High Stacking Fault Energy Metals Research Articles

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.

Direct formation of novel Frank loop and stacking-fault tetrahedron complex

Stacking-fault tetrahedron (SFT) is one kind of typical three-dimensional vacancy defect in quenched, deformed or irradiated face-centered cubic (FCC) metals, which can seriously degrade the mechanical properties of materials. It's generally believed that high stacking fault energy (SFE) is unfavorable for the formation of SFTs. Here, we report the first in-situ investigation of irradiation-induced formation of novel Frank loop-SFT complexes in Pd with extremely high SFE. Our findings reveal a new mechanism that vacancy clusters rearrange directionally to form SFTs due to the ambient stress deviations and compressive stress fields induced by interstitial Frank loops. Continuous hydrogen implantation will lead to a synergistic growth of the complex, while direct interaction between the Frank loop and SFT under thermal effect can cause the complex to disappear. These results uncover a unique formation mechanism for SFT and provide a new perspective for understanding nano-defects in high SFE metals.

Synergistic strengthening of Al–SiC composites by nano-spaced SiC-nanowires and the induced high-density stacking faults

Aluminum alloys are widely used in engineering structures due to light weight and corrosion resistance but aluminum has low yielding and flow strengths. Here we reported super-strong Al-30 vol%SiC composites with a flow strength of about 1.18 GPa up to a uniform strain of 16.0%. Micromechanical tests revealed a flow strength of 0.73 GPa associated with the nano-spaced SiC nanowires strengthening, and additional flow strength of 0.45 GPa associated with high-density stacking faults (SFs) that are rarely formed and stabilized in Al due to high stacking fault energy (SFE). More surprisingly, SFs possess excellent thermal stability up to 320 °C and can be regained by thermal cooling even after they are eliminated during annealing at 600 °C. Microscopy characterizations and theoretical analysis revealed that thermal mismatch induced high stress during cooling promotes the formation of SFs, and the segregation of Si into SFs and dislocation cores enables the thermal stability of wide SFs. This work demonstrated an approach to creating high-density and thermo stable SFs in high SFE metals via microstructure-enabled thermal stress.

Mechanistic modelling of cryo-deformation and post-annealing of aluminium

Dynamic recovery (DRV) occurs through severe plastic deformation (SPD) of high-stacking fault energy metals, even at room temperature. SPD at sub-zero or relatively low temperatures, namely cryo-deformation, has long been employed to confer enhanced mechanical properties due to the suppression of DRV. In this paper, mechanistic models with a metallurgical sense are investigated for low- and room-temperature SPD (200 and 300 K) and post-annealing of aluminium. Cellular substructure as well as cross-slip and climb effects are considered in the modelling of SPD. A model based on stored energy is also used for the contribution of dislocation density to post-annealing behaviour. The predicted flow stress curve is fitted the experimental results perfectly. The mechanistic modelling of SPD has demonstrated a multi-stage work-hardening behaviour, which entails more details such as the mutual effects of DRV and dynamic recrystallisation. The annealing experimental results are in line with the post-annealing model outputs.

Extraordinary Strain Hardening from Dislocation Loops in Defect-Free Al Nanocubes.

The complex interaction of crystalline defects leads to strain hardening in bulk metals. Metals with high stacking fault energy (SFE), such as aluminum, tend to have low strain hardening rates due to an inability to form stacking faults and deformation twins. Here, we use in situ scanning electron microscopy (SEM) mechanical compressions to find that colloidally synthesized defect-free 114 nm Al nanocubes combine a high linear strain hardening rate of 4.1 GPa with a high strength of 1.1 GPa. These nanocubes have a 3 nm self-passivating oxide layer that has a large influence on mechanical behavior and the accumulation of dislocation structures. Postcompression transmission electron microcopy (TEM) imaging reveals stable prismatic dislocation loops and the absence of stacking faults. MD simulations relate the formation of dislocation loops and strain hardening to the surface oxide. These results indicate that slight modifications to surface and interfacial properties can induce enormous changes to mechanical properties in high 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.

