Dislocation Barriers Research Articles

Structure of Ion-Implanted Alloys with Long-Range Order in a Field Ion Microscope

In the present review we describe the series of investigations in which field ion microscopy is used to study the structural and phase changes in alloys with long-range order and in pure metals after ion implantation by different gas ions. It is demonstrated that ion implantation induces defects of different types spread to considerable depths from the irradiated surface that exceed many times the estimated ion mean free path. It is established that disordering and generation of various defects can be observed under irradiation of the ordered alloy surfaces. In PdCuAg alloys being supersaturated solid solutions, the irradiation provokes the intermittent decomposition. The structure of defects induced by ion implantation including disordered regions, dislocations, dislocation configurations, dislocation barriers, vacancy clusters, and segregations of one of the components is analyzed. The structure and sizes of these defects inside single cascades of displacements are determined.

Effect of annealing twins on Hall–Petch relation in polycrystalline materials

We analyze theoretically, how the annealing twin boundaries formed in a material during grain growth, affect its yield stress. For this propose, the twin boundary was considered as a barrier to dislocation motion as in case of grain boundaries. The Hall–Petch relation gets modified due to the presence of twin boundaries. This relation is shown to be in good fit for the only available experimental result. Our analysis also indicates that Hall–Petch relation is more likely to be due to grain boundaries as a dislocation barrier rather than as a dislocation source.

Superdislocation core structure in pyramidal slip planes and temperature anomalies of Ti3Al intermetallic deformation behavior

For Ti3Al intermetallic with the D019 superlattice, the computer-aided simulation of the superdislocation core structure in the basal, prismatic, and type I and type II pyramidal planes is performed. A model of thermally activated transformation of 2c + a glissile edge superdislocations into dislocation barriers in the pyramidal planes is used to account for the anomalous temperature dependence of the yield stress of single-crystal Ti3Al compressed in the [0001] direction.

A nonplanar Peierls–Nabarro model and its application to dislocation cross-slip

A novel semidiscrete Peierls–Nabarro model is introduced which can be used to study dislocation spreading at more than one slip plane, such as dislocation cross-slip and junctions. Within this essentially continuum model, the nonlinearity which arises from the atomic interaction across the slip plane is dealt with atomistic and/or ab initio calculations. As an example, we study the dislocation cross-slip and the constriction process in two contrasting fcc metals, Al and Ag. We find that the screw dislocation in Al can cross-slip spontaneously in contrast with the screw dislocation in Ag, which splits into two partials, and thus cannot cross-slip without first being constricted. The response of the dislocations to an external stress is examined in detail. The dislocation constriction energy and the critical stress for cross-slip are determined, and, from the latter, we estimate the cross-slip energy barrier for the straight screw dislocations.

On stress assisted dislocation constriction and cross-slip

A novel semidiscrete Peierls–Nabarro model is introduced which can be used to study dislocation spreading at more than one slip plane, such as dislocation cross-slip and junctions. The strength of the model, when combined with an atomistic simulation for dislocation core properties, is without suffering from the uncertainties associated with empirical potentials. Therefore, this method is particularly useful in providing insight into alloy design when empirical potentials are not available or not reliable for such multi-element systems. The model is applied to study the external stress assisted dislocation cross-slip and constriction process in two fcc metals, Al and Ag, exhibiting different deformation properties. We find that the screw dislocation in Al can cross-slip spontaneously in contrast with that in Ag, where the screw dislocation splits into two partials that cannot cross-slip without first being constricted. The dislocation response to an external stress is examined in detail. The dislocation constriction energy and the critical stress for cross-slip are determined, and from the latter, we estimate the cross-slip energy barrier for straight screw dislocations.

