Double Twins Research Articles

Nucleation and electromigration-induced grain rotation in an SABI333 solder joint

Single-grain or tricrystal joints are often observed in Sn-based Pb-free solder alloys, such as Sn3.5Ag and Sn3.0Ag0.5Cu, and Sn dendrites tend to grow in the [110] and [\( 1\bar{1}0 \)] directions in these solder joints. In this study, electron backscattering diffraction was used to quantitatively characterize the grain orientations of a reflowed line-type Sn3.0Ag3.0Bi3.0In (SABI333) solder joint. The SABI333 solder joint was composed of polycrystals, and polycrystals with misorientations of more than 15° were separated by high-angle grain boundaries. It was reasonable to assume that these orientations arose from Sn dendrite deviations from the preferred directions. In addition, double twinning of Sn was observed in the SnAgCu solder alloy, and a polycrystal SABI333 solder joint was formed by spreading from the predominant double tricrystals (with five orientations in the preferred [110] and [\( 1\bar{1}0 \)] growth directions). Additionally, during electromigration (EM), the near-pad-interface small grains separated by high-angle grain boundaries in the SABI333 solder joint tended to rotate, and the rotation caused the c-axis to be perpendicular to the current direction. Furthermore, no detectable segregation of Ag or increase in the thickness of the interfacial intermetallic compounds (IMCs) occurred in the SABI333 solder joint after EM for 168 h. These results confirm that the Sn grain orientations in the joints significantly influence the reliability of the solder joints under service conditions that promote EM and that polycrystal SABI333 solder joints may have longer EM service lifetimes than Sn3.5Ag or Sn3.0Ag0.5Cu joints.

Comparative relationship between twin boundary migration and double twinning of a Mg nanotwinned structure with molecular dynamics simulations

In this study, we perform molecular dynamics simulations of plastic deformation of a magnesium (1011) nano-twin structure. Twin boundary migration (TBM) or double twin (DT) occur depending on distance of twin boundaries.

Deformation Behavior of Pure Zinc Single Crystals during a single pass of ECAP

A single pass of equal channel angular pressing (ECAP) at 223 K was applied to six kinds of Zn bulk single crystals with different crystal orientations, and the deformation behavior was investigated, such as grain refinement and texture development. Zn bulk single crystals with several millimeters can be divided into a number of grains with several tens micron meters by a single pass of ECAP. Also, texture formed by activation of basal and second order pyramidal slips. Increasing twin boundaries by double twinning, and occurrences of recrystallization at the twin boundaries are required for grain refinement before passing through the theoretical shear plane in the ECAP die.

A one-step mechanism for new twinning modes in magnesium and titanium alloys modelled by the obliquity correction of a (58°, a + 2b) prototype stretch twin.

The \{ 11{\overline 2}2\} and \{ 11{\overline 2}6\} twinning modes were recently discovered by Ostapovets et al. [Philos. Mag. (2017), 97, 1088-1101] and interpreted as \{ {10{\overline 1}2} \}-\{ {10{\overline 1}2} \} double twins formed by the simultaneous action of two twinning shears. Another interpretation is proposed here in which the two conjugate twinning modes result from a one-step mechanism based on a (58°, a + 2b) prototype stretch twin and differ from each other only by their obliquity correction. The results are also compared with the classical theory of twinning and with the Westlake-Rosenbaum model.

Twin-twin interactions and contraction twin formation in an extruded magnesium alloy subjected to an alteration of compressive direction

Twinning and detwinning represent major deformation mechanisms in hexagonal close-packed (hcp) metals. The aim of this study was to identify twin-twin interactions and contraction twin formation in an AZ31 magnesium alloy when the compressive direction was changed from the extrusion direction (ED) to the normal direction (ND) via electron backscatter diffraction (EBSD) and Schmid factor analysis. {101¯2} extension twins of multiple variants were observed after compressive deformation of 4.3% along ED. The detwinning of {101¯2} extension twins occurred along with the formation of {101¯1} and {101¯3} contraction twins, when the compressive direction was changed to ND. The extension twins in some grains almost fully vanished, making the grains back to a twin-free state. A new twin-twin interaction mechanism being different from double twinning (defined as twins within a twin) was identified, due to the impingement of {101¯1} contraction twins nucleated in the matrix grain on a pre-existing {101¯2} extension twin.

