Mechanism of formation of shear bands upon cold deformation of a commercial Fe-3% Si alloy

A model of the formation of shear bands upon cold deformation of {111}〈112〉 crystals of a commercial Fe-3%Si alloy is proposed. The model suggests a two-stage mechanism for the formation of a shear band and its fine structure. At the first stage, there occurs an anomalous twinning in the {114}〈221〉 system; at the second stage, an almost complete secondary twinning of the band in two standard {112}〈111〉 systems occurs. As a result of these transformations, the shear band consists of regions with an almost cube-on-edge orientation {11 11 1}〈1 1 22〉 and an “octahedral” {111}〈112〉 orientation, which is symmetrical with respect to the initial {111}〈112〉 orientation. A crystallographic mechanism responsible for an anomalous twinning in the {114}〈221〉 system by the slip of (8a/18)〈221〉 partial dislocations is proposed. It is supposed that the shear bands observed consist of groups of different numbers of shear microbands.     

Investigation of the structure of compacts and sheets of an Al-Cu-Li alloy strengthened by Al<Subscript>2</Subscript>CuLi (T<Subscript>1</Subscript>) particles

X-ray diffraction and electron microscopy have been used to investigate structural states of the material of compacts and sheets of an aluminum-copper-lithium alloy with a lithium content corresponding to the field of equilibrium of a solid solution with the phase T ₁ (Al₂CuLi). In pressed strips, there arises a multicomponent texture (Bs “110”〈112〉, Cu “211”〈111〉, S “123”〈hkl〉) typical of pressed articles of aluminum-lithium alloys. Their microstructure was characterized by the presence of slip bands oriented in the rolling direction. The formation of lamellar precipitates of the T ₁ phase at slip-band boundaries in the process of deformation is supposedly stimulated by Shockley partial dislocations. In hot-rolled sheets, there arose an unusually intense nearly single-component texture of the Ex ₁ “011”〈111〉 type, whose appearance is mainly caused by the cross rolling of the sheets. The reduced strength characteristics of the sheets examined are connected with a sharply oriented character of the structure of both the matrix phase and the strengthening T ₁ phase. A quasicrystalline phase T ₂ present in the material and the precipitates of the δ′ phase exert no marked effect on the level of mechanical properties.     

The effect of precipitation on the evolution of recrystallization textures in an AA 8011 aluminum alloy sheet

The texture of an AA 8011 aluminum alloy sheet cold rolled by 95% showed a typical β-fiber, which runs from the copper orientation [C={112}〈111〉] over S [{123}〈634〉] to brass [B={011}〈112〉]. The development of annealing textures depended on annealing temperatures due to the interaction between precipitation and recrystallization. Upon annealing at a low temperature of 275°C, precipitation took place before recrystallization. This led to a weak recrystallization texture consisting of {011}〈122〉, {001∼〈100〉, and {hk0}〈001〉, among which the {011}〈122〉 orientation developed near large FeAl₃ particles as the main orientation and the cube [{001}〈100〉] orientation originating from the matrix was relatively weak. After annealing at 350 and 500°C, a strong cube texture developed along with a weak {011}〈122〉 orientation. When the cube orientation developed, the copper orientation disappeared most rapidly. These results were discussed based on the interaction between precipitation and recrystallization.     

Deformation twinning of titanium β-alloys of transition class

The structure of deformed titanium β-alloys of transition class is studied. A model of anomalous twinning over the {332} 〈113〉 system typical for these alloys is developed. An explanation of the low stress of nucleation and growth of such twins, the ease of their intersection with each other, and the blocking action of the twin boundaries in slip deformation is given within the framework of the model. The effect of aging after cold plastic deformation on the yield strength of titanium β-alloys strained by twinning over the {332} 〈113〉 system is studied. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 14–21, June, 2006.     

Twinning stress in shape memory alloys: Theory and experiments

Utilizing first-principles atomistic simulations we present a twin nucleation model based on the Peierls–Nabarro formulation. We investigated twinning in several important shape memory alloys, starting with Ni₂FeGa (14M modulated monoclinic and L1₀ crystals) to illustrate the methodology, and predicted the twin stress in Ni₂MnGa, NiTi, Co₂NiGa, and Co₂NiAl martensites, all of which were in excellent agreement with the experimental results. Minimization of the total energy led to determination of the twinning stress accounting for the twinning energy landscape in the presence of interacting multiple twin dislocations and disregistry profiles at the dislocation core. The validity of the model was confirmed by determining the twinning stress from experiments on Ni₂FeGa (14M and L1₀), NiTi, and Ni₂MnGa and utilizing results from the literature for Co₂NiGa and Co₂NiAl martensites. This paper demonstrates that the predicted twinning stress can vary from 3.5 MPa in 10M Ni₂MnGa to 129 MPa for the B19′ NiTi case, consistent with the experimental results.     