Size-induced enhanced mechanical properties of nanocomposite copper/niobium wires: nanoindentation study

The effect of microstructure dimension δ on plasticity of high strength multifilamentary nanocomposite copper/niobium wires was studied by nanoindentation. Two structures were tested: a Cu matrix containing Nb filaments and a Cu matrix containing Nb tubes filled with Cu. For δ>10 μm, no size effect on the composite hardness is observed. In the 1–10 μm range, a strong increase in hardness indicates a change in plasticity mechanism, attributed to the classical Hall–Petch grain size strengthening. In the nanometre range, the hardness of the nanocomposite Cu/Nb regions exceeds that of nanocrystalline Cu or Nb, reaching 5.8 GPa for the finest conductors. The observed size effect on the plasticity of Cu/Nb nanostructures added to the dislocation barrier role of Cu/Nb interfaces confirms previous analyses based on the occurrence of a single dislocation regime at nanometre scale associated with impenetrable Cu/Nb interfaces.

Dislocation constriction and cross-slip: Anab initiostudy

A novel model based on the Peierls framework of dislocations is developed. The new theory can deal with a dislocation spreading at more than one slip planes. As an example, we study dislocation cross-slip and constriction process of two fcc metals, Al and Ag. The energetic parameters entering the model are determined from ab initio calculations. We find that the screw dislocation in Al can cross-slip spontaneously in contrast with that in Ag, which splits into partials and cannot cross-slip without first being constricted. The dislocation response to an external stress is examined in detail. We determine dislocation constriction energy and critical stress for cross-slip, and from the latter, we estimate the cross-slip energy barrier for the straight screw dislocations.

The influence of grain size on the indentation hardness of high-purity copper and aluminium

Abstract Quasistatic microindentation hardness studies of specimens of high-purity polycrystals and single crystals of copper and aluminium have been made at room temperature using a Vickers diamond and a tungsten carbide-cobalt sphere of radius 200 μm. The polycrystalline specimens of copper were prepared by heavy deformation of as-received specimens followed by thermal annealing at different temperatures and for different times; the grain sizes produced were in the range 15-520 μm. The polycrystalline samples of the as-received aluminium had a grain size of 330 ± 40 μm. As-formed specimens of copper of different grain sizes were found to have different dislocation densities; the smaller the grain size, the higher was the dislocation density. The Vickers hardness for a given indenter load, determined using the projected contact areas of indentations, was found to increase with decreasing grain size. Similarly, the Meyer hardness increased with decreasing grain size. It is argued that the observed increase in hardness of the polycrystals with decreasing grain size is due to the initial dislocation densities in the grains and not due to the grain boundaries which, because of the lack of impurities at them, do not appear to act as dislocation barriers in these high-purity fcc metals. Moreover, as the indentation hardness values of single crystals of copper of different orientations are very similar, any differences in the orientations of contiguous grains will not affect the hardness values of polycrystals. Thus for large single crystals of copper (10 mm × 10 mm × 10 mm), having the same dislocation density as that of the polycrystalline specimens of copper of grain size 15 ± 7 μm, the indentation hardness values were quite similar. Experimental results from the indentation hardness tests, made using both Vickers and spherical indenters, on polycrystalline aluminium specimens and relatively large single crystals showed that there was insignificant influence of the grain size.

