Strong Obstacles Research Articles

Dislocation-grain boundary interactions in ice crystals

Abstract Dislocation-grain-boundary (GB) interactions in polycrystalline ice Ih during creep have been studied in situ using synchrotron X-ray topography. The basal slip system with the highest Schmid factor was found to be the most active in polycrystalline ice whereas the GB orientation relative to the loading direction seemed unimportant. GBs act both as effective sources of lattice dislocations and as strong obstacles to dislocation motion. The observations revealed pile-up formation upon loading and pile-up relaxation after unloading. Non-basal segments of lattice dislocations can be generated from GBs in ice. However, they neither noticeably decrease stress concentrations nor contribute significantly to the overall plastic deformation. It was found that dislocations can be generated from both free-surface-GB intersections and from the interiors of GBs, indicating that the dislocation generation mechanism presented in our 1993 paper is not a surface artefact. Evidence is also presented that, because ...

An obstacle in a quantum film: the density dipole and extra resistance

The additional resistivity of a quantum film due to an obstacle is investigated. The film is treated by a semi-classical formalism whilst the obstacle is characterized by its quantum mechanical scattering cross-section. The film intrinsic scattering mechanism allows for the adaptation of an arbitrary density distribution to a characteristic 'ideal' distribution over the channels. Special attention is paid to the multiple-scattering cycles between the obstacle and its surroundings, i.e., the scattering background of the film. The analysis of these backscattering processes leads to a self-consistent equation for the current density incident on the scatterer. For the general case of an arbitrarily strong obstacle and many conducting channels, this equation system can be solved only numerically. However, the formalism becomes handy if the obstacle scatters only weakly. A condition is found for the obstacle to be considered as weak. On the other hand, if one considers only one conducting channel it is possible to solve the transport problem analytically even for a strong obstacle. In this case, we find an expression for the resistivity which contains the scattering cross-section in a non-linear manner. This non-linearity was already predicted in 1957 by Landauer.

Institutional barriers to policy reform in Egypt: The case of the agricultural machinery industry

Abstract Economic reform has been on the Egyptian policy agenda since the mid-1970s. But progress has been slow in part due to strong regulatory and institutional obstacles. The experience of the agricultural machinery manufacturing industry displays this problem. An economic analysis of manufacturing threshers shows that while the impact of price policies was about neutral, institutional barriers effectively taxed the industry 15–22%. The greatest obstacle was access to marketing credit from the state-owned Principal Bank for Development and Agricultural Credit. Access to materials and difficulties with tax authorities also hampered the industry.

On the twinning initiation criterion in CuAl alpha single crystals—II. Correlation between the microstructure characteristics and the twinning initiation

The influence of a prestrain on the subsequent plastic response of f.c.c. single crystals is investigated both experimentally and numerically in terms of slip and dislocation microstructures evolutions, and in relation to the twinning initiation process. Proposals available from the literature on twinning initiation criterion are considered and the more relevant expression with respect to the here reported observations and analyses is discussed. It has been shown in the first part that, as long as plastic straining operates by slip, the numerical modelling used for the simulations gives predictions in qualitative agreement with the observed crystallographic and microstructural characteristics of the performed experiments on CuAl alpha samples. It mainly comes out from this second part that: (1) The dislocation microstructure built up during the pretwinning slip stage in tension is strongly dependent on the previous compression amplitude, and so are the twinning initiation conditions. (2) Local stress effects ahead of dislocations piled up on strong obstacles, as proposed in recent twin nucleation mechanisms, are more likely to justify the prestrain dependancy of the critical twinning stress and the preferred twinning mode experimentally obtained than the simple estimate of the macroscopic shear stresses on the twinning systems.

Stress corrosion microcleavage in a ductile f.c.c. alloy

Several f.c.c. alloys exhibit cleavage-like fracture under stress corrosion. A model taking into account the interactions of corrosion with the crack tip, dislocation pile-ups and strong obstacles of the microstructure is proposed. Pile-ups of dislocations emitted at a crack tip are compared with classical Frank-Read pile-ups. It is shown that they are similar and that Stroh's analysis can be applied. The consequences for cleavage are discussed.

