Point-defect-driven dislocation patterning during deformation under irradiation

Dislocation patterning driven by interactions of dislocations with deformation-induced point defects is considered. The effect of concurrent irradiation-induced production of point defects is also included. The uniform time-dependent solution of the set of equations describing the evolution of the system is probed by small periodic perturbations. A linear stability condition obtained in this way as well as the preferred wavelength of the emerging pattern depend on the values of the parameters reflecting biases in the production and annihilation of vacancies and interstitial atoms. It is proposed that by studying the effect of different types of radiation and different irradiation intensities on the occurrence and the wavelength of the dislocation pattern information about the deformation-induced point defect production bias may be obtained.     

Dislocation dynamics in cyclic plastic deformation

The paper presents a theoretical investigation of the slip avalanches (so-called strain bursts) which occur in single-glide-orientated face-centered cubic or hexagonal close-packed metals during stress-amplitude-controlled cyclic plastic deformation. The study is based on a model of the dynamics of dislocations that has been developed in a companion paper (Part I). It is shown that this model allows for a quantitative treatment of the strain-burst phenomenon. In particular, the scaling relations between different strain-burst-characteristic parameters which have been found by experiment are connected to the evolution of the dislocation microstructure and thus find a natural explanation.     

On the foundations of stochastic dislocation dynamics

A stochastic approach to dislocation dynamics is proposed that starts off from considering the geometrically necessary fluctuations of the local stress and strain rate caused by long-range dislocation interactions during plastic flow. On a mesoscopic scale, a crystal undergoing plastic deformation is thus considered an effective fluctuating medium. The auto- and cross-correlation functions of the effective stress and the plastic strain rate are derived. The influences of dislocation multiplication, storage and cross slip on the correlation functions are discussed. Various analogies and fundamental differences to the statistical mechanics of thermodynamic equilibrium are outlined. Application of the theory of noise-induced transitions to dislocation dynamics gives new insight into the physical origin of the spontaneous formation of dislocation structures during plastic deformation. The results demonstrate the importance of the strain-rate sensitivity in dislocation patterning.     

Statistical theory of slip channels in body-centered cubic metals

Received: 26 July 1996/Accepted: 7 October 1996     

Statistical theory of slip channels in neutron-irradiated metals

Accepted: 6 January 1997     

Temperature-induced rearrangement of the dislocation pattern of Persistent Slip Bands in copper single crystals

Experimental investigations are reported on mechanisms by which dislocation arrangements of Persistent Slip Bands (PSBs) respond to changes of the deformation temperature. Copper single crystals orientated for single slip were cyclically deformed well into saturation at 300 K at an applied resolved plastic shear-strain amplitude, , such that the plastic strain became localized in PSBs. The spacings of the dislocation walls in these PSBs are about 1.4 m. After the temperature had been lowered to 77 K, cyclic deformation was continued with unchanged . A transformation of the dislocation pattern started. A certain fraction of the PSBs produced at 300 K finally showed a mean wall spacing of about 0.7 m which is typical for PSBs formed at 77 K. The remaining PSBs did not finish the transformation and became obviously inactive. In the state of cyclic saturation reattained at 77 K 50% of the PSBs, which had been formed at 300 K, show the dislocation pattern characteristic of 77 K. It is concluded that the amplitude of the resolved plastic shear strain localized in a PSB, , must be twice as large at 77 K as at 300 K. In an additional series of experiments crystals were cyclically deformed at constant temperatures of 430 K, 300 K, 190 K, and 77 K. In the temperature range covered by these experiments, the amplitude of the saturation flow stress, _S, appears to be proportional to the intrinsic amplitude of the PSBs, .     

Effects of an inhomogeneous elliptic insert on the elastic field of an edge dislocation

The problem of an edge dislocation in the vicinity of an elliptical inhomogeneity is solved analytically in the form of infinite series. The inhomogeneous inclusion is assumed to be either perfectly bonded or sliding along the interface with the surrounding matrix. The problem is formulated in terms of Papkovich-Neuber displacement potentials and the results for the perfectly bonded and sliding interface are compared. The effects of the inclusion/matrix shear moduli ratio, inclusion/dislocation relative distance and inclusion aspects ratio on the interface stresses and the dislocation force are evaluated.     

