Low Stacking Fault Energy Metals Research Articles

Void and helium bubble interactions with dislocations in an FCC stainless steel alloy: anomalous hardening and cavity cross-slip locking

The critical stress for cutting of a void and He bubble (generically referred to as a cavity) by edge and screw dislocations has been determined for FCC Fe0.70Cr0.20Ni0.10—close to 300-series stainless steel—over a range of cavity spacings, diameters, pressures, and glide plane positions. The results exhibit anomalous trends with spacing, diameter, and pressure when compared with classical theories for obstacle hardening. These anomalies are attributed to elastic anisotropy and the wide extended dislocation core in low stacking fault energy metals, indicating that caution must be exercised when using perfect dislocations in isotropic solids to study void and bubble hardening. In many simulations with screw dislocations, cross-slip was observed at the void/bubble surface, leading to an additional contribution to strengthening. We refer to this phenomenon as cavity cross-slip locking, and argue that it may be an important contributor to void and bubble hardening.

Effect of prefatigue deformation on the uniaxial tensile properties of a dilute Cu-Al alloy

To examine the influence of prefatigue on the uniaxial tensile properties of a Cu-3at%Al alloy with a high stacking fault energy (SFE), the prefatigued tests are carried out at different total strain amplitudes ranging from 1.0 × 10−3 to 5.0 × 10−3 up to the same accumulated plastic strain of 31, and uniaxially tensile tests are subsequently conducted on these prefatigued alloy specimens. With increasing strain amplitude, the induced fatigue dislocation structures change from elongated veins and planar stacking faults to dislocation walls and cells. For the Cu-3at%Al alloy specimen prefatigued at an intermediate Δεt/2 of 2.3 × 10−3, the induced microstructure consists of loose cells, walls and few ill-developed persistent slip bands, which allows for an increase in dislocation storage rate but nearly does not affect dislocation annihilation rate during tensile deformation, thus exhibiting a higher strain hardening capability; therefore, the tensile strength can be improved nearly without loss of uniform elongation compared to the unfatigued specimen. In a word, an appropriate low-cycle prefatigue treatment can cause, to some extent, a strengthening effect on the static mechanical properties of high SFE metals, but such a strengthening effect is not as significant as the case for low SFE metals.

Early Stages of Plastic Deformation in Low and High SFE Pure Metals

Severe plastic deformation (SPD) techniques are known to promote exceptional mechanical properties due to their ability to induce significant grain and cell size refinement. Cell and grain refinement are driven by continuous newly introduced dislocations and their evolution can be followed at the earliest stages of plastic deformation. Pure metals are the most appropriate to study the early deformation processes as they can only strengthen by dislocation rearrangement and cell-to-grain evolution. However, pure metals harden also depend on texture evolution and on the metal stacking fault energy (SFE). Low SFE metals (i.e., copper) strengthen by plastic deformation not only by dislocation rearrangements but also by twinning formation within the grains. While, high SFE metals, (i.e., aluminium) strengthen predominantly by dislocation accumulation and rearrangement with plastic strain. Thence, in the present study, the early stages of plastic deformation were characterized by transmission electron microscopy on pure low SFE Oxygen-Free High Conductivity (OFHC) 99.99% pure Cu and on a high SFE 6N-Al. To induce an almost continuous rise from very-low to low plastic deformation, the two pure metals were subjected to high-pressure torsion (HPT). The resulting strengthening mechanisms were modelled by microstructure quantitative analyses carried out on TEM and then validated through nanoindentation measurements.

Atomistic simulations for the effects of stacking fault energy on defect formations by displacement cascades in FCC metals under Poisson’s deformation

We performed molecular dynamics simulations of displacement cascades in FCC metals under Poisson’s deformation using interatomic potentials differing in stacking fault energy (SFE), in order to investigate the effect of tensile strain on the SFE dependence of defect formation processes. There was no clear SFE dependence of the number of residual defects and the size distribution of defect clusters under both no strain and the applied strain, while the strain enhanced the defect formation to a certain extent. We also observed that the strain affected the formations of self-interstitial atom (SIA) clusters depending on their size and the Burgers vector. These results were consistent with the analysis based on the defect formation energies. Meanwhile, the number of SIA perfect loops was higher at lower SFE under both no strain and the applied strain, leading to an increase in the ratio of glissile SIA clusters with a decrease in SFE. Further, the absolute number of SIA perfect loops was increased by the applied strain, while the SFE dependence of the number of SIA perfect loops was not affected. These findings were associated with the difference in formation energy between an SIA perfect loop and an SIA Frank loop. The insights extracted from this study significantly contribute to the modeling of microstructural evolution in nuclear materials under irradiation, especially for low SFE metals such as austenitic stainless steels.

