Haitham El Kadiri

Anisotropy in hexagonal close-packed structures: improvements to crystal plasticity approaches applied to magnesium alloy

Abstract Due to its polarity, twinning in strongly textured hexagonal close packed (HCP) structures can be maximized or minimized under particular loading conditions. The resulting anisotropy can be dramatically demonstrated for magnesium with a fibre, for example. The stress–strain behaviour from compression loading parallel to the fibre produces a ‘parabolic’ stress–strain curve, but a ‘sigmoidal’ curve when loaded normal to the fibre. When modelling anisotropy in HCP structures with crystal plasticity, contemporary researchers usually fit hardening parameters to only these two extreme cases, i.e., maximized or minimized twinning activity, presuming that the same parameters would interpolate the correct behaviour under any other transitional stress direction. A comparison with experiments presented in this paper demonstrates that this assumption is not fully accurate, whether using the phenomenological Voce hardening model or the dislocation density based hardening model in the VPSC (visco-plastic self-consistent) framework. This indicates that slip-twin interactions are not properly captured in these models. Through a simple phenomenological implementation, we show that dislocation transmutation by twinning is an important aspect of slip-twin interactions that improve the predictability of the above crystal plasticity models for HCP structures.

The effects of water and microstructure on the mechanical properties of bighorn sheep (Ovis canadensis) horn keratin

The function of the bighorn sheep horn prompted quantification of the various parametric effects important to the microstructure and mechanical property relationships of this horn. These parameters included analysis of the stress-state dependence with the horn keratin tested under tension and compression, the anisotropy of the material structure and mechanical behavior, the spatial location along the horn, and the wet-dry horn behavior. The mechanical properties of interest were the elastic moduli, yield strength, ultimate strength, failure strain and hardness. The results showed that water has a more significant effect on the mechanical behavior of ram horn more than the anisotropy, location along the horn and the type of loading state. All of these parametric effects showed that the horn microstructure and mechanical properties were similar to those of long-fiber composites. In the ambient dry condition (10 wt.% water), the longitudinal elastic modulus, yield strength and failure strain were measured to be 4.0 G Pa, 62 MPa and 4%, respectively, and the transverse elastic modulus, yield strength and failure strain were 2.9 GPa, 37 MPa and 2%, respectively. In the wet condition (35 wt.% water), horn behaves more like an isotropic material; the elastic modulus, yield strength and failure strain were determined to be 0.6G Pa, 10 MPa and 60%, respectively.

The candidacy of shuffle and shear during compound twinning in hexagonal close-packed structures

This paper proposes a systematic generalized formulation for calculating both atomic shuffling and shear candidates for a given compound twinning mode in hexagonal closed-packed metals. Although shuffles play an important role in the mobility of twinning dislocations in non-symmorphic crystals, their analytical expressions have not been previously derived. The method is illustrated for both flat planes and corrugated planes which are exemplified by {112¯2} and {101¯2} twinning modes, respectively. The method distinguishes between shuffle displacements and net shuffles. While shuffle displacements correspond to movements between ideal atom positions in the parent and twin lattices, net shuffles comprise contributions from shear on overlying planes which can operate along opposite directions to those of shuffle displacements. Thus, net shuffles in the twinning direction can vanish in a limiting case, as is interestingly the case for those needed in the second plane by the b4 dislocation candidate in {112¯2} twinning. It is found that while shuffle displacement vectors can be irrational when K1 is corrugated, net shuffle vectors are always rational.

Acoustic Emission of Deformation Twinning in Magnesium

The Acoustic Emission of deformation twinning in Magnesium is investigated in this article. Single crystal testing with combined full field deformation measurements, as well as polycrystalline testing inside the scanning electron microscope with simultaneous monitoring of texture evolution and twin nucleation were compared to testing at the laboratory scale with respect to recordings of Acoustic Emission activity. Single crystal testing revealed the formation of layered twin boundaries in areas of strain localization which was accompanied by distinct changes in the acoustic data. Testing inside the microscope directly showed twin nucleation, proliferation and growth as well as associated crystallographic reorientations. A post processing approach of the Acoustic Emission activity revealed the existence of a class of signals that appears in a strain range in which twinning is profuse, as validated by the in situ and ex situ microscopy observations. Features extracted from such activity were cross-correlated both with the available mechanical and microscopy data, as well as with the Acoustic Emission activity recorded at the laboratory scale for similarly prepared specimens. The overall approach demonstrates that the method of Acoustic Emission could provide real time volumetric information related to the activation of deformation twinning in Magnesium alloys, in spite of the complexity of the propagation phenomena, the possible activation of several deformation modes and the challenges posed by the sensing approach itself when applied in this type of materials evaluation approach.