Modeling of Screw Dislocation Dynamics in Crystalline Polyethylene

Recently [1], we studied the plastic deformation of crystalline polyethylene (PE) by molecular-dynamic modeling and showed that the major mechanism of initiation of this process in the presence of only edge dislocations is dissociation of dislocation dipoles formed by dislocations whose Burgers vectors are parallel to the axes of polymer chains. The contribution of screw dislocations to plastic deformation of PE has not been studied. As follows from experimental data [2], this contribution is significant. Indeed, the screw dislocations parallel to polymer chains in crystalline PE have a minimum line energy density; thus, they should dominate plastic deformation. The mechanisms of screw dislocation mobility in PE are still poorly understood. The Peierls stress for these dislocations was found by Peterson [3]; however, the moduli of elasticity used in [3] differed considerably from later values [4], which has cast some doubt on the validity of these findings. At least, we can state with assurance that the Peierls barrier for the [001] screw dislocations (we use the coordinate system with the z axis aligned with the polymer chains) is several orders of magnitude higher than the corresponding barrier for edge dislocations, because the motion of the [001] screw dislocations results in the displacement of polymer chains relative to one another rather than in their deformation. Hence, this process is dominated by the interchain interaction. The Young modulus in the [001] direction thereby has no effect on the mobility of the [001] screw dislocations, and, hence, the small parameter equal to the ratio of the shear modulus to the Young modulus is absent in the expressions describing the screw dislocation dynamics. As was shown in [5], the presence of this small parameter is responsible for the small height of the Peierls barrier for edge dislocations in PE. Therefore, plastic deformation associated with the motion of screw dislocations in polymer crystals differs considerably from plasticity caused by edge dislocation dynamics, because, for edge dislocations with the Burgers vector parallel to the axes of the polymer chains, the Peierls barrier is nine orders of magnitude lower than the shear modulus and can be neglected. For the [001] screw dislocations, competition between two mechanisms of initiation of plastic flow might be expected: overcoming of the Peierls barrier and dissociation of dislocation dipoles (i.e., dissociation of pairs of dislocations whose Burgers vectors are opposite in sign). Herein, we elucidated the role of these processes in the initiation of plastic deformation of crystalline PE and determined the yield stress of crystalline PE containing rectilinear [001] screw dislocations.

Propagation and interaction of {111} planar defects in the SrRuO3buffer layer in SrTiO3/SrRuO3two-layer films on LaAlO3substrates

Abstract We studied the microstructure of {111} planar defects, including stacking faults and nanotwins, in the SrRuO3 buffer layer in SrTiO3/SrRuO3 two-layer films on LaAlO3 substrates by means of high-resolution transmission electron microscopy. Most of the defects stop propagation in the buffer layer by mutual intersection with one another as well as by themselves without intersection. Single Frank or Shockley partial dislocations are found at the terminating end of an isolated stacking fault. We show several types of dislocation barrier formed by the intersection of two {111} stacking faults as well as when an extended dislocation changes its gliding-climbing plane. The formation mechanism of the dislocation barriers is discussed on the basis of reactions between the partial dislocations bounding the stacking faults. We also show a {111} nanotwin and its transition to an extrinsic stacking fault by a Frank partial dislocation. The contribution of the planar defects to the strain relaxation in the SrRu...

Effect of grain size on creep properties of type 316LN stainless steel

The effect of grain size on creep properties of type 316LN stainless steel has been investigated at 600°C under different stresses. The initial strain at the beginning of creep tests decreased with the decrease of grain size. This was confirmed by the Hall-Petch relationship. The steady state creep rate decreased to a minimum value at the intermediate grain size (dm=80–130 μm) and then increased with the further increase of grain size. This result agreed with Garofalo's model stating that grain boundaries act simultaneously as both dislocation sources and barriers to dislocation movement. The rupture elongation at the intermediate grain size was minimal due to the cavity formed easily by carbide precipitates in the grain boundaries.

Theory of plastic vortex creep

We develop a theory for plastic vortex creep in a topologically disordered (dislocated) vortex solid phase in type-II superconductors in terms of driven thermally activated dislocation dynamics. Plastic barriers for dislocations show a power-law divergence at small driving currents j, U(pl)( j) approximately j(-&mgr;), with &mgr; = 1 for a single dislocation and &mgr; = 2/5 for creep of dislocation bundles. This implies a suppression of the creep rate at the transition from the ordered vortex phase ( &mgr; = 2/11) to the dislocated glass and can manifest itself as an observed increase of the apparent critical current (second peak). Our approach applies to general dynamics of disordered elastic media on a random substrate.

Branching of electron current and quantum effects in two-dimensional dislocation barrier of n-type semiconductor

Electron current through the dislocation wall of the bicrystal boundary flows through saddle points of the 3D-potential relief. At low-enough temperatures this leads to current branching, i.e. to the separation of the current into numerous channels. Under some conditions, these channels make up 1D-conductors with the ballistic transport. As this takes place, effects associated with the quantum nature of the channel conductance could be observed experimentally.