Insights into the plasticity of Ag3Sn from density functional theory

The intermetallic Ag3Sn is a strengthening phase in both silver amalgams as well as Sn–Ag lead-free solders, but is also a relatively ductile compound. Experimental reports have suggested that the compound can both readily slip via dislocations as well as deform via twinning. To better understand the deformation mechanisms in Ag3Sn, we investigate the energetics of slip via generalized stacking fault (GSF) curves as well as the structure and energetics of the twins using both density functional theory and analytical modeling. Our results suggest that [100](001) slip, which is equivalent to 〈a〉hcp slip on a prism plane in the hcp structure, is favorable due to a very low antiphase boundary energy. Slip on the (010) plane, which is equivalent to the basal plane in hcp metals, can occur in either the [100] or [001] directions via the formation of four superpartials. Our results further suggest that slip in the 〈102〉 direction could occur easily on either the {201} or {211} planes with the former being more likely since it is similar to prism planes in the hcp crystal. The overall values of the GSF curves are quite low, similar to high stacking fault energy metals, resulting in a rather ductile intermetallic. Simulations of the twins in Ag3Sn demonstrate that they are structurally similar to twins in hcp metals with similar energy magnitudes. However, the experimentally observed activation of the {011} and {211} is likely a result of the small shear required to form the twins, and the suppression of other twin types is a likely a result of chemical ordering. Finally, our results point out that twinning is likely a necessary deformation mode to account for a general state of plastic deformation.

Growth twins in high stacking fault energy metals: Microstructure, texture and twinning

This work explores the formation of twin boundaries in high stacking fault energy metals (SFE >125mJ/m2) by synthesizing thick films (>10µm) of Al, Al-5.3wt%Mg, and Ni using magnetron sputtering. We report the observation of twin boundaries that are inclined with respect to the film growth direction across the entire thickness of the films. The formation of these inclined twin boundaries results in localized changes in the texture of the columnar grains. Microstructural analysis revealed that the fraction of twinned grains in the Al film (46%) is four times higher compared to other similar studies in Al, while the Al-5.3wt% Mg and Ni films can be considered highly twinned since the fraction of twinned grains is 70% and 90%, respectively. The experimental observations provide an explanation on the formation of twin boundaries during the synthesis of the films, and emphasize that, in addition to stacking fault energy restrictions, high grain boundary mobility is a limiting factor on the nucleation of twin boundaries.

Microstructural evolution of Ti-added interstitial free steel in high strain deformation by hot torsion

The dynamically evolved microstructure under high strain deformation condition does still have many debatable aspects, particularly in the case of easy-recovery metals like bcc-iron. In this research, microstructural evolution in high strain deformation by hot torsion of Ti-added interstitial free (IF) steel was systematically investigated. Torsion specimens were deformed up to an equivalent strain of ∼ 7 at different temperatures (650 °C - 850 °C) and strain rates (0.01 s-1 - 1.0 s-1), i.e., under various values of the Zener-Hollomon (Z) parameter. Immediately after the deformation, samples were water-quenched and microstructures were investigated by electron backscattering diffraction (EBSD) measurements and electron channelling contrast imaging (ECCI). Flow stress-strain curves of the IF steel under various deformation conditions showed typical flow curves of high stacking fault energy metals at low Z values, i.e., a peak stress followed by slight softening. On the other hand, under the high-Z deformation conditions, the specimens showed a larger stress drop after a certain amount of deformation. EBSD-based quantitative analysis was used to study the microstructural transition between high and low Z values. At low Z values, the occurrence of strain induced boundary migration (SIBM) as an initiation of dynamic recrystallization (DRX) was clearly observed. On the other hand, at high Z values, grain subdivision phenomena led to very fine and elongated structures.

Crystallographic aspects of nucleation and grain growth during recrystallization of high stacking fault energy metals as characterized on model Al-1%wt.Mn alloy crystals

The objective of this paper is to identify the predominant crystallographic relations between deformed state and recrystallized grains during the early stages of recrystallization of the Goss{110}<001> and brass{110}<112> oriented single crystals of Al-1%Mn. The analysis was based on high resolution local orientation measurements in scanning electron microscopy. After annealing the disorientation across the recrystallization front 'defines' the final rotation by angles in the ranges of 35o-50o around axes located near the normals of all four {111} slip planes. Although the rotation axes approach the normal vector of the active slip planes during deformation, they only rarely coincide with the exact location of the <111> direction. For both initial orientations, preferred grain growth occurred along the {111} planes, the most active during strain.

