Simulations of Mechanical Behavior of Polycrystalline Copper with Nano-Twins

Mechanical behavior and microstructure evolution of polycrystalline copper with nano-twins were investigated in the present work by finite element simulations. The fracture of grain boundaries are described by a cohesive interface constitutive model based on the strain gradient plasticity theory. A systematic study of the strength and ductility for different grain sizes and twin lamellae distributions is performed. The results show that the material strength and ductility strongly depend on the grain size and the distribution of twin lamellae microstructures in the polycrystalline copper.

Alternating Starvation Of Dislocations During Plastic Yielding In Metallic Nanowires

Using molecular dynamics simulations, we show that the mechanical deformation behaviors of single-crystalline nickel nanowires are quite different from their bulk counterparts. Correlation between the obtained stress-strain curves and the visualized defect evolution during deformation processes clearly demonstrates that a sequence of complex dislocation slip events results in a state of dislocation starvation, involving the nucleation and propagation of dislocations until they finally escape from the wires, so that the wires deform elastically until new dislocations are generated. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A finite deformation analysis of polycrystal by considering the influence of grain boundary

By combining grain boundary (GB) and its influence zone, a micromechanic model for polycrystal is established for considering the influence of GB. By using the crystal plasticity theory and the finite element method for finite deformation, numerical simulation is carried out by the model. Calculated results display the microscopic characteristic of deformation fields of grains and are in qualitative agreement with experimental results.

An analysis of the effects of symmetrical tilt grain boundaries on the plastic deformation

A numerical investigation on the simple polycrystals containing three symmetrical tilt grain boundaries (GBs) is carried out within the framework of crystal plasticity which precisely considers the finite deformation and finite lattice rotation as well as elastic anisotropy. The calculated results show that the slip geometry and the redistribution of stresses arising from the anisotropy and boundary constraint play an important role in the plastic deformation in the simple polycrystals. The stress level along GB is sensitive to the load level and misorientation, and the stresses along QB are distributed nonuniformly. The GB may exhibit a softening or strengthening feature, which depends on the misorientation angle. The localized deformation bands usually develop accompanying the GB plastic deformation, the impingement of the localized band on the GB may result in another localized deformation band. The yield stresses with different misorientation angles are favorably compared with the experimental results.

Mechanical constraint and loading on ferroelectric memory capacitors

The influence of mechanical constraint imposed by device geometry upon the switching response of a ferroelectric thin film memory capacitor is investigated. The memory capacitor was represented by two-dimensional ferroelectric islands of different aspect ratio, mechanically constrained by surrounding materials. Its ferroelectric non-linear behaviour was modeled by a crystal plasticity constitutive law and calculated using the finite element method. The switching response of the device, in terms of remnant charge storage, was determined as a function of geometry and constraint. The switching response under applied in-plane tensile stress and hydrostatic pressure was also studied experimentally. Our results showed that (1) the capacitor's aspect ratio could significantly affect the clamping behaviour and thus the remnant polarization, (2) it was possible to maximise the switching charge through the optimisation of the device geometry, and (3) it is possible to find a critical switching stress at zero electric field and a critical coercive field at zero residual stress. © 2009 Materials Research Society.

Deformation and geochronology of syntectonic granitoids emplaced in the Major Gercino Shear Zone, southeastern South America

The Major Gercino Shear Zone is one of the NE-SW lineaments that separate the Neoproterozoic Dom Feliciano Belt, of Brazil and Uruguay, into two different domains: a northwestern supracrustal domain from a southeastern granitoid domain. The shear zone, striking NE, is composed of protomylonites to ultramylonites with mainly dextral kinematic indicators. In Santa Catarina State, southern Brazil, the shear zone is composed of two mylonite belts. The mylonites have mineral orientations produced under greenschist fades conditions at a high strain rate. Strong flattening and coaxial deformation indicate the transpressive character, while the role of pure shear is emphasized by the orientation of the mylonite belts in relation to the inferred stress field component. The quartz microstructures point out that different dynamic recrystallization regimes and crystal plasticity were the dominant mechanisms of deformation during the mylonitization process. Additionally, the fabrics suggest that the glide systems are activated for deformation conditions compatible with the metamorphism in the middle greenschist facies. Elongated granitoid intrusions belonging to two petrographically, geochemically and isotopically distinct rock associations occur between the two mylonite belts. The structures observed in the granites result from a deformation range from magmatic to solid-state conditions points to a continuum of magma straining during and just after its crystallization. Conventional U-Pb analysis of multi-crystal zircon fractions yielded essentially identical ages of 609 +/- 16 Ma and 614 +/- 2 Ma for the two granitic associations, and constrain the transpressive phase of the shear zone. K-Ar ages of biotites between 585 and 560 Ma record the slow cooling and uplift of the intrusions. Some K-Ar ages of micas in regional mylonites are similar, suggesting that thermo-tectonic activity was intense up to this time, probably related to the agglutination of the granite belt to the supracrustal belt NW of the MGSZ. (C) 2009 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Texture, twinning and uniform elongation of wrought magnesium

A small number of crystal plasticity simulations and tensile tests are carried out with the aim of demonstrating that control of twinning can improve the uniform elongation of magnesium based alloys, It is suggested that this can be accomplished through texture manipulation because texture influences both the fraction of grains that undergo twinning and the strain required for the twinning reaction to go to completion.

