Three-dimensional character of the deformation twin in magnesium.

Deformation twins are three-dimensional domains, traditionally viewed as ellipsoids because of their two-dimensional lenticular sections. In this work, we performed statistical analysis of twin shapes viewing along three orthogonal directions: the 'dark side' (DS) view along the twin shear direction (η1), the twinning plane normal (TPN) view (k1) and the 'bright side' (BS) view along the direction λ(=k1 × Î·1). Our electron back-scatter diffraction results show that twins in the DS and BS views normally exhibit a lenticular shape, whereas they show an irregular shape in the TPN view. Moreover, the findings in the TPN view revealed that twins grow faster along λ the lateral direction than along η1 the forward propagation direction at the initial stages of twin growth. These twin sections are irregular, indicating that growth is locally controlled and the overall shape is not perfectly ellipsoidal. We explain these findings using atomistic models, and ascribe them to differences in the mobility of the edge and screw components of the twinning dislocations.

Characterizing the role of adjoining twins at grain boundaries in hexagonal close packed materials.

Hexagonal close packed (HCP) Mg and Zr are being used in transportation and nuclear industries, respectively. The ductility and formability of these materials is significantly limited by the activation of prevalent deformation twinning. Twins in HCP polycrystals usually nucleate at grain boundaries (GBs), propagate into the grain, and they either terminate at opposing GBs (isolated-twins) or transmit into a neighboring grain (adjoining-twin-pairs: ATPs). Because twin interfaces provide a path for crack propagation, twin transmission is relevant to material ductility. This study combines electron backscatter diffraction (EBSD) based statistical analysis of twinning microstructures and crystal plasticity modeling, to characterize twin thickening processes away from and near GBs. Analysis of deformed Mg and Zr microstructures reveals that local twin thicknesses at GBs are statistically larger for ATPs compared to isolated-twins. Further, thicknesses are found to decrease with increasing GB misorientation angle. Full-field Fast-Fourier-Transform micromechanics modeling shows that shear-transformation induced backstress are locally relaxed at GBs for ATPs, but not for isolated-twins. As a consequence, ATPs can thicken locally at GBs and the preferential site for twin thickening shifts from the middle of the twin to common GB.

Deformation twinning and grain partitioning in a hexagonal close-packed magnesium alloy.

Pervasive deformation twinning in magnesium greatly affects its strength and formability. The local stress fields associated with twinning play a key role on deformation behavior and fracture but are extremely difficult to characterize experimentally. In this study, we perform synchrotron experiments with differential-aperture X-ray microscopy to measure the 3D stress fields in the vicinity of a twin with a spatial resolution of 0.5 micrometer. The measured local stress field aids to identify the sequence of events involved with twinning. We find that the selected grain deforms elastically before twinning, and the twin formation splits the grain into two non-interacting domains. Under further straining one domain of the grain continued to deform elastically, whereas the other domain deforms plastically by prismatic slip. This heterogeneous deformation behavior may be mediated by the surrounding medium and it is likely to lead to asymmetric twin growth.

Grain neighbour effects on twin transmission in hexagonal close-packed materials.

Materials with a hexagonal close-packed (hcp) crystal structure such as Mg, Ti and Zr are being used in the transportation, aerospace and nuclear industry, respectively. Material strength and formability are critical qualities for shaping these materials into parts and a pervasive deformation mechanism that significantly affects their formability is deformation twinning. The interaction between grain boundaries and twins has an important influence on the deformation behaviour and fracture of hcp metals. Here, statistical analysis of large data sets reveals that whether twins transmit across grain boundaries depends not only on crystallography but also strongly on the anisotropy in crystallographic slip. We show that increases in crystal plastic anisotropy enhance the probability of twin transmission by comparing the relative ease of twin transmission in hcp materials such as Mg, Zr and Ti.

Characterizing the boundary lateral to the shear direction of deformation twins in magnesium.

The three-dimensional nature of twins, especially the atomic structures and motion mechanisms of the boundary lateral to the shear direction of the twin, has never been characterized at the atomic level, because such boundary is, in principle, crystallographically unobservable. We thus refer to it here as the dark side of the twin. Here, using high-resolution transmission electron microscopy and atomistic simulations, we characterize the dark side of deformation twins in magnesium. It is found that the dark side is serrated and comprised of coherent twin boundaries and semi-coherent twist prismatic-prismatic boundaries that control twin growth. The conclusions of this work apply to the same twin mode in other hexagonal close-packed materials, and the conceptual ideas discussed here should hold for all twin modes in crystalline materials.

Automatic twin statistics from electron backscattered diffraction data.

A new computer code has been developed to automatically extract quantitative twin statistics from electron backscatter diffraction data. The new code is an improvement upon previous codes in that it handles materials of any crystal symmetry, type I, Type II and compound twins, and general stress states. Moreover, accuracy of the results has been greatly improved. In addition, twin statistics including number, area fraction, twin thickness and twinning dependencies on orientation, grain size and neighbourhood effects can be routinely analysed. The new code has been applied to scan data from deformed magnesium, zirconium and uranium, and can potentially be used for any twinning material for which reliable electron backscatter diffraction results can be obtained.

