Nucleation Of Deformation Twins Research Articles

On the nucleation of deformation twins at the early stages of plasticity

Understanding the deformation mechanisms of hexagonal close-packed (HCP) polycrystals at the grain scale is crucial for developing both macro and micro scale predictive models. Slip and twinning are the two main deformation mechanisms of HCP polycrystals at room temperature. In this paper, the development of grain-level stress tensors during nucleation and growth of twins is investigated. A pure zirconium specimen with HCP crystals is deformed in-situ while the centre-of-mass, orientation, elastic strain, and stress of individual grains are measured by three-dimensional synchrotron X-ray diffraction (3D-XRD). The observed microstructure is subsequently imported into a crystal plasticity finite element (CPFE) model to simulate the deformation of the polycrystal. The evolution of stress in twin-parent pairs at the early stages of plasticity, further into plasticity zone, and unload is studied. It is shown that twins do not relax very much at the nucleation step, but the difference between the measured stress in the twin and parent increases further into plastic zone where twins relax. While at the early stages of plasticity all six twin variants are active, a slightly better estimation of active variants is obtained using the measured grain-resolved stress tensors.

On the deformation twinning mechanisms in twinning-induced plasticity steel

Deformation twinning in coarse-grained fcc metals results from the highly coordinated glide of Shockley partial dislocations with the same Burgers vector on successive {111}-type twinning planes. The main issue of the formation mechanism of deformation twinning is how the arrangement of Shockley partials required for twinning evolves. Here, we present the deformation twinning mechanism that operates in Fe-17Mn-0.45C-1.5Al-1Si TWIP steel by means of in-situ TEM deformation tests and post-mortem TEM characterization. The in-situ TEM observation shows the formation of a thin twin by Shockley partial dislocations generated in a grain boundary, which indicates the nucleation of deformation twins at grain boundary defect sites. The observed twinning process is similar to deformation twinning in nano-crystalline materials through successive emission of partial dislocations from grain boundaries. In addition, a high density of Frank partial dislocations is observed inside deformations twins. These defects affect the growth of deformation twins and could contribute to the high work hardening of TWIP steel.

Effect of triple junctions on deformation twinning in a nanostructured Cu-Zn alloy: A statistical study using transmission Kikuchi diffraction.

Scanning electron microscopy transmission Kikuchi diffraction is able to identify twins in nanocrystalline material, regardless of their crystallographic orientation. In this study, it was employed to characterize deformation twins in Cu/10 wt % Zn processed by high-pressure torsion. It was found that in 83% of grains containing twins, at least one twin intersects with a triple junction. This suggests that triple junctions could have promoted the nucleation of deformation twins. It should be cautioned that this technique might be unable to detect extremely small nanoscale twins thinner than its step size.

Nucleation of deformation twins in nanocrystalline fcc alloys

An earlier dislocation model for predicting the grain size effect on deformation twinning in nanocrystalline (nc) face-centred-cubic (fcc) metals has been found valid for pure metals but problematic for alloys. The problem arises from the assumption that the stacking-fault energy (γSF) is twice the coherent twin-boundary energy (γfcc), which is approximately correct for pure fcc metals, but not for alloys. Here we developed a modified dislocation model to explain the deformation twinning nucleation in fcc alloy systems, where γSF ≠ 2γtwin. This model can explain the differences in the formations of deformation twins in pure metals and alloys, which is significant in low stacking-fault energy alloys. We also describe the procedure to calculate the optimum grain size for twinning in alloy systems and present a method to estimate γtwin.

Nucleation of deformation twins on gliding grain-boundary dislocations in nanomaterials

A new micromechanism of nucleating deformation twins in nanocrystalline and ultrafine-grained materials under action of severe mechanical stresses has been proposed and theoretically described. The mechanism is a subsequent splitting of grain-boundary dislocations into lattice partial and sessile grain-boundary dislocations. Ensembles of gliding partial dislocation forms deformation twins. The energy characteristics of this process are calculated. The nucleation of the twins is shown to be energetically profitable and can be athermic (without an energy barrier) under conditions of severe mechanical stress. The dependence of a critical stress at which the barrier-less nucleation of twins took place on the widths of these twins is calculated.

Dislocation mechanics of copper and iron in high rate deformation tests

Different dislocation processes are shown to be operative under high rate loading by impact-induced shock tests as compared with shockless isentropic compression experiments (ICEs). Under shock loading, the plastic deformation rate dependence of the flow stress of copper is attributed to dislocation generation at the propagating shock front, while in shockless ICEs, the rate dependence is attributed to drag-controlled mobile dislocation movement from within the originally resident dislocation density. In contrast with shock loading, shockless isentropic compression can lead to flow stress levels approaching the theoretical yield stress and dislocation velocities approaching the speed of sound. In iron, extensive shock measurements reported for plate impact tests are explained in terms of plasticity-control via the nucleation of deformation twins at the propagating shock front.

Nucleation of deformation twins in nanocrystalline face-centered-cubic metals processed by severe plastic deformation

Nanocrystalline (nc) materials are known to deform via mechanisms not accessible to their coarse-grained counterparts. For example, deformation twins and partial dislocations emitted from grain boundaries have been observed in nc Al and Cu synthesized by severe plastic deformation (SPD). This paper further develops an earlier dislocation-based model on the nucleation of deformation twins in nc face-centered-cubic (fcc) metals. It is found that there exists an optimum grain-size range in which deformation twins nucleate most readily. The critical twinning stress is found determined primarily by the stacking fault energy while the optimum grain size is largely determined by ratio of shear modulus to stacking fault energy. This model formulated herein is applicable to fcc nanomaterials synthesized by SPD techniques and provide a lower bound to the critical twining stress.

Nucleation of deformation twins in zinc bicrystals

Abstract The effects of stress concentration on the nucleation of deformation twins have been investigated in zinc bicrystals grown with controlled orientations. It is found that the apparent resolved shear stress for twinning varies from 25 to 4900 g/mm2 depending upon the orientations of the individual crystal grains of the bicrystals as well as their relative orientations. The maximum stress concentration for the bicrystals twinned at the lowest r.s.s.t. is estimated to be approximately 15 Kg/mm2, a value too low for a homogeneous nucleation model, but high enough to form two coherent twin interfaces. A model of twin nucleation based on the movement of pre-existing twin partials in a twin embryo associated with the strain field of a total slip dislocation to form a thin twin lamella which grows to a stable thickness of approximately 400 A is proposed. It appears that the twin fault energy is the controlling factor of the nucleation of twins in zinc.

Nucleation of Deformation Twins in F.C.C. Metals

Nucleation of Deformation Twins in F.C.C. Metals

Nucleation of deformation twins at slip plane intersections in B.C.C. metals

Abstract The formation and dissociation of dislocation barriers at slip plane intersections in b.c.c. metals are considered, where one or both of the slip planes are of the {112} form. It is found that barriers may be weak or strong, depending on the characteristics of the glide dislocations which contributed to the formation of the barrier, and on the stress state at the barrier. Weak barriers dissociate by a low energy path to produce twin lamellae in the intersecting {112} planes, at stress concentrations of less than 50 times the applied stress. Evidence obtained by transmission electron microscopy on shock-loaded iron-base alloys shows that twin nucleation can occur at slip plane intersections in b.c.c. metals. If dislocation dissociation producing twinning dislocations is prohibited, sufficiently strong barriers could exist to allow local stresses to reach critical values for nucleation of cleavage cracks. It is proposed that twinning and cleavage stresses are of similar magnitudes because both phenomena are heterogeneously nucleated at similar sites.