Principal Slip Planes Research Articles

Structure and mobility of -type screw dislocation in presence of H in α-Ti from first-principles

Hydrogen has both negative and positive effects on the mechanical properties of titanium and titanium alloys. It has been reported that the plastic deformation of titanium is controlled by the <a>-type screw dislocation and the principal slip plane is the prismatic plane. Herein, we employ first-principles calculations to investigate the modification of the <a>-type screw dislocation core structures caused by addition of H and the interactions between them. It reveals that H is attracted to the pyramidal dislocation partial and tends to segregate to the dislocation region. Then, the slip paths of the <a> screw dislocations along different slip planes in the H inserted Ti system are studied. Furthermore, the mechanism on how H affects the strength and plasticity of Ti under different H concentrations through the change of dislocation mobility is discussed.

Analytical elastoplastic analysis of heteroepitaxial core-shell nanowires

Semiconductor nanowires, grown heteroepitaxially, have many unique properties compared to heteroepitaxial thin films: e.g., the possibility of lateral relaxation, high surface to volume ratio and lower strain energy. While the onset of plastic deformation in flat thin films has been studied extensively, much less is understood about this phenomenon in the nanowire geometry. Here, we report development of a continuum analytical model that predicts not only the onset of plastic deformation for core-shell structures with anisotropic slip system, but also the evolution of stress and strain fields beyond the initial yield. This is the first analytical elastoplastic study of heteroepitaxial core-shell nanowires. Our model is verified against finite element simulations. To illustrate trends predicted by the model, we choose InGaAs for core-shell system as an example. We find that most energetically favorable positions for formation of the first dislocations in the heterostructure have misorientation of 0, π/2, π, and 3/2π from the principal slip planes in zinc-blend structures. We demonstrate that anisotropy in slip systems of the heterostructure reduces the critical misfit strain. Finally, we find that there is a critical ratio (χ) of shell thickness to core radius that maximizes the thickness of the elastoplastic region. This critical ratio is independent of geometry and depends only on material properties such as elastic moduli and yield strength of the heterostructure.

Structure Fragmentation in a Subsurface Nickel Titanium Layer Caused by its Irradiation with Pulsed Silicon ion Fluxes

Using the methods of electron backscatter diffraction, an investigation of variations in microstructure of the subsurface nickel titanium layer after its irradiation with pulsed, medium-energy silicon ion fluxes is performed. It is shown that following this ion-beam irradiation of the specimen surfaces, the subsurface-layer structure changes and undergoes fragmentation down to as deep as 5–15 μm, which is smaller than the average grain size of the initial alloy. It is found out that the fragmented-structure layer is characterized by the presence of a martensitic В19′ phase and a high concentration of interfaces and grain boundaries; the linear dimensions of the fragments exceed 1 μm, the structure refinement in the layer below the irradiated surface is nonuniform and depends on crystallographic orientation of the initial grain. A reason for intensive fragmentation of individual grains of the initial B2 phase after ion-beam treatment is assumed to be the proximity of the orientation axes of the principal slip planes to that of the incident ion beam flux. This might have resulted in an earlier, compared to other grains, onset of plastic deformation in these grains and, as a result, partial fragmentation of their structure.

Atomic-level computer simulation of the interaction between dislocations and interstitial loops in α-zirconium

Zirconium is an important metal for internal components of nuclear reactors, yet there have been few computer simulation studies of the interaction between dislocations and interstitial dislocation loops that are created in this metal by radiation damage. Reaction mechanisms have been simulated in this work by using an interatomic potential developed by Mendelev and Ackland (2007 Phil. Mag. Lett. 87 349) that has been shown to provide a good description of the core structure and glide resistance of dislocations on the principal slip plane . The interaction of both edge and screw dislocations of the slip system with small prismatic loops containing up to 156 interstitial atoms has been considered. If a loop intersects the dislocation glide plane it becomes absorbed on the dislocation line in most situations. If it does not intersect the glide plane but has a Burgers vector inclined to that of the dislocation, it glides to and is absorbed by the line in most cases. The obstacle resistance of loops is relatively strong for screw dislocations in comparison with edges, but loop absorption by screws is only temporary.