Atomistic Simulations of Steps in Bimetallic Interfaces as Barriers to Interface Slip Transmission

ABSTRACTAtomistic models of coherent interfaces in the CuNi system with and without (111)-steps were used to study slip transmission across interfaces in CuNi metallic bilayers. The lattice mismatch of the CuNi system results in large coherency stresses at the interface. The (111)-steps afford a larger barrier to slip than the flat, coherent interface. The coherent flat interface dislocation barrier is largely due to the large compressive stresses in the Cu layer that must be overcome by applied tensile stresses. Additional Koehler forces are present as the dislocation in the elastically softer Cu approaches the stiffer Ni layer. The step, however, possesses a small residual edge dislocation with a Burgers vector equal to the difference of bCu and bNi times the height of the (111)-step in (111)-layers. We find that these steps are potent slip barriers, which suggests that homogeneous slip is preferred in such systems.

A dislocation barrier model for fatigue limit : as determined by crack non-initiation and crack non-propagation

Fatigue damage is caused by cyclic slip, and cyclic slip is driven by dislocation glide force. In order to cause fatigue damage, the cyclic glide force has to overcome the resistance of the primary and secondary dislocation barriers. Based on this cyclic damage process, the following diverse fatigue phenomena are synthesized into an integral and self-consistent analysis: Fatigue damage occurs in persistent slip bands (Hempel, 1956; Smith, 1957; Forsyth, 1957, 1961, 1963), and a nucleated fatigue micro crack is a shear crack (Forsyth, 1961). Pre-cracking fatigue damage is confined to the surface layer of a stressed metal (Wood et al., 1963). Fatigue limit is inversely related to grain size as shown in brass (Sinclair and Craig, 1952), in mild steels (Klesnil, Holzmann, Lukas and P. Rys 1965; Yoshikawa and Sugeno, 1965; and Taira, Tanaka, and Hoshina, 1979), and in ferritic-martensitic steel, (Kunio, Shimizu, and Yamada, 1969). Forrest and Tate (1964) found fatigue cracks in fine-grained brass at an alternating stress even below the fatigue limit. The cracks were within the boundaries of single grains. But they found no cracks in coarse-grained brass below the fatigue limit. The analysis synthesizes all of these experimental observations. The analysis is based on a realistic physical model. With a better understanding of the model and an improved calculation of the glide force, quantitative evaluations of the resistance of the dislocation barriers would eventually be possible. The needs for additional research are pointed out. A number of means of improving fatigue strength, based on the analysis, are suggested or explained.

Branching of electron current and quantum effects in two-dimensional dislocation barrier of n-type semiconductor

The current flow through the dislocation wall of the bicrystal (or intergrain) boundary in semiconductor is considered. It is known that the broken bonds along the dislocations make up the acceptor levels (being offset by energy ϵ d from the bottom of the conductance band) whose population by electrons is defined by the bulk donor concentration n D, the ϵ d-value and the dislocation spacing D y . At low enough n D-value ( n D< e 2/ ϵ d D y 4), electrons captured at the dislocation acceptors produce the periodic 2D-lattice with two comparable periods of the order of n s −1/2 where n s∼( ϵ d N D/ e 2) 1/2 is 2D-density of captured electrons. Charges of those electrons result in the formation of the dislocation potential barrier whose height (with mean value in order of ϵ d) is periodically modulated in the wall plane. Naturally, the electron current prefers to flow along the paths with the lowest barrier height, i.e. through the saddle points of the 3D-potential relief positioned in the dislocation wall plane. At low enough temperatures that leads to separation of the electron current into numerous branches. Provided both 2D-lattice periods of the captured charges are smaller as compared with the electron mean free path, all those narrow and short branches make up a filamentary 1D-conductors with the ballistic transport. As this takes place, effects associated with the 1D-conductance quantization define the whole system resistance and could be observed experimentally. In particular, exponentially strong temperature dependence of the whole system resistance vanishes.