Crystallographic analysis of extra reflections in the electron diffraction patterns of the CuAu alloy ordered at <Emphasis Type="Italic">T</Emphasis> < 350°C

The electron-microscopic examination of the CuAu alloy subjected to rapid quenching from temperatures below T _C revealed in its structure the presence of a phase, whose structure is close (in terms of the lattice parameters) to the L1₀ structure, but is characterized by the presence of uncommon extra reflections in the electron diffraction patterns at distances 1/3〈201〉*, 1/5〈203〉*, 1/7〈304〉*, etc., in the coordinates of the L1₀ structure, and by the simultaneous presence of superlattice reflections of two types in their electron diffraction patterns. A crystallographic analysis showed that the extra reflections observed cannot correspond to the points of the reciprocal lattice of the long-period Z phase [4] or any other phase. The arrangement of these reflections is explained well by the punctures of the plane of diffraction by a set of [$ \bar 1 $ \bar 1 01]* parallel streaks passing through fundamental and superlattice reflections. These streaks are parallel to the (101) plane and to the plates of the phase revealed.     

RETRACTED ARTICLE: A study on the microstructure of precipitation strengthened B2-ordered NiAl

Microstructural control to produce a multiphase structure and there by improve the high temperature strength as well as low temperature ductility of intermetallics has received much attention. A transmission electron microscopy investigation has been performed in the present work on the precipitation of supersaturated B2-ordered (Ni,Co)Al and α-Cr in B2-ordered β-NiAl with different stoichiometry. Precipitation behavior and hardening were investigated by measuring the hardness variation. The hardness of (Ni,Co)Al and β-NiAl increases appreciably by the fine precipitation of (Ni,Co)₂Al and α-Cr, and overage softening occurs after prolonged aging. In the case of B2-ordered (Ni,Co)Al, the (Ni,Co)₂Al phase has a hexagonal structure and takes a rod-like shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appears in the hexagonal (Ni,Co)₂Al phase. The orientation relationship between the (Ni,Co)₂Al precipitates and the B2-(Ni,Co)Al matrix is found to be (0001)_p//(111)B2 and [[`1]\bar 12[`1]\bar 10]_p//[[`1]\bar 110]_B2, where the suffixes p and B2 denote the (Ni,Co)₂Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardens appreciably by fine precipitation of the (Ni,Co)₂Al phase. On the other hand, in the case of B2-NiAl, perfect lattice coherency is retained at the interfaces between the α-Cr particles and the matrix during the initial stage of aging. After prolonged aging, a loss of coherency occurs by the attraction of matrix dislocations to the particle/matrix interface followed by climbing around the particles.     

Texture Evolution During Annealing in Warm-Rolled and Cold-Rolled Ti-IF Steel

The texture characteristics and the recrystallization mechanism during annealing in warm rolled and subsequently cold rolled Ti-IF steel have been investigated by x-ray diffraction and electron backscattered diffraction (EBSD). The results attained by x-ray show that texture changes little until the holding time reaches 4 h. After annealing for 4 h, the orientation density of α-fiber reduces dramatically, but the orientation density of γ-fiber has no obvious change; however, the density of {332}〈113〉 component increases. The EBSD analysis shows that orientated-nucleation dominates during recrystallization, and the orientation of the nuclei includes γ-orientation as well as {332}〈113〉 component.     

First-principles study on the mobility of screw dislocations in bcc iron

The fully two-dimensional Peierls barrier map of screw dislocations in body-centered cubic (bcc) iron has been calculated using the first-principles method to identify the migration path of a dislocation core. An efficient method to correct the effect of the finite size cell used in the first-principles method on the energy of a lattice defect was devised to determine the accurate barrier profile. We find that the migration path is close to a straight line that is confined in a {1 1 0} plane and the Peierls barrier profile is single humped. This result clarifies why the existing empirical potentials of bcc iron fail to predict the correct mobility path. A line tension model incorporating these first-principles calculation results is used to predict the kink activation energy to be 0.73 eV in agreement with experiment.     

