On the atomic structure of an asymmetrical near Σ = 27 grain boundary in copper

The atomic structure of an asymmetrical near Σ = 27 {525} tilt grain boundary (GB) in copper is determined by coupling high-resolution transmission electron microscopy and molecular dynamics simulation. The average GB plane is parallel to {414} in crystal (1) and {343} in crystal (2). The detailed GB structure shows that it is composed of facets always parallel to {101} and {111} in crystals (1) and (2), respectively. The atomic structure of one facet is described using the structural units model. Each facet is displaced with respect to its neighbours by a pure step, giving rise to the asymmetry of the GB plane orientation. The energy of this asymmetrical GB is significantly lower than that of both the {525} symmetrical and the {11,1,11}/{111} asymmetrical Σ = 27 GBs. One GB region displays another atomic structure with a dislocation that accounts for the misfit between interatomic distances in the {414} and {343} GB planes.     

On the interactions between dislocations and a near-Σ=3 grain boundary in a low stacking-fault energy metal

The interactions of dissociated lattice dislocations with a near-Σ = 3 grain boundary (GB) in copper have been investigated by transmission electron microscopy using the weak-beam technique combined with bright-field image matching. From our observations, Shockley partials are required to recombine when entering the GB to form an absorbed perfect lattice dislocation. Then, decomposition of the latter into two displacement shift complete (DSC) products occurs. Complex reactions between DSC dislocations yield further stress relaxation. The role of the stacking-fault energy in the dislocation–GB interactions is pointed out.     

Pinning of grain boundary migration by a coherent particle

We studied single-particle pinning of grain boundary (GB) migration during grain growth. A phase-field model was formulated to simulate the pinning by a coherent particle and validated quantitatively by comparison with analytical prediction. A study of GB migration velocity using this model revealed that second-phase coherent particles have a previously unknown restraining effect over the whole of the GB-particle interaction range, which is qualitatively different from the interaction between GB and incoherent particles.     

Grain boundary phase observed in Al–5 at.% Zn alloy by using HREM

The nature and behaviour of grain boundary (GB) phases is very important since they can control strength, plasticity, resistivity, grain growth, corrosion resistance, etc, especially in nanocrystalline materials. For nanocrystalline Al-based light alloys, extremely high plasticity has been observed in restricted temperature and concentration intervals close to the solidus line. This phenomenon is not fully understood. It can be explained by formation of GB phases not included in the bulk phase diagram. Therefore, the structure of GB phases, as well as thermodynamic conditions for their existence, has to be carefully studied. In this work the structure and composition of GBs and GB triple junctions in Al–5 at.% Zn polycrystals annealed in the temperature region above and below the bulk solidus line were studied by high-resolution electron microscopy and analytical transmission electron microscopy. Evidence has been obtained that a thin layer of a liquid-like phase exists in GBs and GB triple junctions slightly below the bulk solidus line.     

Stress-enhanced grain growth in a nanostructured aluminium alloy during spark plasma sintering

In this study, we report on the influence of high pressure on the microstructure evolution of cryomilled nanostructured Al alloy powders during spark plasma sintering (SPS). Our experimental results suggest that the particular mechanism that governs grain growth during SPS depends on the magnitude of the applied pressure. In the case of material consolidated at a high pressure (e.g. 500 MPa), grain coarsening occurs via a combination of thermally activated grain boundary (GB) migration, stress-coupled GB migration and grain rotation-induced grain coalescence. In contrast, in the case of the material consolidated at a low pressure (50 MPa), grain growth occurs primarily via thermally activated GB migration.     

Interactions of persistent slip bands with a grain boundary on the common primary slip plane in a copper bicrystal

In this letter, the dislocation patterns on the common primary slip plane in a fatigued \[1-34]-\[182-7] copper bicrystal with a Sigma=19b grain boundary (GB) have been investigated using the electron channelling contrast technique in a scanning electron microscope. The results show that the two-phase dislocation structure, such as veins and persistent slip band (PSB) walls, embedded within veins, can be clearly seen on the common primary slip plane. In particular, the interactions of PSBs with the GB are clearly revealed. It is found that there are three kinds of interaction mode between the GB and the dislocations during cyclic deformation, and those are discussed. It is suggested that the dislocations carried by PSBs cannot transfer through the GB continuously even though the bicrystal has a common primary slip plane and its surface slip bands are continuous across the GB.     