Mechanical behavior and microstructural evolution in rolled Mg-3Al-1Zn-0.5Mn alloy under large strain simple shear

Mechanical behavior and microstructural evolution in rolled Mg-3Al-1Zn-0.5Mn (AZ31B) magnesium alloy under in-plane shear (IPS) and through-thickness shear (TTS) were investigated. The newly designed butterfly-shaped specimens were sheared along the rolling direction, and a full field strain analysis was performed by digital image correlation (DIC). The shear stress-strain curves show that the shear strength and strain hardening rate are significantly different between the IPS and TTS. The electron backscatter diffraction (EBSD) observation shows that extension twinning is the dominant twinning mechanism under shear deformations. Moreover, a certain volume of {10-12}-{10-12} double twins appears throughout the deformation process in the TTS, while a few compression twins and {10-11}-{10-12} double twins occur at the early stage in the IPS. The results suggest that crystal orientation plays an important role in determining the twin types and twin volume fraction in the shear deformations. The visco-plastic self-consistent (VPSC) model is also applied to predict the activity of slip/twinning under shear deformations. The VPSC results confirm that the mechanical discrepancy at early shear deformation is mainly due to the activation of extension twin and prismatic slip.

Rate and temperature dependent deformation behavior of as-cast WE43 magnesium-rare earth alloy manufactured by direct-chill casting

In this work, we study the deformation behavior of a direct chill cast WE43 Mg alloy. This material initially has equiaxed grains approximately 40µm in diameter and a random texture. The room temperature, quasi-static response exhibits little plastic anisotropy when evaluated parallel to and normal to the solidification direction and no initial yield tension-compression asymmetry. The deformation at room temperature is accompanied by significant basal texture development and formation of three types of deformation twins: {101̅2}〈1̅011〉, {101̅1}〈1012̅〉, and {112̅1}〈1̅1̅26〉 as well as double twins {101̅1}〈1012̅〉-{101̅2}〈1̅011〉, although each in small amounts < 10% up to failure. We find that in the elevated 250°C ± 20°C regime, the material exhibits a negative strain rate sensitivity, with a decreasing flow stress as the strain rate increases. In most of the high-temperature, 250°C to 350°C, and high-strain-rate, 0.01/s to 10/s, tests, the material failed at moderate compression strains, 0.35–0.45. Subsequent fracture analyses find that the material fractures transgranularly by a typical shear fracture, often with the presence of additional arrested cracks. At elevated temperatures and under strain rates sufficiently low (i.e., 275°C and 0.01/s, 300°C and 0.1/s, 350°C and 1/s, 375°C and 10/s) or at a temperature of 400°C deformation conditions, the material exhibited pseudo-super plastic behavior, experiencing relatively high deformation strains (> 1.0 true strain) without fracturing.

Improved twin detection via tracking of individual Kikuchi band intensity of EBSD patterns

Twin detection via EBSD can be particularly challenging due to the fine scale of certain twin types – for example, compression and double twins in Mg. Even when a grid of sufficient resolution is chosen to ensure scan points within the twins, the interaction volume of the electron beam often encapsulates a region that contains both the parent grain and the twin, confusing the twin identification process. The degradation of the EBSD pattern results in a lower image quality metric, which has long been used to imply potential twins. However, not all bands within the pattern are degraded equally. This paper exploits the fact that parent and twin lattices share common planes that lead to the quality of the associated bands not degrading; i.e. common planes that exist in both grains lead to bands of consistent intensity for scan points adjacent to twin boundaries. Hence, twin boundaries in a microstructure can be recognized, even when they are associated with thin twins. Proof of concept was performed on known twins in Inconel 600, Tantalum, and Magnesium AZ31. This method was then used to search for undetected twins in a Mg AZ31 structure, revealing nearly double the number of twins compared with those initially detected by standard procedures.