Temperature dependence of the radiation damage microstructure in V–4Cr–4Ti neutron irradiated to low dose

Transmission electron microscopy (TEM) was performed on a V–4Cr–4Ti alloy irradiated to damage levels of 0.1–0.5 displacements per atom (dpa) at 110–505°C. A high density of small faulted dislocation loops were observed at irradiation temperatures below 275°C. These dislocation loops became unfaulted at temperatures above ∼275°C, and a high density of small Ti-rich defect clusters lying on {0 0 1} planes appeared along with the unfaulted loops at temperatures above 300°C. Based on the TEM and tensile measurements, the dislocation barrier strengths of the faulted dislocation loops and {0 0 1} defect clusters are ∼0.4–0.5 and 0.25, respectively. This indicates that both types of defects can be easily sheared by dislocations during deformation. Cleared dislocation channels were observed following tensile deformation in a specimen irradiated at 268°C.

Extended dislocation barriers in tilt boundaries in fcc crystals

Extended dislocation barriers at tilt boundaries in fcc crystals are considered. For boundaries with a high degree of lattice coincidence, stable extended dislocation barriers, analogous to those occurring within a single crystal, are shown to exist. Other boundaries can have components equivalent to the high coincidence boundaries. The boundary barriers are shown to be possible nucleation sources for mechanical twins or martensite phase transformations.

The evolution of deformation substructure in a creep deformed fully-lamellar TiAl alloy

Abstract The evolution of deformation substructure in a powder metallurgy (P/M) TiAl alloy creep deformed at 760°C has been examined using transmission electron microscopy. Since the multiplication of lattice dislocations within γ lamellae becomes limited as a result of a refined fully-lamellar (FL) microstructure (i.e. refined lamellar spacing) within the P/M TiAl alloy, the deformation of the alloy at the primary creep stage is mainly accommodated by soft lamellar grains through the movement of interfacial dislocations in the γ–α2 and γ–γ interfaces. The mobility of interfacial dislocations is primarily impeded by grain boundaries, interface ledges and dislocation barriers formed by the impingement of lattice dislocations to the lamellar interfaces. When the alloy is deformed into the secondary creep stage, the density of interfacial dislocations increases and deformation twins nucleate and grow from the lamellar interfaces. It is suggested that deformation twinning in FL-TiAl results from a stress-relief process due to the pile-up of interfacial dislocations during deformation. The alloy becomes more resistant to creep deformation as a result of the deformation twinning because the deformation twins further restrict the dislocation motion within the γ lamellae, and the formation of sessile dislocations at the intersections between the deformation twins and the γ or α2 lamellae provides effective barriers for the movement of interfacial dislocations.

Cl and EBIC analysis of a p + -InGaAs/n-InGaAs/n-InP/n + -InP heterostructure

A p + -In 0.56Ga 0.44As/n-In 0.56Ga 0.44As/n-InP/n + -InP heterostructure has been realized by chemical beam epitaxy and characterized using cathodoluminescence (CL) and electron beam-induced current (EBIC) techniques. These techniques allowed the visualization, location and density measurement of a misfit dislocation network due to the compressive strain between the epitaxial layers. The structure was found to be partly relaxed with the dislocation network located in the InP buffer under the hetero-interface and with a dislocation density between 1.9 × 10 7 and 5.3 × 10 7 cm −2 according to the area chosen; no threading dislocations were found in the InGaAs layer, which demonstrates the efficient role of the buffer as a dislocation barrier. Moreover, some important diffusion-recombination (DR) parameters, such as the minority carrier diffusion length, and the surface and interface recombination velocities, have been evaluated throughout the whole structure by fitting a CL theoretical model to experimental measurements. Photon recycling in the highly doped substrate has been evidenced by spectroscopic and semiquantitative measurements.