[1] Interaction of a dislocation with a crack tip: From stimulated emission to avalanche generation

G Michot

Stress relaxation at a crack tip relies on the material’s ability to generate dislocations. Despite the extensive literature devoted to crack–dislocation interaction, no one has yet explained how dislocations appear and multiply in order to build a fully plastic zone. Here we will show how a simple event, such as the intersection of a unique incoming dislocation with a crack front, induces the generation of new dislocations: this effect is called “stimulated emission”. Submitted to the applied crack stress field, these dislocations can repeat the stimulation process step by step all along the crack front, through a cross-slip mechanism. Such a rapidly increasing rate of dislocations nucleation leads to a sudden growth of the plastic zone (avalanche).

[2] Modeling the recrystallized grain size in single phase materials

S Wang et al

A model is proposed for post-recrystallization grain size. The model is based on the coarsening of subgrain networks as present after deformation and recovery. It is shown that the orientation spread in the subgrain network is the key variable in predicting the density of abnormal subgrains and, hence, the recrystallized grain size. The model explains the strong dependence of the post-recrystallization grain size on prior strain and the lack of a dependence on the annealing temperature.

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[1] Formation mechanism of coarse columnar γ grains in as-cast hyperperitectic carbon steels

S Tsuchiya et al

Abstract

The formation mechanism of as-cast coarse columnar γ grain (CCG) structure in hyperperitectic carbon steels is investigated by means of a rapid unidirectional solidification method. This method achieves cooling conditions similar to those in the vicinity of a practically continuously cast slab surface. The microstructural observation of the quenched samples indicates that the CCG structure develops from the mold side along the direction of the temperature gradient. In the solidifying samples, fine columnar γ grains (FCG) always exist ahead of the CCG region. Instead of continuous growth into large grains, FCG always shrink and vanish as a result of the growth of CCG initially formed near the mold side. Therefore, the grain size at a fixed point in the ingot discontinuously changes from the FCG to the CCG. The validity of this process is supported by numerical analyses. This finding is in marked contrast to the assumption made in conventional grain growth analysis on the CCG structure.
Highlights

► We examine the formation process of as-cast coarse columnar γ grains (CCG) in steels. ► Fine columnar γ grains (FCG) exist ahead of the CCG region during the solidification. ► The FCG do not continuously grow into the CCG and they always shrink. ► We find that the CCG develop by the mechanism of the discontinuous grain growth.

[2] Dislocation junction formation and strength in magnesium

L Capolungo et al

Adaptative meshing finite-element-based discrete dislocation dynamics simulations are employed to predict dislocation junction formation in magnesium as well as their resulting strength. Apart from coplanar and collinear interactions, all possible interactions between basal, prismatic and pyramidal slip are considered. Among others it is found that while non-coplanar prismatic junctions are more likely than basal–prismatic junctions, the latter are more stable. However, pyramidal–prismatic junctions appear more stable than pyramidal–basal junctions. Finally, non-coplanar pyramidal junctions are more likely than any other junction formation, and these junctions also appear to be amongst the strongest.

Carbide grain growth in cemented carbides

K Mannesson et al

Abnormal grain growth is often observed in cemented carbides during sintering, but cannot be understood in terms of the classical LSW theory. In this work the grain growth behavior during sintering at 1430 °C is studied both experimentally and by means of computer simulations. A model based on several processes—2-D nucleation of growth ledges, mass transfer across the interface and long-range diffusion coupled in series—is formulated and the equations are solved numerically. Both computer simulations and experimental studies reveal that the grain growth behavior is strongly influenced by the initial size distribution.

In situ TEM observation of stress-induced martensitic transformations and twinning processes in CuAlNi single crystals

N Zarubova et al

Stress-induced martensitic transformations and twinning processes were studied in thin foils of CuAlNi single crystals strained in situ in a transmission electron microscope. The nucleation and growth of the martensite plates were monitored for three transformation processes known from bulk experiments: (i) the transformation of austenite into 2H martensite at low-stress levels; (ii) the twinning/detwinning processes in 2H martensite; and (iii) the transformation between austenite and 18R martensite at higher stress levels. The morphology of the austenite/martensite habit planes was examined, and the existence of planar interfaces between a single variant of 2H martensite and austenite on the microscopic level was proven.