Neutron scattering study of metastable high pressure ice VIII

Abstract It is well known that among the different polymorphic modifications of ice, only phases II and VIII are fully ordered. The disorder in the protonic system of other phases of ice put strong obstacles in the way of theoretical models. On this reason the investigation of the ordered ice represent special interest. Ice VIII has more simple lattice structure compared to ice I1 and investigation of ice VIII are preferable.

Simulation of ion acceleration in a charged dust cloud

The interaction of a plasma flow with a cloud of charged dust particles is studied using a one‐dimensional hybrid simulation model. Depending on the dust charge density and the cloud diameter, this cloud is either a weak or a strong obstacle in the plasma flow. For a weak dust obstacle a super‐magnetosonic flow is decelerated monotonically inside the cloud whereas a sub‐magnetosonic flow is accelerated. For a strong dust obstacle the flow configuration in the cloud changes dramatically: Deceleration and acceleration may change across the cloud and strong proton heating and shock generation may occur with the stationary regime being discarded. An interpretation of the proton dynamics in weak clouds is given on the basis of a simple analytic multifluid model.

A numerical study on the collision of a filled projectile with a strong obstacle

Unsymmetrical interaction is considered between a shaped cylinder containing a powdered ceramic and a planar undeformable obstacle. An elastoplastic model is used with a finite-element method to derive the deformation in the projectile with and without the filler. Chronograms and graphs are given to characterize the dynamic and kinematic parameters.

Quantitative investigation of the role of internal stresses and of dislocation pile-ups in tension and fatigue deformation and fracture using a unidirectionally solidified composite

Abstract Owing to its simplicity, a unidirectionally solidified composite, which combines the presence of strong non-shearable obstacles (the fibres) with the possibility of deriving from observations the same quantitative information as for a single crystal, has been used as a prototype material to study fundamental deformation and fracture mechanisms. This has made possible a quantitative explanation of both the existence of strong unrelaxed internal stresses in a ductile material, the matrix, and the hardening effect (Mott and Nabarro 1940) of the resulting pseudoperiodical shear stress field. With regard to the role of dislocation pile-ups in deformation and fracture, performing either longitudinal or transverse tensile tests has permitted us to obtain, in the presence of an obstacle, different shear-stress normal-stress combinations which cannot be obtained in a single crystal, but may be found in a given grain of a polycrystal where a crack may thus nucleate. In the case of a high shear stress, dislocation pile-ups containing more than 100 dislocations may lead to the fracture of the obstacle; moreover, these observations have shown that the role of dislocation pile-ups is not restricted to the nucleation of the crack itself, but may be extended to the nature of its propagation in the obstacle, thus explaining the paradoxical propagation in a plane containing the direction of the applied stress. In a situation combining a high shear stress with a high normal stress, the presence of the pile-up may lead to the nucleation of a crack in the glide plane itself, in a material as ductile as the nickel-base matrix. In fatigue, the observation of fibre failure at a much lower stress level than in monotonic tensile tests has suggested a deformation model typical of fatigue and involving two double dislocation pile-ups of opposite sign, located in parallel slip planes, in which case the back-stresses exerted on the sources by the emitted dislocations are opposite in sign and thus mutually annihilate, thereby permitting the sources to emit a sufficiently large number of dislocations to break the obstacle.

Role of a complex carbonitride of niobium and vanadium in creep strength of 9% Cr ferritic steels

Complex precipitates of Nb and V in ferritic steels have been considered to act as strong obstacles against dislocation glide. In the previous report, the authors investigated a complex precipitate of Nb and V in 9% Cr ferritic steel and showed that plate-like V-nitrides were formed adhering to spherical Nb (C,N) during tempering. These peculiar shaped precipitates are likely to trap dislocations during high temperature deformation such as creep. In this paper, the effect of the sizes of the complex carbonitrides on the creep strength of 9% Cr ferritic steels was investigated. The sizes of the carbonitrides were varied by changing the amount of N.

