Dislocation Slip Bands Research Articles

Local decomposition induced by dislocation motions inside precipitates in an Al-alloy

Dislocations in crystals are linear crystallographic defects, which move in lattice when crystals are plastically deformed. Motion of a partial dislocation may remove or create stacking fault characterized with a partial of a lattice translation vector. Here we report that motion of partial dislocations inside an intermetallic compound result in a local composition deviation from its stoichiometric ratio, which cannot be depicted with any vectors of the primary crystal. Along dislocation slip bands inside the deformed Al2Cu particles, redistribution of Cu and Al atoms leads to a local decomposition and collapse of the original crystal structure. This finding demonstrates that dislocation slip may induce destabilization in complex compounds, which is fundamentally different from that in monometallic crystals. This phenomenon of chemical unmixing of initially homogeneous multicomponent solids induced by dislocation motion might also have important implications for understanding the geologic evolvement of deep-focus peridotites in the Earth.

New discoveries in ultrasonic consolidation nano-structures using emerging analysis techniques

This paper examines the influence of ultrasonic consolidation (UC) on the bonding interface and challenges existing opinion regarding the mechanism of oxide dispersal during UC bond formation. The findings presented in this paper were achieved through the use of dual beam focused ion beam (DBFIB) etching and transmission electron microscopy (TEM), enabling the close examination of the weld interface, sub-grain morphology, and dislocation structure in ultrasonically consolidated aluminium (Al) 3003-T0 samples. As a result of this investigation, it has been established that a persistent oxide layer between bonded foils often remains after UC with this material. The level of sub-grain refinement in ultrasonically consolidated components was seen to decrease as the distance from the weld interface increases. This trend was characterised by the nanoscale sub-grain and gradient sub-grain regions, between which a distinct stepped transition was apparent. Direct contact with the sonotrode surface during the deposition of each new layer was also seen to cause significantly more sub-grain refinement than interface dynamics alone. Nano-grain colonies were also discovered in areas surrounding material flaws, manganese particles, or non-planar interface regions, and persistent dislocation slip bands were seen near the UC weld interface. In light of these results, the context of previously proposed methods of bond formation in UC is discussed.

The Kinetics of Phase Transformations During Tempering in Laser Melted High Chromium Cast Steel

The precipitation of secondary carbides in the laser melted high chromium cast steels during tempering at 300-650 °C for 2 h in air furnace was characterized and the present phases was identified, by using transmission electron microscopy. Laser melted high chromium cast steel consists of austenitic dendrites and interdendritic M23C6 carbides. The austenite has such a strong tempering stability that it remains unchanged at temperature below 400 °C and the secondary hardening phenomenon starts from 450 °C to the maximum value of 672 HV at 560 °C. After tempering at 450 °C fine M23C6 carbides precipitate from the supersaturated austenite preferentially. In addition, the dislocation lines and slip bands still exist inside the austenite. While tempering at temperature below 560 °C, the secondary hardening simultaneously results from the martensite phase transformation and the precipitation of carbides as well as dislocation strengthening within a refined microstructure. Moreover, the formation of the ferrite matrix and large quality of coarse lamellar M3C carbides when the samples were tempered at 650 °C contributes to the decrease of hardness.

Dislocation dynamics and slip band formation in silicon: In-situ study by X-ray diffraction imaging

White beam X-ray diffraction imaging (topography) with an optimised CCD-detector system is used to monitor in-situ and in real time the nucleation, growth and movement of dislocations in silicon at high temperatures. It can be shown, that damage like microcracks and the surrounding strain fields in a wafer act as sources for dislocation loops, which end in slip bands far away from the source. The dislocations are arranged in channels of parallel {111} glide planes, which become visible as bands of parallel surface steps when the dislocations slip out on the back or front sides of the wafer. The width of such a channel or band depend on the dimensions of the damaged volume where the dislocations nucleate. This can be explained with a simple geometrical model.

Dislocation sources and slip band nucleation from indents on silicon wafers

The nucleation of dislocations at controlled indents in silicon during rapid thermal annealing has been studied byin situX-ray diffraction imaging (topography). Concentric loops extending over pairs of inclined {111} planes were formed, the velocities of the inclined and parallel segments being almost equal. Following loss of the screw segment from the wafer, the velocity of the inclined segments almost doubled, owing to removal of the line tension of the screw segments. The loops acted as obstacles to slip band propagation.