The role of self-shadowing on growth and scaling laws of faceted polycrystalline thin films

C Ophus et al

We investigate, via both experiment and simulation, the effects of self-shadowing on the growth of faceted polycrystalline thin films. Faceted aluminum thin films were sputtered and the anomalous scaling behaviour of their surfaces was characterized. To understand the causes of this anomalous behavior, growth of faceted thin films was simulated by coupling a level set construction to a ballistic deposition model. The angular distribution function of deposition flux was varied to control the degree of self-shadowing. We show how differing degrees of self-shadowing strongly modify film surface morphologies and compare these results with experimental findings.

[1] Enhanced multiferroic properties and domain structure of La doped BiFeO3 thin films

F Yan et al

BiFeO3 (BFO) and La-doped BFO (BLFO) thin films are grown on Pt/TiO2/SiO2/Si substrate using pulsed laser deposition. The domain structure of BFO and BLFO are investigated via piezoresponse force microscopy. Highly enhanced ferroelectric properties with great remanent polarization (Pr) of 102 μC/cm2 and decreased leakage current density are obtained via La-doping. Magnetic property is also increased by La doping ascribed to spatial homogenization of spin arrangement. The mechanisms for the enhancement of ferroelectric and ferromagnetic characteristics are discussed.

[2] Experimental method for true in situ measurements of shear-coupled grain boundary migration

T Gorkaya et al

A novel set-up developed to continuously observe and measure stress-driven grain boundary migration is presented. A commercial tensile/compression scanning electron microscope hot stage was utilized for in situ observations of mechanically loaded samples at elevated temperatures up to 850 °C by recording orientation contrast images of bicrystal surfaces. Two sample holders were designed and fabricated for applying a shear stress to the boundary in bicrystals of various geometries. The results of the first measurements in Al bicrystals are presented.

[1] On grain growth in the presence of mobile particles

V.Yu. Novikov

The ability of second phase particles to migrate along with grain boundaries is shown to be determined not only by the particle mobility but also by the migration rate of the grain boundary where they locate. This leads to a duality in the mobile particle behaviour: they behave as either movable or immovable depending on the boundary migration rate. In the first case, they reduce the boundary mobility; in the second one they decrease the driving force for boundary migration. It is demonstrated by numerical modeling that mobile particles with low mobility can suppress grain growth even in nanocrystalline material, the limiting grains size being several times smaller than in the case of randomly distributed immobile particles. It is also shown that the Zener solution to the problem of the grain growth retardation by disperse particles is a specific case of the proposed approach.

[2] Neutron Larmor diffraction measurements for materials science

J. Repper et al

Neutron Larmor diffraction (LD) is a high-resolution diffraction technique based on the Larmor precession of polarized neutrons. In contrast to conventional diffraction, LD does not depend on the accurate measurement of Bragg angles, and thus the resolution is independent of the beam collimation and monochromaticity. At present, a relative resolution for the determination of the crystal lattice spacing d of Δd/dnot, vert, similar10-6 is achieved, i.e. at least one order of magnitude superior to conventional neutron or X-ray techniques. This work is a first step to explore the application of LD to high-resolution problems in the analysis of residual stresses, where both the accurate measurement of absolute d values and the possibility of measuring type II and III stresses may provide additional information beyond those accessible by conventional diffraction techniques. Data obtained from Inconel 718 samples are presented.

[1] The effects of grain grooves on grain boundary migration in nanofilms

A Novick-Cohen et al

Using numerical computations and asymptotic analysis, we study the effects of grain grooves on grain boundary migration in nanofilms, focusing for simplicity on axisymmetric bicrystals containing an embedded cylindrical grain located at the origin. We find there is a critical initial grain radius, R*, such that if RR*, groove growth during grain shrinkage leads to film break-up. The central cross-section of the grain boundary profile is seen to be parabolic, and an ordinary differential equation which depends on the tilt angle and the groove depth is seen to govern the location of the groove root. Near the annihilation–pinch-off transition, temporary stagnation occurs; thereafter, the shrinking grain accelerates rapidly, then disappears.

