Growth Of Slip Bands Research Articles

Enhancing fatigue crack propagation resistance of heterostructured Al composites and multistage crack mechanisms

High tensile strength and low fatigue crack propagation (FCP) rate are hard to achieve simultaneously in aluminium (Al) based materials, which has been a long-lasting topic. It is because the traditional strengthening mechanisms may lead to the increase in FCP rate. In this work, we developed dual-level heterostructures by incorporating the in-situ synthesized TiB2 particles into Al matrix, to create particle-lean zones (PLZs) and particle-rich zones (PRZs) by extrusion. Fine grains were introduced by particle-associated local recrystallization in PRZs. By means of particle and grain size distribution, a heterostructured Al composite featuring with the coarse grains in PLZs and fine grains in PRZs was fabricated. It was found that simultaneous enhancement of both the strength and FCP resistance of Al composite was achieved through the development of heterostructures. During FCP, the PRZs can retard the growth of slip bands and increase crack deflection frequency while the PLZs increase the crack deflection distance and plastic deformation capability at crack tip. The fracture behavior of composite during FCP depended on grain characteristics, particles and stress intensity range. The detailed cracking behavior for typical <100>Al and <111>Al grains in different FCP stages was identified. The associated models were developed for different FCP behaviors. Particularly, the quantitative relationship between Pairs parameters and microstructure features was established, which was critical to understand fatigue properties of Al composite reinforced by small particles. These findings can provide a strategy to design metal materials with an excellent combination of both static and dynamic mechanical properties.

Influence of pulse current density on suppression of persistent slip band growth

Influence of pulse current density on suppression of persistent slip band growth

Mechanical responses of Al20.4Mo10.5Nb22.4Ta10.1Ti17.8Zr18.8 nanopillar under uniaxial compression

The mechanical responses of crystallographic orientations [001], [110], and [111] of Al 20.4 Mo 10.5 Nb 22.4 Ta 10.1 Ti 17.8 Zr 18.8 refractory high entropy alloy (RHEA) nanopillars under the uniaxial compression are investigated by the MD simulation using 2NN MEAM potential. The [001]-orientated nanopillar shows the widest elastic deformation regime with the yielding strain about 0.075, compared to the other two nanopillars with the yielding strains about 0.03~0.04. The strength of [111]-oriented nanopillar is the highest, which is about 36.3% and 117% higher than those of [001]- and [110]-oriented nanopillars, respectively. After the strain exceeds the ultimate one, the stress gradually declines with the increasing strain, and then displays small flow stress oscillation during plastic deformation, indicating the stresses saturate at the quasi-steady flow stress. For the [001]-orientated nanopillar, the extent of BCC-HCP phase transition is the most significant at strains below the flow stress regime. From strain 0 to the beginning of the flow stress regime, the local structural transition is the main deformation mechanism, and the dislocation slip becomes the dominant deformation mechanism within the flow stress regime. For the [110]-orientated nanopillar, the dislocation nucleates at the strain between the yielding and ultimate strains, and then the dislocation density increases sharply with the increasing strain until the occurrence of the dislocation slip. The dislocation slip and deformation twinning are two main deformation mechanisms for the [110]-orientated nanopillar. For the [111]-orientated nano-pillar, the dislocation nucleates at the strain larger than the ultimate strain when the stress undergoes an abrupt drop. After the dislocation nucleation, the dislocation density shows a distinct increase during the stress drop, leading to the appearance of dislocation lines. In the flow stress regime, the dislocation density gradually decreases from the maximum value about 600.17 10 19 m -2 to zero with the increasing strain. Local shear strain of [110]-oriented Al20.4Mo10.5Nb22.4Ta10.1Ti17.8Zr18.8 nanopillar during the compression • The mechanical responses of crystallographic orientations [001], [110], and [111] of Al 20.4 Mo 10.5 Nb 22.4 Ta 10.1 Ti 17.8 Zr 18.8 refractory high entropy alloy nanopillars under the uniaxial compression • 2NN MEAM potential parametrization process for the Al-Mo-Nb-Ta-Ti-Zr system using DFT calculation data. • Polyhedral template matching (PTM) method for monitoring the local structural transformation. • Atomic local shear strain for observing dislocation slip band growth • dislocation extraction algorithm (DXA) results for dislocation nucleation and evolution

TEM Study of Microstructure Changes, Formation and Distribution of Slip Bands in Austenitic Steels after Low-Cycle Fatigue (LCF) Deformation - II

