Slip Steps Research Articles

Spatial variation of the surface state onset close to three types of surface steps on Ag(111) studied by scanning tunnelling spectroscopy

A regular step, a dislocation slip step and a step formed by the emergence of a split edge dislocation (SED) to the surface influence the local density of states close to the onset of the surface state as investigated by scanning tunnelling spectroscopy at low temperature. The onset of the surface state shifts close to the regular step and the dislocation slip step by approximately 15 meV towards the Fermi energy. Additional maxima above the onset are only observed if a second step leads to confinement. In both cases, the conductivity decreases close to the step. However, an increase in conductance above the surface state onset is observed close to the SED step. Furthermore, a variety of additional states are discernable. Thus, different types of steps lead to markedly different changes in the local electronic structure on surfaces.

Study on dislocation slips in ferrite and deformation of cementite in cold drawn pearlitic steel wires from medium to high strain

The deformation of cementite was studied via optical microscopy and SEM in the longitudinal sections of pearlitic steel wires from medium to high strain. The cementite shows good deformability, the angle between the deformation direction of cementite and drawing direction decreases with increasing strain, and finally the deformation directions of cementite turn to the drawing axis at high strains. The deformation of the cementite is strongly related to plastic deformation in the ferrite, with coarse slip steps, S-bands and cracks across cementite observed parallel to either {110}α-Fe or {112}α-Fe plane traces determined by the largest Schmid factors.

Experimental Evaluation of the Cyclic Slip Irreversibility Factor

Cyclic slip irreversibility is one of the most important features of fatigue processes in ductile metals because it induces surface relief evolutions during cycling which are mainly responsible for crack initiation. The reversible and irreversible parts of the slip within persistent slip bands (PSBs) in polycrystalline nickel are measured directly after half-cycle deformation and one full cycle on specimen surfaces once more well-polished after 60% of fatigue life using atomic force microscopy (AFM) and different techniques of scanning electron microscopy as electron channelling contrast imaging and electron backscattered diffraction. Using AFM measures on the same slip steps after half-cycle and full cycle, the cyclic slip irreversibility factor is directly evaluated and discussed with respect to the literature.

Effect of Strain Rate on Cathodic Reaction During Stress Corrosion Cracking of X70 Pipeline Steel in a Near-Neutral pH Solution

The effect of strain rate on cathodic reactions of X70 pipeline steel during stress corrosion cracking in a near-neutral pH solution was investigated by electrochemical impedance spectroscope and potentiodynamic polarization curve measurements as well as slow strain rate tests. A local additional potential model was used to understand mechanistically the role of strain rate in electrochemical cathodic reaction. It was found that an application of elastic stress would not affect the electrochemical stable state of the steel specimen at a macroscopic scale. Under a weak cathodic polarization, the interfacial charge-transfer process occurring on steel contains both cathodic and anodic reactions. Since the anodic reaction process is still significant, localized dissolution could occur even at such a cathodic potential, resulting in generation of corrosion pits. These pits could be the start sites to initiate stress corrosion cracks. Strain rate affects the corrosion reaction, which is associated with the generation of dislocation emergence points and slip steps on the specimen surface, resulting in a negative local additional potential to enhance the cathodic reaction locally.

Role of Surfaces and Interfaces in Controlling the Mechanical Properties of Metallic Alloys

This article explores the subtle effects of surfaces and interfaces on the mechanical properties of bulk metallic alloys using three examples: environmental effects on fatigue life, hydrogen embrittlement effects on the ductility of intermetallics, and the role of coherent precipitates in the toughness of steels. It is demonstrated that the marked degradation of the fatigue life of metals is due to the strong chemisorption of adsorbates on exposed slip steps that are formed during fatigue deformation. These adsorbates reduce the reversibility of slip, thus accelerating fatigue damage in a chemically active gas environment. For certain intermetallic alloys such as Ni(3)Al and Ni(3)Fe, the ductility depends on the ambient gas composition and the atomic ordering in these alloys, both of which govern the complex surface chemical reactions taking place in the vicinity of crack tips. Finally, it is shown that local stresses at a coherent precipitate-matrix interface can activate dislocation motion at low temperatures, thus improving the fracture toughness of bulk alloys such as steels at cryogenic temperatures. These examples illustrate the complex interplay between surface chemistry and mechanics, often yielding unexpected results.

