Orientation dependence of dislocation structure evolution during cold rolling of aluminum

A well-organized dislocation structure forms in many polycrystalline metals during plastic deformation. This structure is described qualitatively with no explanation of the quantitative characterization. In this work, the evolution of dislocation structure in commercial purity aluminum is described by means of the excess dislocation density and by quantitative characterization of the cell structure as seen on a plane surface. The measurements were performed on a pseudo-internal surface of a split specimen deformed by channel die deformation. The results show a clear dependence of cell structure formation on orientation of the crystallite with respect to the imposed deformation gradient with the largest excess dislocation density occurring in grains of {0 1 1}[1 2 2] orientation for plane strain deformation. Neighboring grain and non-local effects are shown to be of importance in the type of dislocation structures that evolve.     

Grain size, grain boundary sliding, and grain boundary interaction effects on nanocrystalline behavior

A dislocation–density grain boundary (GB) interaction scheme, a GB misorientation dependent dislocation–density relation, and a grain boundary sliding (GBS) model are presented to account for the behavior of nanocrystalline aggregates with grain sizes ranging from 25 nm to 200 nm. These schemes are coupled to a dislocation–density multiple slip crystalline plasticity formulation and specialized finite element algorithms to predict the response of nanocrystalline aggregates. These schemes are based on slip system compatibility, local resolved shear stresses, and immobile and mobile dislocation–density evolution. A conservation law for dislocation–densities is used to balance dislocation–density absorption, transmission and emission from the GB. The relation between yield stresses and grain sizes is consistent with the Hall–Petch relation. The results also indicate that GB sliding and grain-size effects affect crack behavior by local dislocation–density and slip evolution at critical GBs. Furthermore, the predictions indicate that GBS increases with decreasing grain sizes, and results in lower normal stresses in critical locations. Hence, GBS may offset strength increases associated with decreases in grain size.     

Synthesis of bulk nanocrystalline nickel by pulsed electrodeposition

Square-wave cathodic current modulation was used to produce nanocrystalline nickel electrodeposits with grain sizes in the range 40–10 nm from saccharin-containing Watts-type baths. The optimum plating conditions to synthesize nanocrystals, namely pulse on- and off-time and peak current density, as well as bath pH and temperature, were identified. At these plating conditions, the grain size of the electrodeposits was found to decrease with increasing saccharin concentration in the bath. The preferred orientation of the deposits progressively changed from a strong (2 0 0) fibre texture for a saccharin-free bath to a (1 1 1) (2 0 0) double fibre texture for a bath containing 10 gl^–1 saccharin. Transmission electron microscopy showed that the electrodeposits consist of uniform structure with narrow grain-size distribution. These deposits, as expected, were found to contain co-deposited sulphur and carbon impurities.     

Dislocation structures in cyclically deformed nickel polycrystals

The effect of the grain orientation and the plastic strain amplitude _pa on the saturated dislocation structure was studied on individual grains of cyclically deformed nickel polycrystals by means of scanning electron microscopy using the electron back scattering pattern technique and the channelling contrast of back scattered electrons. The main features of the dislocation configuration in a grain were found to be essentially determined by the crystallographic axial orientation of the grain. A labyrinth-like dislocation pattern is typical for grains with axial orientations near [001], a patch pattern exists in grains with a loading axis (LA) near [011] and fragmented dislocation walls are dominant in grains with LA near [ 11]. Grains with axial orientations in the central part of the stereographic standard triangle contain a bundle arrangement of dislocation structures. All four types of dislocation structures, but mostly the bundle type, can occur together with the ladder structure of persistent slip bands. Cell patterns were found to be a result of a modification of the bundle and patch configuration at high deformation amplitudes. The mesoscopic dimensions of the dislocation patterns turned out to depend on _pa in the same way for all grain orientations: while the thickness of regions with high dislocation density is reduced with increasing _pa, the width of regions with low dislocation density remains roughly constant.     