The microstructure changes, development of micro plastic deformation and formation and distribution of slip bands were studied. It is shown that development of micro deformation during LCF depends on loading conditions (amplitude and number of cycles) and microstructureIt is shown that as non-localized as well as localized micro plastic deformation takes place because of structural inhomogeneity. Supposedly, the localized deformation is related to the sites of internal stress concentration accumulated during the LCF.The effect of microstructure of structural steels on the rate of local cyclic deformation, leading to nucleation and growth of slip bands of fatigue cracks, was studied. The interaction of slip bands with precipitates, grain boundaries and low-angle boundaries were also analyzed.The sites of nucleation of primary and secondary slip bands were identified, and the following aspects were considered: 1. the possibility of microcrack nucleation on (or in) slip bands, 2. The kind of slip bands the slip bands may nucleate in, 3. The potential sites (except the slip bands) and reasons of nanocrack formation are specified.

On slip band growth in neutron-irradiated copper single crystals

AbstractA transmission electron microscopy (TEM) study of the dislocation structure in slip bands at the Lüders band front in neutron-irradiated copper (Cu) single crystals reveals bands in various stages of development. This permits to specify the dislocation processes in particular for termination of slip band growth in the yield region. The important processes occurring during slip band growth are obstacle destruction by moving dislocations, formation of dynamic pile-ups, frequency of microscopic cross-slip and climb processes, formation of multipoles and of clusters of heavily jogged edge dipoles, which ultimately stop the local deformation in the cleared slip band channel. The formation of a dislocation group in the stress gradient within the Lüders band region is considered and it is shown that the group behaves like a single dislocation placed in the “center of gravity” of the quasi-stationary distribution which is the same as in the static case.

Orientation dependence of slip band formation in single-crystal aluminum studied by photoelectron emission

We report the application of a photoemission technique to study slip band formation in single-crystal aluminum (99.995%) during uniaxial tensile deformation. Light with a single 253 nm wavelength was isolated from a mercury lamp and used as the source for photoemission experiments. Deformation of aluminum single crystals with three orientations (near [1 1 2], near [2 1 5] and near [2 3 ¯ 3 ¯ ]) was conducted with a tensile stage in ultrahigh vacuum at an initial strain rate of about 5 × 10 −4 s −1. We show that photoelectron emission is sensitive to the changes in the surface morphology accompanying deformation, including slip line and slip band formation, and that the photoemission intensity both increases with strain and exhibits a strong orientation dependence. Moreover, some time-resolved photoelectron measurements show discontinuous increases in intensity that are consistent with the heterogeneous nucleation and growth of slip bands during tensile deformation. The in situ photoelectron data and real-time stress vs. strain from the three single crystals validate the classical crystallographic relationships governing slip band formation and suggest that dislocation dynamics during deformation of aluminum may demonstrate a percolation-like behavior. The slip bands on the deformed surfaces were examined with both optical microscopy and atomic force microscopy.

“Observation” of dislocation motion in single crystal and polycrystalline aluminum during uniaxial deformation using photoemission technique

We report measurements of photostimulated electron emission (PSE) from single-crystalline aluminum (99.995%) and high-purity polycrystalline aluminum (>99.9%) during uniaxial tensile deformation. Photoelectron intensities are sensitive to changes in surface morphology accompanying deformation, including slip line and slip band formation. In the single crystalline material, the PSE intensity increases linearly with strain. In the polycrystalline material, the PSE intensity increases exponentially with strain. In both materials, time-resolved PSE measurements show step-like increases in intensity consistent with the heterogeneous nucleation and growth of slip bands during tensile deformation. In this sense, we have “observed” dislocation motion by this technique. Slip bands on the surfaces of deformed samples were subsequently imaged by atomic-force microscopy (AFM). Photoelectron measurements can provide reliable, quantitative information for dislocation dynamics.

740 α黄銅の繰返しせん断応力下における疲労すべり帯の成長とき裂発生条件の AFM による観察

740 α黄銅の繰返しせん断応力下における疲労すべり帯の成長とき裂発生条件の AFM による観察

AFM characterization of the evolution of surface deformation during fatigue in polycrystalline copper

Atomic force microscopy (AFM) is a relatively new tool that readily provides high resolution digitized images of surface features. AFM is used here to study the development of slip bands and protrusions in strain controlled fatigue tests on polycrystalline copper at 0.161 and 0.255% strain amplitudes. The average slip band heights at failure for both strain amplitudes conditions are comparable, implying that the growth of slip bands saturates at a specific height. A parameter, γ irrev, is defined that is a measure of the local slip irreversibility at the surface and is applicable to any type of surface deformation feature, independently of the size of the fields of view. Thus, estimates of surface deformation developed in regions where fatigue crack nucleation is likely to occur can be obtained, from which a fatigue crack nucleation criterion is defined.

