[1] Nucleation, growth and impingement modes deduced from isothermally and isochronally conducted phase transformations: Calorimetric analysis of the crystallization of amorphous Zr50Al10Ni40

F Liu et al

The crystallization kinetics of amorphous Zr50Al10Ni40, as measured by means of both isothermal and isochronal differential scanning calorimetry, were evaluated using a new procedure involving application of a modular analytical model to provide a complete description of the phase-transformation kinetics, in combination with a preceding analysis of the transformation-rate maximum. The power of detailed analysis of the position of the transformation-rate maximum, as a function of the transformed fraction, was demonstrated by identification of the operating impingement mode. On this basis, the kinetic parameters governing the crystallization kinetics could then be determined quantitatively using the modular analytical model. The crystallization governing mechanisms could be varied by appropriate control of the crystallization conditions. The results obtained are consistent with the microstructural evolution, as observed by transmission electron microscopy.

[2] Simulations of stress-induced twinning and de-twinning: A phase field model

S Hu et al

Twinning in certain metals or under certain conditions is a major plastic deformation mode. Here we present a phase field model to describe twin formation and evolution in a polycrystalline fcc metal under loading and unloading. The model assumes that twin nucleation, growth and de-twinning is a process of partial dislocation nucleation and slip on successive habit planes. Stacking fault energies, energy pathways (γ surfaces), critical shear stresses for the formation of stacking faults and dislocation core energies are used to construct the thermodynamic model. The simulation results demonstrate that the model is able to predict the nucleation of twins and partial dislocations, as well as the morphology of the twin nuclei, and to reasonably describe twin growth and interaction. The twin microstructures at grain boundaries are in agreement with experimental observation. It was found that de-twinning occurs during unloading in the simulations, however, a strong dependence of twin structure evolution on loading history was observed.

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Some papers from scripta

September 10, 2010

[1] Single-Phase Interdiffusion in Ni3Al–Mo Ternary System

H Wei et al

Interdiffusion behavior of Ni3Al-Mo ternary system at 1423, 1473 and 1523 K were studied using Ni3Al/Ni3Al-Mo single-phase diffusion couples. The concentration dependent interdiffusion coefficients were calculated over whole diffusion range, and average ternary interdiffusion coefficients were carefully determined in the middle of the diffusion zone. These ternary coefficients were also examined to estimate tracer diffusion coefficients of Mo in Ni3Al phase. Furthermore, the results were utilized to explain the diffusion behavior of Mo in Ni3Al-based superalloys IC-6.

[2] Stabilizing force on perturbed grain boundaries using dislocation model

X Zhu and Y Xiang

In this paper, we study the glide force due to stress on the constituent dislocations of slightly perturbed symmetric low angle tilt boundaries. We show that the stabilizing force comes from both the long-range interaction of the constituent dislocations and their local line tension effect. We also present a continuum model for such glide force. The obtained results and continuum model provide a basis for further understanding of the stress-driven migration of distorted grain boundaries.

[3] Stress Driven Grain Growth in Nanocrystalline Pt Thin Films

J A Sharon et al

Micro-tensile experiments and electron microscopy have been utilized to characterize the mechanical behavior of nanocrystalline Pt films. The behavior can be described as high strength with limited strain to failure. The tensile deformation triggers a microstructural evolution increasing the grain size from 20nm in the initial state to 33nm in the deformed state. This observation of grain growth at a homologous temperature of 0.146 provides further evidence of the role of mechanical stress in initiating grain growth in nanocrystalline metals.

[4] Morphological evolution of the interface microstructure in the presence of bubbles during directional solidification

H Xing et al

The evolution of morphology during directional solidification is investigated in terms of the interaction between bubbles and the solid–liquid interface. The results reveal that the solid phase grows along the bubble boundary to form solid envelopes and a liquid gap. As the interface velocity increases, the expansion coefficients of bubbles increase continually, and then decrease. The solidification microstructures of bubbles transform in the sequence water-drop→elongated→irregular with increasing interface velocity.

[5] Direct observations of silver nanoink sintering and eutectic remelt reaction with copper

J W Elmer and E D Specht

Ag nanoink sintering kinetics and subsequent melting is studied using in situ synchrotron-based X-ray diffraction. Direct observations of Ag nanoink sintering on Cu demonstrate its potential for materials joining since the Ag nanoink sinters at low temperatures but melts at high-temperatures. Results show low expansion coefficient of sintered Ag, nonlinear expansion as Ag densifies and interdiffuses with Cu above 500 °C, remelting consistent with bulk Ag, and eutectic reaction with Cu. The results demonstrate the usefulness of Ag nanoink as a high-temperature bonding medium.