Analysis of shear deformation by slip and twinning in low and high/medium stacking fault energy fcc metals using the [formula omitted]-model

Experimental tests involving shear stresses allow material to be deformed to very high plastic strain by overcoming localization phenomena. The simple shear texture development, which is also common near the surface of rolled parts, is important to study since it is directly connected to the metal anisotropy. Crystal plasticity models are used to simulate large deformation plasticity and texture evolution. The main insufficiency of most existing models is that they are, in certain cases, unable to predict all type of experimentally observed textures as well as texture transitions. In this paper, we show that the polycrystalline ϕ-model can be used to compute simple shear crystallographic texture transition for face-centered cubic metals (fcc) at large strains. This model takes into account the grains interaction effects but without the Eshelby inclusion theory. Predicted results are compared to experimental shear textures for medium stacking fault energy (SFE) metals (i.e. copper) and low SFE metals (i.e. silver). We show that the ϕ-model is able to predict a clear shear texture transition characterizing a range of fcc metals having high/medium to low SFE. The twinning mechanism is included in the ϕ-model in order to improve the predicted shear textures for low SFE metals. The effect of twinning on the ideal shear texture components is shown and is consistent with experimental results from the literature.

Role of stacking fault energy in strengthening due to cryo-deformation of FCC metals

The effectiveness of the cryogenic (CT) rolling vis-à-vis room temperature (RT) rolling on strengthening is significantly affected by stacking fault energy (SFE) and there is an optimum SFE at which CT rolling is most effective. Studies on Al, Al alloy AA6061, Cu, Cu–4.6Al, Cu–9Al and Cu–15Al (in at.%) alloys revealed that in metals with very high and very low SFEs, the strength difference between CT and RT rolled samples is <10%. The Cu–4.6Al alloy with an intermediate SFE revealed maximum enhancement of strength (25–30%). These results are explained by changes in deformation mechanisms with SFE and temperature. High SFE metals deformed by dislocation slip and low SFE metals deformed by twinning during CT and RT rolling. Metals with intermediate SFEs deformed by twinning during CT rolling but by dislocation slip during RT rolling, and this makes the CT rolling most effective over RT rolling in enhancing the strength.

Molecular dynamics simulation of the interaction between a mixed dislocation and a stacking fault tetrahedron

A high number-density of nanometer-sized stacking fault tetrahedra are commonly found during irradiation of low stacking fault energy metals. The stacking fault tetrahedra act as obstacles to dislocation motion leading to increased yield strength and decreased ductility. Thus, an improved understanding of the interaction between gliding dislocations and stacking fault tetrahedra are critical to reliably predict the mechanical properties of irradiated materials. Many studies have investigated the interaction of a screw or edge dislocation with a stacking fault tetrahedron (SFT). However, atomistic studies of a mixed dislocation interaction with an SFT are not available, even though mixed dislocations are the most common. In this paper, molecular dynamics simulation results of the interaction between a mixed dislocation and an SFT in face-centered cubic copper are presented. The interaction results in shearing, partial absorption, destabilization or simple bypass of the SFT, depending on the interaction geometry. However, the SFT was not completely annihilated, absorbed or collapsed during a single interaction with a mixed dislocation. These observations, combined with simulation results of edge or screw dislocations, suggest that defect-free channel formation in irradiated copper is not likely by a single dislocation sweeping or destruction process, but rather by a complex mix of multiple shearing, partial absorption and defect cluster transportation that ultimately reduces the size of stacking fault tetrahedra within a localized region.

Strain Path Change Effect on Deformation Behaviour of Materials with Low-to-Moderate Stacking Fault Energy

Stacking fault energy (SFE) plays an important role in face centred cubic (f.c.c.) metals and alloys in determining the prevailing mechanisms of plastic deformation. Low SFE metals and alloys have a tendency to develop mechanical twinning, besides dislocation slip, during plastic deformations. Deformation behaviour and microstructure evolution under simple and complex strain paths were studied in 70/30 brass, with small and intermediate grain sizes, which corresponds to a f.c.c. material with low SFE. Simple (rolling and tension) and complex (tension normal to previous rolling) strain paths were performed. The macroscopic deformation behaviour of materials studied is discussed in terms of equivalent true stress vs. equivalent true strain responses and strain hardening rates normalized by shear modulus (dσ/dε)/G as vs. (σ – σ0)/G (σ0 is the initial yield stress of the material and G is the shear modulus). The mechanical behaviour is discussed with respect to dislocation and twin microstructure evolution developed in both, simple and complex strain paths.

