Twinning

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 S3{111} coherent twin boundaries considerably decreases with deformation due to dislocation/twin boundary interactions while S3{112} inco-herent 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.

Classical theories on deformation twinning in face centered cubic (fcc) metals and alloys suggest that mechanical twinning is initiated in high stress conditions, in stage III hardening, at low temperatures or in high strain rate deformation conditions. The twin orientation is achieved by a homogeneous shearing of the matrix, which requires the highly coordinated glide of a/6 < 1 1 2 >-type partial dislocations with the same Burgers vector, with exactly one dislocation gliding on each successive {111}-type twinning planes. Twinning is initiated after slip on the primary slip system and the conjugate system has occurred and a high stress is available to activate the twin nuclei formed by dislocation interactions.
The twin formation modifies the prior dislocation structure, with some initially glissile dislocations of the parent crystal being transformed to sessile dislocations in the twin. The Burgers vector of some dislocations may change when they are transformed in the twin matrix. This process contributes to the strain hardening of the deformation twins. In addition, the dynamic Hall–Petch effect also contributes to strain hardening in polycrystals undergoing deformation twinning as twin boundaries are strong obstacles to glide and the formation of micro-twins gradually reduces the dislocation mean free path. Mechanical twinning in fcc metals does not twin single crystals or grains entirely, and the deformed microstructure always contains layers with the parent orientation alternating with twinned ones. Several deformation twin formation mechanisms have been proposed for polycrystalline metals and alloys, such as the emission of dislocations from periodic grain boundaries ledges, which assumes that twins can be generated directly, without the need to form suitable dislocation junctions. Using molecular dynamics and discrete dislocation dynamics simulation, Weinberger et al. argued that the emission of surface dislocations, nucleated at atomically sharp corners, compensates for dislocation
starvation during the deformation of micro-pillars in which dislocation starvation occurs. Zhu et al. also argued in favor of twinning by partial dislocation nucleation at surfaces, but they did not propose an actual mechanism for the generation of partial dislocations from surfaces. There are fundamental problems with the assumption that surfaces or grain boundaries can be a source of twinning dislocations, such as the fact that the emission of a partial dislocation loop requires a high critical shear stress, and that twin formation requires a precise partial dislocation sequence, i.e. one single partial dislocation on every {111} type slip plane. Partial dislocation emission from a surface has not conclusively been shown to be compatible with these requirements.
When small scale compression testing (in a nano-indentation setup) is used to analyze the deformation of micro-pillars, a characteristic size effect is observed [10,11]: the shear stress to initiate plastic flow increases with decreasing pillar diameter. Greer proposed a dislocation starvation model to explain the size effect. Dislocation source-hardening, which occurs in small samples due to the physical limitation of the dislocation source size, was proposed to account for the considerable strengthening in micron-sized single crystals. The size dependence of the strength of single-ended dislocation sources was verified by three-dimensional discrete dislocation dynamics simulation. Yu reported that the twinning stress in micro-pillars was also size-dependent. Seo et al. suggested that the strength of Au nano-wires, in which twins were formed, was inversely
related to their diameter. Size effects were also reported for TWIP steel micro-pillars deformed in compression. In the present study, the twinning-induced plasticity mechanism of TWIP steel was studied by means of compression tests
on bulk single crystals and micro-pillars, oriented either for perfect dislocation glide or, respectively, for deformation twinning. Transmission electron microscopy (TEM) was used to investigate dislocation interactions and the evolution of the deformation microstructure in the micro-pillars. The dislocation interactions were characterized at the initial stage of the deformation in which
no microstructural evolution had occurred in order to verify whether the yielding mechanism of the micro-pillars was associated with twin formation.
In summary we here study bulk and micro-pillar single crystals regarding the twinning-induced plasticity mechanism in austenitic Fe-22 wt%Mn-0.6 wt%C TWIP steel. Compression of micro-pillars oriented either for deformation-induced twinning or for perfect dislocation glide was carried out for pillars with diameters
in the range of 600 nm to 4 um. The same size dependence of the critical resolved shear stress was observed for both orientations. The critical micro-pillar diameter for size-independent plasticity was approximately 7.6 um. Partial dislocation-mediated formation of twins and e-martensite was observed in micro-pillars oriented for twinning by transmission electron microscopy. The elastic–plastic transition in micro-pillars oriented for deformation twinning did not involve twinning, and dislocation–dislocation interactions were a necessary precondition for twin formation.