[6] Deviation of the magnetization change from the structural phase transition temperature in polycrystalline Ni–Mn–Sn in low magnetic fields

P J Shamberger et al

Magnetization and resistivity were measured as a function of temperature in polycrystalline Ni–Mn–Sn Heusler alloys with a magnetocrystalline first order phase transition. At external magnetic fields <0.2 T the magnetization change associated with the phase transition happened at a temperature up to not, vert, similar5 K lower than the true phase transition temperature measured by electrical resistivity or predicted by thermodynamic theory. We associate this anomaly with magnetic exchange coupling between austenite and martensite phases coexisting near the transition.

[7] On the retardation of grain boundary motion by small particles

G Gottstein and L S Shvindlerman

The drag effect by second-phase particles on grain boundary motion is considered with regard to the triple line particle–grain boundary that is formed during the interaction between a particle and a grain boundary. A quantitative analysis of this effect has become possible due to recent measurements of the triple line energy. In the limit of large particles (dp > 50 nm) the particle is wet by the boundary (Zener approximation) whereas smaller particles are repelled by the grain boundary.

Couple of papers from Acta

September 10, 2010

[1] Evolution of structure and free volume in symmetric tilt grain boundaries during dislocation nucleation

G J Tucker et al

Grain boundary evolution in copper bicrystals is investigated during uniaxial tension at 10 K. Grain boundary structures are generated using molecular statics employing an embedded atom method potential, followed by molecular dynamics simulation at a constant 1 × 109 s−1 strain rate. Interfacial free volume is continuously measured during boundary deformation, and its evolution is investigated both prior to and during grain boundary dislocation nucleation. Free volume provides valuable insight into atomic-scale processes associated with stress-induced grain boundary deformation. Different boundary structures are investigated in this work to analyze the role of interface structure, stress state and initial free volume on dislocation nucleation. The results indicate that the free volume influences interfacial deformation through modified atomic-scale processes, and grain boundaries containing particular free volume distributions show a greater propensity for collective atomic migration during inelastic deformation.

[2] Segregation-induced grain boundary electrical potential in ionic oxide materials: A first principles model

D Gomez-Garcia et al

A first principles continuum analytical model for cationic segregation to the grain boundaries in complex ceramic oxides is presented. The model permits one to determine the electric charge density and the segregation-induced electric potential profiles through the grain and can be extrapolated to the range of nanostructured grain sizes. The theoretical predictions are compared with existing data for yttria-stabilized tetragonal zirconia polycrystals. The implications for physical properties (mainly high temperature plasticity and hardening behaviour) are then discussed.

Three papers from PRL that discuss some interesting problems:

[1] Rolling Ribbons

P S Raux et al

We present the results of a combined experimental and theoretical investigation of rolling elastic ribbons. Particular attention is given to characterizing the steady shapes that arise in static and dynamic rolling configurations. In both cases, above a critical value of the forcing (either gravitational or centrifugal), the ribbon assumes a two-lobed, peanut shape similar to that assumed by rolling droplets. Our theoretical model allows us to rationalize the observed shapes through consideration of the ribbon’s bending and stretching in response to the applied forcing.

[2] Morphology, Growth, and Size Limit of Bacterial Cells

H Jiang and S X Sun

Bacterial cells utilize a living peptidoglycan network (PG) to separate the cell interior from the surroundings. The shape of the cell is controlled by PG synthesis and cytoskeletal proteins that form bundles and filaments underneath the cell wall. The PG layer also resists turgor pressure and protects the cell from osmotic shock. We argue that mechanical influences alter the chemical equilibrium of the reversible PG assembly and determine the cell shape and cell size. Using a mechanochemical approach, we show that the cell shape can be regarded as a steady state of a growing network under the influence of turgor pressure and mechanical stress. Using simple elastic models, we predict the size of common spherical and rodlike bacteria. The influence of cytoskeletal bundles such as crescentin and MreB are discussed within the context of our model.

[3] Atomically Smooth Stress-Corrosion Cleavage of a Hydrogen-Implanted Crystal

G Moras et al

We present a quantum-accurate multiscale study of how hydrogen-filled discoidal “platelet” defects grow inside a silicon crystal. Dynamical simulations of a 10-nm-diameter platelet reveal that H2 molecules form at its internal surfaces, diffuse, and dissociate at its perimeter, where they both induce and stabilize the breaking up of highly stressed silicon bonds. A buildup of H2 internal pressure is neither needed for nor allowed by this stress-corrosion growth mechanism, at odds with previous models. Slow platelet growth up to micrometric sizes is predicted as a consequence, making atomically smooth crystal cleavage possible in implantation experiments.

