Dislocation structure in a single-crystal nickel-base superalloy during low cycle fatigue

An investigation of dislocation structure in a single crystal nickel-base superalloy during low cycle fatigue (LCF) at 760 °C has been conducted. Dislocation bands are found to be produced first in the matrix in some defined directions. With an increase in cycle numbers, there is an increase in dislocation density in the bands and a decrease in the spacing between the bands, leading to the formation of the dislocation walls or cells. Sometimes, three-dimensional (3-D) networks are formed also by the interaction between two sets of parallel dislocations. The Burgers vectors of the dislocations in the network are 1/2 〈110〉. Clustering of dislocations eventually occurs at γ′/γ interfaces because of the obstruction of the γ′-particles to moving dislocations. Most of the dislocations observed in the γ′-phases are in the form of superdislocations. Dislocation shearing through theγ′-phase was found occasionally. Reprecipitation of γ′-phase induced by strain was also observed in the present study.     

Atomic structure of interphase boundary enclosing bcc precipitate formed in fcc matrix in a Ni-Cr alloy

The atomic structure of the interphase boundaries enclosing body-centered cubic (bcc) lath-shape precipitates formed in the face-centered cubic (fcc) matrix of a Ni-45 mass pct Cr alloy was examined by means of conventional and high-resolution transmission electron microscopy (HRTEM). Growth ledges were observed on the broad faces of the laths. The growth ledge terrace (with the macroscopic habit plane ) contains a regular array of structural ledges whose terrace is formed by the (111)_fcc//(110)_bcc planes. A structural ledge has an effective Burgers vector corresponding to an transformation dislocation in the fcc → bcc transformation. The side facet (and presumably the growth ledge riser) of the bcc lath contains two distinct types of lattice dislocation accommodating transformation strains. One type is glissile dislocations, which exist on every six layers of parallel close-packed planes. These perfectly accommodate the shear strain caused by the stacking sequence change from fcc to bcc. The second set is sessile misfit dislocations (∼10 nm apart) whose Burgers vector isa/3[111]_fcc =a/2[110]_bcc. These perfectly accommodate the dilatational strain along the direction normal to the parallel close-packed planes. These results demonstrate that the interphase boundaries enclosing the laths are all semicoherent. Nucleation and migration of growth ledges, which are controlled by diffusion of substitutional solute atoms, result in the virtual displacement of transformation dislocations accompanying the climb of sessile misfit dislocations and the glide of glissile dislocations simultaneously. Such a growth mode assures the retention of atomic site correspondence across the growing interface. formerly Graduate Student, Kyoto University, Kyoto 606-01, Japan This article is based upon a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18–22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.     

Distortion correction of tubular crystals: improvements in the acetylcholine receptor structure

Biological molecules often crystallize either as tubes, having helical symmetry, or as two-dimensional sheets. Both sorts of crystal are potentially suitable for structure determination to atomic resolution by electron crystallography, but their lattice distortions must first be corrected. We have developed a procedure for tubular crystals, based on independent alignment of very short segments against a reference structure, that allows accurate determination and correction of distortions in all three dimensions. Application of this procedure to images used previously to determine the 9 A structure of the acetylcholine receptor showed that about half of the signal loss caused by the distortions arises from effects correctable in the image plane (bending, changes in scale) and half from effects requiring out-of-plane correction (variations in tilt and in twist around the tube axis). By dividing the tubes into short segments (of lengths about equal to their diameter) it became possible to recover almost all of this loss without reducing appreciably the accuracy in the segmental alignments. The signal retention improved by only 10% at low resolution (20 A), but by progressively greater amounts at higher resolutions, up to approximately 40% at 9 A. As a result the finer structural details were more clearly resolved. With images of better electron-optical quality, much greater gains in signal retention should be obtained.     

Correction: Snoek-Dominated Internal Friction Response in bcc Steel: Relating Experiments With a Multi-scale Atomistic Computational Framework

Metallurgical and Materials Transactions A -     

Computer simulation of a random distribution of nonequiaxial inclusions in a two-phase matrix structure

A program for the computer simulation of a random distribution of constant-size nonequiaxial rectangular inclusions (l _x , l _y , l _z ) arranged in a specified volume with sides X, Y, and Z in length is described in order to subsequently determine the dependence of the projection (P) of inclusions onto the orthogonal planes on the thickness (L) of the analyzed layer of the form P = V + AL ^B , where V is the volume number of inclusions and A and B are the empiric coefficients and to evaluate other parameters of the structure. The use of this program yielded the acquisition of the P−L curves for the representative volume and various combinations of inclusion numbers and sizes for the first time. The results will be further applied in order to investigate the relationship between new (P, A, B) and conventional (V, l _i , L _i , S) characteristics of the structure, as well as to consider their relations with properties.     

Snoek-Dominated Internal Friction Response in bcc Steel: Relating Experiments With a Multi-scale Atomistic Computational Framework

Metallurgical and Materials Transactions A - Internal friction is often sensitive to microstructural features. However, there is a clear absence of rational approach for decoupling internal...     

Home vaccination for children behind in their immunisation schedule: a randomised controlled trial

OBJECTIVE: To ascertain the effectiveness of a home vaccination service for children behind in their vaccination schedule. DESIGN: Randomised controlled trial of nurse-administered vaccination at home. Children were allocated at random to the intervention or the control group before any contact with the parents was made. SETTING: 10 council areas in north-west metropolitan Melbourne defined by 56 postcode zones. Six-week intervention period from November 1996. PARTICIPANTS: 405 children--all those in the study area (n = 2610) 90 days late (age 9 months) for their third diphtheria-tetanus-pertussis/poliomyelitis/Haemophilus influenzae type B (DTP/OPV/Hib) vaccination, or 120 days late (age 16 months) for their measles-mumps-rubella (MMR) vaccination, according to the Australia Childhood Immunisation Register. MAIN OUTCOME MEASURES: Number of children completing DTP/OPV/Hib or MMR during the intervention period, and number up to date before intervention. RESULTS: Verification of vaccination status with the parents revealed that 123 (60%) of the children in the intervention group and 113 (56%) of those in the control group were up to date with their vaccinations, leaving a study population of 81 (intervention group) and 88 (control group). Vaccination was achieved in 46 (57%) intervention children and 24 (27%) control children (risk ratio [RR], 2.08; 95% CI, 1.4-3.1; P < 0.001). For DTP/OPV/Hib, 18/32 (56%) intervention children and 12/36 (33%) control children were vaccinated, (P = 0.06). For MMR, 28/49 (57%) and 12/52 (23%) children were vaccinated, respectively (P < 0.001). Home vaccinations were completed with 26 families (including five siblings). The average cost per child vaccinated as a result of the home program was $92.52. CONCLUSION: Home vaccination for children behind in their immunisation schedule is an effective, acceptable and relatively cheap method of completing recommended vaccinations. We recommend that a home vaccination program be widely implemented and made available, particularly for disadvantaged families.