Influence of crystallography upon critical nucleus shapes and kinetics of homogeneous f.c.c.-f.c.c. nucleation—V. The origin of GP zones in AlAg and AlCu alloys

GP zone formation in aluminum-base alloys has been proposed, by previous investigators, to take place by spinodal decomposition or by homogeneous nucleation and growth. The possibilities of these two mechanisms are examined here for two alloy systems, AlAg and AlCu, representing two extreme cases, a narrow and a (potentially) very wide miscibility gap, respectively, resulting in low and high interphase boundary energies. The coherent spinodal and the temperature-composition curve corresponding to abundant homogeneous f.c.c.-f.c.c. nucleation were calculated for both systems. In AlAg alloys, the nucleation barrier is so low that negligible undercooling is needed to achieve abundant nucleation. Hence cooling into the spinodal region with ordinary quenching rates is virtually impossible. In AlCu, however, the inverse situation is present and GP zone formation by (or aided by) spinodal decomposition should be feasible if the diffusivity remains sufficiently high in the spinodal region. Detailed evaluation of the latter proposal is prevented, however, by the incompleteness of thermodynamic information on f.c.c. AlCu alloys.     

Influence of crystallography upon critical nucleus shapes and kinetics of homogeneous f.c.c.-f.c.c. nucleation—III. The influence of elastic strain energy

The influence of crystallography upon critical nucleus shapes and kinetics of homogeneous f.c.c.-f.c.c. nucleation was studied by combining the discrete lattice point non-classical model utilized in paper II of this series with the microscopic theory of strain energy as applied by Cook and de Fontaine to compositional fluctuations in cubic lattices. This permits simultaneous consideration of the influence of anistropic interfacial energy and anisotropic strain energy upon nucleation. This is extremely difficult to do using other models of strain energy because of mathematical problems. The model is applied to f.c.c. nuclei in AlCu and CuCo alloys. Nuclei in AlCu are plates because coherency strain energy predominates in this system. In CuCo alloys, the misfit is smaller and the distortion tensor is cubic; thus the nuclei are essentially spherical. Despite this lesser effect, the influence of strain energy upon the nucleation kinetics of Co-rich precipitates in a Cu-rich CuCo alloy is found to be very important, reducing these kinetics by as much as five orders of magnitude.     

Influence of crystallography upon critical nucleus shapes and kinetics of homogeneous f.c.c.-f.c.c. nucleation—IV. Comparisons between theory and experiment in CuCo alloys

A discrete lattice point model (Cook, de Fontaine and Hilliard) which incorporates strain energy (Cook and de Fontaine), described in earlier papers, has been used to determine the ranges of temperature and composition at which homogeneous nucleation kinetics of f.c.c. precipitates in CuCo alloys would be neither too fast nor too slow to be measured. These predictions proved successful and it was possible to measure experimentally nucleation kinetics in CuCo alloys containing from 0.5 to 1.0 at.% Co within 50°C temperature ranges. Experimental results were compared with theoretical values obtained from the discrete lattice point, the Cahn-Hilliard continuum and the classical theories of homogeneous nucleation. Very good agreement was obtained between the experiments and all three theories. Although surprising at first, the good matching between classical theory and experiments was explained by showing that the calculated concentration profiles of critical nuclei at the temperatures and alloy compositions experimentally studied did show distinct “volumes” and “interfaces” i.e. the solute concentration did not vary continuously throughout the nuclei. In this case, as pointed out in effect by Cahn and Hilliard, classical nucleation theory indeed applies. These findings provide the first strong support for the essential correctness of homogeneous nucleation theory.     

Fracture toughness of f.c.c. nickel and strain ageing b.c.c. iron in the temperature range 77–773 K

Ductile initiation fracture toughness J_IC of b.c.c. Armco iron and f.c.c. nickel has been measured in the temperature range 77–773 K. Armco iron exhibits dynamic strain ageing (DSA) in the temperature range 383–573 K while nickel of the purity used does not evince DSA. Load vs load line displacement (LLD) plots during fracture toughness testing of Armco iron show serrations in the temperature range 383–573 K similar to those observed in the tensile stress-strain curves. DSA is found to have a beneficial effect on the fracture toughness J_IC. A marked increase in tensile strength and fracture toughness occurs in Armco iron in the DSA regime. The strain hardening exponent, known to have a bearing on the plastic zone size and the void growth rate, seems a clear parameter in terms of which the observed J_IC variation with temperature can be understood. Remarkably, the variation of n with temperature is found to closely follow the observed trend in _IC. However, the slope of the J-R curve, dJ/da, decreases in the DSA regime with a minimum at 423 K. The decrease has been related to the fracture propagation process which is shown to occur by an alternate fast fracture and the ductile dimpled mode in the DSA regime. In the case of nickel, free from DSA, J_IC or dJ/da are found to be largely unaffected by the test temperature. At room temperature f.c.c. nickel, at closely matching strength levels, possesses higher fracture toughness as compared to b.c.c. Armco iron. The crystal structure effect is more pronounced at temperatures below the ambient. At 77 K, the fracture toughness of iron is drastically reduced due to the onset of cleavage while nickel, not prone to a change in the fracture mode, maintains the same level of J_IC, as at the ambient.     