Nanoindentation-induced phase transformation in (1 1 0)-oriented Si single-crystals

Pressure-induced plastic deformation and phase transformations manifested as the discontinuities displayed in the loading and unloading segments of the load–displacement curves were investigated by performing the cyclic nanoindentation tests on the (1 1 0)-oriented Si single-crystal with a Berkovich diamond indenter. The resultant phases after indentation were examined by using the cross-sectional transmission electron microscopy (XTEM) technique. The behaviors of the discontinuities displayed on the loading and re-loading segments of the load–displacement curves are found to closely correlate to the formation of Si-II metallic phase, while those exhibiting on the unloading segments are relating to the formation of metastable phases of Si-III, Si-XII, and amorphous silicon as identified by TEM selected area diffraction (SAD) analyses. Results revealed that the primary indentation-induced deformation mechanism in Si is intimately depending on the detailed stress distributions, especially the reversible Si-II ↔ Si-XII/Si-III phase transformations might have further complicated the resultant phase distribution. In addition to the frequently observed stress-induced phase transformations and/or crack formations, evidence of dislocation slip bands was also observed in tests of Berkovich nanoindentation.

Temperature dependence of tensile behavior of Ni-based superalloy M951

Tensile properties of M951 alloy are shown to be significantly dependent on temperatures in this study. Like in other superalloys, the yield and tensile strengths decreased slightly between room temperature and 600 °C, then both strengths showed peak values at intermediate temperatures, and both decreased sharply above 850 °C. The elongation fluctuated with temperature with its minimum at 800 °C. The TEM observations showed that at low temperature the dominant deformation mechanism was γ′ shearing by dislocation pairs and slip bands. At high temperature, the deformation was dominated by γ′ by-pass. At intermediate temperature it showed a transition from γ′ shearing to by-passing.

Stress–strain behavior on tensile and low cycle fatigue tests of JLF-1 steel at elevated temperature in vacuum

In this paper, the tensile and fatigue properties of JLF-1 steel were studied from room temperature to 873 K in vacuum with engineering size round bar specimens. Weak serration occurred in the stress–strain curve at elevated temperature, which is considered as the dynamic strain ageing effect. The serration pattern for the tensile specimens was different from that for the fatigue ones. This was due to the difference in the location of dislocation slip band relative to the position of beams of the gage. The fatigue life at elevated temperature was almost the same as that at RT when the life was plotted against the total strain range. Cyclic softening was observed in fatigue tests at elevated temperature.

Crack opening due to deformation twin shear at grain boundaries in near-γ TiAl

Microcrack nucleation has been observed at apparent deformation twin interactions with grain boundaries in a duplex near-gamma TiAl specimen deformed to surface tensile strain of about 1.4%. To prove that these microcracks are a result of twins and not dislocation slip bands, detailed characterization of the surface topography and crystallography associated with the microcracks was conducted and analyzed. Electron backscatter diffraction patterns were used in conjunction with selected area channeling patterns to determine the crystallography near the observed microcracks. Transmission electron microscopy of a selected twin, extracted using a focused ion beam, and atomic force microscopy were used to show that the observed microcracks could only have been caused by local strain heterogeneities caused by twin interactions with grain boundaries and not by dislocation slip bands.

On dislocation patterning: Multiple slip effects in the rate equation approach

Strain localization and dislocation pattern formation are typical features of plastic deformation in metals and alloys. Glide and climb dislocation motion, along with accompanying production/annihilation processes, lead to the occurrence of instabilities of initially uniform dislocation distributions. These instabilities result to the development of various types of dislocation microstructures (dislocation cells, slip and kink bands, persistent slip bands, labyrinth structures, etc.), depending on the externally applied loading and the intrinsic lattice constraints. The term “dislocation patterning” was introduced over 20 years ago by the third author and a corresponding “gradient dislocation dynamics” framework was suggested to describe such phenomena. In the W–A model proposed at that time by the last two authors, it was shown how coupled nonlinear evolution equations of the reaction-diffusion type for the forest (immobile) and gliding (mobile) dislocation densities can generate dislocation microstructures which correspond to walls perpendicular to the slip direction for Cu-crystals oriented for single slip under cyclic loading conditions. This model is adapted to the multiple slip case here. Weakly nonlinear analysis predicts that dislocation patterns should correspond to domains of walls perpendicular to each slip direction and separated by domain walls in the same orientations. This result is confirmed by numerical analysis and experimental observations. The present model generalizes the original W–A model to the case of multiple slip and considers also explicitly gradient effects by allowing for non-uniform dislocation velocities and internal stress effects.