[2] Misfit strain–film thickness phase diagrams and related electromechanical properties of epitaxial ultra-thin lead zirconate titanate films

Q Y Qiu et al

The phase stability of ultra-thin (0 0 1) oriented ferroelectric PbZr1–xTixO3 (PZT) epitaxial thin films as a function of the film composition, film thickness, and the misfit strain is analyzed using a non-linear Landau–Ginzburg–Devonshire thermodynamic model taking into account the electrical and mechanical boundary conditions. The theoretical formalism incorporates the role of the depolarization field as well as the possibility of the relaxation of in-plane strains via the formation of microstructural features such as misfit dislocations at the growth temperature and ferroelastic polydomain patterns below the paraelectric–ferroelectric phase transformation temperature. Film thickness–misfit strain phase diagrams are developed for PZT films with four different compositions (x = 1, 0.9, 0.8 and 0.7) as a function of the film thickness. The results show that the so-called rotational r-phase appears in a very narrow range of misfit strain and thickness of the film. Furthermore, the in-plane and out-of-plane dielectric permittivities ε11 and ε33, as well as the out-of-plane piezoelectric coefficients d33 for the PZT thin films, are computed as a function of misfit strain, taking into account substrate-induced clamping. The model reveals that previously predicted ultrahigh piezoelectric coefficients due to misfit-strain-induced phase transitions are practically achievable only in an extremely narrow range of film thickness, composition and misfit strain parameter space. We also show that the dielectric and piezoelectric properties of epitaxial ferroelectric films can be tailored through strain engineering and microstructural optimization.

[3] A more accurate two-dimensional grain growth algorithm

E A Lazar et al

We describe a method for evolving two-dimensional polycrystalline microstructures via mean curvature flow that satisfies the von Neumann–Mullins relation with an absolute error O(Δt2). This is a significant improvement over a different method currently used that has an absolute error O(Δt). We describe the implementation of this method and show that while both approaches lead to indistinguishable evolution when the spatial discretization is very fine, the differences can be substantial when the discretization is left unrefined. We demonstrate that this new front-tracking approach can be pushed to the limit in which the only mesh nodes are those coincident with triple junctions. This reduces the method to a vertex model that is consistent with the exact kinetic law for grain growth. We briefly discuss an extension of the method to higher spatial dimensions.

[4] Point defects in multicomponent ordered alloys: Methodological issues and working equations

R Besson

The aim of this work is to give the independent-point-defect thermodynamics of ordered compounds a sufficiently general flavour, adapted to and working for multicomponent alloys. Generalizing previous approaches, we first show that an appropriate description for a crystal with point defects allows treatment of the practically important pressure and defect volume parameters in the grand canonical framework, the equivalence of which is explicited with the closer to experiments isothermal–isobaric conditions. Since industrial applications often involve multialloyed compounds, we then derive an operational tool for atomic-scale investigations of long-range order alloys with complex crystallographies and multiple additions.

[5] Misorientation texture development during grain growth. Part II: Theory

J Gruber et al

A critical event model for the evolution of number- and area-weighted misorientation distribution functions (MDFs) during grain growth is proposed. Predictions from the model are compared to number- and area-weighted MDFs measured in Monte Carlo simulations with anisotropic interfacial properties and several initial orientation distributions, as well as a dense polycrystalline magnesia sample. The steady-state equation of our model appears to be a good fit to all data. The relation between the grain boundary energy and the normalized average boundary area is discussed in the context of triple junction dynamics.

[6] Spatial correlations in symmetric and asymmetric bicontinuous structures

A L Genau and P W Voorhees

Spatial correlations of interfacial curvature are compared for symmetric and asymmetric two-phase mixtures produced following spinodal decomposition as given by a numerical solution to the Cahn–Hilliard equation in three dimensions. By calculating radial distribution functions of the density of interfacial area as a function of the mean interfacial curvature of these bicontinuous microstructures, it is found that long-range diffusive interactions, in combination with the morphology of the system, yield a variety of correlations and anticorrelations over a range of length scales. The asymmetric mixtures show some similarities to the symmetric mixtures, as well as other unique features.