Deposition of Brownian particles during evaporation of two-dimensional sessile droplets

Abstract Evaporation of sessile droplets gives rise to internal flow that, in turn, affects drastically the deposition of the suspended particles on the substrate. Analytical expressions for the flow field are used to compute the local velocity inside an evaporating two-dimensional droplet as a function of position and time of evaporation. Trajectories of a sample of suspended Brownian particles are computed along the progress of the evaporation process and terminate upon collision with the solid substrate. Traveling particles are allowed to interact with the evaporating free surface and are either rebounced into the main body of the droplet or float on the surface until they are collected on the substrate. The dependence of the diffusion coefficient on the distance from the substrate is taken into account giving rise to anisotropic diffusional effects that, however, are shown to affect slightly the deposition rate profile except in the case of flow-dominated transport. Results for hydrophilic and strongly hydrophobic substrates are presented and the conditions for the development of the two-dimensional variant of the famous “coffee stain phenomenon” are investigated. Comparison of the simulated deposition profiles with the corresponding predictions of a convection-diffusion lattice–Boltzmann model revealed excellent agreement for various Peclet number values. Simulation of stick-slip evaporation leads to well-defined multiple stripes, the density of which depends strongly on the difference between the initial contact angle and the angle at which the slip step is activated.

Influence of bulk damage on crack initiation in low‐cycle fatigue of 316 stainless steel

ABSTRACTTo investigate the effect of bulk damage on fatigue crack initiation, crack initiations due to low‐cycle fatigue of Type 316 stainless steel were observed by electron backscatter diffraction (EBSD) and scanning electron microscopy. The EBSD observations showed that local misorientation developed inhomogeneously due to the cyclic strain, and many cracks were initiated from the slip steps and grain boundaries where the local misorientation was relatively large. The crack initiations could be categorized into two types: enhancement of the driving force by geometrical discontinuity (slip steps and notches), and reduction of material resistance against crack initiation caused by accumulated bulk damage at grain boundaries. In particular, more than half of the cracks were initiated from grain boundaries. However, in spite of the significant bulk damage, the fatigue life was extended by removing the surface cracks under strain of 1 and 2% amplitude. The stress state at the microstructural level was changed by the surface removal, and the damaged portion did not suffer further damage. It was concluded that although bulk damage surely exists, the fatigue life can be restored to that of the untested specimen by removing the surface cracks.

A study aimed at determining and understanding the fracture behaviour of an Al–Li–Cu–Mg–Zr alloy 8090

The S-L fracture toughness of aluminium–lithium based alloys is generally poor and this has limited their applicability, particularly in aerospace where good damage tolerance is required. The low S-L toughness has been attributed variously to grain boundary precipitation, planar slip and lithium segregation. This work examined the S-L fracture behaviour of an 8090 Al–Li-based alloy in 34–45mm plate. The results confirmed that the S-L fracture toughness decreases from the centre to the surface of plate material and increases with double ageing. Fracture is principally intergranular, with both ductile and brittle components occurring, but some transgranular fracture also occurs and this produces steps in the fracture plane. Changes in the relative proportions of brittle and ductile intergranular fracture, as well as in the amount of transgranular fracture, accompany the changes in toughness. However, the decrease in fracture toughness across the plate is accompanied principally by an increase in the relative proportion of brittle intergranular fracture, while the toughening produced by double ageing is accompanied principally by an increase in the amount of transgranular fracture. Evidence of coarse slip, indicative of slip planarity, was seen from slip steps and dislocation structures. However, planar slip was seen only towards the centre of the plate and not towards the surface. The level of planar slip was not reduced markedly by double ageing. Texture varied with position across the plate. There were also a larger number of low angle boundaries towards the centre of the plate than towards the surface. The hardness did not change across the plate. The results could not be explained fully in terms of either the grain boundary precipitate theory or the planar slip model, but were generally consistent with the lithium segregation model. However, the basic tenet of this model is that the level of embrittlement is influenced by the grain boundary structure, and the results did not indicate a substantial difference between the boundaries that failed in a ductile manner and those which failed by brittle fracture. This suggests that the factors which affect lithium segregation may be more complex than originally envisaged.