Formula omitted] double tensile twinning in a Mg-3Al-1Zn alloy sheet during cyclic deformation

The typical fracture morphology of the Mg-3Al-1Zn alloy after cyclic deformation was investigated using optical microscopy (OM), and an electron back-scatter diffraction (EBSD). A novel finding of this study is that large number of stripe-like laminas were found within the primary {101¯2} tensile twins in the crack initiation and crack propagation regions. It is shown that these laminas primarily result from {101¯2}−{101¯2} twinning during cyclic deformation. According to the results and combined with theoretical analysis, the formation of {101¯2}−{101¯2} twins are largely contributing to the local strain accommodation caused by different twin variants.

Deformation mechanism of highly textured AZ31 sheet under high strain rates

AbstractIn this study, in‐plane tension behavior of AZ31 magnesium sheet was measured in rolling direction, transverse direction and 45° direction under high strain rates of 600 s‐1, 1200 s‐1 and 2400 s‐1 at room temperature using a Split Hopkinson Tension Bar (SHTB). The microstructures after high strain rates deformation were observed by optical micrography. The anisotropic behavior was negligible and marginally positive strain rate effects were observed in all deformation directions. Texture analysis of the sheet demonstrated that three possible deformation mechanisms could apply to AZ31 magnesium alloy sheet under high strain rates tension, those are contraction twinning and/or ‐ double twinning, prismatic slip and &lt;c+a&gt; pyramidal slip. The volume fraction of twinning was decreased with strain rate increasing. Dynamic recrystallization took place upon the strain rate of 2400 s‐1 and the dynamic recrystallization mechanism was attributed to the twinning‐induced dynamic recrystallization.

TEM analysis of the deformation microstructure of polycrystalline cobalt plastically strained in tension

The microstructure evolution of hexagonal close-packed cobalt at different stages of work hardening has been studied. The deformation microstructure of polycrystalline samples upon a tension test was investigated by Transmission Electron Microscopy (TEM). Corresponding straining mechanisms were also identified by acoustic emission measurements. It was shown that dislocation glide is the predominant deformation mechanism during the first stage of plastic deformation, while twinning becomes the prominent strain accommodation mechanism in the later stage. A detailed TEM study using electron diffraction allowed identifying different types of twins activated during tensile testing. Four single twinning modes were identified, from which two types correspond to tensile strain and two types to compressive strain (with respect to the c direction), double twins were also observed. A relative importance of various systems is estimated and it is shown that the vast majority of twins correspond to the 101¯2 twinning system.

Competitive twinning behavior in magnesium and its impact on recrystallization and texture formation

Rolled pure Mg and Mg-1wt% Gd alloy were subjected to room temperature in-plane compression along the rolling direction, followed by isochronal annealing treatments for 1h. The results of deformation texture and microstructure showed substantial differences due to rare earth alloying. In spite of imposed c-axis extension during deformation, the Mg-1Gd alloy retained the initial texture with the majority of basal poles concentrated near the longitudinal direction of the used channel-die tool. Electron back scatter diffraction analysis of the deformation microstructure revealed a predominance of {101̅1} compression and {101̅1}-{101̅2} double twins relative to coexisting {101̅2} tension twins. This behavior was significantly contrasting in comparison with that of pure Mg, wherein first and second generation {101̅2} tension twins were observed in profuse quantities. Continuous dynamic recrystallization took place inside compression and double twins by means of slip assisted subgrain rotation about the [0001] axis giving rise to a sharp prismatic fiber of recrystallized orientations. This fiber was transformed into a randomized texture pattern during subsequent static recrystallization and grain growth due to a different discontinuous recrystallization mechanism. This resulted in a significant annealing texture weakening and an increase of the overall Schmid factor for basal slip.

Static recrystallization behavior of hot-rolled Mg-Zn-Ce magnesium alloy sheet

Static recrystallization behavior of Mg-1.5Zn-0.2Ce (wt%) alloy rolled at 480 °C was investigated at different annealing stages using quasi-in-situ electron backscattered diffraction (EBSD) analyses, which were carried out repeatedly in the same region to trace microstructural evolution. In the as-rolled condition, double twins exhibit the highest fraction compared with tensile and compression twins. Double twins mostly exist in deformed grains with basal orientations, and statically recrystallized grains mainly nucleate at the intersections of double twins and pre-existing grain boundaries. Statically recrystallized grains tend to exhibit TD-tilted orientations and grow along double twins to consume adjacent basal-oriented grains. This is the main reason for inducing the change from a RD-split texture to a well-weakened TD-split texture during annealing. The residual unrecrystallized grains with TD-tilted orientations and the growth advantage of TD-tilted grains also enhance the formation of TD-split texture.