Electrodeposition of nanocrystalline nickel by using rotating cylindrical electrodes

In this research, nanocrystalline nickel (14–25 nm) was electrodeposited on rotating cylindrical electrodes in a modified Watts bath. Saccharin was used as a grain refiner. The effect of cathode rotation speed and saccharin concentration on the grain size was studied by transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis. The preferred orientation of deposits progressively changed from a (2 2 0), (2 0 0), and (1 1 1) fiber texture for a saccharin free bath to a (1 1 1) and (2 0 0) double fiber texture for a bath containing 5 g l⁻¹ saccharin. Cathode rotation enhanced the intensity of (1 1 1) peak relative to (1 0 0). The effect of cathode rotation speed, current density, and saccharin concentration on the coating microhardness was investigated. The maximum recorded hardness was 620 HV for 14 nm grain size. The effect of current density and saccharin concentration on morphology was observed by scanning electron microscopy (SEM). The current efficiency changes were studied as a result of saccharin concentration.     

Orientations of nuclei during static recrystallization in a cold-rolled Mg-Zn-Gd alloy

The static recrystallization process of a cold-rolled Mg-Zn-Gd alloy was tracked by a quasi-in-situ electron backscatter diffraction method to investigate the orientations of nuclei.The results show that orientation distribution of nuclei is associated with nucleation mechanism.The continuous static recrystallization nuclei display similar orientations to the parent grains with TD orientation.Differently,discontinuous static recrystallization nuclei formed within the parent grains(TD-45~0 orientation) show random orientations and a variety of misorientation angles but preferred axes <5273> or <5270>.Interestingly,a special oriented nucleation is found.Discontinuous static recrystallization nuclei originated from boundaries of the parent grain(TD-70° orientation) show concentrated TD orientations in another side due to the preferred misorientation relationship 70°<1120>(∑18 b).It is speculated that these two special misorientation relationships are related to the dislocation type.     

Cryogenic deformation microstructures of 32Mn–7Cr–1Mo–0.3N austenitic steels

The cryogenic deformation microstructures of impact and tensile specimens of 32Mn–7Cr–1Mo–0.3N austenitic steel were investigated using light microscopy and transmission electron microscopy. The results show that the deformation microstructures of the impact specimens are mainly composed of stacking faults, network dislocation, slip bands, and a few mechanical twins and -martensite. These microstructures cross with each other in a crystal angle. The deformation microstructures of the tensile specimens consist only of massive slip bands, in which a few mechanical twins and -martenite are located. Because of the larger plastic deformation the slip band traces become bent. All the deformation microstructures are formed on the {111} planes and along the <110> orientation.     

Texture evolution analysis of warm-rolled Fe–28Mn–6Si–5Cr shape memory alloy

Since texture control tends to be a promising way to improve the shape memory effect (SME) of polycrystalline Fe–Mn–Si shape memory alloys, rolling texture evolution of an Fe–28Mn–6Si–5Cr shape memory alloy was systematically investigated with orientation distribution functions (ODFs) and electron backscattering diffraction (EBSD) analysis. At the rolling temperature of 873 K, Copper-type texture components, including D, S, Goss, as well as a weak Brass, obviously develop before 44% rolling reduction. With increased rolling reduction to 57%, D orientation abruptly disappears, which indicates a texture transition has occurred. S orientation and α fiber texture except the Goss orientation undergo a decrease accompanying the intensification of γ fiber texture. In the whole deformation processes, Goss orientation is the dominant texture component while no pronounced Brass component is observed. The dominant Goss component can be attributed to the preferred Goss orientation both in shear bands and in matrix. When the rolling temperature is decreased to 573 K, even at the early deformation stage, 42% rolling reduction, both D and Brass orientations are not observed. EBSD analysis confirms that the texture evolution is promoted to the early deformation stages at lower rolling temperature.     

