Slip Steps Research Articles

A study relating slip steps and substructure produced during creep of an AlZn alloy

A study relating slip steps and substructure produced during creep of an AlZn alloy

Heteroepitaxy of ferrite‐spinel layers by CVD method

AbstractThe nucleation mechanism, the substructure, the inner stresses, and the formation of different dislocation structures were studied in the epitaxial ferrospinel layers. Monocrystalline layers of Mg‐Mn‐, Min‐ and Li‐ferrites were grown by the CVD method in a small gap on MgO substrates. The initial growth stage begins with the formation of three‐dimensional nuclei, but after their coalescence the mechanism of layer‐to‐layer growth is realized. X‐ray topographical study shows that the layer structure up to 6 μm thick was formed by slip steps but above 10 μm by cleavage steps. The residual elastic compressive stresses studied by X‐ray tensometry are about 6 × 108 N/m2. The development of dislocation structure of layers depending on their thickness and growth conditions was studied by means of optical and scanning electron microscopy: for synthesis temperatures up to 1200 K only the dislocation sliping was observed, above 1370 K their overcreeping occurred.

A STUDY OF SYNTHETIC DIAMOND SURFACE USING REFLECTION ELECTRON MICROSCOPY

The surface of synthetic diamond have been observed employing reflection electron microscopy with both TBM and STEM instrument. It is found that there are slip steps of screw dislocations, growth steps and microcrystals on the surface. The result shows that the surface of sythetie diamonds is characteristic of crystallization from solution.

Slip structures and cross-slip of screw dislocations during the deformation of NaCl single crystals at low temperatures

NaCl single crystals were deformed at liquid helium temperature and at liquid nitrogen temperature. The slip structure was investigated by the gold decoration technique. At liquid helium temperature, relatively pure crystals show a predominance of sharp narrow slip bands with a high internal strain. Localized displacements of the bands are frequently indicated by microcracks. At liquid nitrogen temperature the slip is homogeneous in most cases. In the work reported in this paper, the slip steps of individual dislocations are imaged, and the cross-slip processes of screw dislocations were investigated as a function of the concentration of divalent cationic impurities. Both the cross-glide events were almost identical with the corresponding room temperature data. The results suggest that cross-slip is initiated by long-range internal stresses.

Theoretical Considerations on Corrosion Fatigue Crack Initiation

Based on corrosion kinetics and fracture mechanics, it has been possible to determine quantitatively the corrosion fatigue life for three different types of corrosion behavior. Under general, active corrosion, corrosion fatigue life is controlled by the corrosion rate and the applied alternating stress range. If pitting corrosion occurs, corrosion fatigue life depends on the incubation time for nucleating a pit, the pit growth kinetics, and a critical pit depth, which is a function of the applied stress range. It has been assumed that under passive corrosion conditions, the passive layer has to be penetrated by slip steps, to form corrosion fatigue cracks. The corrosion fatigue crack initiation in this case is controlled by the repassivation kinetics of the material and also by a critical notch depth, depending on the applied stress range. It has been found that a critical current density exists, below which no corrosion fatigue cracks can initiate. Comparison of the theoretically calculated life times with experimental results showed a quite good correlation.

Effect of Nitrogen and Phosphorus on the Stress Corrosion Cracking of Austenitic Steels in MgCl2Solutions

The influence of phosphorus and nitrogen on the stress corrosion cracking behaviour of austenitic stainless steels similar to Type 316 (but with variations in nickel and molybdenum contents) has been investigated. Nitrogen additions decreased the critical stress for stress corrosion crack (SCC) nucleation but did not increase the maximum crack growth rate in Type 316 tested in MgCl2 at 154°c. In a molybdenum-free steel, phosphorus additions were far more deleterious than nitrogen additions and gave increased cracking rates and nucleation frequencies.The stacking fault energy was only decreased a little by the phosphorus addition and nitrogen tended to increase the stacking fault energy, so that changes in SCC cannot be attributed simply to stacking fault energy changes. An ordered carbide type structure was observed in a slow-cooled specimen containing a very high nitrogen content. Only slight freckling contrast and no precipitate diffractions were observed in the range of nitrogen contents usedfor the SCC tests. The strengthening by nitrogen and the slip planarity observed were similar from 13 wt% nickel to 30 wt% nickel. However no cracking was observed in the alloy with 30% nickel. Whilst the change in dislocation behaviour may account for some increase in SCC susceptibility at 13 and 20 wt% nickel, slip planarity is not sufficient to produce crack growth at high nickel contents.The phosphorus addition gave little hardening and little increase in slip planarity. It does, however, have a marked effect on the slip step oxide growth characteristics, producing tunnels rather than slots. It seems, therefore, that the effect of phosphorus is mainly a chemical one, that is, on the rate of dissolution or on the nature of the oxide film, rather than on the dislocation behaviour.