Carbonate pseudotachylytes: evidence for seismic faulting along carbonate faults

The Canalone Porta fault, developed within dolomitic and marly limestones in the Grigna Massif (Southern Alps, Italy), shows two types of reddish veins: one parallel to the principal slip plane (fault-veins)andtheotherintrudingthefaulthostrock(injectionveins). The veins have a dominant carbonate composition and show a microtexture with millimetre-sized clasts embedded in a fine-grainedmatrix. Thismatrixiscomposed ofmicrometre-sized globular clusters of micrometre- to nanometre-sized calcite and dolomite crystals, bound together by K-bearing aluminosilicate glass. Field and petrological data suggest that this carbonaterich matrix represents an undercooled melt-bearing assemblage produced at T 700 C and P 0.1‐0.2 GPa. The Canalone Porta veins are therefore carbonate pseudotachylytes, i.e. the friction-induced melting product of carbonate rocks. Herein, we discuss the scarcity of these rocks in general and criteria to find and recognize them along relatively deep fault zones.

Subsurface Stress Fields in Face-Centered-Cubic Single-Crystal Anisotropic Contacts

Single-crystal superalloy turbine blades used in high-pressure turbomachinery are subject to conditions of high temperature, triaxial steady and alternating stresses, fretting stresses in the blade attachment and damper contact locations, and exposure to high-pressure hydrogen. The blades are also subjected to extreme variations in temperature during start-up and shutdown transients. The most prevalent high-cycle fatigue (HCF) failure modes observed in these blades during operation include crystallographic crack initiation/propagation on octahedral planes and noncrystallographic initiation with crystallographic growth. Numerous cases of crack initiation and crack propagation at the blade leading edge tip, blade attachment regions, and damper contact locations have been documented. Understanding crack initiation/propagation under mixed-mode loading conditions is critical for establishing a systematic procedure for evaluating HCF life of single-crystal turbine blades. This paper presents analytical and numerical techniques for evaluating two- and three-dimensional (3D) subsurface stress fields in anisotropic contacts. The subsurface stress results are required for evaluating contact fatigue life at damper contacts and dovetail attachment regions in single-crystal nickel-base superalloy turbine blades. An analytical procedure is presented for evaluating the subsurface stresses in the elastic half-space, based on the adaptation of a stress function method outlined by Lekhnitskii (1963, Theory of Elasticity of an Anisotropic Elastic Body, Holden-Day, Inc., San Francisco, pp. 1–40). Numerical results are presented for cylindrical and spherical anisotropic contacts, using finite element analysis. Effects of crystal orientation on stress response and fatigue life are examined. Obtaining accurate subsurface stress results for anisotropic single-crystal contact problems require extremely refined 3D finite element grids, especially in the edge of contact region. Obtaining resolved shear stresses on the principal slip planes also involves considerable postprocessing work. For these reasons, it is very advantageous to develop analytical solution schemes for subsurface stresses, whenever possible.

Dislocation generation, slip systems, and dynamic recrystallization in experimentally deformed plagioclase single crystals