Room-temperature tensile properties of a directionally solidified magnesium alloy and its deformation mechanism dominated by contraction twin and double twin

A Mg–4.78Zn–0.45Y–0.10Zr (wt%) alloy with specifically oriented columnar grain structures (the preferential growing plane was the prism (101̄2) plane and the growth direction was 〈011̄0〉) was prepared using a directional solidification technique. The columnar grain structures had parallel-growing primary arms and straight longitudinal grain boundaries. Room-temperature tensile tests were carried out along the growth direction of the columnar grains. The results showed that the tensile strength (σb) of the alloy was 198MPa and the elongation (δ) was 20.5%. The fractography results showed that the fracture surface had many dimples of different sizes, indicating its ductile nature. The electron backscatter diffraction (EBSD) analysis results showed that in the beginning of the tensile deformation, a bamboo leaf-shaped {101̄1} contraction twin was activated and then a {101̄2} secondary extension twin was activated within the {101¯1} contraction twin, forming a double twin. As the deformation continued, several bamboo leaf-shaped {101̄1} contraction twins joined together, forming a contraction twin band cluster. And because the {101̄2} secondary extension twinning, subgrains and recrystallized grains afterwards formed inside the contraction twin variants, boundaries of these contraction twins were not flat or smooth anymore. Twins were then gradually split to form fragmentation, and newly formed twins would nucleate and grow alongside the twin boundaries. The main reason for the good room-temperature plasticity and specific grain orientation of this alloy was the plastic deformation mechanism dominated by the contraction and double twins.

Individual effect of recrystallisation nucleation sites on texture weakening in a magnesium alloy: Part 1- double twins

Recrystallised grain nucleation, grain growth and corresponding texture evolution in a cold-rolled rare earth containing WE43 Mg alloy during annealing at 490 °C was fully tracked using a quasi-in-situ electron backscatter diffraction method. The results show nucleation sites, such as double twins, can weaken the deformed texture and for the first time provide direct evidence that recrystallised grains originating from double twins can form the rare earth texture during annealing. Precipitation and recrystallisation occurred concurrently during most of the annealing period, with precipitates forming preferentially along prior grain and twin boundaries. These precipitates effectively retard the recrystallisation due to particle pinning leading to an excessively long time for the completion of recrystallisation. A large portion of recrystallised grains were observed to have 〈0001〉 poles tilted 20–45° away from the normal direction. The RE texture emerges during the nucleation of recrystallised grains and is maintained during subsequent uniform grain growth, which results in a stable RE texture being developed as recrystallisation progresses. The uniform grain growth could be attributed to solute drag suppressing the grain boundary mobility of those grains that had recrystallised with a basal texture and precipitate pinning restricting potential orientated grain growth.

Crystallographic Analysis of Fatigue Crack Initiation Behavior in Coarse-Grained Magnesium Alloy Under Tension-Tension Loading Cycles

Plane bending fatigue tests are conducted to investigate fatigue crack initiation mechanisms in coarse-grained magnesium alloy, AZ31, under the stress ratios R = −1 and 0.1. The initial crystallographic structures are analyzed by an electron backscatter diffraction method. The slip or twin operation during fatigue tests is identified from the line angle analyses based on Euler angles of the grains. Under the stress ratio R = −1, relatively thick tension twin bands are formed in coarse grains. Subsequently, compression twin or secondary pyramidal slip operates within the tension twin band, resulting in the fatigue crack initiation. On the other hand, under R = 0.1 with tension-tension loading cycles, twin bands are formed on the specimen surface, but the angles of those bands do not correspond to tension twins. Misorientation analyses of c-axes in the matrix grain and twin band reveal that double twins are activated. Under R = 0.1, fatigue crack initiates along the double twin boundaries. The different manners of fatigue crack initiation at R = −1 and 0.1 are related to the asymmetricity of twining under tension and compression loadings. The fatigue strengths under different stress ratios cannot be estimated by the modified Goodman diagram due to the effect of stress ratio on crack initiation mechanisms.