On the study of the f.c.c.-h.c.p. martensitic transformation using a diffraction approach—II. H.c.p. → f.c.c. transformation

Theory of diffraction from h.c.p. crystals undergoing martensitic transformation to the f.c.c. structure through a non-random insertion of deformation faults is developed. From a comparison of the theoretically predicted diffraction effects with those experimentally observed on single crystals of ZnS, it is shown that the thermally induced transformations in this material are growth controlled while the deformation induced transformation is nucleation controlled.     

On the study of the f.c.c.-h.c.p. martensitic transformation using a diffraction approach—I. F.c.c. → h.c.p. transformation

A new approach based on diffraction evidence for the involvement of shear at the atomic level is proposed to elucidate the mechanism of the f.c.c. to h.c.p. martensitic transformation. This approach is based on the analysis of the intensity distribution along streaked reciprocal lattice rows observed on diffraction patterns recorded from partially transformed f.c.c. crystals. The relevant theory of diffraction from crystals undergoing f.c.c.-h.c.p. transformation by non-random insertion of deformation faults is developed. This approach, as distinct from the conventional microstructural approach, can be especially useful in situations where the transformation strains are self accommodating in nature, such that there is no observable surface relief effect.     

On kinked screw dislocations in the b.c.c. lattice—I. The structure and peierls stress of isolated kinks

The core structures and lattice friction stresses of single kinks on a screw dislocation in the b.c.c. lattice have been calculated by means of computer simulation. Interatomic binding potentials derived from first principles for potassium and by empirical methods for an iron-like material have been used. It is shown that a wide variety of kinks can exist on the dislocation, with core structures largely independent of the potential. The width of the kinks, i.e. the spatial extent of the kink displacement field parallel to the dislocation line, is found to vary strongly with potential, as does the kink Peierls stress. The calculated Peierls stresses, however, are found to be too small to explain the observed temperature dependence of the flow stress in real materials.     

Theoretical analyses of 〈111〉 pencil glide in b.c.c. crystals

A new theoretical analysis of 〈111〉 pencil glide in b.c.c. crystals is proposed. Complete sets of equations are given for one, two, three or four active slip systems. The yield surface for b.c.c. single crystals deforming by pencil glide is described, and the closed subspaces are listed. The cases where stress or shear rate and rotation ambiguities occur are also detailed. A correspondence is shown to exist between the plastic deformation rates that are induced by the classical eight solutions in pencil glide, and those associated with the Bishop and Hill vertices for {110}〈111〉 slip. Each six-fold vertex in groups B and D is related to a particular three-system pencil glide solution, and each eight-fold vertex in groups A, C, and E with one four-system solution as well as four three-system solutions. The analysis shows that the cones of normals of the Bishop and Hill vertices have very different extensions, the solid angle pertaining to the group C vertices being an order of magnitude greater than those of group A. The Taylor (prescribed strain) problem for b.c.c. crystals is treated in this way and it is shown that the necessary calculations are reduced by factors of three to six when compared with the previous methods. In particular the number of three-system solutions to be analyzed, which require a special numerical procedure, is reduced to a minimum. The Taylor factor for isotropic aggregates of b.c.c. crystals with pencil glide is also calculated for various imposed strain rates. Finally, the well known analyses of pencil glide available in the literature are compared; the correspondence between the notations is tabulated, and the approaches and sets of results obtained are evaluated and discussed in turn.     

Kinetics of the martensitic f.c.c.→ h.c.p. transformation in CoCrMo alloy powders

The kinetics of isothermal martensìtìc nucleation in CoCrMo alloy powders is monitored. It is shown that a linear relation exists between the activation energy for martensite formation and the chemical free energy for the f.c.c. → h.c.p. transition. The nucleation kinetics is governed by the thermally activated motion of partial dislocations and the interaction between the Shockley partials and short range obstacles (stacking faults intersections and solute clusters).     

Structure-energy correlation for grain boundaries in f.c.c. metals—IV. Asymmetrical twist (general) boundaries

The zero-temperature energies and equilibrium volume expansions of pont-defect free asymmetrical grain boundaries (GBs) in f.c.c. bicrystals have been determined using both an embedded-atom-method potential fitted to Au and a Lennard-Jones potential fitted to Cu. The boundaries chosen involve one of the four densest planes on one side of the interface and a higher-index plane on the other. For both potentials it is found that the two asymmetrical tilt GBs at the endpoints of the GB-energy vs twist-misorientation curves give rise to pronounced energy cusps. These cusps are probably the main reason for the dominating appearance of tilt boundaries in rotating sphere-on-a-plate experiments, in spite of the fact that these boundaries represent an infinitely small subset in the total misorientation space comprised by the five macroscopic parameters needed to specify a general grain boundary. A comparison with earlier work on symmetrical GBs suggests that, except for the two densest lattice planes, symmetrical configurations may have higher energies than asymmetrical GBs with a densest plane on one side of the interface. A practically linear correlation between the energy of a GB and its volume expansion is observed, similar to that for the symmetrical GBs investigated earlier.     