Microstructural effects on crack path selection in bending and fatigue in a Nb–19Si–5Cr–3.5Hf–24Ti–0.75Sn–1W alloy

Nb–Si alloy containing 19Si, 5Cr, 3.5Hf, 24Ti, 0.75Sn and 1W (in at.%) has been tested by three-point bending and fatigue at both room temperature and 400 °C. The microstructure and fracture surface associated with crack growth were investigated by scanning electron microscopy (SEM) and electron backscattering diffraction (EBSD). The alloy contains Nb solid solution (Nb(ss)) and Nb- or Ti-rich silicides. Some pre-existing microcracks were found in the silicides. EBSD Analyses on the cast and HIPped Nb–Si alloy revealed a preferred orientation in the Nb(ss) similar to that of a unidirectionally solidified alloy. The fatigue crack path preferentially propagated either along the phase boundaries between the silicide and Nb(ss) or through the pre-existing silicide microcracks. Slip steps were observed inside the Nb(ss) during crack propagation, indicating deformation in the alloy occurred in the Nb(ss) while the silicides assisted the crack propagation.

Evolution of cementite morphology in pearlitic steel wire during wet wire drawing

The evolution of the cementite phase during wet wire drawing of a pearlitic steel wire has been followed as a function of strain. Particular attention has been given to a quantitative characterization of changes in the alignment and in the dimensions of the cementite phase. Scanning electron microscope observations show that cementite plates become increasingly aligned with the wire axis as the drawing strain is increased. Measurements in the transmission electron microscope show that the cementite deforms plastically during wire drawing , with the average thickness of the cementite plates decreasing from 19 nm ( ε = 0) to 2 nm ( ε = 3.7) in correspondence with the reduction in wire diameter. The deformation of the cementite is strongly related to plastic deformation in the ferrite, with coarse slip steps, shear bands and cracks in the cementite plates/particles observed parallel to either {110} α or {112} α slip plane traces in the ferrite.

Angular Distribution of Slip Steps by Three-Dimensional Polycrystalline Model for Stainless Steel

Localized deformation plays an important role in the initiation of irradiation-assisted stress corrosion cracking, and it can be characterized by the spacing and inclination of slip steps observed on the surface. In order to examine the contribution of mechanical factors (stress) to slipping, the angular distribution of slip steps was evaluated by using a three-dimensional polycrystalline model generated by Voronoi tessellation. An elastic finite element analysis was performed to derive the resolved shear stress (RSS) acting on each slip system. Random crystallographic orientations assigned to each grain caused the microstructural inhomogeneity of local stress due to anisotropic elastic properties. The angular distribution obtained was compared with the experimental results observed in irradiated and deformed stainless steel. It was shown that mechanical factors dominate the slipping behavior of the irradiated stainless steel and that not only RSS but also normal stress acting on the slip plane affects the crystallographic slip. The angular distribution was also evaluated from the maximum Schmid factor of individual grains, in which the inhomogeneous local stress was not taken into account. The angular distributions obtained using RSS and the Schmid factor were almost the same. This suggests that the inhomogeneity of local stress negligibly affects the angular distribution of slip steps, although the change in the local stress in the polycrystalline material is significant.

Persistent Slip Bands

Abstract The slip activity and shear strain of persistent slip bands (PSBs) in polycrystalline nickel were studied after half-cycle deformation on well-polished specimen surfaces at different stages of fatigue life using the combination of atomic force microscopy (AFM) and scanning electron microscopy (SEM). Although the local shear strain of PSBs seems to be nearly independent of the stage of fatigue life, the half-cycle slip activity of PSBs significantly depends on it. In-situ deformation experiments in the scanning electron microscope (SEM) showed that during half-cycle loading the slip steps do not proportionally develop to the applied plastic strain and not the whole accumulated PSB volume is active. The reactivation of the cumulative PSB volume is different for individual PSBs and grains, and evolves more slowly than the reactivation of grains. PSBs in polycrystals are subjected to a development history — their activity changes both temporally as well as spatially. Possible reason for this behaviour could be hardening effects due to secondary slip within the PSBs. Detailed investigations of the dislocation structure developed at different stages of fatigue life using the electron channelling contrast (ECC) in the SEM give more information about it.