RETRACTED ARTICLE: Dislocation-particle interaction in precipitation strengthened Ll<Subscript>2</Subscript>-ordered Ni<Subscript>3</Subscript>Al

Transmission electron microscope investigation has been performed on the particle-dislocation interactions in Ni₃Al-based intermetallics containing various types of fine precipitates. In an Ll₂-ordered Ni₃Al alloy with 4 mol.% of chromium and 0.2–0.5 mol.% of carbon, fine octahedral precipitates of M₂₃C₆ type carbide, which has a cube-cube orientation relationship with the matrix, appear during aging. Typical Orowan loops are formed in Ni₃Al containing fine dispersions of M₂₃C₆ particles. In the alloys with appropriate titanium content, fine precipitates of coherent disordered γ are formed during aging. The γ precipitates are initially spherical or rounded cubic in shape and grow into platelets as aging proceeds. Loss of coherency is initiated by the introduction of dislocations at the γ/γ′ interface and results in step formation at the dislocations. The γ precipitates become globular after the loss of coherency. In the γ′ phase hardened by the precipitation of the disordered γ phase, dislocations are attracted into the disordered γ phase and cut through the particles during deformation at any stage of aging. In Ni₃Al containing a fine dispersion of disordered γ, superdislocations are strongly attracted to the disordered particles and dissociate on the (111) plane in the γ particles, while they dissociate on the (010) plane in the matrix. It is shown by comparison that the strengthening due to attractive interaction is more effective than that due to repulsive interaction. The roles of the variation of the interaction modes and of the dissociation of superdislocations in the matrix and particles are discussed in connection with the optimum microstructures of Ll₂-ordered intermetallics as high temperature structural materials.     

Mechanisms of plastic deformation in microcrystalline and nanocrystalline TiNi-based alloys

Mechanisms of plastic deformation of a high-temperature B2 phase that act upon tension, compression, and high-pressure torsion in TiNi-based single crystals have been studied depending on the crystal orientation. For the crystals with orientations located near the [$ \bar 1 $ \bar 1 11] and [$ \bar 1 $ \bar 1 12] poles in the standard stereographic triangle, multiple dislocation slip prevails upon both compression and tension. In “hard” crystals with the deformation axis close to the [001] direction, in which the Schmid factors for dislocation slip are close to zero, the main deformation mechanisms are the mechanical twinning in the B2 phase and the stress-assisted B2 → B19′ martensitic transformation. All the above listed mechanisms take part in the formation of the {111}〈hkl〉 texture. The mechanism of the change in the orientation of “hard” polycrystalline grains upon the formation of a nanocrystalline and amorphous-crystalline state has been demonstrated on the example of the evolution of the structure of [001] crystals upon severe plastic deformation in a Bridgman cell.     

Study of the texture of aluminum alloys after cold rolling,annealing, and irradiation by Ar<Superscript>+</Superscript> ions

The crystallographic texture of samples (1–3 mm thick) of AMg6, VD1, and 1441 aluminum alloys has been studied after cold rolling, post-rolling annealing, and irradiation by accelerated Ar⁺ ions (E = 20–40 keV). A feature in common for the {200} and {111} pole figures of samples rolled to a medium degree of reduction (35–72%) is the absence of the pole density at the center of these figures. When the alloys under study undergo post-rolling technological furnace annealing at high temperatures, a texture is formed with a scattered main component of the {001}〈100〉 type, which also occasionally contains orientations from scatter regions of the rolling-texture components. An exposure of the cold-deformed samples to a beam of accelerated Ar⁺ ions generally leads to analogous textural changes (in the whole volume of the samples irradiated from one side) in a much shorter time and, most frequently, at temperatures 100–200 K lower than in the case of the furnace annealing. In the 1441 alloy, the formation of an unusual two-component cube {001}〈210〉 texture has been noted.     

Pressure Dependence of the Peierls Stress in Aluminum

The effect of pressure applied normal to the {111} slip plane on the Peierls stress in Al is studied via atomistic simulations. Edge, screw, 30°, and 60° straight dislocations are created using the Volterra displacement fields for isotropic elasticity. For each dislocation character angle, the Peierls stress is calculated based on the change in the internal energy, which is an invariant measure of the dislocation driving force. It is found that the Peierls stress for dislocations under zero pressure is in general agreement with previous results. For screw and 60° dislocations, the Peierls stress versus pressure relationship has maximum values associated with stacking fault widths that are multiples of the Peierls period. For the edge dislocation, the Peierls stress decreases with increasing pressure from tension to compression. Compared with the Mendelev potential, the Peierls stress calculated from the Mishin potential is more sensitive to changes in pressure.     