Studies on twin pregnancy are uniquely important to Africa and particularly Nigeria where the highest incidence in the world exists. This study was designed to determine the trend, rate, and obstetric outcomes of twin deliveries in the University of Abuja Teaching Hospital, Gwagwalada. This was a retrospective study of twin deliveries in the hospital over a period of 10 years. During the study period, there were 349 twin births out of 10,739 deliveries, giving an overall twining rate of 32.5 per 1,000 deliveries. Preterm delivery occurred in 39.7% cases and was, therefore, the most common complication. Mode of delivery was vaginal in 72.7% while 27.3% were delivered by caesarean section. Emergency caesarean section for delivery of both the babies was carried out in 22.3% while elective caesarean section for both the babies accounted for 1.0 %. Combined vaginal and abdominal delivery occurred in 4.0% of deliveries. The stillbirth rate was 102 per 1,000 births. There were 24 (8.0%) and 37 (12.3%) stillbirths among the first and the second baby respectively. The mean foetal weight was 2.395±0.63 kg while the female-to-male ratio was 1:1.1. The rate of twin deliveries in our centre is high. Successful vaginal delivery of twins is high when the mothers are booked and the presentations of the twins are favourable. The use of antenatal care services and good intrapartum management will help improve outcome in twin pregnancies.

Hydrogen embrittlement of austenitic steels is of high interest because of the potential use of these materials in hydrogen-energy related infrastructures. In order to elucidate the associated hydrogen embrittlement mechanisms, the mapping of heterogeneities in strain, damage (crack/void), and hydrogen and their relation to the underlying microstructures is a key assignment in this field.
Specifically mapping the connection between microstructure heterogeneity
and the associated hydrogen trapping at similar spatial resolution opens a novel pathway to identify hydrogen embrittlement mechanisms in complex alloys.
One of the materials classes that is expected to be applied for energy-related structure parts are austenitic steels with high Mn content. In particular, twinning-induced plasticity (TWIP) high Mn austenitic steels are well known for an exceptional balance of ductility and strength with less hydrogen susceptibility compared to ferritic steels with a similar strength.
The hydrogen embrittlement phenomenon has been observed under severe mechanical deformation and hydrogen charging conditions such as delayed fracture testing in a deep drawn cup or tensile testing during hydrogen charging at a high current density. Fracture was in such cases caused by various metallurgical factors e.g. by the formation and failure of deformation twins. The importance of deformation twins on hydrogen embrittlement of austenitic steels has been recently studied. It was found that they can act as crack initiation sites and enable crack propagation. Microstructure-sensitive hydrogen mapping in a TWIP steel has been conducted by using microprinting experiments, which enable visualization of hydrogen emission from a sample through reduction of silver ions in gelatin-based AgBr emulsion. The microprinting technique demonstrated that hydrogen is indeed localized at deformation twins, hence, promoting the initiation of hydrogen embrittlement. Also, from our previous experiments we suggested that the hydrogen localization at/near deformation twins requires local plastic straining at/before deformation twins. These facts indicate that the hydrogen-assisted twin boundary cracking of TWIP steels is a complex phenomenon including both, local plastic straining and local hydrogenation. To better understand the influence of deformation twins on the hydrogen embrittlement phenomenon, it is thus essential to map the spatial hydrogen distribution through a high resolution detection approach that is sufficiently microstructure and concentration sensitive. It should be noted here that the microprinting technique is only sensitive to hydrogen at relatively
high concentration levels, or more precisely at relatively high emission rates. It has, therefore, to be applied directly after a sample is charged with hydrogen. It will be shown here that hydrogen is critical for prolonged times after charging. Hence, the findings obtained by microprinting needed to be confirmed also for longer times after charging by a more sensitive technique. In the last years several promising novel approaches have been reported for the localized resolved and sensitive detection of hydrogen.
One is a direct electrochemical detection via a capillary cell, developed by Suter et al. which, however, does not provide the resolution required here. Another approach is the use of Kelvin probe techniques, which allow detection of hydrogen in a material by means of the change ofwork function caused by the hydrogen entering the oxide at the surface. Studies at quite high resolution have been carried out with Scanning Kelvin Probe ForceMicroscopy (SKPFM), where diffusion profiles of hydrogen have been mapped successfully at relatively high
resolution at cross section of samples after hydrogen charging.
However, a direct quantification is not possible by this method, due to the complex dependence of the work function of oxide on different defect states in the oxides. For the same reason this approach is not suitable to provide reliable information on hydrogen at different features of the microstructure, because different oxides show a different dependence on hydrogen. A new approach by applying a thin palladium layer eliminates this problem. More specifically, the SKPFM with a thin palladium layer is a novel and efficient method to very sensitively analyze local hydrogen concentrations down to levels well below 0.01 atom ppm at spatial resolutions as small as several tens of nanometers. The SKPFM can hence be used for detecting the hydrogen distribution with high sensitivity, since the potential measured by the SKPFM on the palladium that has been deposited as a thin layer on the hydrogen charged sample surface correlates logarithmically with the hydrogen content in the palladium. Because of this Nernstian-like behavior, which is similar to a hydrogen electrode, as also reported for immersed palladium metal, we also refer to the work function as electrode potential. In fact, as discussed previously, the surface of the palladium is, even in dry nitrogen atmosphere, still covered by an ultrathin water layer, which together with the hydrogen in the palladium leads to the formation of an hydrogen electrode “in the dry”. By exploiting this effect, SKPFM was successfully applied to the spatially resolved detection of hydrogen in ferrite/austenite duplex stainless steels. More specific in this project the hydrogen distribution in a hydrogen-charged Fe-18Mn-1.2C (wt%) twinning-induced plasticity austenitic steel was studied by Scanning Kelvin Probe Force Microscopy (SKPFM). We observed that 1–2 days after the hydrogen-charging, hydrogen showed a higher activity at twin boundaries than inside the matrix. This result indicates that hydrogen at the twin boundaries is diffusible at room temperature, although the twin boundaries act as deeper trap sites compared to typical diffusible hydrogen trap sites such as dislocations. After about 2 weeks the hydrogen activity in the twin boundaries dropped and was indistinguishable from that in the matrix. These SKPFM results were supported by thermal desorption spectrometry and scanning electron microscopic observations
of deformation-induced surface cracking parallel to deformation twin boundaries. With this joint approach, two main challenges in the field of hydrogen embrittlement research can be overcome, namely, the detection of hydrogen with high local and chemical sensitivity and the microstructure-dependent and spatially resolved observation of the kinetics of hydrogen desorption.