3604 ナノ-メゾ-マクロ強度解析による疲労特性の解析(S13-1 組織因子,S13 実用構造材料の疲労問題と健全性評価)

Fully reversed total-axial-strain-controlled low cycle fatigue (LCF) tests have been conducted in air at room temperature on fine- and coarse-grain low-carbon steels, SUS304 steel and SUS316L steel. Three types of hardness measurements, namely nano-indentation, micro-indentation and conventional Vickers hardness tests, with the indentation size ranging from several tens of nm to several tens of μm, were performed for both virgin and fatigued specimens to evaluate the mechanical behaviour at different scales of the cyclically deformed steel during LCF. OM and TEM observation showed that the microstructural parameters are the dislocation cell size, d_<cell>, slip spacing, w_<slip>, and austenitic grain size, d_γ. Referring 10 d_<cell> and 10w_<slip>. Vickers hardness, HV, corresponding to macro strength was expressed as HV=Hv^*_<base>+Hv^*_<sol>+Hv^*_<cell>+Hv^*_<slip>. Hv^*_<base> is the base hardness, Hv^*_<sol> is the solid solution strengthening hardness, and Hv^*_<cell> and Hv^*_<slip> are the fine grain strengthening hardness due to the dislocation cell and slip band.

Multiscale Deformation of Open Cell Aluminum Foams

This article reports the results of experimental studies of the deformation of open cell aluminum foams under monotonic and cyclic compression. The multiscale nature of compressive deformation is examined from individual struts to overall foam deformation. Stress-strain curves of individual struts are investigated using micro-tensile testing. Deformation localization (dislocation slip bands) in individual struts is discussed along with evidence of deformation bands at the macro-scale. The onset and propagation of deformation localization bands are elucidated via in situ imaging and digital image correlation (DIC) techniques that provide continuous mapping of strain fields across sample sections; the Gibson-Ashby model is then used to estimate the dependence of foam strength and stiffness on relative density and strut properties. Finally, foam fatigue behavior under cyclic behavior is also studied, and attributed to surface crack nucleation and propagation in individual struts at different fatigue stages. Micro-fatigue testing is also carried out on individual struts that are extracted from foam blocks.

Twin–dislocation interaction in monazite (monoclinic LaPO4)

Dislocation-twin interactions were identified by TEM in monazite (monoclinic LaPO4) that was spherically indented at room temperature. Emissary dislocations in front of twin tips were observed. Propagation of dislocation slip bands through twins was also observed. Where possible, slip systems are identified. Formation mechanisms are discussed.

Redistribution of Dislocations in Silicon near Stress Concentrators

The distribution of defects in dislocation tracks in silicon plates was studied for various indentation angles. The regularities of variations in the linear density and maximum path of dislocations in slip bands are established. A model is proposed to describe the distribution of dislocations in the dislocation tracks. By fitting the theory to the experimental data, the dependence of this distribution on the energy relaxation time is determined.

Mechanisms and mechanics of compressive deformation in open-cell Al foams

The paper presents the results of experimental study of compressive deformation in open-cell 6101-O aluminum alloy foams with three different pore sizes (10, 20 and 40 pores per inch or PPI). Samples of two dimensions (10 mm × 10 mm × 10 mm and 24.5 mm × 24.5 mm × 17.0 mm) were investigated. Linear elastic deformation was first observed in a small 10 PPI foam sample at small enough stress level (0.11 MPa). The onset and spread of plasticity in individual struts were studied by the observation of dislocation slip bands, which were examined during ex situ scanning electron microscopy (SEM). An in situ optical microscope, connected to a video camera, was also used to study the progressive deformation of the foam structure under compressive loading. Both plastic buckling and plastic bending of struts were observed to occur during compressive deformation of the foam blocks. However, plastic bending of struts was the dominant deformation mechanism.

