Microband evolution during large plastic strains of stable {1 1 0}〈1 1 2〉 Al and Al–Mn crystals

The deformation microstructures of Al and Al–Mn {1 1 0}〈1 1 2〉 single crystals have been characterized after room temperature channel-die compression up to true strains of 2.1. The evolution of local misorientations and microband structures were quantified by high-resolution electron backscatter diffraction in a field emission gun scanning electron microscope and their alignments compared with the traces of active slip planes and macroscopic shear stress planes. During plane-strain compression these “Brass” oriented crystals remain stable in terms of the final, average, orientation, with a small orientation spread. However, the microband alignment varies with strain and also with solute content. There is a general tendency for the microbands to be both crystallographic and non-crystallographic at low strains, then crystallographic, and finally mixed again at high strains (with some lamellar banding).     

Evolution of deformation microstructures of cold-rolled Ta–2.5W alloy with coarse grains at low to medium strains

Ta–2.5W alloy with coarse grains was cold-rolled to reductions ranging from 5 to 40%. The evolution of the microstructure was investigated by optical microstructure, electron backscatter diffraction (EBSD). A few microbands appear when the reduction reaches 20%. The density of microbands increases with increasing reduction. When the reduction reaches 40%, grains are composed of one or two groups of microbands except the {001}<110 > orientations. Most of the inclination angle between microbands and RD in this condition is 20–35°. As the strain increases, the inclination angle between microbands and RD gets smaller. The habit plane of microbands can be {110} plane. The microbands and matrix usually share a common < 110 > or < 111 >. The mature body-centered cubic rolling texture, including α and γ fibers, is not developed until the reduction reaches 40%. Meanwhile, shear bands appear. New grains can be seen in shear bands and a model is proposed to explain this process.     

Microbands and crystal orientation metastability in cold rolled interstitial-free steel

At low cold rolling reductions microbands form in grains which have either 〈1 1 0〉 or 〈1 1 1〉 parallel to the transverse direction. They appear to form by crystallographic slip and existing theory is sufficient to explain their morphology. Detailed analysis of the mechanics of crystal rotation shows that the orientations belonging to the sets 〈1 1 0〉//TD and 〈1 1 1〉//TD can rotate between two unstable orientations before a stable orientation is reached. This crystallographic metastability is shown to be responsible for the formation of either one or two sets of microbands, and theory and measurement agree that they should form at 20–40° to the rolling direction when measured in the longitudinal section.     

Evolution of microstructure and microtexture during the hot deformation of Mg–3% Al

The orientation relationships associated with different mechanisms of new grain formation during the hot deformation of a Mg–3.4% Al–0.33% Mn alloy were investigated using electron back-scattered diffraction (EBSD) techniques. Compression tests were carried out at 350 °C with a strain rate of 0.001 s^?1 on samples machined from extruded tubes. Three types of microstructural features were produced at this temperature: (i) microbands (MBs); (ii) bulged regions; (iii) new grains formed by continuous dynamic recrystallization (cDRX). The formation of the MBs is attributed to the collection of basal dislocations in the MB boundaries. Both the bulges as well as the new cDRX grains are formed as a result of dislocation-based processes that produce c-axis rotations toward the loading axis (i.e. away from the radial direction–transverse direction (RD–TD) plane). Once nuclei have formed, however, the new grains have their c-axes located fairly close to the RD–TD plane. In this way dynamic recrystallization leads to the retention of the main characteristics of the initial RD–TD texture.     

The evolution of dislocation microstructure in electron beam melted Ta-2.5W alloy during cold rolling

The evolution of dislocation microstructure in electron beam melted Ta-2.5W alloy was investigated by transmission electron microscope (TEM). Long straight dislocations and dislocation loops are formed in Ta-2.5W alloy cold-rolled by 5%. A set of long, continuous extending planar boundaries (EPBs) are formed when the reduction reaches 20%. In the early stage of development, EPBs are fragmented, diffused and curved, which are connected by non-crystallographic cells boundaries to maintain their continuity. The straight segments of EPBs are usually parallel with the trace of {110}, and incline at about 25–35° to the rolling direction (RD). Two groups of EPBs are formed in a grain when the reduction is larger than 30%. The dislocations within EPBs tend to rearrange themselves with increasing strain in a sequence, from tangled dislocations, followed by parallel long straight screw dislocations and finally into dislocation nets, which are composed by 1/2 < 111 > and [100] type dislocations. The relaxation process of dislocations and the interaction of dislocations with EPBs make EPBs appear wavy and deviate from the trace of slip planes.     