The presence and interaction of nanotwins, geometrically necessary dislocations, and grain boundaries play a key role in the mechanical properties of nanostructured crystalline materials. Therefore, it is vital to determine the orientation, width and distance of nanotwins, the angle and axis of grain boundary misorientations as well as the type and the distributions of dislocations in an automatic and statistically meaningful fashion in a relatively large area. In this paper, such details are provided using a transmission electron microscope-based orientation microscopy technique called ASTAR™/precession electron diffraction. The remarkable spatial resolution of this technique (~ 2 nm) enables highly detailed characterization of nanotwins, grain boundaries and the configuration of dislocations. This orientation microscopy technique provides the raw data required for the determination of these parameters. The procedures to post-process the ASTAR™/PED datasets in order to obtain the important (and currently largely hidden) details of nanotwins as well as quantifications of dislocation density distributions are described in this study.

Extrusion of the DC-cast alloy AZ80 has been performed at three extrusion ratios (12, 22 and 44) in the temperature range 175 • C ≤ T ≤ 350 • C. Upon extrusion, the as-cast coarse grains undergo pronounced grain refinement and the ␤-phase constituents form stringers in the extrusion direction. The activation of twinning at low extrusion temperatures promotes nucleation of the recrystallized grains at the twin interfaces and refines the grains size. The higher extrusion ratio increases the extrusion velocity and therefore raises the die exit temperature, which results in a slightly coarsen grain size. The unidirectional deformation and DRX during extrusion leads to a fibrous type of crystallographic texture, which results in anisotropy of the yield stress under tension and compression, usually called "strength differential effect" (SDE). The impact of the SDE weakens by lowering the extrusion temperature and remains unaffected by the variations in the extrusion ratio.

In this study genetic parameters , ( co )variance components and genetic trends were estimated for live weight at birth ( B.W, n = 4844 ) and at weaning ( 3month of age = W.W, n = 4496 ) , 6month of age (6M.W, n = 3544 ) , 9month of age (9M.W, n = 2767) , yearling (12month of age = Y.W, n = 1948), average daily gain of birth to weaning (ADG1, n = 4495) , average daily gain of weaning to 6month of age (ADG2, n = 3530) , average daily gain of 6mpnth to 9month of age (ADG3, n = 2763) and average daily gain of 9month to yearling (ADG4, n = 1902) for flock of breeding station of Makuie sheep. Direct heritability estimates from single-trait analysis were 0.30 for B.W, 0.19 for W.W, 0.41 for 6M.W, 0.38 for 9M.W, 0.33 for Y.W, 0.37 for ADG1, 0.15 for ADG2, 0.15 for ADG3 and 0.18 for ADG4 . Estimates of direct genetic correlation from two-trait analysis were 0.46 between B.W and W.W, 0.38 between B.W and 6M.W, 0.42 between B.W and 9M.W, 0.35 between B.W and Y.W, 0.93 between W.W and 6M.W, 0.93 between W.W and 9M.W, 0.85 between W.W and Y.W, 0.93 between 6M.W and 9M.W, 0.79 between 6M.W and Y.W, 0.88 between 9M.W and Y.W, 0.046 between B.W and ADG1, 0.27 between W.W and ADG1, -0.02 between W.W and ADG2, 0.19 between 6M.W and ADG2, -0.09 between 6M.W and ADG3, 0.09 between 9M.W and ADG3, -0.03 between 9M.W and ADG4, 0.18 between Y.W and ADG4 . For estimating of breeding values ( B.V ) used from single-trait and five-trait analyses ( five-trait analysis only formed for main traits ; B.W, W.W, 6M.W, 9M.W and Y.W). Genetic trends over time was -0.016kg, 0.15kg, 0, 0, 0.15kg, 0, 0.0017kg, 0, and 0 for B.W, W.W, 6M.W, 9M.W, Y.W, ADG1, ADG2, ADG3 and ADG4, respectively.