Determination of dislocation damping in copper single crystals with stress pulse method

Dislocation damping in copper single crystals was determined by the stress pulse method. For this purpose very short stress pulses were used, maximum length 30 s, by which loading conditions of a metallic matrix in the frontal region of a propagating brittle crack were simulated. At loading by these very short stress waves, strain rates above 10⁴ s^–1 are achieved, however, the strain rate during loading is not constant. Therefore a technique was elaborated that enables to compare the measured results of mechanical characteristics with very short stress pulses ( const.) with results obtained on a device with a constant strain rate of the specimen. With the use of the dependence = ( ) the existence of damping mechanisms of a viscous character was proved and the damping coefficientB determined. These results agree with theoretical assumptions in [10,20] about the probable mechanical behaviour in the region of the propagating brittle crack.ikova 22, Brno, Czechoslovakia.     

The effect of bloch-wall motion on the flow stress of nickel single crystals

Experimental evidence is presented that moving Bloch-walls decrease the flow stress of nickel single crystals. Via their strain fields, Block-walls interact with dislocations.     

Restricted dislocation motion in crystals of colloidal dimer particles

At high area fractions, monolayers of colloidal dimer particles form a degenerate crystal (DC) structure in which the particle lobes occupy triangular lattice sites while the particles are oriented randomly along any of the three lattice directions. We report that dislocation glide in DCs is blocked by certain particle orientations. The mean number of lattice constants between such obstacles is Z[over](exp)=4.6+/-0.2 in experimentally observed DC grains and Z[over](sim)=6.18+/-0.01 in simulated monocrystalline DCs. Dislocation propagation beyond these obstacles is observed to proceed through dislocation reactions. We estimate that the energetic cost of dislocation pair separation via such reactions in an otherwise defect free DC grows linearly with final separation, hinting that the material properties of DCs may be dramatically different from those of 2-D crystals of spheres.     

Peculiarities of the dislocation ensemble motion in doped alkali halide crystals

Abstract

Correlation between the dislocation mobility in the stress field of a concentrated load and other plastic deformation parameters for doped NaCl crystals was studied. The good correlation between all parameters under investigation was found in the case of the impurity hardening and the disturbance of this correlation was established in the case of the impurity softening. The influence of impurity state on the low-temperature anomaly of dislocation mobility was revealed.     

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).     

Post-yielding dislocation retraction of nano-lamellar TiAl single crystals

The perfect single crystal has ultra-high strength but is often accompanied by catastrophic failures after yielding. This study reveals that nano-lamellar TiAl single crystals alleviate the catastrophic failure due to a post-yielding dislocation retraction through atomistic simulations and theoretical analyses. This dislocation retraction leads to a retained post-yielding strength of1.03 to 2.33 GPa(about 50% of the yielding strength). It is shown that this dislocation retraction is caused by local stress relaxation and interface-mediated image force. The local stress relaxation is due to successive dislocation nucleation in different slip systems, and the interface-mediated image force is caused by the heterogeneous interface. Based on dislocation theory, this study demonstrates that the size effect also plays a vital role in dislocation retraction. Theoretical modeling shows that the dislocation retraction occurs when the lamellar thickness is less than approximately 12 nm. Additionally, the post-yielding dislocation retraction is more pronounced at higher temperatures, making it more effective in alleviating catastrophic failures.These findings demonstrate a viable option for avoiding catastrophic failure of single crystals through nanoscale-lamellar design.     

Optical inhomogeneities in ruby crystals

The optical inhomogeneities in ruby laser crystals, resulting from inhomogeneity of the chromium concentration, from various glide paths, and from the presence of disoriented blocks, are investigated. It is shown that the most harmful influence on the characteristics of laser damage is exerted by the axially asymmetrical part of the refractive index distortion due to residual mechanical stresses. A correlation is obtained between the inhomogeneity of the refractive index and the laser beam divergence. A method is proposed for improving the optical inhomogeneity of the ruby laser crystals by high-temperature (diffusion) annealing.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva, Vol. 101, pp. 130–147, 1978.     