Some recent papers from Acta

September 4, 2010

[1] Coupling of grain boundary sliding and migration within the range of boundary specialness

A D Sheikh-Ali

Stress-induced behavior of high-angle near-coincidence symmetric tilt boundaries has been examined in bicrystal specimens of zinc. Parameters of coupling between boundary sliding and migration were determined. The angular deviation from the coincidence misorientation within the range of boundary specialness has a noticeable effect on the sliding-to-migration ratio, called “coupling factor”. Mechanisms of coupled boundary sliding and migration based on the motion of edge-type extrinsic and intrinsic grain boundary dislocations are discussed. It has been demonstrated that the observed alteration of the coupling factor with the change in boundary misorientation is due to the change of the parameters of extrinsic secondary grain boundary dislocations. The obtained results have also shown the limitation of the coincidence site lattice/displacement shift complete lattice model for the quantitative description of the structure of near-coincidence boundaries.

[2] Diffusion-controlled peritectic reaction process in carbon steel analyzed by quantitative phase-field simulation

M Ohno and K Matsuura

The peritectic reaction process in carbon steel, L + δ → γ, has been analyzed by a quantitative phase-field simulation. The calculated moving velocities of the γ–L and γ–δ planar interfaces in the isothermal peritectic transformation precisely agree with the corresponding experimental data, which strongly supports the accuracy of the present simulation. The diffusion-controlled peritectic reaction rate and the growth velocity of the γ phase along the δ–L interface obtained by the present simulation were fairly consistent with the experimentally measured values. This indicates that recent experimental findings can be explained by a diffusion-controlled mechanism. This is in marked contrast to the claims made on the basis of the experimental data and an analytical model that the peritectic reaction is not controlled by the diffusion of carbon.

[3] Spacing characterization in Al–Cu alloys directionally solidified under transient growth conditions

M Amoorezaei et al

We study spacing selection in directional solidification of Al–Cu alloys under transient growth conditions. New experimental results are presented which reveal that the mean dendritic spacing vs. solidification front speed exhibits plateau-like regions separated by regions of rapid change, consistent with previous experiments of Losert and co-workers. Quantitative phase-field simulations of directional solidification with dynamical growth conditions approximating those in the experiments confirm this behavior. The mechanism of this type of change in mean dendrite arm spacing is consistent with the notion that a driven periodically modulated interface must overcome an energy barrier before becoming unstable, in accord with a previous theory of Langer and co-workers.

[4] Microstructure evolution during dewetting in thin Au films

C M Mueller and R Spolenak

Thin metal films can degrade into particles in a process known as dewetting. Dewetting proceeds in several stages, including void initiation, void growth and void coalescence. Branched void growth in thin Au films was studied by means of electron backscatter diffraction (EBSD). The holes were found to protrude into the film predominantly at high angle grain boundaries and the branched shape of the holes can be explained by surface energy minimization of the grains at the void boundaries. (1 1 1) Texture sharpening during dewetting was observed and quantified by EBSD and in situ X-ray studies.

[5] Effect of heat treatment temperature on the microstructure and actuation behavior of a Ti–Ni–Cu thin film microactuator

M Tomozawa et al

Ti–Ni–Cu/SiO2 two layer diaphragm-type microactuators were fabricated by sputter deposition and micromachining. The influence of heat treatment temperature on the actuation behavior was investigated under quasi-static conditions. The interfacial structure of Ti–Ni–Cu/SiO2 and internal structure of the Ti–Ni–Cu layer were also investigated using transmission electron microscopy. The reaction layer formed between the Ti–Ni–Cu and SiO2 layers, and preferentially grew into the SiO2 side. The reaction layer formed at 1023 K mainly consisted of Ti4(Ni,Cu)2O. The maximum height of the diaphragm decreased with increasing heat treatment temperature. The growth of the reaction layer also affected the microstructure of the Ti–Ni–Cu layer. The density of fine platelets and Ti2Ni precipitates decreased with increasing heat treatment temperature from 873 to 923 K, and they disappeared at 973 K due to the fact that the reaction layer mainly consisted of a Ti-rich phase. The microactuator heat treated at 973 K showed the highest transformation temperature with the lowest transformation temperature hysteresis, which is attractive for high speed actuation.