Recrystallization mechanisms of low stacking fault energy metals as characterized on model silver single crystals

The microstructural evolution during light annealing of a representative low stacking fault energy metal has been characterized by detailed electron microscopy orientation measurements. High-purity silver single crystals with initial C(112)[111¯] orientation were channel-die deformed to reductions of 32% and 67%; the crystals first developed twin–matrix layers and then compact clusters of shear bands. The latter are the nucleation sites for recrystallization. Microtexture analysis of partially recrystallized samples indicates a simple 25–40°〈111〉 or 〈112〉 relation between isolated nuclei and one of the two as-deformed groups of components (twins or matrix). This implies the existence of a second misorientation with respect to the other component, usually described as 50–55°〈uvw〉. During the rapid growth stage, recrystallization twinning radically increases. This twinning is considered to operate after the formation of the primary nuclei and, in C-oriented crystals, also plays a critical role in the formation of the cube orientation.

The role of shear banding on deformation texture in low stacking fault energy metals as characterized on model Ag crystals

The microstructure and texture evolution during deformation of a representative low-stacking fault energy (SFE) metal was characterized by detailed local scanning electron microscopy and transmission electron microscopy orientation measurements. High purity silver single crystals with an initial (1 1 2) [ 1 1 1 ¯ ] orientation were channel die deformed to total reductions of 88% (logarithmic strains 2.1), first developing twin-matrix layers and then compact clusters of two families of shear bands. It was shown that twinning and shear banding caused several important transitions of the deformation textures. The as-deformed shear bands exhibited large orientation spreads of up to 40° with respect to the adjacent twinned areas. Most of these misorientations occurred by rotations about the transverse direction ∥〈1 1 0〉 axis with significant further rotations about 〈1 1 2〉 poles. These two rotations explained the influence of the shear bands on the formation of Goss{1 1 0}〈0 0 1〉 and brass{1 1 0}〈1 1 2〉-S{1 2 3}〈6 3 4〉 texture components, which are clearly observed in highly deformed low-SFE metals. Symmetrically equivalent crystal lattice rotations inside narrow areas led to the formation of the positive and negative macroscopic shear bands for high deformations.

A dislocation-based description of grain boundary dissociation: application to a 90° 〈110〉 tilt boundary in gold

High resolution transmission electron microscopy (HRTEM) observations and atomistic simulations of {1 1 ̄ 1}/{121} facets in a gold 90° tilt boundary show the presence of a ∼10 Å wide layer with stacking faults distributed one to every three close-packed planes. This interfacial reconstruction, which forms the rhombohedral 9R stacking arrangement, is similar to that found previously for near−Σ=3{112} boundaries in low stacking fault energy metals. We discuss a general approach for partitioning grain boundary orientation into a set of Shockley partial dislocations and then apply this description to the {1 1 ̄ 1}/{121} interface. The analysis explains both the distribution of faults and the geometry of the local plane bending and shows, further, that the 9R stacking occurs in both the Σ=3{112} and {1 1 ̄ 1}/{121} interfaces due to the similar ratios of 30° and 90° Shockleys in both cases. Finally, we discuss limitations of this description, in particular concerning a 1/8[ 1 ̄ 01] relaxation that is predicted by the atomistic simulations.

〈110〉 symmetric tilt grain-boundary structures in fcc metals with low stacking-fault energies

Twenty-one {l_angle}110{r_angle} symmetric tilt grain boundaries (GB{close_quote}s) are investigated with atomistic simulations, using an embedded-atom method (EAM) potential for a low stacking-fault energy fcc metal. Lattice statics simulations with a large number of initial configurations are used to identify both the equilibrium and metastable structures at 0 K. The level of difficulty in finding the equilibrium structures is quantitatively assessed. The stability of the structures at an elevated temperature is investigated by Monte Carlo annealing. A form of GB dissociation is identified in a number of the boundaries. These structures are used to develop a dislocation model of GB dissociation by stacking-fault emission. Also, an attempt is made to apply the structural unit model (SUM) to the simulated boundaries and problems that are encountered for GB structures in low stacking-fault energy metals are enumerated and discussed. {copyright} {ital 1996 The American Physical Society.}

Grain-boundary dissociation by the emission of stacking faults.

A range of $〈110〉$ symmetric tilt grain boundaries (GB's) are investigated in several fcc metals with simulations and high-resolution electron microscopy. Boundaries with tilt angles between 50.5\ifmmode^\circ\else\textdegree\fi{} and 109.5\ifmmode^\circ\else\textdegree\fi{} dissociate into two boundaries 0.6 to 1.1 nm apart. The dissociation takes place by the emission of stacking faults from one boundary that are terminated by Shockley partials at a second boundary. This is a general mode of GB relaxation for low stacking fault energy metals. The reasons for the occurrence of this relaxation mode are discussed using the theory of GB dislocations.