Three samples of gem quality plagioclase crystals of An60 were experimentally deformed at 900 °C, 1 GPa confining pressure and strain rates of 7.5–8.7×10 −7 s −1. The starting material is effectively dislocation-free so that all observed defects were introduced during the experiments. Two samples were shortened normal to one of the principal slip planes (010), corresponding to a “hard” orientation, and one sample was deformed with a Schmid factor of 0.45 for the principal slip system [001](010), corresponding to a “soft” orientation. Several slip systems were activated in the “soft” sample: dislocations of the [001](010) and 〈110〉(001) system are about equally abundant, whereas 〈110〉{111} and [101] in (1̄31) to (2̄42) are less common. In the “soft” sample plastic deformation is pervasive and deformation bands are abundant. In the “hard” samples the plastic deformation is concentrated in rims along the sample boundaries. Deformation bands and shear fractures are common. Twinning occurs in close association with fracturing, and the processes are clearly interrelated. Glissile dislocations of all observed slip systems are associated with fractures and deformation bands indicating that deformation bands and fractures are important sites of dislocation generation. Grain boundaries of tiny, defect-free grains in healed fracture zones have migrated subsequent to fracturing. These grains represent former fragments of the fracture process and may act as nuclei for new grains during dynamic recrystallization. Nucleation via small fragments can explain a non-host-controlled orientation of recrystallized grains in plagioclase and possibly in other silicate materials which have been plastically deformed near the semi-brittle to plastic transition.

Structural geology and kinematic history of rocks formed along low-angle normal faults, Death Valley, California

Several late Cenozoic low-angle normal, or detachment, faults on the western flank of the Black Mountains are each characterized by the following, in descending structural order: (1) a hanging wall of upper Tertiary to Quaternary sedimentary and volcanic strata that displays little evidence for fault-related damage other than widely spaced planar or listric normal faults; (2) a sharp, planar, and locally striated principal slip plane forming the lower boundary of the hanging wall; (3) an upper zone—zone I—of very fine grained, fault-generated rocks composed dominantly of gouge; (4) a lower zone—zone II—of coarser-grained fault rocks consisting chiefly of foliated breccia; and (5) a variably damaged footwall, consisting of partly mylonitic Precambrian units or Tertiary plutonic rocks, that has been exhumed from depths of 10–12 km since late Miocene time. The fault rocks, which were mostly derived from the footwalls, preserve evidence for cataclastic and particulate flow. Fault rocks contain authigenic minerals but lack the cyclically deformed, mineral-filled syntectonic veins that are abundant in some other late Cenozoic high-angle and strike-slip faults. Meso- and microscale fabrics in zones I and II indicate that finite shear strains increase progressively upward, toward the principal slip plane. In a conceptual kinematic model of a shear box, displacements of the hanging wall produced a shearing flow in the fault rocks below. Some of the slip on the principal slip plane was also partitioned into localized slip on discrete sliding surfaces in zones I and II. Since 770 ka, during the latest stages of incremental deformation in the brittle shear zones, distributed flow in the fault rocks alternated with slip that was chiefly localized on the principal slip planes. In this respect, the detachment faults differ from inactive segments of the San Andreas system, along which displacements were progressively and irreversibly localized onto a single principal slip surface. The strain gradients and corresponding changes in grain size in the shear zones resemble those in mylonitic shear zones except in their symmetry. Strain gradients in the Black Mountains are notably asymmetric: they are present only below the principal slip planes, not above.

Scales of structural heterogeneity within neotectonic normal fault zones in the Aegean region

Structural heterogeneities within neotectonic normal fault zones in the Aegean region are important controls on the geomorphic expression of their uplifted footwall blocks. Contrasting fault rocks and slip-plane phenomena (corrugations, comb fractures, slip-parallel fractures and pluck holes) are responsible for mesoscopic inhomogeneities within fault scarps. Along-strike disruptions in the continuity of fault scarps (step-over zones, step-up zones, step-down zones, footwall cross-faults and hangingwall cross-faults) are products of perturbations in slip-plane geometry or intersections with transverse faults. Large-scale variations in range-front morphology reflect contrasting patterns of deformation within fault zones. Some major fault zones, and many minor fault zones, are characterized by a migration of the active slip plane towards the hangingwall, resulting in a distributed network of high-angle faults and a range front which is stepped in profile. The presence of a localized mass of uneroded bedrock (hangingwall salients) protruding from the hangingwall of some major fault zones, results in intense deformation along a principal slip plane, and a range front which is ramp-like in profile. As a consequence of structural heterogeneities within neotectonic normal fault zones, fault scarps and range fronts in the Aegean region are subject to a temporally and spatially variable pattern of degradation.