Compact and dissociated dislocations in aluminum: implications for deformation.

Atomistic simulations, confirmed by electron microscopy, show that dislocations in aluminum can have compact or dissociated cores. The calculated minimum stress (sigma(P)) required to move an edge dislocation is approximately 20 times smaller for dissociated than for equivalent compact dislocations. This contradicts the well accepted generalized stacking fault energy paradigm that predicts similar sigma(P) values for both configurations. Additionally, Frank's rule and the Schmid law are also violated because dislocation core energies become important. These results may help settle a 50-year-old puzzle regarding the magnitude of sigma(P) in face-centered-cubic metals, and provide new insights into the deformation of ultra-fine-grained metals.

Diagnosis in adolescent inpatients: diagnostic confidence and comparison of diagnoses using ICD-9 and DSM-III.

In a prospective Multi-Centre research study involving four British Adolescent Psychiatry Inpatient Units, all 276 adolescent admissions were diagnosed using both ICD-9 and DSM-III. Ratings of diagnostic confidence for each diagnostic group varied considerably. Clinicians recorded high levels of confidence in the diagnosis of both Schizophrenia and Anorexia nervosa, while considerably lower scores were noted for Adjustment Disorders (ICD-9 3-digit code 309.), and for Emotional Disorders (ICD-9 3-digit code 313.). Exact concordance in diagnoses made for each case using both ICD-9 and DSM-III occurred in 72% of the entire cohort. The dissimilar concordance rates comparing ICD-9 and DSM-III diagnoses for the same patient admissions suggest differences in validity measures, such as face validity and content validity, in the reliability of diagnostic criteria between similarly named disorders across the two systems, and in the clinician's training and attitude to diagnosis. With the recent introduction of DSM-IV following upon ICD-10, further comparative studies are needed to examine both reliability and validity issues into diagnosis and classification in child and adolescent psychiatry.

Psychosocial adversity in an adolescent psychiatry inpatient cohort: WHO ICD-10 findings from a multicentre prospective study.

Sixteen Axis 5 ICD-10 codes were applied to a prospective multicentre study of 276 adolescent psychiatric inpatients. Frequency distributions were obtained for the whole cohort and separately for each of the seven main ICD-9 psychiatric diagnoses. Certain abnormal psychosocial situations were associated with particular diagnoses, and many significant correlations were found between Axis 5 codes. Factor analysis with varimax rotation produced four factors accounting for 46% of the total variance.

Psychosocial consequences of head injury in children and adolescents: implications for rehabilitation.

Studies measuring psychosocial outcome in children and adolescents have shown that head injury leads to cognitive impairment which is directly related to the severity of injury in those with very severe head injury. Psychiatric disorders are also related to the severity of injury but here the relationship suggests that mediating factors are involved. No specific pattern of post-traumatic psychological/psychiatric dysfunction emerges from the studies, but it is clear that, as with adults, psychosocial recovery lags behind physical. Head injury affects the functioning of the young person in the family, at school, and within the wider community, often resulting in a secondary handicap of low self-esteem. The multitude of deficits which are a consequence of severe head injury present a challenge for rehabilitation specialists. A multi-disciplinary, multi-specialist, and multi-agency response is required. As a result, families are often presented with a bewildering array of treatments and programmes at different agencies. A case manager can be helpful in ensuring the appropriate use of available resources and can be the one professional in charge of a coordinating case record.

Rehabilitating severely head-injured adolescents: three case reports.

The case reports of 3 severely head-injured adolescents, who developed psychiatric sequelae after the trauma, are presented. All 3 cases displayed various socially disinhibited behaviours. Individually tailored treatment programmes helped these young people manage their maturational tasks of adolescence. Several issues, central to this rehabilitative work, are discussed, including the personal significance of the adolescent's injury, identity issues and defence mechanisms; a family perspective on coping with the injured adolescent and the components of a therapeutic package.

Alcohol-dependent individuals sixteen years on.

Fifty-seven married individuals were followed up 16 years after treatment for alcohol dependence in 1965. The tracing located 96% of subjects. At interview an assessment of past alcohol-related problems between 1965 and 1981, and of recent possible alcohol dependence was made. No current psychiatric morbidity was found in 54% of interviewees. Fewer alcohol-related problems between 1965 and 1981 were experienced by older individuals and those of higher social class. Subjects classified as problem drinkers (non-alcohol-dependent individuals with significant alcohol-related problems) by 1981 had experienced more alcohol-related accidents during follow-up than those of other outcome groups. Individuals classified as bout drinkers in 1965 had the poorest 1981 outcomes. Outcome classification and results involved 14.5% abstinent, 20% controlled drinkers 7.4% problem drinkers, 14.5% alcohol-dependent and 43.6% deceased. The observed to expected mortality ratio of the subjects was 1.6, with the female ratio twice that of the males.