Relationship between the fatigue cracking probability and the grain-boundary category

In this paper, the probability of fatigue cracking along different kinds of grain boundary (GB) and persistent slip bands (PSBs) is considered in the light of data obtained by cyclic deformation of copper bicrystals and columnar crystals. It is found that, in copper bicrystals, fatigue cracks always nucleate and propagate along large-angle GBs, irrespective of whether the GB is perpendicular, parallel or inclined to the stress axis. On the contrary, for columnar copper crystals containing small-angle GBs, PSB-matrix interfaces become the preferential sites for initiation of fatigue crack; fatigue cracking along the small-angle GBs was never observed. For a special [1-34]/[182^-7] copper bicrystal with a Sigma = 19b GB and a common primary slip plane, GB cracking also results in fatigue failure. Based on the results above, the interactions of dislocations carried by PSBs with GBs, including 'pile-up of dislocations', 'passing through of dislocations' and 'partial passing-through dislocations', are discussed. It is suggested that the probability of fatigue cracking in fatigued copper crystals increases in the order of small-angle GBs, PSBs and large-angle GBs.     

Comments on ‘A model of diffusion-induced grain boundary migration’ by T. K. Chaki

Abstract

Chaki (1988) has recently proposed a model for diffusion-induced grain boundary migration (DIGM). In his Letter he concluded that the interface energy of a grain boundary and the free energy of mixing are responsible for DIGM and, from the equations he derived, it was claimed that they can explain many DIGM experimental results. However, on examining his model closely there appear to be some fundamental difficulties. The following are comments on his Letter:

(1) In Chaki's calculation of ΔG_s he assumed that, after interface 1 had moved a distance δx, the radius of curvature of this interface increased from R to R + δx, which gives a surface energy drop of 2γV_m δx/R². However from many DIGM observations (for example Balluffi and Cahn (1981)) the curvature of a migrating boundary is increased rather then decreased, that is, the surface free- energy term actually prevents a grain boundary from migrating rather than helping it!

(2) Chaki considered ΔG_cryst during the migration of interface 2. It is also necessary to consider AGcrys, during the migration of interface 1, since within the migration distance δx the structure is changed from a crystalline structure to a grain boundary core structure. ΔG_m should also be considered during the migration of interface 2 since the concentration of the area swept by interface 2 will not be the same as that of a grain boundary core.     

Schedules of reward shift in the double runway

Eighty food deprived rats received 62 trials in a double runway. On Trials 1-30, reward in the first goal box (GB1) was either always two food pellets or always zero pellets. All subjects received two pellets in the second goal box (GB2). On Trials 31-62 subjects in each preshift group (GB1 reward or GB1 nonreward) were shifted to the opposite GB1 reward level on 0, 25, 50, 75 or 100% of occasions. GB2 reward remained unaltered in all cases. For subjects experiencing reward decrease, second runway (A2) run and goal speeds after nonreward were generally enhanced, both within-group and in comparison with never rewarded controls. No such effect was evident on A2 start speed, nor was there any evidence to suggest that A2 performance after decreased reward was a function of the schedule of decrease. Increased GB1 reward resulted in general within-group impairment of A2 start and run speeds, with no effect on A2 goal performance. However, comparisons of speeds after increased reward with those of always rewarded controls revealed no difference on A2 start or run but indicated impairment of A2 goal performance. With the 50% schedule of reward increase, A2 run speeds after nonreward (the training level) exceeded those of never rewarded controls. Results are discussed with reference to McHose's contrast account of double runway phenomena and Amsel's frustration theory.     

Self-diffusion activation energies in α-Al2O3 below 1000°C – measurements and molecular dynamics calculation

Results from impedance spectroscopy measurements at temperatures between 400 and 1000°C, for single crystal and highly pure and dense polycrystalline α-Al₂O₃ samples with well-defined grain size, are compared with that from molecular dynamics calculation. Between 650 and 1000°C, the measured activation energy for conductivity is 1.5?eV for the single crystal, and increases from 1.6 to 2.4?eV as the grain size decreases from 15 to 0.5?µm. The molecular dynamics calculation leads to the conclusion that the self-diffusion activation energy is about 1.5?eV for O and 1.0?eV for Al in single crystal α-Al₂O₃. The much higher mobility of O ions makes the O ions responsible for the conductivity of the single crystal oxide. It seems that the grain boundary leads to an increase in the activation energy. However, the quantitative influence of grain boundary still needs to be explained. Between 400 and 650°C, the measured activation energy is about 1.0?eV and independent of the grain size.     

Impact of coherent and incoherent twin boundaries on the microhardness of annealed nanocrystalline Ni–W alloys

The microstructural evolution of nanocrystalline Ni–W alloys with annealing temperature and more specifically grain boundary (GB) character is investigated through several techniques and correlated with the hardening behaviour. It is shown that two distinct regions can be identified in relation to the annealing temperature and the microstructural evolution. At temperatures below 550 °C (Regime I), a small increase in grain size is observed and is accompanied by a significant hardening and an increase in the fraction of Σ3 incoherent twin boundaries. At temperatures above 550 °C (Regime II), the thermal stability is overcome and important grain growth occurs with a decrease in both the volumic fraction of GBs and the microhardness. It is suggested that the microhardness evolution during heat treatment is influenced by two opposing processes: an increase in the fraction of incoherent twin boundaries (hardening effect) and grain growth (softening effect). Both aspects are directly associated with the mean free path of mobile dislocations.     