Effect of stacking fault energy on the dynamic recrystallization during hot working of FCC metals: A study using processing maps

The influence of stacking fault energy (SFE) on the mechanism of dynamic recrystallization (DRX) during hot deformation of FCC metals is examined in the light of results from the power dissipation maps. The DRX domain for high SFE metals like Al and Ni occurred at homologous temperature below 0·7 and strain rates of 0·001 s−1 while for low SFE metals like Cu and Pb the corresponding values are higher than 0·8 and 100 s−1. The peak efficiencies of power dissipation are 50% and below 40% respectively. A simple model which considers the rate of interface formation (nucleation) involving dislocation generation and simultaneous recovery and the rate of interface migration (growth) occurring with the reduction in interface energy as the driving force, has been proposed to account for the effect of SFE on DRX. The calculations reveal that in high SFE metals, interface migration controls DRX while the interface formation is the controlling factor in low SFE metals. In the latter case, the occurrence of flow softening and oscillations could be accounted for by this model.

Development of Texture and Microstructure During Rolling of the Copper - 8 Wt PCT Germanium Alloy

A comprehensive description of processes occurring during the plastic deformation requires confrontation of the variation of texture with the accompanying changes in microstructure and physical properties. However, the variation of texture and microstructure in low stacking fault energy f.c.c, metals and alloys has not been till now well recognized as investigations were generally limited to texture analysis at high rolling reductions Not enough attention has also been paid to the effect of initial texture and grain size, and to the influence of geometry of the rolling gap on inhomogeneity of texture and microstructure. Equally, the commonly used method of series expansion for the quantitative description of texture does not allow a satisfactory identification of texture details, especially in lower levels of orientation density. The orientation distribution function is then burdened with truncation errors and those ensuing from the phenomenon of ghosts; the ghosts are observed in the ODF in the positions which are in twin relations with the pronounced components, and mechanical twinning is a very important process in the deformation of low stacking fault energy metals and alloys. In the present research the rolling has been carried on with unit draughts generally not much smaller than 0.5 and not greater than 5. In these conditi.ns the texture does not exhibit the through-thickness inhomogeneity-. Texture analysis has been carried out on samples from the central layer of the rolled material when applying the direct ADC method bae on discretization, in which the above described errors do not appear For all experiments the copper 8 wt pct germanium alloy was used, rolled from the annealed state up to 98 pct reduction. In the initial state the alloy was characterized by a rather sharp texture, and the mean grain size was 95 /m. The stacking fault _energy determined by the dislocation nodes method was equal to 10 mJ/m-.

Texture and formability in fcc metals and alloys

Abstract Plane stress yield loci are derived from an extensive bank of textural data which covers a range of fcc metals and alloys subjected to a wide range of thermomechanical processing operations. Results are given in terms of a parameter Y which describes the shape of the yield locus and corresponds to the ratio of the yield stress at balanced biaxial tension to the yield stress at pure shear. It is found that the yield loci indicative of good formability occur in metal sheets in which textural components are carefully balanced. For high stacking fault energy metals this corresponds to a mixture of cube texture with retained rolling texture components. For low stacking fault energy metals, the mixture is in the form of the brass annealing texture supplemented by components of grain growth texture. Improvements which can be gained in the formability with the latter texture are quite significant, while the former texture is capable only of restoring the formability to isotropic levels.MST/869

Texture and formability in fcc metals and alloys

Plane stress yield loci are derived from an extensive bank of textural data which covers a range of fcc metals and alloys subjected to a wide range of thermomechanical processing operations. Results are given in terms of a parameter Y which describes the shape of the yield locus and corresponds to the ratio of the yield stress at balanced biaxial tension to the yield stress at pure shear. It is found that the yield loci indicative of good formability occur in metal sheets in which textural components are carefully balanced. For high stacking fault energy metals this corresponds to a mixture of cube texture with retained rolling texture components. For low stacking fault energy metals, the mixture is in the form of the brass annealing texture supplemented by components of grain growth texture. Improvements which can be gained in the formability with the latter texture are quite significant, while the former texture is capable only of restoring the formability to isotropic levels.MST/869

The role of glide and twinning in the final separation of ruptured gold crystals

After polycrystalline gold specimen of high purity were ruptured, their crack flanks were investigated by scanning and transmission electron microscopy. While 0.15cm diameter wire developed a macroscopic chisel point, higher magnifications showed that the final separation occurred along chisel edges. Flat specimens, 0.05 cm thick, developed a localized neck consisting of a number of thin sheets 0.1 μm thick and only 0.5 tot 1 μm wide lying parallel to {110}. Rupture occurred by first thinning the sheets parallel to 〈110〉 in numerous areas which were located 1 μm apart. After developing microcracks along 〈110〉 by slip on two systems, glide and twinning parallel to 〈112〉 interconnected the first set of microcracks through fracturing along this direction. The result is a zig-zag fracture flank. The role of dislocation cell walls and voids in the rupture process has been discussed. The sequence of events is considered typical for the rupture of pure, low stacking fault energy metals.