The application of equal-channel angular pressing to an aluminum single crystal

An investigation was conducted to examine the nature of the deformed microstructure when an aluminum single crystal of known orientation is subjected to equal-channel angular pressing (ECAP). The experiment was performed using a single crystal that was initially oriented within the entrance channel of the die so that the (1 1 1) slip plane was parallel to the theoretical shear plane and the [1 1 0] slip direction lay parallel to the direction of shear. The crystal was subjected to a single pass at room temperature and then examined using various microscopic techniques including orientation imaging microscopy and transmission electron microscopy. It is shown that the detailed experimental observations are fully consistent with the expectations from crystallographic considerations except only in the vicinity of the lower die wall where frictional effects are present.     

Mechanisms for initial grain refinement in OFHC copper during equal channel angular pressing

The stages of microstructural evolution during the first pass of equal channel angular pressing in polycrystalline oxygen-free, high conductivity (OFHC) copper are identified using transmission electron microscopy (TEM). Microstructural features are generated in the following order: randomly distributed dislocations, dislocation cell structures, elongated laminar substructures (ELSs), and if a transition in activated slip systems takes place, secondary ELSs and/or microbands. TEM analysis suggests that primary and secondary ELSs form along certain {1 1 1} slip planes via a self-organized gliding of dislocations. Prior to reaching the main shear plane (MSP), many ELS boundaries are nearly perpendicular to the MSP. After crossing it, they are most often nearly parallel to it (±15°). The initial grain orientation determines if such a transition in slip pattern occurs. Mechanisms for initial grain refinement are proposed and the final dimension of refined grains is found to be directly associated with some initial substructure characteristics prior to reaching the MSP.     

Deformation-induced grain coalescence in an electrodeposited pure iron sheet studied by in situ neutron diffraction and electron backscatter diffraction

The plastic deformation behavior of an ultrafine-grained electrodeposited pure iron sheet with a strong {1 1 1}〈h k l〉 texture was studied by in situ neutron diffraction during tensile deformation at room temperature and by electron backscatter diffraction (EBSD). The combination of volume-averaged crystallographic orientation changes determined by neutron diffraction and the local orientation relationship determined by EBSD reveals a texture change to {1 1 1}〈1 1 0〉 and corresponding microstructural changes with tension deformation. Related to such grain rotation, grain coalescence on deformation was found using semi in situ EBSD. The results obtained are explained using a characteristic slip model, which also gives a reason for the ultrahigh Lankford value of this material.     

Three-dimensional investigation of particle-stimulated nucleation in a nickel alloy

By combining a focused ion beam (FIB) microscope with a field emission gun scanning electron microscope, it is possible to sequentially mill away ∼50 nm sections of a material by FIB and characterize, at high resolution, the crystallographic features of each new surface by electron backscatter diffraction (EBSD). The successive EBSD maps are subsequently combined to generate three-dimensional crystallographic maps of the microstructure. FIB-EBSD tomography was used to investigate the deformation and recrystallization behaviour of a nickel alloy containing coarse silica particles. The technique demonstrated unambiguously the influence of particle diameter on the misorientations generated within particle deformation zones and showed that particle-stimulated nucleation (PSN) of recrystallization occurred at particles greater than ∼1 μm. PSN also often generated groups of contiguous grains separated by both coherent and incoherent twin boundaries. It was found that much of the behaviour observed using FIB-EBSD tomography is not clearly evident in two-dimensional EBSD micrographs.     

Examination of the energy phase space of mixed copper grain boundaries by EBSD and the sphere-on-a-plate method

This article reports on an experimental study of the energy phase space of mixed copper grain boundaries by a combination of electron backscatter diffraction (EBSD) and the so-called sphere-on-a-plate method. Single crystal copper spheres with diameters of a few microns were sintered onto flat single crystal {1 1 1} copper plates, resulting in random initial grain boundary configurations. EBSD measurements together with an assumption about the grain boundary plane orientation were used for the determination of the five macroscopic degrees of freedom of the grain boundaries. The tilt and twist components of the grain boundaries were calculated making use of the interface plane scheme representation of grain boundaries. Upon annealing, the spheres rotated along gradients in the grain boundary energy phase space. Thus, points of the trajectories of single spheres could be recorded between the single annealing steps, allowing for tracing the path of single spheres towards and into energy minima regions. The results gathered from 13 spheres underline a strong complexity of the grain boundary energy phase space.     

