[1] Cluster type grain interaction model including twinning for texture prediction: Application to magnesium alloys

Mu et al

[2] Twinning and grain subdivision during dynamic deformation of a Mg AZ31 sheet alloy at room temperature

Dudamell et al

[3] Self-energy of elliptical dislocation loops in anisotropic crystals and its application for defect-free core/shell nanowires

Chu et al

Simulating creep of snow based on microstructure and the anisotropic deformation of ice

T Theile et al

It is generally agreed that the creep of low-density snow is accompanied by drastic microstructural changes, which are a major source of macroscopic strain hardening. There is, however, no agreement about the dominating mechanism which mediates structural mobility at the microscale. A widely used model of creeping snow allows for intercrystalline deformations at the grain boundaries but neglects intracrystalline deformations in the grains. Here we show that the opposite scenario, which solely uses intra-crystalline deformations, while neglecting grain boundary sliding, is in better agreement with experiments. To this end we have conducted in situ, microtomography measurements of snow microstructure during creep experiments. 3-D tomography images are used to simplify the full microstructure to a 3-D beam-network. This reduces the number of degrees of freedom drastically, which enables us to carry out creep simulations by finite-element methods. We use Glen’s law for secondary creep of ice as the material model and account for the anisotropic creep behaviour of single crystals by assigning individual network strands a random orientation of the c-axis. The results suggest a separation of time scales between creep stress relaxations and slow microstructural changes make the key contribution to snow hardening. Although open-cell foam models clearly fail in predicting the observed viscosity–density relations, they are interestingly suggested as a potential limiting behaviour in our experiments.

Mechanics and chemical thermodynamics of phase transition in temperature-sensitive hydrogels

Here; interesting!

Deformation mechanisms, length scales and optimizing the mechanical properties of nanotwinned metals

Wu et al

Refinement of microstructural length scales and modification of interface character offer opportunities for optimizing material properties. While strength and ductility are commonly inversely related, nanotwinned polycrystalline copper has been shown to possess simultaneous ultrahigh strength and ductility. Interestingly, a maximum strength is found at a small, finite twin spacing. We study the plastic deformation of nanotwinned polycrystalline copper through large-scale molecular dynamics simulations. The simulations show that plastic deformation is initiated by partial dislocation nucleation at grain boundary triple junctions. Both pure screw and 60° dislocations cutting across twin boundaries and dislocation-induced twin boundary migration are observed in the simulation. Following twin boundary cutting, 60° dislocations frequently cross-slip onto {0 0 1} planes in twin grains and form Lomer dislocations. We further examine the effect of twin spacing on this Lomer dislocation mechanism through a series of specifically designed nanotwinned copper samples over a wide range of twin spacings. The simulations show that a transition in the deformation mechanism occurs at a small, critical twin spacing. While at large twin spacings, cross-slip and dissociation of the Lomer dislocations create dislocation locks that restrict and block dislocation motion and thus enhance strength, at twin spacings below the critical size, cross-slip does not occur, steps on the twin boundaries form and deformation is much more planar. These twin steps can migrate and serve as dislocation nucleation sites, thus softening the material. Based on these mechanistic observations, a simple, analytical model for the critical twin spacing is proposed and the predicted critical twin spacing is shown to be in excellent agreement both with respect to the atomistic simulations and experimental observations. In addition, atomistic reaction pathway calculations show that the activation volume of this dislocation crossing twin boundary process is consistent with experimental values. This suggests that the dislocation mechanism transition reported here for the first time can be a source of the observed transition in nanotwinned copper strength.

Extraction of fibre network architecture by X-ray tomography and prediction of elastic properties using an affine analytical model

D. Tsarouchas and A.E. Markaki

This paper proposes a method for extracting reliable architectural characteristics from complex porous structures using micro-computed tomography (μCT) images. The work focuses on a highly porous material composed of a network of fibres bonded together. The segmentation process, allowing separation of the fibres from the remainder of the image, is the most critical step in constructing an accurate representation of the network architecture. Segmentation methods, based on local and global thresholding, were investigated and evaluated by a quantitative comparison of the architectural parameters they yielded, such as the fibre orientation and segment length (sections between joints) distributions and the number of inter-fibre crossings. To improve segmentation accuracy, a deconvolution algorithm was proposed to restore the original images. The efficacy of the proposed method was verified by comparing μCT network architectural characteristics with those obtained using high resolution CT scans (nanoCT). The results indicate that this approach resolves the architecture of these complex networks and produces results approaching the quality of nanoCT scans. The extracted architectural parameters were used in conjunction with an affine analytical model to predict the axial and transverse stiffnesses of the fibre network. Transverse stiffness predictions were compared with experimentally measured values obtained by vibration testing.

Some papers of interest

August 26, 2011

[1] A dislocation density-based crystal plasticity constitutive model for prismatic slip in α-titanium

Alankar et al

[2] Structure and energetics of the coherent interface between the θ′ precipitate phase and aluminium in Al–Cu

Bourgeois et al

[3] Transient solute drag in migrating grain boundaries

Svoboda et al

[4] An extended Mori–Tanaka model for the elastic moduli of porous materials of finite size

Gong et al

[5] Shape evolution by surface and interface diffusion with rigid body rotations

Klinger and Rabkin

[6] Dendritic morphology of α-Mg during the solidification of Mg-based alloys: 3-D experimental characterization by X-ray synchrotron tomography and phase-field simulations

Wang et al

[7] Thermodynamics and kinetics of nanovoid nucleation inside elastoplastic material

Levitas and Altukhova

[8] A unified mechanistic model for size-dependent deformation in nanocrystalline and nanotwinned metals

Gu et al

[1] Tensile behavior of columnar grained Cu with preferentially oriented nanoscale twins

You et al

By means of direct current electrodeposition nanoscale twins confined within microsized columnar grains of bulk Cu samples have been synthesized which are preferentially oriented parallel to the growth plane. Tensile tests of the as-deposited Cu samples showed that yield strength increased with decreasing twin thickness, while the work hardening capacity and the uniform tensile ductility decreased at smaller grain sizes. Detailed microstructure investigations suggest that columnar grained Cu samples exhibit inhomogeneous deformation during uniaxial tension, where grain boundaries take much larger plastic strain than that sustained by grain interiors.