The effect of deformation twinning on stress localization in a three dimensional TWIP steel microstructure

We present an investigation of the effect of deformation twinning on the visco-plastic response and stress localization in a low stacking fault energy twinning-induced plasticity (TWIP) steel under uniaxial tension loading. The three-dimensional full field response was simulated using the fast Fourier transform method. The initial microstructure was obtained from a three dimensional serial section using electron backscatter diffraction. Twin volume fraction evolution upon strain was measured so the hardening parameters of the simple Voce model could be identified to fit both the stress-strain behavior and twinning activity. General trends of texture evolution were acceptably predicted including the typical sharpening and balance between the 〈1 1 1〉 fiber and the 〈1 0 0〉 fiber. Twinning was found to nucleate preferentially at grain boundaries although the predominant twin reorientation scheme did not allow spatial propagation to be captured. Hot spots in stress correlated with the boundaries of twinned voxel domains, which either impeded or enhanced twinning based on which deformation modes were active locally.

Stress-induced roughness development during oxide scale growth on a metallic alloy for SOFC interconnects

The metallic alloys employed in solid oxide fuel cell (SOFC) interconnects rely on the growth of a protective oxide scale to sustain the aggressive functioning environment at high temperature. The morphology of the oxide/metal interface is critical to the mechanical resistance of the material system upon cooling and therefore to the component lifetime. This work investigates the development of a roughness at the oxide/metal interface, under the influence of a non-uniform stress field along the phase boundary. A small initial roughness is assumed along with a mechanism of growth stress generation with time, and a previously derived formulation accounts for the influence of the stress and strain situation at the sharp metal/oxide phase boundary on the local oxidation kinetics. A continuum mechanics/diffusion framework is formed to complete the model, and a specifically developed simulation tool is used, based on a finite element method coupled with an external routine for the phase boundary propagation. The non-uniform growth of a chromia scale over a ferritic stainless steel used in a SOFC interconnect is simulated, and coupled stress and roughness developments are investigated. The consequences regarding mechanical failure and the influence of the main parameters are discussed.

The Role of Compositional Strain in the Instability of Solid-Fluid Thin Film Interfaces

Consider a binary substitutional solid solution not in equilibrium with a fluid rich in either solid component. At the interface, depending on the chemical energy, the solid may selectively lose or gain atoms from the fluid. Any atom exchange upsets the nominal composition; thereby stresses are generated in the solid by simultaneous action of lattice dilation and interdiffusion. As stresses build up, the local changing volume tends to split up into islands in an effort to escape from stresses, while curvature tends to amalgamate the islands in an effort to minimize surface energy. This competition could lead to substantial corrugations of the solid-fluid interface. These corrugations and the thresholds for noticeable competition between stresses and curvatures had an old tradition in chemo-mechanical models of sharp interfaces beginning with Srolovitz (1989). However, stresses were generally considered as a mere cause of external loads, so a pure solid was tacitly asserted. This paper couples elasticity with a conserved-phase field model to mainly investigate the effect of compositional strain on the interface instability process between a binary solid selectively interacting with a fluid. The chemical energy barrier is considered, however, uncoupled from stress, and the mobility quadratically dependent on the gradient of composition. Under initial enforced sinusoidal perturbations, results of simulations showed that the compositional strain parameter can dramatically alter the effect of the chemical energy barrier on the critical wavelength of perturbations that trigger interface instability.

Crystal Plasticity Modeling of the Dynamic Behavior of Magnesium Alloy, WE43-T5, Plate

Understanding the high strain rate behavior of Mg alloys is of interest for applications ranging from armor to automobile crash worthiness. Toward this end, the viscoplastic self-consistent (VPSC) polycrystal plasticity code, including the recently developed twinning-detwinning (TDT) model, is used to describe the homogeneous plastic flow of the rare earth element containing Mg alloy, WE43-T5, plate. The model accounts for the presence of an initial, moderate texture and its evolution as during deformation. It reveals that the moderate texture is responsible for the difference between the plate through-thickness and in-plane behaviors. The model also helps to reconcile why the in-plane response is nearly isotropic, despite the presence of orthotropic (not radially symmetric) texture. Note that a single set of parameters was used to fit the entire set of results, i.e. it is a model, which can describe all of the observed strength, strain, and strain hardening anisotropies and asymmetries.

Unraveling Recrystallization Mechanisms Governing Texture Development from Rare-Earth Element Additions to Magnesium

Abstract The origin of texture components often associated with rare-earth element (REE) additions in wrought magnesium alloys is a long-standing problem in magnesium technology. While their influence on the texture is unquestionable, it is not yet clear why certain texture components, such as