects. Production, annihilation and migration of point defects, preferably vacancies, is quantitatively characterized. The effect of vacancy production and migration on the extrusion growth is treated under simplified conditions. At low temperatures when vacancies are immobile, a static extrusion is produced. At elevated temperature the diffusion equation is solved, a general expression for stabilized rate of extrusion growth is obtained and temperature dependence of the growth rate can be predicted. The predictions of the model are compared with experimental observations.     

Self-Diffusion in 3He crystals

Measurements of self-diffusion in bcc ³ He 24,20-24,80 cm ³/mole were carried out by a new method in the temperature range 0.4 - 0.8 K. The vacancy diffusion coefficient was obtained by comparison of the self-diffusion data and the vacancy specific heat. It is found out that the vacancy diffusion is independent of temperature, because of spin disorder in this region. The data obtained shows that vacancies in bcc ³ He are wide-gap quasiparticles.     

Dynamic aging of 18 wt-%Ni maraging 250 steel,based on studies of serrated flow

Abstract

The dynamic aging of an 18 wt-%Ni 250 maraging steel was investigated. The first stage of dynamic aging is suggested to be the formation of Mo atmospheres on dislocations, which occurs at approximately 204°C. The activation energy for Mo-atmosphere formation was found to be 1·01–1·38 eV. It is also suggested that Mo clustering in the matrix independent of the atmosphere–dislocation interaction occurs in the first stage of dynamic aging. In the later stage of dynamic aging, the Mo atmospheres are depleted from the dislocation lines by a precipitation reaction which occurs during the arrest period of the dislocations at the matrix Mo clusters. The second stage of dynamic aging (depletion of Mo atmospheres and growth of matrix Mo clusters), occurs at and above 316°C. As the growth of the Mo clusters progresses, a driving force for Ni diffusion to the clusters occurs in order to form matrix Ni₃Mo precipitates. Ni diffusion to the Mo clusters occurs with an activation energy of 2·7-3·2 eV, and is considered to be the rate-controlling process sustaining the later stage of dynamic aging. As the reaction in the matrix begins to saturate, the dislocations retain the remaining Mo as atmospheres. At this point, Ni begins to diffuse to the dislocation lines, where a precipitation reaction to form Ni₃Mo directly on the dislocation lines occurs.

MST/109     

Vacancy induced brittle-to-ductile transition of Nb5Si3 alloy from first-principles

The effect of vacancy on mechanical properties of α-Nb₅Si₃ is systematically investigated by first-principles calculations. Four different mono vacancies in this alloy are considered in detail. The vacancy formation energy, formation enthalpy, elastic modulus, hardness, B/G ratio, thermodynamic properties and electronic structure of α-Nb₅Si₃ with different vacancies are discussed. The calculated vacancy formation energies show that Nb vacancies are more stable than that of Si vacancies, and α-Nb₅Si₃ prefers to form the Nb_-va2 vacancy. Those vacancies weaken the volume, shear deformation resistances and reduce the elastic stiffness. However, those vacancies result in brittle-to-ductile transition and α-Nb₅Si₃ with Si_-va1 mono vacancy exhibits ductile behavior. The calculations of electronic structure reveal that these vacancies change the localized hybridization between Nb–Si and Nb–Nb atoms, which are the origin of brittle-to-ductile transition. Finally, we conclude that vacancy is beneficial for improving the ductility of Nb₅Si₃.     

Edge-Mediated Annihilation of Vacancy Clusters in Monolayer Molybdenum Diselenide (MoSe2) under Electron Beam Irradiation

Annihilation of vacancy clusters in monolayer molybdenum diselenide (MoSe₂) under electron beam irradiation is reported. In situ high-resolution transmission electron microscopy observation reveals that the annihilation is achieved by diffusion of vacancies to the free edge near the vacancy clusters. Monte Carlo simulations confirm that it is energetically favorable for the vacancies to locate at the free edge. By computing the minimum energy path for the annihilation of one vacancy cluster as a case study, it is further shown that electron beam irradiation and pre-stress in the suspended MoSe₂ monolayer are necessary for the vacancies to overcome the energy barriers for diffusion. The findings suggest a new mechanism of vacancy healing in 2D materials and broaden the capability of electron beam for defect engineering of 2D materials, a promising way of tuning their properties for engineering applications.     

Formation of vacancies in a nanocluster-strengthened ferritic steel

The effect of vacancy on the microstructure of the ferritic steel with a nominal composition of Fe-14Cr-3W-0.4Ti-0.4Y₂O₃ was investigated. By using the resistivity measurements, the vacancy formation enthalpy of the alloy is calculated to be 1.11 ± 0.16 eV, which is lower than that of α-Fe. The Ti-Y-O nanoclusters in the alloy are responsible for stabilizing the vacancies. The hardness of the steel increases with the heat treating temperature. The hardening mechanism in the ferrite matrix can be attributed to the interaction between dislocations and vacancies or vacancy clusters.