[2] A more accurate three-dimensional grain growth algorithm

E A Lazar et al

In a previous paper, the authors described a simulation method for the evolution of two-dimensional cellular structures by curvature flow that satisfied the von Neumann–Mullins relation with high accuracy. In the current paper, we extend this method to three-dimensional systems. This is a substantial improvement over prior simulations for two reasons. First, this method satisfies the MacPherson–Srolovitz relation with high accuracy, a constraint that has not previously been explicitly implemented. Second, our front-tracking method allows us to investigate topological properties of the systems more naturally than other methods, including Potts models, phase-field methods, cellular automata, and even other front-tracking methods. We demonstrate this method to be feasible in simulating large systems with as many as 100,000 grains, large enough to collect significant statistics well after the systems have reached steady state.

[1] Creep deformation and rafting in nickel-based superalloys simulated by the phase-field method using classical flow and creep theories

Y Tsukada et al

A phase-field model has been developed to simulate the evolution of both (γ + γ′) microstructure and inelastic strain between γ′ phases (i.e. γ channel) during high-temperature creep in nickel-based superalloys. Inelastic strain is defined as the sum of time-independent and time-dependent components. Previously reported mechanical properties of single-phase γ alloys are considered in the calculation of inelastic strain evolution. A two-dimensional phase-field simulation is performed, and the results of microstructure evolution and the creep rate vs. time curve are fitted to the experimental data of the high-temperature creep of CMSX-4. The slope of the creep rate vs. time curve during the initial stage of transient creep, the plasticity preference in different types of γ channels, and the rafting phenomenon are reproduced well by the simulation. Furthermore, it is demonstrated that the creep rate increases locally at γ/γ′ interfaces when the rafted structure is formed.

[2] Dislocation and twin substructure evolution during strain hardening of an Fe–22 wt.% Mn–0.6 wt.% C TWIP steel observed by electron channeling contrast imaging

Gutierrez-Urrutia and Raabe

We study the kinetics of the substructure evolution and its correspondence to the strain hardening evolution of an Fe–22 wt.% Mn–0.6 wt.% C TWIP steel during tensile deformation by means of electron channeling contrast imaging (ECCI) combined with electron backscatter diffraction (EBSD). The contribution of twin and dislocation substructures to strain hardening is evaluated in terms of a dislocation mean free path approach involving several microstructure parameters, such as the characteristic average twin spacing and the dislocation substructure size. The analysis reveals that at the early stages of deformation (strain below 0.1 true strain) the dislocation substructure provides a high strain hardening rate with hardening coefficients of about G/40 (G is the shear modulus). At intermediate strains (below 0.3 true strain), the dislocation mean free path refinement due to deformation twinning results in a high strain rate with a hardening coefficient of about G/30. Finally, at high strains (above 0.4 true strain), the limited further refinement of the dislocation and twin substructures reduces the capability for trapping more dislocations inside the microstructure and, hence, the strain hardening decreases. Grains forming dislocation cells develop a self-organized and dynamically refined dislocation cell structure which follows the similitude principle but with a smaller similitude constant than that found in medium to high stacking fault energy alloys. We attribute this difference to the influence of the stacking fault energy on the mechanism of cell formation.

[3] Precipitates in Al–Cu alloys revisited: Atom-probe tomographic experiments and first-principles calculations of compositional evolution and interfacial segregation

A Biswas et al

Atom-probe tomography, transmission electron microscopy, X-ray diffraction and first-principles calculations are employed to study: (i) compositional evolution of GPII zones and θ′ precipitates; and (ii) solute segregation at α-Al/θ′ interfaces in Al–1.7 at.% Cu (Al–4 wt.% Cu) alloys. GPII zones are observed after aging at 438 K for 8 h, whereas higher aging temperatures, 463 K for 8 h and 533 K for 4 h, reveal only θ′ precipitates. Most GPII zones and θ′ precipitates are demonstrated to be Cu-deficient at the lower two aging temperatures; only the 533 K treatment resulted in θ′ stoichiometries consistent with the expected Al2Cu equilibrium composition. For alloys containing not, vert, similar200 at. ppm Si we find evidence of Si partitioning to GPII zones and θ′ precipitates. Significant Si segregation is observed at the coherent α-Al/θ′ interface for aging at 533 K, resulting in an interfacial Si concentration more than 11 times greater than in the α-Al matrix. Importantly, the Si interfacial concentration undergoes a transition from a non-equilibrium delocalized profile to an equilibrium localized profile as the aging temperature is increased from 463 to 533 K. Consistent with these measurements, first-principles calculations predict a strong thermodynamic driving force favoring Si partitioning to Cu sites in θ′. Silicon segregation at, and partitioning to, θ′ precipitates results in a decrease in interfacial free energy, and concomitantly an increase in the nucleation current. Our results suggest that Si catalyzes the early stages of precipitation in these alloys, consistent with the higher precipitate number densities observed in commercial Al–Cu–Si alloys.