On the role of interphase barrier and substructural strengthening in deformation processed composite materials

Deformation processed composites are complex materials, and it is unlikely that any single strengthening mechanism/model can account for all aspects of their behavior. The work hardening model is consistent with some aspects of microstructural evolution and can be used to predict composite strengths as they depend on interphase spacing, phase volume fraction and deformation strain. In addition, the model predicts other important qualitative aspects of DPCM behavior. The work hardening model, however, can not be relied on to provide a detailed picture of dislocation substructure.Recent attempts to model DPCM strengths solely in terms of interphase barrier strengthening appear implausible. This is a consequence of disregarding substructural development and the importance of substructure in determining composite strengths. Nor is such a mechanism able to explain many other aspects of DPCM behavior. The interphase barrier mechanism is most plausible when processing produces a composite without strong substructural obstacles.

Percolation theory for the plastic yield transition

It will be shown how the percolation threshold in a standard continuum percolation model corresponds to the plastic yield transition in a dislocation model with infinitely strong point obstacles and with no dislocation cancellation allowed. The cluster properties of areas defined to be ‘‘open’’ to dislocation loop expansion will be simulated using a lattice model and Monte Carlo methods. The critical exponents ν, γ, and β for these areas will be given. Only the value of β, the exponent corresponding to the infinite cluster, is the same as that for the standard continuum percolation model.

The Perseus Myth as Defined by Calderón and Corneille

That myths are capable of multiple uses and interpretation is demonstrated by two palace plays based on the same Ovid story of Perseus. Corneille's Andromède and Calderón's Las fortunas de Andrómeda y Perseo are entirely different in their affective impact. Corneille's play celebrates Perseus as heroic bridegroom; Calderón celebrates him as heroic saviour. The reason for such a divergence in interpretation and style can be partly explained by the basic difference between two concepts of romance. While Corneille uses romance in a limited sense, as adventure dependent on love interest (with social values as a strong obstacle), Calderón makes full use of the romance quest as defined by Northrop Frye. This makes it possible for Calderón to use the monster, for instance, not only poetically but dramatically, while Corneille is obviously bothered by the monster's incredibility. While Corneille's play is no more than a palace entertainment, Calderón succeeds in preserving, re-expressed for a Christian audience, the universal significance and dramatic impact of the Greek myth. (MH)

HVEMin situobservation of dislocation processes in the γ′-hardened superalloy NIMONIC 105

Abstract The critical dislocation configurations in the γ′-hardened nickel-base superalloy NIMONIC 105 have been investigated by in situ deformation experiments. Single-crystal thin foils were deformed in a high-voltage electron microscope operated at 300 kV. Two types of specimens which differed in their γ′-particle dispersion, were studied: S1 contained γ′ precipitates of 52 nm diameter; S2 was bimodal. The diameters of the γ′ particles of the two classes were 52 nm and 263 nm. The most important results are: (a) In S1 as well as in S2, dislocations cut through the γ′ precipitates. (b) Dislocations glide in pairs. (c) In both types of specimens the γ′ particles of 52 nm diameter cause the leading dislocation of a pair to bow out very far; sometimes the critical configuration resembles the Orowan configuration. The trailing dislocation of a pair is far less bent. (d) The large γ′ precipitates present in S2 are very strong obstacles to the glide of dislocations. They first bypass these γ′ particles and cut through them later. (e) No Orowan loops were found.

Influence of grain size and strain on the barkhausen noise power of non-oriented silicon steel

Barkhausen noise powers (BNP) were measured to examine the relationship between microstructures and macroscopic properties such as Br, Hc, and μ d , of non-oriented silicon steels. The results show that the volume of Barkhausen jumps increases and a jump number decreases with the increase of grain size as the grain boundary works as strong obstacle to wall motions, however, the volume comes to be broken into fractions when precipitates are induced. With the increase of strain, the number of Barkhausen jumps increases and the volume decreases as the induced dislocations interfere with 180° wall motions. BNP, μ d and Br decrease remarkably and then gradually with the increase of strain.

The radius of curvature of dislocation segments in MgO crystals stressed in the high-voltage electron microscope

Abstract MgO single crystals have been deformed inside the high-voltage electron microscope. The local radius of curvature of dislocation segments bowing out between strong obstacles was determined by photometric measurement of the dislocation coordinates and by piecewise fitting of third-order polynomials to these coordinates. While individual segments show very different dependences of the local radius of curvature on the dislocation orientation, averages over greater numbers of segments agree fairly well with theoretical results obtained using the line tension model with isotropic elasticity. During unloading, the segments have the form of almost straight sections connected by strongly curved regions, probably as a result of obstacles to their back-motion. The bowing out of the segments is influenced by interactions with neighbouring segments. The radii of curvature depend on the length of the segments. The dependence found experimentally is much stronger than is predicted theoretically; this would con...