Cold work hardening in stages IV and V of F.C.C. metals—I. Experiments and interpretation

The paper presents numerous measurements which confirm stages IV and V to be general ranges of cold work deformation. Analogous to stage II, stage IV exhibits a linear athermal hardening with constant strain rate sensitivity and activation enthalpy. In stage IV the dislocation cell size is constant, while the dislocation density growth rate is markedly reduced compared with stages II and III. Features of stage V are analogous to stage III, the increase of strain rate sensitivity (decrease of activation enthalpy) indicating the onset of thermally activated dislocation annihilation. In stage V, the mechanism is identified as dislocation climb from observing subgrain formation and saturation in density of deformation induced vacancies. Comparisons with recent investigations of stage IV and V at high temperatures suggest a common picture of low and high temperature deformation which only requires principles of storage and annihilation for both screw and edge dislocations.     

Monte-carlo simulations of interstitial diffusion and trapping—I. One type of traps and dislocations

The long-range tracer diffusion coefficient of interstitials their jump rate and their distribution among different sites in one-, two- and three-dimensional lattices have been determined by Monte-Carlo calculations. In one part of this study traps or antitraps were introduced with equilibrium site energies being lower or higher, respectively, and energies of the surrounding saddle points being higher or lower when compared with the normal sites. Already a small decrease of the saddle point energies around the trap sites, reduces the diffusion coefficient remarkably. For an overall constant saddle point energy the results could be compared with various trapping models, but for lower or higher energy barriers around the trap sites new analytical solutions for the site occupancy and the diffusivity had to be derived which are in agreement with the computer calculations. According to the theoretical results the free energy of a site is also determined by its accessibility where lower saddle points decrease the free energy and vice versa. In a second part of this study a site energy distribution was constructed which corresponds to the elastic interaction energy between an interstitial solute and an edge dislocation. Again the segregation of solute atoms in trap sites of the dislocation is strongly dependant on the distribution of saddle point energies and remarkable deviations from the treatment of Cottrell and Bilby occur if the saddle point energy is lowered in the same way as the energy of the equilibrium points.     

Older adults’ recognition of bodily and auditory expressions of emotion.

This study compared young and older adults’ ability to recognize bodily and auditory expressions of emotion and to match bodily and facial expressions to vocal expressions. Using emotion discrimination and matching techniques, participants assessed emotion in voices (Experiment 1), point-light displays (Experiment 2), and still photos of bodies with faces digitally erased (Experiment 3). Older adults’ were worse at least some of the time in recognition of anger, sadness, fear, and happiness in bodily expressions and of anger in vocal expressions. Compared with young adults, older adults also found it more difficult to match auditory expressions to facial expressions (5 of 6 emotions) and bodily expressions (3 of 6 emotions). (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Friction properties of composites of the system TiN−AlN. I. Effect of structure and phase composition of friction and wear of materials of the system TiN−AlN

The tribological properties of ceramic composites of the system TiN?AlN are studied in the concentration range 10–90% AlN. It was found that materials which contain 25, 50, and 75% AlN have a low friction coefficient. Their rate of wear when paired with steel is neglible—2.9–6.0 μm/km (the wear rate of a steel-steel 024 couple is 1000 μm/km). Thin oxide films are formed during friction at high velocities and pressures. These films have relatively high adhesion with respect to materials of the TiN?AlN system and relatively low adhesion with respect to steel. The films may act as a solid lubricant, thus reducing the friction coefficient and wear. This is particularly true of the materials 25% TiN?75% AlN and 50% TiN?50% AlN.     

Life manifestations and nursing. A discussion of K.E. L?gstrup''s ethics with starting point of Kari Martensen''s and Patricia Benner''s dialogs

This article discusses philosophical questions of ethics dealing with nursing as well as other human activities. The purpose is: 1) To communicate the content, status and function of the phenomenon "manifestation of life", as described by the Danish philosopher and theologian K.E. L?gstrup. 2) To discuss what L?gstrup has to teach us about ethics of care and from this point of view to discuss ethics in relation to the development of nursing. Inspired by the ongoing dialogue between the two nurses and professors Patricia Benner and Kari Martinsen about ethics in nursing the article deals in depth about with the central phenomenon "manifestation of life" as seen in relation to the positions of Benner and Martinsen. The relevance of this theme in relation to nursing is considered, and the article argues, that nurses have an obligation to matter-of-factness, and to develop nursing not only from practical experiences and reflections on concrete actions, but also by scientifically based activities. Both approaches are necessary. They are mutual conditions for development of good nursing practice.