Twinning of CaMgSi phase in a cast Mg–1.0Ca–0.5Si–0.3Zr alloy

Two types of twins and an orientation domain (orientation twin) were found in CaMgSi particles formed in a Mg–Ca–Si alloy by transmission electron microscopy. The two types of twins were shown to be a (1 0 2) twin with the (1 0 2) plane serving as the twinning plane and a (0 1 1) twin with the (0 1 1) plane serving as the twinning plane. In addition to the frequently observed two-fold twin, two different three-fold twins were also observed in the (1 0 2) twinned CaMgSi particles; their crystallographic features are discussed in detail. The boundaries of both the (1 0 2) and (0 1 1) twins were found to be coincident with the twinning plane. The orientation domains were formed by one segment rotating 60° about the [1 0 0] axis relative to the other. The possible formation mechanisms of the twins and orientation domain are discussed based on a crystallographic consideration.     

Relationships between rolled grain shape,deformation bands,microstructures and recrystallization textures in Al–5%Mg

In this study, the textures and microstructures of Al–5%Mg after cold rolling 95% and recrystallization at 300 and 450 °C have been investigated. The cold rolling textures were typical but the recrystallization textures at 300 °C comprised Cube and S and at 450 °C, Cube, ND Rotated Cube plus a weak Q component. The deformed grains were wavy in the longitudinal section as a result of composite-like behaviour, and the narrowed regions had scattered orientations which weakened the final annealing texture. Another feature was grains dividing into their respective crystallographic complements. S grains frequently behaved in this way and nucleation of S-oriented recrystallized material at low temperature was common. Cube-oriented volumes were often found between complementary S volumes, and this favoured micro-growth selection and a detailed discussion of the Cube origins are given. Remarkably, recrystallization texture is largely determined by the behaviour of S- and Cube-oriented material.     

Crystallographic fatigue crack initiation in nickel-based superalloy René 88DT at elevated temperature

The fatigue behavior of a polycrystalline nickel-based superalloy René 88DT was examined in the lifetime regime of 10⁵–10⁹ cycles at 593 °C in air using an ultrasonic fatigue apparatus operating at frequencies close to 20 kHz. Three experimental techniques were combined to obtain new insights into the crack initiation process: serial sectioning, electron backscatter diffraction and quantitative fractographic analysis. Most fatigue failures initiated from internal microstructural sites comprised of large grains. Large crystallographic facets formed at crack initiation sites due to cyclic strain localization on {1 1 1} slip planes in the region close to Σ3 twin boundaries in large grains having high Schmid factors. The micromechanical mechanism of crystallographic fatigue crack initiation was analyzed in terms of both resolved shear stress and elastic incompatibility stresses in regions close to Σ3 twin boundaries. The influence of critical microstructure features on fatigue crack initiation and fatigue life variability is discussed.     

Orientations of recrystallization nuclei developed in columnar-grained Ni at triple junctions and a high-angle grain boundary

A directionally solidified, high-purity nickel sample with a strong 〈0 0 1〉 fiber texture was cold rolled to 40% reduction. Electron backscattered diffraction (EBSD) was used to identify triple junctions (grain edges) before annealing the sample at 430 °C, after which further EBSD was performed to identify 17 recrystallization nuclei at the initially characterized triple junctions on the rolling plane. In addition, a longitudinal section was cut on which a further seven recrystallization nuclei were found and characterized. This characterization of all nuclei revealed that the majority of nuclei could be broadly associated with a 40° 〈1 1 1〉 misorientation to the matrix in which they formed, while a much lower percentage were found to have orientations as those of the local deformation substructure. However, a substantial fraction (around one-third) could not be associated with either of these types. Mechanisms for the formation of nuclei with new orientations are discussed, as is evidence of reorientation of material ahead of the recrystallization front.     

Nucleation and variant selection of secondary α plates in a β Ti alloy

The microtexture of secondary α plates in Ti–4.5Fe–6.8Mo–1.5Al has been investigated by electron backscatter diffraction (EBSD) to obtain more insight in the nucleation and variant selection of these α plates. A statistical analysis of the EBSD data shows that for most β grain boundaries the variant selection of the α plates is in agreement with a commonly used variant selection criterion yielding that the α-{0 0 0 1} pole is nearly parallel to the closest β-{1 1 0} poles of the two adjacent β grains. For a small angle between the β-{1 1 0} poles nucleation is predominantly observed at both sides of the grain boundary, while with increasing angle some β grain boundaries exhibit nucleation of α plates at only one side. In the β grain interior many so-called Type 2 α–α grain boundaries are observed which are thought to originate from autocatalytic nucleation when a new α plate is formed at an existing α–β interface.     