Hydrogen Effects on Localized Plasticity in SUS310S Stainless Steel Investigated by Nanoindentation and Atomic Force Microscopy

The effect of hydrogen on the localized plasticity of an industrial material, SUS310S stainless steel, has been investigated using a combination of nanoindentation and atomic force microscopy (AFM). The load versus displacement curves show that 73.4 wt.ppm hydrogen in the SUS310S sample causes a 41% decrease in the first excursion load but has little effect on the first excursion depth, which indicates that hydrogen enhances dislocation nucleation and promotes dislocation emission and multiplication. The AFM images of the pile-ups around the indentation indicate that hydrogen causes two types of slip step in the {111} slip planes, indicating that hydrogen promotes slip planarity and localized plasticity.

Verhakungen, dislocations, solitons, and kinks

Abstract The paper retraces, from a personal viewpoint, the development of atomistic models of dislocations in crystals from the model of Prandtl, Dehlinger, Frenkel, and Kontorova, first conceived in 1912, to recent work on kinks in dislocations. Among the topics discussed in some detail are the emergence of the theory of solitons, the rate theory of kink-pair generation, and the interplay of experiment and theory in the quantitative investigation of kinks by mechanical relaxation and flow-stress measurements. An outcome of this interplay, the determination of the planes of the elementary slip steps of screw dislocations in refractory body-centred cubic metals, has become the ‘open sesame!’ of quite a number of puzzling phenomena in which the plastic deformation of bcc metals differs from that of fcc metals and alloys. These phenomena include non-uniformities in the flow-stress – temperature relationship, anomalous slip, tension – compression asymmetry in uniaxial straining tests, alloy softening, and the enigma of ‘reversible’ vs. ‘irreversible’ γ-relaxation. The key for their understanding is a first-order transition transforming the cores of a0<111>/2 screw dislocations from a low-temperature configuration capable of slipping on {110} planes to a configuration with a {112} slip plane at elevated temperatures. Various problems needing further research are pointed out.

Angular Distribution of Slip Steps by Three-Dimensional Polycrystalline Model for Stainless Steel

Localized deformation plays an important role in the initiation of irradiation-assisted stress corrosion cracking, and it can be characterized by the spacing and inclination of slip steps observed on the surface. In order to examine the contribution of mechanical factors (stress) to slipping, the angular distribution of slip steps was evaluated by using a three-dimensional polycrystalline model generated by Voronoi tessellation. An elastic finite element analysis was performed to derive the resolved shear stress (RSS) acting on each slip system. Random crystallographic orientations assigned to each grain caused the microstructural inhomogeneity of local stress due to anisotropic elastic properties. The angular distribution obtained was compared with the experimental results observed in irradiated and deformed stainless steel. It was shown that mechanical factors dominate the slipping behavior of the irradiated stainless steel and that not only RSS but also normal stress acting on the slip plane affects the crystallographic slip. The angular distribution was also evaluated from the maximum Schmid factor of individual grains, in which the inhomogeneous local stress was not taken into account. The angular distributions obtained using RSS and the Schmid factor were almost the same. This suggests that the inhomogeneity of local stress negligibly affects the angular distribution of slip steps, although the change in the local stress in the polycrystalline material is significant.