Application of radial return mapping algorithm for finite strain elastoplasticity in a hybrid finite element and particle-in-cell model

The radial return mapping algorithm within the computational context of a hybrid Finite Element and Particle-In-Cell (FE/PIC) method is constructed to allow a fluid flow FE/PIC code to be applied solid mechanic problems with large displacements and large deformations. The FE/PIC method retains the robustness of an Eulerian mesh and enables tracking of material deformation by a set of Lagrangian particles or material points. In the FE/PIC approach the particle velocities are interpolated from nodal velocities and then the particle position is updated using a suitable integration scheme, such as the 4th order Runge-Kutta scheme[1]. The strain increments are obtained from gradients of the nodal velocities at the material point positions, which are then used to evaluate the stress increment and update history variables. To obtain the stress increment from the strain increment, the nonlinear constitutive equations are solved in an incremental iterative integration scheme based on a radial return mapping algorithm[2]. A plane stress extension of a rectangular shape J2 elastoplastic material with isotropic, kinematic and combined hardening is performed as an example and for validation of the enhanced FE/PIC method. It is shown that the method is suitable for analysis of problems in crystal plasticity and metal forming. The method is specifically suitable for simulation of neighbouring microstructural phases with different constitutive equations in a multiscale material modelling framework.

Characteristics of the contraction twins formed close to the fracture surface in Mg-3A1-1Zn alloy deformed in tension

Characteristics of the “contraction” twins, formed close to the fracture surface in Mg–3Al–1Zn alloy deformed in tension approximately perpendicular to the grain c-axes, are investigated using transmission electron microscopy. The grain c-axis contractions were largely accommodated by {1011}-{1012} source double-twins in a variant characterized by 38° ⟨1210⟩ source twin/matrix misorientation in conjunction with dislocation slip. A possible interpretation of the observed preference for this variant formation is given and some crystal plasticity modelling is performed to elucidate the matter.

Identification and rationalization of secondary twin variants in a magnesium alloy

The present work combines electron backscatter diffraction, transmission electron microscopy and Schmid analysis to investigate secondary twinning in the magnesium alloy Mg-3AI-1Zn. Inspection of the misorientations between the parent matrix and {1011} - {1012} doubly twinned volumes reveals that there are four possible variants. One of these variants characterized by 38°< 1210 > misorientation with the matrix is favoured much more than the others. This variant involves activation of the secondary twinning systems that are quite inconsistent with the Schmid-type behaviour. For the secondary twin to grow significantly it must take on a shape enforced by the primary twin, however, this is not optimal for strain compatibility. It is argued that the 38° < 1210 > variant occurs most frequently because it provides the closest match between the primary and secondaty twinning planes, thus minimizing the compatibility strain. This conjecture is confirmed by the simulations of twin activity m ellipsoidal grains performed using the visco-plastic self-consistent crystal plasticity model.

Finite element simulations of the cyclic elastoplastic behaviour of copper thin films

A large-scale computational and statistical strategy is presented to investigate the development of plastic strain heterogeneities and plasticity induced roughness at the free surface in multicrystalline films subjected to cyclic loading conditions, based on continuum crystal plasticity theory. The distribution of plastic strain in the grains and its evolution during cyclic straining are computed using the finite element method in films with different ratios of in-plane grain size and thickness, and as a function of grain orientation (grains with a {1 1 1} or a {0 0 1} plane parallel to the free surface and random orientations). Computations are made for 10 different realizations of aggregates containing 50 grains and one large aggregate with 225 grains. It is shown that overall cyclic hardening is accompanied by a significant increase in strain dispersion. The case of free-standing films is also addressed for comparison. The overall surface roughness is shown to saturate within 10 to 15 cycles. Plasticity induced roughness is due to the higher deformation of {0 0 1} and random grains and due to the sinking or rising at some grain boundaries.

Modelling the combined effect of grain size and grain shape on plastic anisotropy of metals

Within each columnar grain of a metallic film, the resistance to dislocation glide varies in function of the orientation of the slip plane with regard to the grain long axis. Plastic slip is impeded across grain boundaries and this contributes to the anisotropy of the overall mechanical response. A simplified (Taylor-type) crystal plasticity model is proposed that accounts for such effect of grain shape on the slip system selection. Assuming that dislocation density gradients are normal to the grain boundaries, backstresses developed at the onset of plasticity are estimated based on two definitions of the effective grain boundary spacing ‘‘seen’’ by individual slip systems. The first one reduces to the mean area-to-perimeter ratio of cross-sections of the grain cut parallel to the slip plane. Closed-form expressions of the average backstresses developed inside grains with spheroidal shapes are introduced in the crystal hardening law. The model reproduces the very high plastic anisotropy of electro-deposited pure iron with a strong c-fiber and a refined columnar grain structure [Yoshinaga, N., Sugiura, N., Hiwatashi, S., Ushioda, K., Kada, O., 2008. Deep drawability of electro-deposited pure iron having an extremely sharp h111i//ND texture. ISIJ Int. 48, 667–670]. It also provides valid estimates of the texture development and the influence of grain size on the yield strength.