Tem evaluation of ion-implanted and fatigued metal surface layers

The fatigue deformation was characterized by transmission electron microscopy in unimplanted and in ion-implanted metal surface layers. The metals investigated were austenitic 304-stainless steel and copper, annealed and implanted at 3 MeV with nitrogen or neon to a dose of 10 21m −2. The implanted surface layers contain a high density of lattice defects, most likely being Frank-dislocation loops. They are obstacles against glide dislocations. After flexural fatigue at ±0.3% cyclic strain, dislocation channels were formed heterogeneously along crystallographic {111} slip planes. Inside the channels the Frank loops were annihilated. These areas became thus mechanically soften relative to their surrounding volume. The channels assume the role of the well known persistent slip bands in unimplanted metals and lead to extrusions at the free surface. They are the precursors of fatigue crack nucleation. The fatigue lives of the as-implanted samples were not improved. However, an improvement by a factor of two to three was obtained after aging of the nitrogen-implanted stainless steel. Electron diffraction evidence suggests that this improvement was due to the formation of beginning precipitation ( = stronger obstacles against glide dislocations).Stress-induced martensitic transformation and twinning in the stainless steel were also modified by the implantation. Implanted nitrogen prevented the formation of α′-martensite, whereas in neon-implanted samples the martensite formed freely.

Recovery of edge-defined film-fed grown silicon Dislocation/twin boundary interaction and mechanisms for twin-induced grain boundary formation

Abstract Transmission electron microscopy and the electron-beam induced-current technique have been used to study changes in the defect structure of rapidly grown, edge-defined film-fed growth silicon ribbons, processed at high temperatures. Twin boundaries are the major structural defects in the ribbons, and these were found to act as strong obstacles to dislocation glide. This barrier eifect, possible mechanisms for slip transfer, and consequences for the mechanical and electrical properties of the silicon ribbons are discussed. Models are proposed to explain both twin-induced grain boundary formation during recovery of the microstructure and the generation of microtwins and second-order twin joins by glide-induced twinning processes.

The role of elastic interaction stresses on the onset of slip in polycrystalline alpha brass—II. Rationalization of slip behavior

A finite element method analysis, extended to three dimensions, was used to calculate the stresses in 11 grains of an as-cast, 2.4 mm grain size α-brass specimen strained to the proportional limit. Resolved shear stresses (RSS) were not uniformly distributed. However, the predicted slip system based on highest total RSS was found to operate in 9 of the 11 grains. In the other two cases it was not possible to predict which of the two highest RSS slip systems would operate because the RSS were too close. RSS at which slip occurred were compatible with those found by other investigators and measured slip offsets were generally higher where the calculated stresses were higher. Second highest or lower RSS slip systems, which did not form strong obstacles with the primary slip systems, were found to operate. The range of stresses in a given grain, over which slip occurred increased with grain size. On the basis of available evidence, it must be considered that elastic interaction stresses play an important role in the yield process and may exercise influence for strains of at least several percent.

The strengthening effect of voids

The strengthening effect due to voids was calculated using contemporary, computer-based dislocation theory methods that allow dislocation self interaction and elastic anisotropy to be properly taken into account. Void-dislocation interaction was included by using an approximation based on Lothe's solution for a dislocation terminating on the boundary of an elastically anisotropic half-space. The results show that voids are very strong obstacles to dislocation motion, and the void bypassing stress approaches the Orowan stress for impenetrable obstacles obtained under similar conditions. A correlation based on the isotropic-elastic, line-tension approximation was developed to describe the void bypassing stresses in quantitative fashion. With an appropriate choice of the several arbitrary parameters in the line tension description, elastic anisotropy and the various interactions can be included. The resistance due to surface-step formation on voids during dislocation penetration was also included in the calculations and the correlation. Limited experimental data available in the literature is in agreement with the quantitative predictions, and supports the fact that voids must be regarded as strong obstacles to dislocation motion.