Characterization of the orientation structure and distribution in rolled polypropylene

The polymer orientation structure in rolled polypropylene sheets was studied by polarized-light microscopy, scanning electron microscopy, and wide-angle x-ray diffraction. The initial spherulites became deformed to a pancake shape as rolling deformation proceeded and became more perfectly aligned as the rolling procedure continued. Most polymer crystallites were oriented with theirb- axis nearly perpendicular to the rolling direction, as shown by x-ray pole figures. Even-order 〈P_n(cosχ)〉_c coefficients of the Legendre polynomial series (n = 2 to 10) describing thec-axis or orientation distribution of the molecular chains were determined from measurements of polar angle scan using the transmission technique. The 〈P _n 〉 _c (n = 2 to 10) curves calculated from the pseudoaffine deformation theory agreed with the experimental data.     

Deformation twinning in TiAl: Effects of defect clustering

Possible roles of point defect clustering in the formation of deformation twins in γ-TiAl are critically assessed by reviewing the available models of dislocation-assisted twin nucleation and experimental data on deformation twinning in Ti-56 at.% Al single crystals and two-phase Ti-47 at.% Al alloys. According to the pole mechanism for twinning in the Ll₀ structure, a reasonable combination of the stress concentration (n≈r27) and the vacancy supersaturation (c/c₀≈13) is needed to overcome the critical stages of twin formation. The so-called radiation-induced ductility reported in Ti-47 at.% Al alloys is attributed to the effective formation of twin embryos in the presence of interstitial-type Frank loops and the subsequent nucleation and growth of twins during plastic deformation.     

Structure and mechanical properties of an Ni<Subscript>3</Subscript>Al single crystal upon high-temperature deformation

Structure and strength properties of single-crystal 〈001〉 samples of Ni₃Al have been studied in the as-grown and homogenized state during tensile tests in the temperature range of 1150–1250°C. At the strain rate of 1.32 mm/min (2 × 10⁻⁵ m/s), the samples are in the state of superplasticity. The basic mechanism of relaxation is dynamic recovery; in some regions of the sample, recrystallized grains are formed. At 1250°C, coarse twins are observed in the zone of fracture, which indicates the “switching on” of additional slip systems necessary to guarantee the relaxation process.     

Laser annealing of flash-evaporated CuInSe2 thin films

In this paper, the impact of laser annealing on the structural, electrical, and optical properties of CuInSe₂ (CIS) thin films has been investigated. The films were deposited using a modified flash evaporation system onto glass substrates. Structural analysis using x-ray diffraction (XRD) showed that the films have a strong preferred growth direction in the 〈112〉 plane. After laser annealing with a diffused beam of 20 ns width, the structure was relaxed and an increase in the intensity of 〈112〉 diffraction line was observed. A gas-microphone-type, high-resolution, near-infared (IR) photoacoustic spectrometer was used for the analysis of nonradiative defect states in as-grown and laser-annealed CIS thin film samples at room temperature. The absorption coefficient has been derived from photoacoustic spectra to establish activation energies for several defect-related energy levels. The calculated intrinsic defect ionization energies were also compared with the existing data available in the literature. The changes in the optical properties of the films have been explained in terms of the variations in the structural characteristics within the material. This paper was presented at the fourth International Surface Engineering Congress and Exposition held August 1–3, 2005 in St. Paul, MN.     

Multiscale modeling of plastic deformation of molybdenum and tungsten: I. Atomistic studies of the core structure and glide of 1/2〈1 1 1〉 screw dislocations at 0 K

Owing to their non-planar cores, 1/2〈1 1 1〉 screw dislocations govern the plastic deformation of body-centered cubic (bcc) metals. Atomistic studies of the glide of these dislocations at 0 K have been performed using Bond Order Potentials for molybdenum and tungsten that account for the mixed metallic and covalent bonding in transition metals. When applying pure shear stress in the slip direction significant twinning–antitwinning asymmetry is displayed for molybdenum but not for tungsten. However, for tensile/compressive loading the Schmid law breaks down in both metals, principally due to the effect of shear stresses perpendicular to the slip direction that alter the dislocation core. Recognition of this phenomenon forms a basis for the development of physically based yield criteria that capture the breakdown of the Schmid law in bcc metals. Moreover, dislocation glide may be preferred on {1 1 0} planes other than the most highly stressed one, which is reminiscent of the anomalous slip observed in many bcc metals.