Studying grain boundary regions in polycrystalline materials using spherical nano-indentation and orientation imaging microscopy

In this article, we report on the application of our spherical nanoindentation data analysis protocols to study the mechanical response of grain boundary regions in as-cast and 30% deformed polycrystalline Fe–3%Si steel. In particular, we demonstrate that it is possible to investigate the role of grain boundaries in the mechanical deformation of polycrystalline samples by systematically studying the changes in the indentation stress–strain curves as a function of the distance from the grain boundary. Such datasets, when combined with the local crystal lattice orientation information obtained using orientation imaging microscopy, open new avenues for characterizing the mechanical behavior of grain boundaries based on their misorientation angle, dislocation density content near the boundary, and their propensity for dislocation source/sink behavior.     

The effect of orientation on the shock response of a carbon fibre–epoxy composite

The effect of fibre orientation on the shock response of a two-dimensional carbon fibre–epoxy composite has been studied using the technique of plate impact. In the through-thickness orientation, it appears that the material behaves as though it is a simple polymer. When one of the fibre directions is orientated parallel to the loading axis, very different behaviour is observed. The stress pulse has a pronounced ramp, before at sufficiently high stresses, a much faster rising shock occurs above it. Examination of the wave velocities suggests that the start of the ramp travels at a near constant velocity of ca. 7.0 mm μs⁻¹, whilst the shock velocity in this orientation converges with that of the shock velocity of the through-thickness orientation. Therefore, we believe that the stress pulse is separated into a fast component that travels down the fibres, with the rest travelling at the shock velocity in the matrix between the 0° fibres (epoxy plus fibres normal to the loading axis). Finally, from the Hugoniot, we observed that at low shock intensities, the 0° orientation was significantly stiffer than the through-thickness orientation. As the severity of the shock increased, the Hugoniots of the two orientations converged. Therefore, it would appear that orientation only effects the shock equation of state at lower shock stresses.     

Effects of thermo-mechanical processing parameters on the special boundary configurations of commercially pure nickel

The roles of plastic strain, annealing temperature, and annealing time in affecting the fraction of special boundaries (Fsp), and twin densities of thermo-mechanically processed commercially pure nickel are examined. Different strain levels were achieved by cold rolling the material at different amounts. One-step low strain-recovery processing with strain levels in the range of 3.0–7.5% and annealing temperatures in the range 800–1000 °C were conducted in order to ensure that recrystallization did not occur. From orientation image microscopy analysis it was found that the fraction of special boundaries increased from about 30% for the as-received material to almost 80% for plastically deformed and annealed material. This showed that material strained in the range from 3.0% to 7.5% and annealed at 800 °C for different times all reached Fsp values in the range 75–80%, a considerable increase over the as-received material. Various multi-processing cycle treatments did not increase the fraction of special boundaries to above the value of 80% achieved in single processing cycles.TEM observations indicated dislocation tangles/cells occurred near grain boundaries in material strained at 6%. The density of these dislocation tangles decreased with annealing time at 800 °C and was reduced considerably after 20 min.Experimental results for the variation of twin density with grain size showed that the twin density decreased with increase in grain size. However, there was a tendency for the twin density to decrease more slowly with increase in grain size in the more highly strained samples. The experimental data for twin density were compared with values calculated from the equation suggested by Pande et al. [1]. With appropriate choice of the two variable parameters in the equation, a good “average” fit for all the data was obtained. However, the effect of plastic strain on the twin density could not be accommodated in the model.The experimental twin density–grain size relationship was also compared to values calculated from the formulation developed by Gleiter [2] modified to accommodate the effects of prior plastic train on the twin densities of annealed samples. Calculations from the modified model followed the observed trend that twin densities for the more highly deformed samples decreased more slowly with grain size than those for lesser strained samples. Good quantitative agreement between the experimental values of twin density and those calculated from the modified Gleiter formulation was achieved.     

High-temperature deformation behaviour of YBa2Cu3O7−x

High-temperature compression tests were performed in air for YBa₂Cu₃O_7–x polycrystals with grain sizes of 3 and 7 m at various strain rates between 1.3×10^–5 and 4×10^–4s^–1 and at temperatures between 1136 and 1253 K. Steady state deformation appeared above 1203 K for both samples. A stress exponent of 1.3 and an activation energy of 150 kJ mol^–1 were evaluated. The compression tests and microstructural observations revealed that there was a difference in deformation mechanism above and below 1203 K. The dominant mechanism was diffusional creep associated with grain-boundary sliding above 1203 K, and dislocation glide accompanied with grain-boundary sliding below 1203 K. The growth of anisotropic grains and their preferred arrangement were enhanced by deformation.     