Recent Progress of Experimental and Measuring Technology. Quantitative Evaluation of Slip-Band Growth and Crack Initiation in Fatigue of 70-30 Brass by Means of Atomic-Force Microscopy.

Since the surface morphology of materials can be observed with atomic-scale resolution, scanning atomic force microscopy (AFM) in a powerful technique to study mechanisms of fatigue and fracture of solid materials. In the present study, slip-band formation and fatigue crack-initiation processes in 70-30 brass were observed by means of AFM. The slip-band angle relative to the stress-axis at the specimen surface varied from 15 to 90°, and it appeared most frequently around 60°. The depth of the intrusion drastically increased with its outgrowth to a crack, and with coalescence of cracks, the width of cracks increased rapidly. The critical value of the intrusion depth for crack initiation was given as a function of the slip-band angle through a geometrical model for the slip-direction and the slip-step, and the critical value was independent of stress amplitude. From presice observations of slip-band tips, slip-bands had steep slope when they were blocked by grain boundaries, and the slip-bands descended by gradual slopes to plain surfaces when they terminated within grains.

Slip band formation during bending of GaAs wafers

Slip bands which appear after 4-point bending of liquid encapsulated Czochralski grown zinc doped GaAs (001) wafers at (412…447)°C have been characterized quantitatively both by reflection and transmission microscopy. Slip is initiated at the edges of the tensile part of the specimen. Deformation proceeds by the growth of slip bands in length through the whole specimen and in height up to ≈ 200 nm. Their average distance goes down to ≈ 20 μm within the bending angle interval (0.2…5.9)° investigated. Residual stresses σ1 – σ2 ≈ (8…30) MPa have been detected by stress-induced birefringence measurements in the bent crystals.

Growth of slip bands during fatigue of 6061-T6 aluminum

The growth of persistent slip bands (psb), the initial surface manifestation of metal fatigue, was measured with a photoelectron microscope equipped with a fatigue stage. Once the psb had been identified and located, the specimen was subjected to a final detailed examination by scanning electron microscopy. In 6061-T6 aluminum psb initiated within a grain, usually at the site of an inclusion. The psb appeared as a small extrusion and elongated across the grain by the sequential addition of new extrusions. As the psb elongated, the initial extrusions became more pronounced and eventually microcracks developed. Under constant amplitude loading, the rate of elongation of a psb in polycrystalline material varied inversely as the length, whereas in a large grain specimen the rate remained constant. This difference is attributed to the constraints imposed upon a small grain by the surrounding material. The growth laws can be accounted for in terms of a simple two phase model in which the psb has a much lower yield stress than the matrix of the grain.

Slip band growth and dislocation velocities in thin flat crystals of n‐irradiated Cu and Cu‐30% Zn

AbstractExtending earlier work (POTTHOFF) the dependence of the arrangement and development of slip bands on the thickness of thin flat single crystals of neutron‐irradiated copper and Cu‐30% Zn orientated for singe glide is studied at room temperature. Distinct differences in the behaviour of thin (20 … 200 μm thickness) and thick (≧ 500 μm thickness) crystals are found. Partly they are connected with the bending stresses in the region of the Lüders band front. The velocities of dislocation groups in the first stages of slip band growth are estimated. The deformation kinetics is discussed in the concepts of viscous glide and nucleation rate of dislocations for both materials.

Cathodoluminescence studies of dislocation motion in IIb?VIb compounds deformed in SEM

Continuous observation of dislocation motion on the specimen surface of IIb–VIb semiconducting compounds (CdS and CdTe) has been made using a scanning electron microscope (SEM) with the cathodoluminescence (CL) by use of a deformation apparatus installed in the SEM chamber. With the application of stress, SEM-CL patterns revealed an increasing density of dark spots that formed dark stripes along slip traces. For both CdS and CdTe crystals, regardless of the slip system involved, the dark spots corresponding to individual dislocations appeared in the SEM field without displaying their moving state and seldom disappeared, indicating that dislocations are immobilized after they travel a certain distance from the source with a high velocity. The investigation of slip-band growth in CdS showed that screw bands on the basal plane exhibit the photoplastic effect, whereas those on the prismatic plane do not, in accordance with the macroscopic deformation tests.