有向性加工層の残留応力に関する研究 フライス加工層における切れ刃作用密度の効果

The feed per tooth fz for slab-milling was varied to observe effects of the density dz (=1/fz) of cutting edges acting on machined surface in unit length. As the dz is increased, macroscopic residual stresses σxv and σyv translate into a compressive stress and microscopic residual shear stress τzxw decreases. Decreasing dz results in growing fiber texture in a machined layer and the principal slip planes α·Fe {110} in medium carbon steel concentrate along a direction inclined to the machined surface. The microscopic τzxw is generated by the slip on the planes. The effects of dz, on τzxw for up-milling are stronger than for down-milling. The effects of dz appears also in a carbide phase in high speed steel.

Work hardening in ionic crystals

Abstract The work-hardening characteristics of two ionic materials with different structures and slip systems, CaF2 and NaCl, are studied and compared. It is found that the work hardening is similar in both, and athermal in origin. Stage I work hardening is due to interactions with blocking obstacles and forest dislocations, and the bowing out at the sessile jogs formed by multiple cross glide. The latter is the principal contribution except at low strain-rates or high temperatures. Stage II work hardening is due to interactions with blocking obstacles, forest dislocations and the debris formed on planes inclined to the principal slip planes. An important contribution is provided by one or both of the last two.

Plastic deformation of alpha-uranium; twinning and slip

Data on the crystallography of twinning, slip and kinking in α-uranium are reported. In default of single crystals, very coarse grained uranium was used for the experiments, in conjunction with a new design of X-ray camera. The techniques, some of them new, used for finding the slip, twinning and kink hand planes, the slip directions and the directions and magnitudes of the twinning shears are fully set out. A discussion follows of the observed intersections of twins and their application to the confirmation of twin directions. The principal slip plane is (010). the slip direction [100], and the kink bands are normal to [100]. {110} is a subsidiary slip plane. The twinning elements found are tabulated. One of the four observed twin types is of the “second kind” (i.e. with an irrational twinning plane and rational twinning direction); it is the only one of its kind hitherto observed in a metal. Observations on the influence of the temperature on slip and twinning, on the effect of annealing on twins and on the pseudo-cleavage sometimes associated with twins, are set forth. The observations are discussed in relation to the crystal structure of uranium; in particular, the probable atomic motions in twinning are derived graphically.

The behaviour of a single crystal of zinc subjected to alternating torsional stresses

The present experiment has been carried out in the course of a general investigation into the causes and characteristics of the failure of metals under repeated cycles of stress which has been in progress for some years past at the National Physical Laboratory. As this research proceeded, one definite and promising line of investigation suggested itself in the study of the behaviour of single metallic crystals under repeated stress systems, and, as a commencement, attention is being directed to typical metals crystallising in the cubic (face-centred, also body-centred), close packed hexagonal, and rhombohedral space lattice systems. The investigation, when completed, should not only give certain fundamental data concerning fatigue phenomena, but also, when studied in conjunction with the classical researches of Mark, Polanyi and Schmid, Taylor, Elam, and Farren—who have studied in detail, the characteristics of deformation of single crystals under static stressing—afford some contribution towards the discovery of the general laws of deformation of metals subjected to any stress system. Previous reports have described experiments on single crystals of aluminium (face-centred cubic) and iron (body-centred cubic). The present paper describes an experiment on a single crystal of zinc, a metal crystallising in the hexagonal (close-packed) lattice. The deformation of zinc crystals under statical direct stress has received much attention from Mark, Polanyi and Schmid. They showed clearly that the principal slip plane is the basal (0001) plane, the direction of slip being that of the most highly stressed (shear stress) primitive direction (normal to the 1̄21̄0, 1̄1̄20, or 21̄1̄0 plane) contained by the basal plane. In the last stages of deformation, however, they found that deformation was consistent with slip occurring on planes apparently perpendicular to the basal planes, and they suggested that prismatic planes were also slip planes; they were unable, however, to establish this identity.