Microstructure evolution during dewetting in thin Au films

Thin metal films can degrade into particles in a process known as dewetting. Dewetting proceeds in several stages, including void initiation, void growth and void coalescence. Branched void growth in thin Au films was studied by means of electron backscatter diffraction (EBSD). The holes were found to protrude into the film predominantly at high angle grain boundaries and the branched shape of the holes can be explained by surface energy minimization of the grains at the void boundaries. (1 1 1) Texture sharpening during dewetting was observed and quantified by EBSD and in situ X-ray studies.     

Orientation dependence of nanoindentation pile-up patterns and of nanoindentation microtextures in copper single crystals

We present a study about the dependence of nanoindentation pile-up patterns and of microtextures on the crystallographic orientation using high purity copper single crystals. Experiments were conducted on a Hysitron nanoindentation setup using a conical indenter in order to avoid symmetries others than those of the crystal structure. Orientation measurements were conducted using a high resolution electron back-scatter diffraction technique for the automated acquisition of texture mappings around the indents. Simulations were carried out by means of a 3D elastic–viscoplastic crystal plasticity finite element method which takes full account of crystallographic slip and orientation changes during indentation. The experiments as well as the simulations show that the pile-up patterns on the surfaces of (0 0 1)-, (0 1 1)- and (1 1 1)-oriented single crystals have four-, two-, and sixfold symmetry, respectively. The different pile-up patterns can be explained in terms of the strong crystallographic anisotropy of the out-of-plane displacements around the indents. Pronounced accumulation of material entailing characteristic pile-up patterns occurs along the intersection vectors between the primary crystallographic slip planes and the indented surface planes.     

Dynamic characteristics of nanoindentation using atomistic simulation

Atomistic simulations are used to investigate how the nanoindentation mechanism influences dislocation nucleation under molecular dynamic behavior on the aluminum (0 0 1) surface. The characteristics of molecular dynamics in terms of various nucleation criteria are explored, including various molecular models, a multi-step load/unload cycle, deformation mechanism of atoms, tilt angle of the indenter, and slip vectors. Simulation results show that both the plastic energy and the adhesive force increase with increasing nanoindentation depths. The maximum forces for all indentation depths decrease with increasing multi-step load/unload cycle time. Dislocation nucleation, gliding, and interaction occur along Shockley partials on (1 1 1) slip planes. The indentation force applied along the normal direction, a tilt angle of 0°, is smaller than the force component that acts on the surface atoms. The corresponding slip vector of the atoms in the (1 1 1) plane has low-energy sessile stair-rod dislocations in the pyramid of intrinsic stacking faults.     

Retention of the Goss orientation between microbands during cold rolling of an Fe3%Si single crystal

An FeSi single crystal with an initial {1 1 0}〈0 0 1〉 orientation, also referred to as Goss orientation, was cold rolled up to a thickness reduction of 89%. Most of the crystal volume rotated into the two symmetrical {1 1 1}〈1 1 2〉 orientations. However, a weak Goss component remained in the highly strained material, even though the Goss orientation is mechanically unstable under plane strain loading. Two types of Goss-oriented regions were discernable in the material subjected to 89% reduction. It appeared that these two types of Goss regions have different origins. Goss grains that were found aligned in shear bands form during straining. A second type of Goss region was found between microbands where the initial Goss orientation was retained.     

Five-parameter grain boundary distribution of commercially grain boundary engineered nickel and copper

The five-parameter grain boundary distributions of grain boundary engineered nickel and copper specimens have been analyzed in detail. The relative areas of {1 1 1} planes in the entire population did not increase as a result of grain boundary engineering (GBE) and, in the Σ3-excluded population, decreased after GBE. This decrease occurred because the majority of the newly generated Σ3 grain boundaries were not coherent twins with {1 1 1} grain boundary plane orientations. GBE increased the proportion of Σ3 boundary length that was vicinal-to-{1 1 1} and the proportion of asymmetrical 〈1 1 0〉 tilt boundaries. There was a clear propensity for selection of particular planes or plane combinations which were associated with low energy. These plane types were analyzed in some detail, and it was shown that many of these boundaries were asymmetrical tilts comprising (or vicinal to) at least one low-index plane.