Thermal stability of Pb-alloy Josephson junction electrode materials. V. Effects of repeated cycling between 298 and 4.2 K of Pb-Bi counter electrode

Changes in the microstructure of ε-phase Pb-Bi films with various thicknesses that were deposited onto oxidized Si substrates and then repeatedly thermally cycled between 298 and 4.2 K were studied using x-ray diffraction, transmission, and scanning electron microscopy. The level of strain supported elastically by the films had a strong dependence on the film thickness. For 0.2-μm-thick films, almost all of the strain was supported elastically. For 1.0-μm-thick films most of the strain was relaxed upon cooling to 4.2 K, and grain rotation, hillock formation, and dislocation slip steps were observed after repeated cycling. For 0.4-μm-thick films in which about 0.1% of the strain was relaxed upon cooling to 4.2 K, such changes in the microstructure were observed after ∼200 cycles. Although these changes were significantly suppressed by 0.6-μm-thick SiO layers coated onto the films, the grain rotation and hillock formation were observed after 400 cycles for SiO-coated, 0.4-μm-thick films similar to those used for the counter electrodes of Pb-alloy Josephson junctions.

The Effect of Solute Content on the Slip Behavior in 7XXX Series Aluminum Alloys

The microstructures of three high strength, high purity Al-Zn-Mg-Cu alloys, in the T6 temper, were characterized extensively using quantitative optical metallography and quantitative transmission electron microscopy. Only the solute content (Mg + Zn) of these alloys was varied for this study. These alloys were shown to be identical in grain size and shape, dispersoid (E-phase) and grain boundary precipitate (ν) populations, and precipitate free zone widths. The matrix microstructures consisted of ordered GP zones and ν′ and differed only in the volume fraction of these strengthening precipitates. The higher solute alloys had the higher yield strengths and volume fractions of matrix precipitates. Subsequent slip behavior analysis of prestrained tensile specimens demonstrated that slip band spacings and slip step heights increased with increasing solute content for the same macroscopic strain (ep = 0.02). A work softening model by Hornbogen and Gahr was shown to predict this tendency toward increased strain localization with higher solute levels.

Dislocation behavior in fatigue VI: Variation in the localization of strain in persistent slip bands

It is now widely accepted that the strain localized in the persistent slip bands (PSBs) of fatigued pure metals is about 0.01, so much so that the average nature of this strain appears to have been forgotten. Accordingly, we offer interferometric observations on the details of slip offsets within PSBs. These were obtained by repolishing the gauge surfaces of specimens cycled into saturation and restraining to the strain amplitude; they thus represent PSB behavior in transitu. Strain concentrations up to 1000 are shown to exist at the surface well into saturation. The implications of these results for understanding dislocation behavior in fatigue are discussed: the slip step behavior can be explained by the glide of groups of like-handed screw dislocations in the PSBs and is not consistent with a model in which any significant part of the strain is due to edge dislocations bowing out of the PSB walls.

The Role of Defects in the Fracture of Oxide Films on Metals

The extent of fracture of anodic oxide films during tensile deformation of aluminum is shown to be controlled by the density of defects in the oxide. The density of defects was monitored by the “leakage” resistance of the film, while the extent of fracture of the oxide was obtained from measurements of reanodization current transients. Other experiments in a photoelectron microscope showed that these oxides are ruptured at emerging slip steps. This provided the basis of a simple model for oxide fracture, which requires the presence of a defect in the oxide at the site of an emerging slip step. A relationship between the leakage resistance and fracture susceptibility of the oxide was derived and confirmed by experiment, showing that both properties are controlled by the same type of defect. The density of these defects may be controlled by the surface preparation prior to anodization. If the density of defects is greatly reduced, even a thin (28 nm) oxide becomes strong enough to suppress slip step formation during tensile deformation.