True Stress vs Temperature Diagram of Structural States for Polycrystalline bcc Metals

For the first time, a new type of a diagram of structural states is constructed for polycrystalline bcc metals. This true-stress-vs-temperature diagram is a regular collection of temperature dependences of critical stresses corresponding to successive changes in the type of dislocation structure from the onset of plastic deformation to fracture within the temperature range from completely brittle fracture to the formation of a new grain structure. On the basis of analysis of true-stress-vs-temperature diagrams for metals and alloys (Mo, Fe, and Fe–3%Si) with the help of the thermal activation analysis of the temperature dependences of critical stresses, we establish the regularities and distinctive features of the formation and behavior of the temperature–force boundaries and domains of existence of certain structural states and identify the mechanisms of plastic deformation within the limits of each of these domains.     

Hot compressive deformation behavior and microstructure evolution of Ti–6Al–2Zr–1Mo–1V alloy at 1073 K

Hot compressive behaviors of Ti–6Al–2Zr–1Mo–1V alloy at 1073 K, as well as the evolution of microstructure during deformation process, were investigated in this paper. The results shows that flow stress increases up to a peak stress, then decease with increasing strain, and forms a stable stage at last. The grain size also shows an decrease at first and increase after a minimum value. Dislocations are observed to produce at the interface of α/β phase, and the phase interface and dislocation circle play an important role in impeding the movement of dislocation. As strain increase, micro-deformation bands with high-density dislocation are founded, and dynamic recrystallization occurs.     

不同因素下镍铜双层膜表面摩擦行为的研究

材料在摩擦接触过程中的弹塑性变形对基体的力学性能具有重要影响。为研究镍铜双层膜在接触过程中的变形行为和力学特性,本文从原子轨迹、原子晶格结构变化、接触力和基体内部位错等方面,详细研究了表面纹理密度、纹理方向、晶体学方向、磨粒半径和接触深度等因素对摩擦接触过程的影响。结果表明：纳观纹理表面以及镀层的引入对接触力产生影响。镍膜晶体学方向对滑动接触过程影响显著,存在接触力最小的晶体学方向;凸体的分布角度对摩擦过程的影响较小;在界面作用下,特定纹理密度表现出一定的减摩作用;基体材料的接触力随着磨粒半径和接触深度的增大而增大;在不同因素及水平下,基体表现出不同的位错缺陷程度和原子堆积现象。     

Reduction of dislocation density in GaN films on sapphire using AIN interlayers

GaN (00.1) thin films, of thickness 1.25 to 2.25 m grown on sapphire substrate (11.0) by metallo organic chemical vapor phase deposition (MOCVD) with different number of AlN interlayers, were characterized by triple crystal diffractometry and synchrotron white beam x-ray topography (SWBXT). The full width at half maximum (FWHM) of x-ray rocking curves from symmetric and asymmetric reflections was used to estimate the dislocation density in GaN films. It has been found that the edge dislocation density decreased from 1.63 × 10¹⁰ cm^–2 to 1.23 × 10¹⁰ cm^–2 and the screw dislocation density decreased from 2.0 × 10⁸ cm^–2 to 1.1 × 10⁸ cm^–2 when one AlN interlayer was inserted between the high temperature GaN layer. The dislocation density decreased further with the increase in number of interlayers. On the other hand the compressive stress in the GaN film increased from –0.29 GPa to –0.86 GPa. The compressive stress further increased as the number of interlayers increased but no cracking in the GaN film was observed. This could be due to better adhesion between the film and substrate due to interlayers. SWBXT in transmission from a GaN(00.1)/Al₂O₃(11.0) sample confirms the orientation of GaN and indicates that it is a single crystal with high dislocation density. SWBXT from the Al₂O₃ substrate shows cellular structure of dislocations.     