The energetics and interactions of random dislocation walls

Abstract

Regular dislocation walls, where dislocations are spaced equally along the wall direction, are a popular idealization for modelling the arrangement and interactions of excess dislocations in plastic deformation. The assumption of regular walls is motivated by the fact that such walls represent minimum energy arrangements for dislocations of the same sign, and it allows to use the analytically known short-ranged stress fields of such walls for analyzing the structure of plastic boundary layers. In order to critically evaluate the physical robustness of models based on regular walls, we investigate their random counterparts and demonstrate that the energetics and interactions of such non-periodic dislocation arrangements differ completely from the periodic wall arrangements. Implications of our results for the modelling of plastic boundary layers and dislocation-grain boundary interactions are discussed.     

Parsing as deduction: The use of knowledge of language

Hypothesising that the human parser is a specialized deductive device in which Universal Grammar and parameter settings are represented as axioms provides a model of how knowledge of language can be put to use. The approach is explained via a series of model deductive parsers for Government and Binding Theory, all of which use the same knowledge of language (i.e., underlying axiom system) but differ as to how they put this knowledge to use (i.e., they use different inference control strategies). They differ from most other GB parsers in that the axiom system directly reflects the modular structure of GB theory and makes full use of the multiple levels of reprepresentation posited in GB theory.     

A phase-field model for deformation twinning

We propose a phase-field model for modeling microstructure evolution during deformation twinning. The order parameters are proportional to the shear strains defined in terms of twin plane orientations and twinning directions. Using a face-centered cubic Al as an example, the deformation energy as a function of shear strain is obtained using first-principle calculations. The gradient energy coefficients are fitted to the twin boundary energies along the twinning planes and to the dislocation core energies along the directions that are perpendicular to the twinning planes. The elastic strain energy of a twinned structure is included using the Khachaturyan's elastic theory. We simulated the twinning process and microstructure evolution under a number of fixed deformations and predicted the twinning plane orientations and microstructures.     

Loss of coherency of the alpha/beta interface boundary in titanium alloys during deformation

The loss of coherency of interphase boundaries in two-phase titanium alloys during deformation was analyzed. The energy of the undeformed interphase boundary was first determined by means of the van der Merwe model for stepped interfaces. The subsequent loss of coherency was ascribed to the increase of interphase energy due to absorption of lattice dislocations and was quantified by a relation similar to the Read–Shockley equation for low-angle boundaries in single-phase alloys. It was found that interphase boundaries lose their coherency by a strain of approximately 0.5 at T = 800°C.     

Twinning stress prediction in bcc metals and alloys

The development of a twin stress relationship for bcc metals and alloys in agreement with experiments has been both scientifically challenging and technologically vital. A modified approach to Peierls–Nabarro model is formulated that predicts the twinning stress in excellent agreement with experiments. We utilize the first principles energy calculations to extract the energy landscape associated with twinning and obtain the disregistry function to account for the interaction of multiple dislocations comprising the twin. The metals and alloys considered include Fe, V, Nb, Ta, Mo, W, Fe–3at.%V, Fe–35at.%Ni and Fe–3at.%Si. The variation of twinning stress within metals is substantial (90–800 MPa) and depends primarily on the twin boundary migration energy, the shear moduli, the interplanar spacing and the geometrical positions of the fractional dislocations constituting the twin.     

Nucleation of recrystallization in fine-grained materials: an extension of the Bailey–Hirsch criterion

Abstract

A new criterion for nucleation in the case of dynamic recrystallization is proposed in order to include the contribution of the grain boundary energy stored in the microstructure in the energy balance. Due to the nucleation events, the total surface area of pre-existing grain boundaries decreases, leading to a nucleus size smaller than expected by conventional nucleation criteria. The new model provides a better prediction of the nucleus size during recrystallization of pure copper compared with the conventional nucleation criterion.     

The effect of affective bibliotherapy on clients' functioning in group therapy

Abstract The effect of affective group bibliotherapy (GB) was compared to affective group therapy (GT) on patients' functioning in therapy and their session impression. Three small groups totaling twenty-five in-patients in a hospital in Israel participated in the study. Clients concurrently participated in both group types, undergoing three sessions in each condition. In-therapy behaviors were assessed through the Client Behavior System (CBS; Hill & O'Brien, 1999). Results indicated that in the GB condition compared to the GT condition, clients showed less resistance, used simple responses less frequently, and expressed greater affective exploration. The Session Evaluation Questionnaire (SEQ; Stiles et al., 1994) was used to measure clients' impressions of the sessions. Results indicated that patients evaluated the two treatment conditions equally. Overall, the results support earlier findings, suggesting that affective bibliotherapy can be an effective method of treatment.