The study of evaporated antimony cleavage faces by surface decoration

Antimony cleavage faces have been evaporated in vacuum and decorated with platinum by the technique of Bassett. Subsequently the decoration patterns have been studied in the transmission electron microscope. At low evaporation temperatures zig-zag line patterns have been found. The lines form angles of 60° and have been interpreted as slip steps. At higher evaporation temperatures triangular decoration patterns in two orientations, and triangular evaporation spirals have been observed. Furthermore, the interaction of steps, twist boundaries and the path of a moving dislocation have been found. The observed decoration patterns were interpreted. From the decoration pattern of the coalescence of “Lochkeim” steps and steps of screw dislocations, a unimolecular step height (3.76 Å) must be assumed.

Orientations of Transgranular Stress Corrosion Cracks in Austenitic Steels tested in MgCl2Solutions

Abstract Orientations of stress corrosion cracks have been determined in single crystals of Type 316, Type 310 and a few experimental steels. The cracks are more irregular in Type 316 than in Type 310 which shows a tendency to form {100} facets and {100} crack planes. The electrochemical behaviour of crystal slices did not vary systematically with orientation and, although differences were detected, it is considered that faster repassivation on {100} crystal faces is not the explanation of the observed crack plane orientation. Cross slip of dislocations was identified from slip-traces at the surface and indicates a localisation of slip to surface regions which could be associated with local stress relaxation. Such processes would increase the aspect ratio if they occurred in the bulk and increased the number of dislocations operating in a given crack length. Toward the crack tips fairly regular slip steps of about 30nm spaced at 1μm were observed but smaller steps (10nm) and spacings (70nm) could be detec...

X-ray topographic examination of the generation, glide and locking of basal dislocations during the microplastic deformation of zinc and cadmium crystals

Abstract Nearly perfect zinc and cadmium single crystals were subjected to increasing tensile stresses and simultaneously characterized by transmission X-ray topography. Fresh dislocations were emitted in basal slip planes at resolved shear stresses as small as a few 104 Pa; in some crystals, an emission mechanism could be identified as the elongation and/or the expansion of original dipolar loops; in most eases however, the nucleation took place mainly from stress-enhanced parts of the specimens. Once emitted, the dislocations were observed to interact with sub-grain boundaries, prismatic loops of Burgers vector c or c + a, and with the crystal surface. A model is developed for the latter case, based on the dragging force felt by a dislocation at its point of emergence while it develops a slip step on the surface. In the rather thin crystals (80 pm) used for transmission X-ray topography, this surface effect, could be proved to be responsible for the locking of some fresh dislocations in their glide plane.

Microscopic Observation of Cleaved Surface of Stearic Acid Crystal

Cleavage structures on the (001) plane of the monoclinic stearic acid crystal were observed. The cleavage steps detected by optical microscope were giant steps with heights of more than 1000 Å and were composed of short ledges in the <100> and <110> crystallographic directions. Small cleavage steps with heights of less than 200 Å were also observed by electron microscope. These small cleavage steps had two kinds of origin; (1) lattice defects in the crystal and (2) dislocation loops newly nucleated by the cleavage itself. The small cleavage steps generated by dislocation loops showed the line pair feature. These line pair cleavage steps can be explained as the formation of a cleavage step and a slip step. The step directions, which were strongly dependent on the cleavage propagation directions, were considered in terms of the anisotropic surface energies of the monoclinic stearic acid crystal.

Surface decoration: Depletion zone analysis along the steps

A detailed study has been made of the zones depleted of gold crystallites on either side of a cleaved KC1 surface slip step. We have been able to show that step decoration takes place with the first stages of deposition, during nucleation. The depletion zones are to be ascribed to the diffusion halos of the adatoms on either side of the step when the depositions occur at substrate temperatures T ⩽ 20°C. It has also been shown that the roughness of the slip steps at room temperature is too low for measurement by the decoration technique.

Microstructure of cold-rolled copper

Optical and electron metallography have been used to examine the substructures developed in OFHC copper after rolling reductions from 50–97%. These structures are related to the displacements observed at free surfaces after deformation. It is shown that deformation occurs in a series of overlapping stages in each of which a single operating mechanism predominates. At low strains >10% reduction), deformation occurs by slip on planes spaced about 0.03 μm apart to produce an elementary structure. In the range 10–65% reduction, the operative mechanism produces microbands. It is shown that these features correspond to surface slip lines and to particular strain markings in metallographic specimens and that their formation involves the cooperative movement of dislocations. on planes that are much more closely spaced (<0.008 μm) than has been thought previously. At higher levels of strain, instabilities develop in the form of shear bands and these gradually replace microband formation as the dominant deformation mechanism. The equiaxed cell structure formed at low strains cannot be related to the microband structure or the surface slip steps and is apparently a relaxation effect.