Estimating the strength of single-ended dislocation sources in micron-sized single crystals

Three-dimensional (3D) discrete dislocation dynamics simulations were used to calculate the effects of anisotropy of dislocation line tension (increasing Poisson's ratio, ν) on the strength of single-ended dislocation sources in micron-sized volumes with free surfaces and to compare them with the strength of double-ended sources of equal length. Their plastic response was directly modelled within a 1?µm³ volume composed of a single crystal fcc metal. In general, double-ended sources are stronger than single-ended sources of an equal length and exhibit no significant effects from truncating the long-range elastic fields at this scale. The double-ended source strength increases with ν, exhibiting an increase of about 50% at ν?=?0.38 (value for Ni) as compared to the value at ν?=?0. Independent of dislocation line direction, for ν greater than 0.20, the strengths of single-ended sources depend upon the sense of the stress applied. The value for α in the expression for strength, τ?=?α(L)µb/L is shown to vary from 0.4 to 0.84 depending on the character of the dislocation and the direction of operation of the source at ν?=?0.38 and L?=?933b. By varying the lengths of the sources from 933 to 233b, it was shown that the scaling of the strength of single-ended and double-ended sources with their length both follow a ln(L/b)/(L/b) dependence. Surface image stresses are shown to have little effect on the critical stress of single-ended sources at a length of ～250b or greater. This suggests that for 3D discrete dislocation dynamics simulations of the plastic deformation of micron-sized crystals in the size range 0.5–20?µm, image stresses making the surface traction-free can be neglected. The relationship between these findings and a recent statistical model for the hardening of small volumes is discussed.     

Multiphysical simulation analysis of the dislocation structure in germanium single crystals

To grow high-quality germanium crystals is one of the most important problems of growth industry. The dislocation density is an important parameter of the quality of single crystals. The dislocation densities in germanium crystals 100 mm in diameter, which have various shapes of the side surface and are grown by the Czochralski technique, are experimentally measured. The crystal growth is numerically simulated using heat-transfer and hydrodynamics models and the Alexander–Haasen dislocation model in terms of the CGSim software package. A comparison of the experimental and calculated dislocation densities shows that the dislocation model can be applied to study lattice defects in germanium crystals and to improve their quality.     

Singularities of the motion of individual surface dislocation half-loops in sodium chloride crystals

Selective etching was used to study the singularities of the screw segments of individual dislocation half-loops with a preload acting in various ways. In all cases the kinetic curves are observed to be nonlinear when the loads act for a long time. Hypotheses are presented about the possible nature of the effect.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 70–76, March, 1978.In conclusion, the authors take this opportunity to express their heartfelt gratitude to S. V. Lubenets and A. A. Predvoditeelev for their discussion of the avove data as they were obtained.     

Mechanism of formation of the interlamella dislocation network in polyethylene single crystals

It was found experimentally that bilayered polyethylene single crystals with an orientational misfit angle larger than the critical angle, θ*, show moire pattern and only those with an angle smaller than θ* show the interlamella dislocation network. The intermediate pattern, which could neither be classified into the typical moire pattern nor the typical interlamella dislocation network, was found in the vicinity of θ*. Criteria for discrimination between the moire pattern and the interlamella dislocation network are discussed. The crystal with a misfit angle a little larger than θ* also comes to show the feature of the dislocation network during long storage of the crystal in the mother solution at the crystallization temperature. θ* is, therefore, a function of the time of storage. A mechanism is presented such that crystal lattices near the interfacial boundary are distorted to form the dislocation network by intermolecular force between the overlying crystals when the misfit angle is smaller than θ*.     

Determination of the dislocation structure in paratellurite single crystals using the piezo-optic effect

It is shown theoretically and experimentally that dislocations in paratellurite single crystals can be revealed using the piezo-optic effect. The mechanical-stress distribution near dislocations is calculated. Dislocations in the samples are observed using the photoelastic method and chemical etching. Data on the spatial distribution of dislocations in Czochralski-grown paratellurite crystals are reported.