106 超音波スペクトロスコピーによる金属疲労損傷の評価(転位ループ密度とすべり帯面積の相関)

106 超音波スペクトロスコピーによる金属疲労損傷の評価(転位ループ密度とすべり帯面積の相関)

Diffuse-interface modeling of composition evolution in the presence of structural defects

A diffuse-interface phase field model is described for modeling the interactions between compositional inhomogeneities and structural defects. The spatial distribution of these structural defects is described by the space-dependent eigenstrains. It takes into account the effect of the coherency elastic energy of a compositional inhomogeneity, and the elastic coupling between the coherency strains and defect strains. The temporal evolution of composition is described by the Cahn–Hilliard equation. Particularly, the solute segregation, and nucleation and growth around dislocation slip bands and crack-like condensed interstitial dislocation loops are discussed. The effect of nucleated coherent precipitates on the stress field around these defects is analyzed

Non-Planar Slip and Cross-Slip at the Onset of Plastic Deformation in Silicon

The first stages of plastic deformation of FZ silicon single crystals were investigated by in situ synchrotron radiation topography, in creep conditions at temperatures between 975 and 1075 K and applied stresses from 22 to 44 MPa. Silicon samples were initially dislocation-free and deformation started at Vickers indentations and surface damage. Several features relevant for dislocation multiplication were observed: cross-slip in the bulk as well as near the surface, development of prismatic half-loops, non-planar development of dislocations in slip bands, in the wake of leading segments.

Evidence of the existence an α/β interface phase in a near-α titanium alloy

There have been contradictory reports about the existence of an α/β interface phase in titanium alloys over the last two decades. Recently some publications tend to favour the notion that the interface phase is an artifact formed in the preparation of the thin foil by electropolishing. However our studies on an advanced near-α titanium alloy (Ti-5.5Al-3.98Sn-2.16Zr-0.98Mo-0.85Nd-0.28Si) especially by means of High Resolution Transmission Electron Microscopy (HRTEM) showed that the α/β interface phase in the alloy existed before electropolishing and it could decompose during prolonged aging. In this paper, these evidences will be presented. It was observed from a strained region of a tensile sample along the longitudinal direction that the dislocation slip bands exist across the α platelets. At a higher magnification, it was noted that the dislocations that pass through the α platelet boundary regions and also cut through the α/β interface phase. From the HRTEM image of the ledge formed by the dislocations cutting through a platelet boundary, the glide plane of the dislocations is identified. It clearly showed that the α/β interface phase had existed before the tensile test was conducted. Moreover, it was also found that the decomposition of the α platelet occurred during the prolonged aging up to 3000 h at 550°C and the resultant product, silicide, was formed on the platelet boundary after aging. Around the decomposed β phase, the α/β interface phase disappeared, but still existed around the non-decomposed part of the boundary β phase. The evidences imply that the α/β interface phase also underwent decomposition or transformation while the β phase was decomposed. Such evidence would not have existed if the α/β interface phase is formed during electropolishing. The evidence also demonstrated that the α/β interface phase not only is a real phase in the alloy employed in this case but also unstable and may change dramatically under thermal exposure, which should affect the mechanical properties of this alloy.

Gravitational Stress‐Induced Dislocations in Large‐Diameter Silicon Wafers Studied by X‐Ray Topography and Computer Simulation

Dislocations in slip bands introduced under gravitational stress in 200 mm diam Czochralski‐grown silicon wafers were characterized by X‐ray topography, and slip band initiation was estimated by computer simulation for 300 mm diam wafers. Surface scratching on the wafer back by contact with the jig forms an amorphous region, resulting in a collective motion of dislocations during annealing at 1473 K under gravitational stress. The occurrence of slip bands along the 〈110〉 directions was observed with a length of several centimeters from the contact area of the wafer‐supporting jigs. Burgers vectors and slip planes of the gravitational stress‐induced dislocations were determined to extend along the [1¯10] direction in the (111) or (1¯1¯1) plane, terminating at the surface with screw‐type in character. Three types of half loop of dislocations were identified with Burgers vectors of [011] in (1¯1¯1), [101] in (1¯1¯1), and [011] in (111¯) planes. By the reaction , Lomer‐type edge dislocations were found to be created. The gravitational stress in a 300 mm diam wafer was computed by a finite element method. By comparing the gravitational stress with the critical stress to multiply slip dislocations previously obtained, conditions for suppressing slip bands are predicted for wafers larger than 300 mm diam.