The dynamic symmetry and plastic deformation of diamond lattice

An analysis of possible transformations of moments of force by a diamond lattice has been made. The conditions of resonance transformations of moments of force by this lattice have been found. The dynamics symmetry of the diamond lattice has been determined in terms of the Shubnikov group Fd3m'. Besides, it has been found that resonance librationally deformed configurations are oneperiodic and have cores 〈110〉 two times more deformed than the surrounding area. As a result of the action of resonance phonons of the librational mode it is possible to generate a new kind of defects-bidislocations with Burgers vector components+[110]/2 and −[110]/2. It has been shown that bidislocations can play an essential role in plastic deformation of a diamond lattice and other related lattices.     

In-situ NMR study of dislocation motion in Ca-doped NaCl crystals

Using a fast multi-window NMR technique, we have measured in-situ the mean jump width x of mobile dislocations during plastic deformation in a series of NaCl single crystals with varying Ca⁺⁺ content. Aside from immobile forest dislocations, the Ca⁺⁺ impurities form additional obstacles for the moving dislocations thus lowering x. We found that the Ca⁺⁺-related obstacles exhibit a pronounced non-random distribution which results in a corresponding broad distribution of x. We show that the data can be evaluated by means of an appropriate distribution function g(1/x) with an uncommon dependence of the observed fitting parameters on the Ca⁺⁺ content. As expected, quenching of a sample leads to a more uniform distribution of the Ca⁺⁺-related obstacles resulting in a corresponding narrowing of g(1/x).     

Spatio-temporal aspects of low-temperature thermomechanical instabilities: A model based on dislocation dynamics

The occurrence of plastic instabilities which are accompanied by a significant heat release is a typical feature of the plastic behaviour of metals deformed at sufficiently low temperature. This phenomenon may be studied within the framework of a dislocation-dynamical model. The influence of the heat which is released by the deformation process on the dislocation velocity, and thus on the deformation dynamics, is taken into account. In particular, the influence of the spatial coupling which arises from heat conduction on the spatio-temporal behaviour of the deformation process is studied.     

Supersaturated Si-As alloy formation by ion implantation and pulsed electron beam annealing

Supersaturated surface alloys produced by very high dose (0.8–2.6×10¹⁷cm^–2) implantation of As-ions into silicon and subsequent pulsed electron-beam annealing have been investigated by means of the channeling technique. The maximum solubility limit of 7×10²¹ As/cm³ has been determined. It exceeds the equilibrium solubility limit by more than a factor of 4. Angular scan measurements indicated that for doses above 1×10¹⁷cm^–2 As atoms are displaced by about 0.12 Å from the regular lattice sites.     

Electrical activity of dislocations: Prospects for practical utilization

The electrical activity of interfacial misfit dislocations in silicon has been examined using the electron beam induced current technique (EBIC) in a scanning electron microscope. Clean misfit dislocations, i.e. no EBIC contrast, formed during high-temperature Si(Ge) chemical vapor epitaxy were studied. These defects were subsequently decorated with known metallic impurities (Au and Ni) by diffusion at 400° C to 1130° C from a back-side evaporated layer. Qualitative analysis of the electrical activity in relation to the energy levels anticipated for the clean or decorated dislocations is presented. Of particular interest is the case of defect-induced conductivity type inversion which occurred both at the top surface and at the buried dislocated interfaces of the multilayer. The prospects for using dislocations in a beneficial manner as active elements in electronic devices are discussed.     

Pulsed electron-beam annealing of high-dose arsenic implanted silicon

The effects of pulsed electron-beam annealing of high-dose As implanted {111} Si single crystals has been studied. The depth distributions and lattice location of the As atoms were obtained using MeV⁴He⁺ backscattering and channeling technique. The implantation energy was 100 keV with a total dose of 3.5·10¹⁶/cm². Above the threshold energy of 0.9 J/cm² the single-crystal transition was observed with about 95% of the As atoms on substitutional lattice sites. This leads to an As concentration of 2·10²¹/cm³ which was demonstrated to be a metastable one.     