Review on TEM analysis of growth twins in nanocrystalline palladium thin films: Toward better understanding of twin-related mechanisms in high stacking fault energy metals (Phys. Status Solidi B 6/2014)

Review on TEM analysis of growth twins in nanocrystalline palladium thin films: Toward better understanding of twin-related mechanisms in high stacking fault energy metals (Phys. Status Solidi B 6/2014)

Review on TEM analysis of growth twins in nanocrystalline palladium thin films: Toward better understanding of twin-related mechanisms in high stacking fault energy metals

Various modes of transmission electron microscopy including aberration corrected imaging were used in order to unravel the fundamental mechanisms controlling the formation of growth twins and the evolution of twin boundaries under mechanical and hydrogen loading modes in nanocrystalline (nc) palladium thin films. The latter were produced by electron-beam evaporation and sputter deposition and subjected to uniaxial tensile deformation as well as hydriding/dehydriding cycles. The results show that the twins form by dissociation of grain boundaries. The coherency of Σ3{111} coherent twin boundaries considerably decreases with deformation due to dislocation/twin boundary interactions while Σ3{112} incoherent twin boundaries dissociate under hydrogen cycling into two-phase boundaries bounding a new and unstable 9R phase. The effect of these elementary mechanisms on the macroscopic behavior of the palladium films is discussed and compared to recent experimental and simulation works in the literature. The results provide insightful information to guide the production of well-controlled population of growth twins in high stacking fault energy nc metallic thin films. The results also indicate directions for further enhancement of the mechanical properties of palladium films as needed for instance in palladium-based membranes in hydrogen applications.

Basic criteria for formation of growth twins in high stacking fault energy metals

Nanotwinned metals received significant interest lately as twin boundaries may enable simultaneous enhancement of strength, ductility, thermal stability, and radiation tolerance. However, nanotwins have been the privilege of metals with low-to-intermediate stacking fault energy (SFE). Recent scattered studies show that nanotwins could be introduced into high SFE metals, such as Al. In this paper, we examine several sputter-deposited, {111} textured Ag/Al, Cu/Ni, and Cu/Fe multilayers, wherein growth twins were observed in Al, Ni, and face-centered cubic (fcc) Fe. The comparisons lead to two important design criteria that dictate the introduction of growth twins in high SFE metals. The validity of these criteria was then examined in Ag/Ni multilayers. Furthermore, another twin formation mechanism in high SFE metals was discovered in Ag/Ni system.

Validating computed grain boundary energies in fcc metals using the grain boundary character distribution

The grain boundary character distribution (GBCD) is a direct measurement that can be determined from a single planar section. Since the GBCD is inversely related to the grain boundary energy distribution, it offers a useful metric for validating grain boundary energy calculations. Comparisons between the measured GBCD and calculated energies for 388 grain boundaries in Al show that, for boundaries with a statistically reliable number of observations, including general, Σ3, Σ7, Σ11 and 〈1 1 1〉 twist boundaries, the GBCD and calculated grain boundary energy have weighted correlation coefficients of approximately 0.9, reproducing both qualitative and quantitative trends seen in simulations. GBCDs for Ni and Al are positively correlated, as predicted by simulation. By combining GBCD measurements with simulation results, we validate grain boundary energy simulations in both low and high stacking fault energy metals.

Mechanism Responsible for Dynamic Recrystallization for Repeated Plastic Working Deformation Process

The repeated plastic working deformation(RPW) process can reduce grain size of a Mg alloy from 50-100 um to 10-500 nm, but the mechanism responsible for it has not been clear up to now. In the present paper, the effect of RPW deformation process on the grain size of Mg-5Gd-1Er alloy has been studied, and a series microstructural evaluations were performed to investigate the possible mechanism for RPW deformation by using transmission electron microscopy. Although there are no twinning or fibrous microstructures, the dynamic recrystallization, which usually occurs in high stacking fault energy metals, has been found in the alloy deformed by RPW process. The results indicated that the vacancy diffusion played an important role in this kind of dynamic recrystallization. According to the evolution of microstructure, a mechanism was proposed to explain the dynamic recrystallization for RPW deformation process.