Thermally activated growth of slip bands in Cu30%Zn single crystals

From microcinematographic measurements of slip band growth during deformation of quenched and annealed Cu30%Zn single crystals the velocity of dislocation groups is determined directly at different deformation rates and temperatures. The thermal activation analysis is consistent with results from macroscopic stress relaxation measurements and yields a negative activation entropy in the case of the quenched alloy at room temperature. This is interpreted by atomic ordering rearrangements in the dislocation core region supporting observations during stress relaxation and former calorimetric measurements on deformed crystals.

Stress-strain curves in alpha Ti alloys—Some problems and results

Stress-strain curves for alloys in the Ti−(0–15)Al system do not exactly obey a power law. As Al content increases, the uniform strain to ultimate load first decreases, then increases. Uniform strain and the strain hardening exponent correlate well experimentally in accord with mathematical requirements. This fact however does not prove that a power law best represents the data. The same is true of straight lines on plots of logdσ/d∈ vs log ∈ when the strain hardening exponent slightly exceeds unity, as it does for The Ti−(12, 15)Al alloys at large plastic strains. For these alloys, true stress may be described equally well by a relation of the form σ = σ" exp ∈ at large plastic strains. These observations support the view of Cass that new slip bands nucleate while old ones broaden via a cross slip mechanism in such a way that the mobile dislocations are those essentially that are moving into virgin crystal. It is proposed that the “doublem” behavior observed in the Ti−(12–15)Al alloys is a manifestation of slip band nucleation in the low plastic strain region followed by slip band growth in the high plastic strain region.

Dynamic behaviour of planar dislocation pile-ups

Abstract The dynamic behaviour of a twin and a slip band is considered when simulated as the planar pile-ups of partial and complete dislocations, respectively, with a continuum distribution density. A similarity between solutions has been found for the equations describing the twin shrinking under the action of surface tension forces, and the slip band growth under the action of a δ-function type force. It turns out that the twin length decreases according to the law L∼ (τ − t) 2 3 , where τ is the full time of the twin shrinking. It has been also shown that within a small region directly adjacent to the twin center the dislocation density increases according to the law 1 x , where x is the distance to the center. Annihilation of opposite-sign dislocations takes place in the twin center where the twin cannot be considered planar and where a two-dimensional region is formed. An attempt has been made to describe this region and its evolution in the process of the twin shrinking. The theoretical results obtained are in good agreement with the experimental data for the process of elastic twin shrinking and sound emission arising in this case.

Deformation of quench-hardened gold single crystals

The deformation of quench-hardened gold single crystals has been studied by observing stress-strain curves, formation and growth of slip bands, and slip channels at various testing temperatures (4.2° – 575°K). The plastic deformation took place exclusively in distinct slip bands and these slip bands correspond to slip channels which are free from tetrahedral stacking faults. The slip band density and average shear displacement per band were measured as functions of macroscopic deformation and found to satisfy the empirical relationships developed for the case of quenched-hardened aluminum. The nature of plastic deformation in quench-hardened gold is discussed on the basis of these observations.

Dislocation glide in UO2single crystals at 1600°K

Abstract A study has been made of the effects of crystal axis orientation on the slip behaviour, at 1600°K, of compressively deformed uranium dioxide single crystals. It is shown that the critical resolved shear stress for slip is anisotropic, and that the stress is sensitive to ionic configurations at the dislocation core. The morphology of slip on ‘easy’ and ‘difficult’ systems is markedly dissimilar and is a function of the ease with which dislocations are able to cross-slip from the primary glide plane. Slip in many cases is macroscopically non-crystallographic, deformation taking place by a composite slip process. In these cases slip on the primary system may be limited by dislocation interaction and the rate-controlling stress is that on the cross-slip plane. Dislocation multiplication and slip-band growth on {001} planes is limited by the unusual dislocation mobility behaviour on these planes. Fracture can be initiated at the intersection of glide bands on {011}〈011〉 systems in UO2. In this case the fracture plane is {001}.

The effect of stress and temperature on the velocity of dislocations in pure iron monocrystals

The velocity of edge oriented dislocations in pure iron single crystals was determined as a function of stress and temperature. The velocities were determined from measurements of the growth of slip bands which had been subjected to constant amplitude stress pulses of known duration. Slip bands were observed using the Berg-Barrett X-ray technique. Measurements were made for temperatures of 77, 198, 295 and 373°K. Resolved shear stresses covered a range of 10–500 Mdyn/cm 2. Measured velocities ranged from 10 −2 to 1 cm/sec. A strong temperature dependence of the dislocation velocity was observed. The results can only be correlated with theories of a single thermally activated mechanism if a substantial entropy of activation exists. Alternative explanations of the experimental results in terms of multiple processes and differences in slip band structure are proposed.