Variant selection of {10-12}-{10-12} double twins during the tensile deformation of an AZ31 Mg alloy

Multiple primary {101¯2} twins were generated when samples of a hot-rolled AZ31 alloy plate were deformed in tension along the normal direction. Secondary {101¯2} twins also formed within the wider primary twins (PTs). Most of the secondary twins (STs) were located at the intersections of two PTs. The crystallographic orientation relationships (ORs) between 34 STs and their host PTs were analyzed. Although ten different ORs have the potential to form, only two were observed. Here 28 of the STs had misorientations of 49.7°<9¯094¯> with respect to their hosts. The observed variant selection is explained in terms of strain accommodation by basal glide in the host PT.

Pyramidal slips in high cycle fatigue deformation of a rolled Mg-3Al-1Zn magnesium alloy

The high cycle fatigue behavior of a hot-rolled AZ31 magnesium alloy were investigated at high frequencies (97.3Hz) and different stress amplitudes (50MPa, 60MPa, 70MPa, 90MPa, 110MPa) by using a tension-compression fatigue test at room temperature. The results show that the stress amplitude significantly influences the activation of fatigue mechanisms. When the stress amplitudes were 50 and 60MPa, which is close to the fatigue strength, only basal dislocations were observed, no obvious twins and <c + a> dislocations can be observed. Grains were refined under high cyclic numbers led by continuous dynamic recrystallization (CDRX). When the stress amplitudes increased to 70, 90 and 110MPa, which were higher than the fatigue strength, few {10–12} tensile twins, {10–11}-{10–12} double twins and <c + a> pyramidal dislocations were observed in the fatigued samples, and no obvious grain refinement was observed, only a small amount of twins in the material. Pyramidal slip is one of the deformation mechanisms in magnesium alloy during high cycle fatigue deformation when the stress amplitude is higher than the fatigue strength.

Compound cross-grain boundary extension twin structure and its related twin variant selection in a deformed Mg alloy

Interrupted bi-axial compressions are applied to a hot-rolled Mg alloy with a strong basal texture in order to activate {1 0 −1 2}-{1 0 −1 2} double extension twinning. Compound cross-grain boundary (cross-GB) extension twin structures form after the compressions, which consist of cross-GB secondary twin pairs within their host cross-GB primary twin pairs. Shear accommodation of a twin is proven to be proportional to a geometrical m′ factor and the magnitude of its twinning shear. It is found that 60% of cross-GB twin pairs, including both the primary and the secondary ones, have a value of m′>0.7, indicating twinning shear transmission over GB is a major mechanism. Multi-level twinning shear transmissions over GB contribute prominently to the formation of 40% of the observed compound cross-GB twin structures. About 98.3% of the twins are high Schmid factor (SF) ones, while low SF ones appear due to twinning shear transmission. About 81.7% of secondary twins have a misorientation of 〈0 14 −14 1〉60° with respect to their host grains, indicating a strong confining effect of primary twins. A cross-GB twin pair could form through a mechanism of associated nucleation or isolated nucleation. High m′ value is characteristic of associated nucleation through twin-to-twin accommodation mode. Depending on the process of microstructure evolution, SF, shear accommodation and the confining effect of the matrix if necessary should be synthetically considered to predict twin variant selection related to cross-GB twin pairs.

Mechanical Properties and Microstructural Evolution of Variable-Plane-Rolled Mg-3Al-1Zn Alloy

The microstructural evolution and mechanical properties of AZ31 magnesium alloy produced by variable-plane rolling (VPR) were investigated. Two types of weak textures were formed: basal texture in odd pass and double-peak basal texture in even pass. Dynamic recrystallization (DRX) was observed during the VPR treatment, and the nucleation of grains during DRX was dependent on the coalescence of subgrains. Three types of twins were observed in the VPR treatment: {10-12} extension twins, {10-13} contraction twins and {10-11}-{10-12} double twins. The {10-11}-{10-12} double twinning is the underlying mechanism in the formation of the double-peak texture. Tensile testing revealed improved strength without loss of ductility. The Hall–Petch relationship can be used to describe the strengths in any even pass with the same texture. The significant strengthening is ascribed to the refined grain, twin boundaries, texture hardening, and high dislocation density.