The Role of Recrystallization in the Failure of SnAgCu Solder Interconnections Under Thermomechanical Loading

The chip scale packaged/ball grid array component boards were thermally cycled according to the IEC 68-2-14N standard (+125°C/-45°C, 15 min dwells, 5 min ramps). The as-solidified microstructures of Sn-rich solder interconnections were composed of relatively few, typically from two to five, large tin colonies distinguished by high-angle boundaries. However, during the thermal cycling tests the as-solidified microstructures transformed gradually into more or less equiaxed grain structure by recrystallization. It is suggested that cracking of solder interconnections under thermomechanical loadings is enhanced by the recrystallization, because the network of newly formed grain boundaries extending through the interconnections provide favorable paths for cracks to propagate intergranularly. The incubation time of recrystallization was about 50% of the average cycles-to-failure. The decrease of stored energy in high-stacking fault energy metals such as Sn takes place very effective by the recovery. Therefore, microstructures recrystallize only under restricted loading conditions: dynamic loading condition where strain hardening is more effective than recovery. Therefore, the experimentally observable transformation of the microstructures by recrystallization enables us, in principle, to correlate the field use loading conditions with those produced in accelerated reliability tests. Furthermore, the recrystallization provides the means to incorporate the effects of microstructural evolution into the lifetime prediction models.

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.

Twinning in nanocrystalline metals

Twinning is a commonly observed deformation mode in lowsymmetry crystals such as hexagonal-close packed metals, but not in high-symmetry metals that possess many possible slip systems with lower critical resolved shear stress (CRSS) than the twinning stress. However, if the CRSS for slip can be increased to a level greater than the twinning stress, then twinning may be favored as a deformation mechanism even in cubic materials. Nanostructured metals present such a scenario. Following the observation of deformation twinning in nano-crystalline metals, even in high stacking fault energy metals such as aluminum, recently there has been signifi cant interest in studying twinning as a deformation mode in nanocrystalline face-centered cubic (fcc) metals. Another important breakthrough is the fabrication of fcc metals with nanotwinned structures using methods such as electro-deposition or sputter deposition. Nanoscale growth twins have also been observed in nanowires. Nanotwinned fcc metals made via sputtering can have twin boundary spacing as small as 2–4 nm and preferential alignment of {111} twin boundaries normal to the growth direction. These as-grown nanotwinned structures exhibit unusual properties as compared to nanocrystalline metals. Typically, in the equiaxed grain nanocrystalline metals with high-angle grain boundaries the increased strength is accompanied by a loss in ductility, thermal stability, and electrical conductivity. Twinning in Nanocrystalline Metals

Study of the Microtexture of Recrystallized Aluminium

The nucleation of recrystallized grains in deformed, high-purity aluminium were examined in relation to the microstructural heterogeneities in the form of shear bands (SBs) and transition bands (TBs). The experiments were carried out on single crystals deformed in channel-die compression at 77 K to true strains within the range of 1–1.5. The substructures were identified by means of a combination of transmission (TEM) and scanning (SEM) electron microscopes. Local texture measurements in particular places of samples were mostly investigated by means of TEM and convergent beam electron diffraction (CBED), with on-line diffraction pattern analysis. The coarser analyses were performed by SEM and electron back scattered diffraction (EBSD). The \({\rm C}\{112\} \langle 111 \rangle\) and the \({\rm S} \{531\} \langle 123 \rangle\) oriented single crystals are stable in the global sense and form clearly defined SBs. The \({\rm H} \{110\} \langle 110 \rangle\), and the ‘shear’ \(\{100\} \langle 011 \rangle\) orientations are very unstable and tend to form TBs. The nuclei of new grains formed along the band are misoriented by α (\(\langle 111 \rangle or \langle 112 \rangle\)) with respect to the orientations identified within the neighbouring banded material. The author attempts to explain, from the crystallographic point of view, the processes operating within high stacking fault energy metals at the very early stages of recrystallization.

A model of continuous dynamic recrystallization

During continuous dynamic recrystallization (CDRX), which occurs in high stacking fault energy metals, new grains are formed by the progressive increase of low-angle boundary misorientations. A simple CDRX model is proposed, that deals with a set of “crystallites” delimited partly by low-angle (LABs) and partly by high-angle (HABs) boundaries. Strain hardening and dynamic recovery are described by the Laasraoui-Jonas equation, which has been modified to take HAB migration into account. Recovery occurs in two ways, viz. by the condensation of dislocations into new LABs, and the absorption of dislocations in the pre-existing boundaries, which leads to the transformation of LABs into HABs. The predictions of the model are presented for both the transient and steady states, including stress-strain curves at various temperatures and strain rates, as well as the associated evolutions of the microstructural parameters such as crystallite sizes, dislocation densities and LAB misorientation distributions. The effect of HAB migration on these parameters is emphasized.