Defect structure in EuS single crystals grown from the melt

The defect structure in EuS single crystals grown form the melt is studied by etch pitting, scanning and high-voltage electron microscopy. Circular and square etch pits and a second phase in the shape of thin hexagonal platelets are observed by etching. Microprobe analysis indicates the platelets to consist of Eu metal. In the transmission electron microscope, smoothly curved dislocations and helical dislocations, small dislocation loops and inclusions associated with dislocations are observed. The possible origin of the detected dislocation structure is considered with reference to climb and glide processes occurring during cooling down the grown crystals. The results corroborate the glide geometry of the NaCl lattice for EuS. On leave from Institute of Physics, Academic Sinica, Peking, VR China     

Laser-induced defect formation in semiconductors

The electron-deformation-thermal theory of pulsed laser-induced point defect generation in strongly absorbing semiconductors is developed. The theoretical results obtained are in a good agreement with the results of experiments carried out in Ge, GaAs, and GaP.     

Magnetic studies of the dislocation structure of iron single crystals deformed at 195K and 77K

Measurements of the coercive field, the initial susceptibility and the reversible susceptibility in the approach to ferromagnetic saturation show that during low-temperature deformation of iron single crystals mainly screw dislocations are created. Long-range internal stresses are found to be significantly smaller than in crystals deformed at room temperature. Macroscopic slip occurs on several slip systems. In the parabolic region of the work-hardening curves at 195 K the relation is valid, where τ isthe shear stress andN is the dislocation density. In the region of saturation of the shear stress the dislocation density further increases. After room-temperature prestrain the relation appears to hold for 77K-deformation also. Exhaustion hardening of edge dislocation is found at the beginning of the low-temperature deformation.     

Heat “superemission” and nucleation-front propagation under laser-induced crystallization of thin amorphous films

The laser-induced solid-state explosive nucleation in amorphous media is studied analytically. The shapes of the temperature switching wave and that of the nucleation front as well as the formula for the front velocity are derived considering also self-consistent medium deformation. Two conditions of explosive nucleation reflecting the roles of latent heat emission and of deformation are formulated. It is shown that, in explosive nucleation, the rate of internal heat emission is proportional to the square of the latent crystallization heat (superemission) in analogy to photon superradiance in initially inverted two-level atomic systems.     

Asymmetry of quadrupole doublet of Mössbauer spectra in LiNbO3∶Fe

Polycrystalline samples of LiNbO₃ doped by the enriched⁵⁷Fe isotope have been studied at room and liquid nitrogen temperatures by nuclear γ-resonance (NGR). Iron impurity concentrations were 0.07, 0.30, 1.02, 2.06 at.%, the Li/Nb ratio being 0.935, 0.990, 1.000. An asymmetry of the quadrupole doublet of NGR experimental spectra has been found. Acceptable hypotheses are discussed to explain the nature of the asymmetry: the Goldanskii-Karyagin effect, the effect of sample texture, and the electron and nuclear spin-lattice relaxation effect. On the basis of computed NGR spectra and thorough analysis of the models we can conclude that there is tendency to form coupled pairs of Fe²⁺ cations in neighbour lithium and niobium sites. The probability of the formation of such pairs exceeds considerably the statistical value.     

TEM analysis of the influence of dose on damage behavior in MeV Au2+-implanted silicon

Extended lattice damage created by implantation of 3.6 MeV Au²⁺ ions has been investigated using transmission electron microscopy (TEM) and Rutherford backscattering spectrometry (RBS). Systematic observations of damage for Au²⁺ ions implanted with varying doses into silicon are explained in terms of a model. The origin of two distinct bands of extended defects is explained in terms of annealing of the central region of implant-damage, during the course of the implantation. Two distinct bands of Au precipitates are observed in high-dose implanted samples. This observation is explained as being the result, in part, of segregation of gold in front of a recrystallizing front, and in part, of gettering of dopant-atoms to nodes in a dislocation network. The network arises as a result of dynamic annealing of damaged crystalline silicon.