SERT法による304鋼の沸騰42%MgCl&lt;SUB&gt;2&lt;/SUB&gt;中における応力腐食割れ伝ぱに関する研究

Stress corrosion crack velocity (da⁄dt) under the slow extention rate condition has been determined for commercial Type 304 austenitic stainless steel in boiling 42%MgCl2 solution at 143°C. The test specimens were 2.5 and 5 mm thick with round notch.A single crack length was measured by a traveling microscope (×60) in the crosshead speed (CS) range 4×10−5 to 1×10−2 mm/min. Slip lines on the tested specimen surface were examined by a dfferential-interference microscope and the fracture surfaces of all tested specimens were also observed by a scanning electron microscope.Specimens showed a single transgranular cracking in the CS range of 4×10−5∼4×10−3 mm/min. The initial crack propagation rate (da⁄dt) increased with CS in 2.5 mm thick specimens and da⁄dt also increased with crack length at any constant CS when a single transgranular cracking occurred. On the other hand, in 5 mm thick specimens, the dependence of da⁄dt on CS or crack length was not so clear as seen in 2.5 mm thick ones. Whenever a single transgranular cracking was observed, the maximum value of da⁄dt was around 10−2 mm/min.These dependences of da⁄dt on CS, crack length and the thickness of specimen have been discussed from the view point of the formation rate of slip steps and the corrosion rate of metals in slip steps. The relationship between da⁄dt obtained in fracture mechanics by other investigators and those in this study has also been discussed.

The role of oxide films in stress corrosion cracking initiation

In the low-potential transition range, passivatable alloys undergo electrolytic crevice corrosion i.e. localized anodic attack at any area prevented from passivating by shielding effects. A possible role of stress in this range may be seen, accordingly, in producing at every film rupture uncontrollable localized attack in critical underfilm crevices at the feet of emerging slip steps. The fundamental condition for s.c.c. would be that the incubation time, i.e. the time of attack required to form a critical notch, must be shorter than the film lifetime. The resulting model for s.c.c. initiation at the active-passive transition fits in with the mostly accepted slip-dissolution mechanism. A simple mathematical treatment is given for such model and applied to three typical systems, that are the combination of steel with K 2CO 3 KHCO 3, NaOH and NH 4NO 3 solutions, respectively. Calculated incubation times are consistent with experimental times to rupture and cracking rates from s.c.c. tests.

The role of oxide films in stress corrosion cracking initiation

In the low-potential transition range, passivatable alloys undergo electrolytic crevice corrosion i.e. localized anodic attack at any area prevented from passivating by shielding effects. A possible role of stress in this range may be seen, accordingly, in producing at every film rupture uncontrollable localized attack in critical underfilm crevices at the feet of emerging slip steps. The fundamental condition for s.c.c. would be that the incubation time, i.e. the time of attack required to form a critical notch, must be shorter than the film lifetime. The resulting model for s.c.c. initiation at the active-passive transition fits in with the mostly accepted slip-dissolution mechanism. A simple mathematical treatment is given for such model and applied to three typical systems, that are the combination of steel with K 2CO 3-KHCO 3, NaOH and NH 4NO 3 solutions, respectively. Calculated incubation times are consistent with experimental times to rupture and cracking rates from s.c.c. tests.

Localized Corrosion on Slip Steps of Aluminum Straining

The initial steps of localized corrosion of aluminum strained in aggressive solutions were studied by electron microscopy of oxide replicas. Aluminum samples were strained at a constant potential in the following deaerated solutions:; ; and . No pitting was found on the aluminum strained in those solutions below the pitting potential; the slip bands showed the same substructure as that found on aluminum strained in air. At, and above, the pitting potential, pitting was found on the surface of the strained aluminum. Pitting nucleated on the whole surface in neutral solutions. In solution (pH 11) and in solution pitting nucleated preferentially on the slip bands, leading to semicylindrical pits. It was also concluded that mechanical film breakdown is not a sufficient condition for pit initiation. Another process, such as localized acidification, must be assumed to explain pit initiation at pitting potential.