Characterization of deformation structure in ion-irradiated stainless steels

The effects of strain rate and type of stainless steels on the deformation behavior of irradiated stainless steels were investigated. Type 316 and 304 stainless steels irradiated with 200 keV He + ions up to 20 dpa at 300 °C were deformed at 10 −4 and 10 −7/s at 300 °C. The deformation changed from uniform dislocation tangling to dislocation channeling and twinning, and slip step spacing widened due to prevention of dislocation gliding by dislocation loops and cavities. The deformation mode was the same in both stainless steels deformed at both strain rates. The loop and cavity density in type 304 stainless steel was smaller than that in type 316 stainless steel. However, dislocation gliding was more effectively prevented in the case of faster deformation and in type 304 stainless steel when dislocation loops were the dominant microstructure. The difference became small at higher doses when cavities were the dominant microstructure.

Enhanced rate-dependent tensile deformation in equal channel angularly pressed Sn–Ag–Cu alloy

Tensile stress–strain response of an equal channel angularly pressed (ECAPed) Sn–3.8Ag–0.7Cu alloy was investigated to seek a mechanistic understanding of the time-dependent deformation in Sn-rich alloys. It was found that the deformation behavior of the ECAPed alloy had a stronger rate-dependence than the as-cast alloy. At low strain rates, the ECAPed alloy exhibited a higher ductility but a lower flow stress. During tensile deformation, the equiaxed Sn grains were stretched into thin strips. In comparison, the slip steps were primary feature within single grain at high strain rates. The enhanced rate-dependence of the ECAPed alloy was related to greater tendency for grain boundary diffusion due to the increase in the grain boundary density.

Observation of thermal-misfit strain relaxation in a PbTe semiconductor grown on Cd0.96Zn0.04Te(1 1 1)

The thermal-misfit strain relaxation in epitaxial PbTe grown on Cd0.96Zn0.04Te(1 1 1) substrates has been studied by the combined characterization of atomic force microscopy and high-resolution transmission electron microscopy. It is shown that the strain relaxes by the movement of dislocations in the ⟨1 1 0⟩–{1 0 0} primary glide system and leaves straight slip steps on the surface. The thermal-misfit strain relaxation is greatly affected by the growth temperature and post-growth cooling rate. Post-growth treatment with a slower cooling rate improves the crystalline quality of PbTe grown on Cd0.96Zn0.04Te(1 1 1). Because of the easy dislocation glide down to the interface in the ⟨1 1 0⟩–{1 0 0} primary glide system, PbTe/Cd0.96Zn0.04Te(1 1 1) heterostructures exhibit good crystal and optical qualities, which indicate that PbTe/CdZnTe heterostructures may become a promising candidate for the fabrication of PbTe-based mid-infrared optoelectronic devices.

Multiscale modeling of ductile crystals at the nanoscale subjected to cyclic indentation

A multiscale method is applied to study the response of an aluminum single-crystal substrate to cyclic indentation at finite temperature. The evolution of contact-induced deformation on the nanoscale is controlled based on defect nucleation beneath the indenter. The method allows for visualization of atomistic deformation during loading and unloading. Although there are inherent limitations to our two-dimensional model, we have found qualitative similarities to the mechanisms of homogeneous defect nucleation and deformation in three-dimensional face-centered cubic crystals. It is shown that the atomistic surface roughening process mostly arises from homogeneous dislocation nucleation during successive loading/unloading processes. These sub-surface defects cause major permanent deformation of the substrate during indentation. The slip steps forming on the surface of the indented substrate contribute their own dislocation activity, sending dislocations directly from the surface into the crystal, but those activities mostly remain localized near the indented surface. Force–displacement curves and the hysteresis which occurs due to inelastic deformation and heating of the substrate are studied for each cycle, and correlated with sub-surface and surface nucleation of defects.

Effect of carbon on stress corrosion cracking and anodic oxidation of iron in NaOH solutions

Anodic behaviour of decarburised iron and of quenched Fe–C materials with up to 0.875 wt% C was examined in 8.5 M NaOH at 100 °C to explain the role of carbon in caustic stress corrosion cracking (SCC) of plain steels. Removal of carbon from Armco iron strongly reduced its intergranular SCC. Slip steps on grains did not initiate cracks. It has been shown that carbon at low contents deteriorates the passivation of iron, whereas at high contents it promotes the formation of magnetite. High resistance to SCC of high carbon steels can be explained by an intense formation of magnetite on these steels.