压下率对轧制单层晶极薄带晶界滑移特性的影响

采用退火态单层晶轧制铜箔为原料,进行不同压下率的箔轧.以多晶体位错滑移及塑性流动机制为基础,建立了考虑潜在硬化和晶格转动效应的率相关晶体滑移本构模型,分析了压下率对轧制单层晶极薄带晶界附近区域变形分布特性、取向演化和滑移系激活规律的影响,并探讨其机理.确定了合理的材料本构参数,铜箔拉伸实验与晶体塑性有限元模拟得到的应力-应变曲线一致.所建立的晶体塑性有限元模型,可很好的模拟最大压下率达到80%时轧制单层晶铜箔的变形过程.结果表明:1)由于晶粒形状、晶界及晶粒取向的作用导致晶内-晶间变形分布非均匀性;2)由于晶粒间复杂的相互作用导致晶粒取向主要绕横向(TD)进行旋转,且旋转角度和取向分散度随压下率的增加而增大;3)在晶内-晶间不同区域的滑移系启动存在显著差异,启动滑移系随压下率的增加而增多,当压下率小于等于60%时,在晶粒表层和晶界处,滑移系成对发生启动,当压下率达到80%时,表层和晶界处为多滑移系启动情形;4)滑移最先从晶粒表层和晶界处开始,然后向晶粒内部延伸.     

Continuous dynamic recrystallization during severe plastic deformation

Severe plastic deformation (strains > 100%) has been shown to create significant grain refinement in polycrystalline materials, leading to a nanometric equiaxed crystalline structure for such metals as aluminum, copper and nickel alloys. This process, termed continuous dynamic recrystallization, is governed by evolution of the dislocation structure, which creates new grain boundaries from dislocation walls. In the proposed model, plasticity occurs which firstly involves dislocation multiplication, leading to strain hardening limited by dynamic recovery. After a critical dislocation density is reached new grain boundaries are formed by condensation of walls of dislocations, creating a new stable configuration that is favored due to a reduction of the system free energy. This evolution of the microstructure continues to develop, with a consequent progressive decrease in the average grain diameter. The proposed model provides a quantitative prediction of the evolution of the average grain size, as well as the dislocation density, during continued plastic strain. The model can be calibrated by use of results from any experiment that involves large plastic deformation of metals, subject to negligible annealing effects. In this paper, the model has been calibrated, and consequently validated, through experiments on machining of Al 6061-T6.     

Grain boundary statistics in nano-structured iron produced by high pressure torsion

The microstructure and the spectrum of grain boundary misorientations were studied in Armco iron, following high pressure torsion (HPT) deformation, by means of transmission electron microscopy (TEM) and orientation imaging microscopy (OIM). It was found that HPT deformation results in the formation of an equiaxed grain structure with a mean grain size of 270 and 130 nm using a shear strain of γ = 210 and 420, respectively. The misorientation spectra in HPT iron have a bimodal character with maxima in low (at 1–2°) as well as in high misorientation angle ranges. A marked increase in the fraction of special boundaries (Σ3–Σ45) was revealed as a result of HPT. The microstructural changes due to HPT are discussed and compared with those obtained during conventional deformation modes.     

Direct observation of grain-boundary dislocations in the FeCo alloy

A unified dislocation theory of grain boundaries proposed earlier for simple cubic crystals has been extended to include the case of ordered and disordered body-centred-cubic structures. Pure symmetric and asymmetric tilt and pure twist boundaries have been treated in detail and extended to an arbitrary grain boundary. It is shown that a unique set of coincidence site lattices exists for both the symmetric and asymmetric boundaries and that each of these sets, in turn, depends upon the rotation axis which characterizes the boundary. Furthermore, a unique set of grain-boundary dislocations is associated with each of these coincidence site lattices. The results are then applied to the transmission electron microscopy studies carried out in Part 2 of the present study.On leave from The Materials Research Group, Department of Metallurgy, Indian Institute of Science Bangalore — 560012, India.