A.L. Oppedal

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 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.

Characterizing the Local Primary Dendrite Arm Spacing in Directionally Solidified Dendritic Microstructures

Characterizing the spacing of primary dendrite arms in directionally solidified microstructures is an important step for developing process–structure–property relationships by enabling the quantification of (i) the influence of processing on microstructure and (ii) the influence of microstructure on properties. In this work, we utilized a new Voronoi-based approach for spatial point pattern analysis that was applied to an experimental dendritic microstructure. This technique utilizes a Voronoi tessellation of space surrounding the dendrite cores to determine nearest neighbors and the local primary dendrite arm spacing. In addition, we compared this technique to a recent distance-based technique and a modification to this using Voronoi tessellations. Moreover, a convex hull-based technique was used to include edge effects for such techniques, which can be important for thin specimens. These methods were used to quantify the distribution of local primary dendrite arm spacings, their spatial distribution, and their correlation with interdendritic eutectic particles for an experimental directionally solidified Ni-based superalloy micrograph. This can be an important step for correlating processing and properties in directionally solidified dendritic microstructures.

Limitation of current hardening models in predicting anisotropy by twinning in hcp metals: Application to a rod-textured AM30 magnesium alloy

When a strongly textured hexagonal close packed (HCP) metal is loaded under an orientation causing profuse twinning or detwinning, the stress-strain curve is sigmoidal in shape and inflects at some threshold. Authors have largely attributed the dramatic stress increase in the lower-bound vicinity of the inflection point to a combined effect of a Hall-Petch mechanism correlated to grain refinement by twinning, and twinning-induced reorientation requiring activation of hard slip modes. We experimentally and numerically demonstrate that these two mechanisms alone are unable to reproduce the stress-strain behaviors obtained under intermediate loading orientations correlated to in-between profuse twinning and nominal twinning. We argue based on adopting various mechanistic approaches in hardening model correlations from the literature. We used both a physics dislocation based model and a phenomenological Voce hardening model. The HCP material is exemplified by an extruded AM30 magnesium alloy with a � 10¯0� -fiber parallel to the extrusion direction.

Evaluating Local Primary Dendrite Arm Spacing Characterization Techniques Using Synthetic Directionally Solidified Dendritic Microstructures

Microstructure characterization continues to play an important bridge to understanding why particular processing routes or parameters affect the properties of materials. This statement certainly holds true in the case of directionally solidified dendritic microstructures, where characterizing the primary dendrite arm spacing is vital to developing the process–structure–property relationships that can lead to the design and optimization of processing routes for defined properties. In this work, four series of simulations were used to examine the capability of a few Voronoi-based techniques to capture local microstructure statistics (primary dendrite arm spacing and coordination number) in controlled (synthetically generated) microstructures. These simulations used both cubic and hexagonal microstructures with varying degrees of disorder (noise) to study the effects of length scale, base microstructure, microstructure variability, and technique parameters on the local PDAS distribution, local coordination number distribution, bulk PDAS, and bulk coordination number. The Voronoi tesselation technique with a polygon-side-length criterion correctly characterized the known synthetic microstructures. By systematically studying the different techniques for quantifying local primary dendrite arm spacings, we have evaluated their capability to capture this important microstructure feature in different dendritic microstructures, which can be an important step for experimentally correlating with both processing and properties in single crystal nickel-based superalloys.

Crystal Plasticity Modeling of Anisotropic Hardening and Texture Due to Dislocation Transmutation in Twinning

In crystalline materials, dislocations are three-dimensional lattice distortions that systematically distort twin interfaces that they encounter. This results in dislocation dissociation events and changes in the atomic structure of the interface. The manner in which the interface distorts drive the product of the dissociation event, and consequently, the incident dislocation core and the magnitude and relative direction of the Burgers vector govern these slip-twin interaction phenomena. Recent characterization studies using transmission electron microscopy as well as advanced molecular dynamic simulations have shown that slip dislocations, whether striking or struck by a {101¯2} twin boundary, dissociate into a combination of twinning disconnections, interfacial disclinations (facets), jogs, and other types of dislocations engulfed inside the twin domains, called transmuted dislocations. While twinning disconnections were found to promote twin propagation, the dislocations incorporated inside the twin are of considerable importance to hardening and damage initiation as they more significantly obstruct slip dislocations accommodating plasticity of the twins. In this work, the dislocation transmutation event and its effect on hardening is captured using a dislocation density based hardening model contained in a visco-plastic self-consistent mean-field model. This is done by allowing the twins to increase their dislocation densities, not only by virtue of slip inside the twin, but also through dislocations that transmute from the parents as the twin volume fraction increases. A correspondence matrix rule is used to determine the type of converted dislocations while tracking and parameterizing their evolution. This hypothesis provides a modeling framework for capturing slip-twin interactions. The model is used to simulate the mechanical response of pure Mg and provides a more physically based approach for modeling stress-strain behavior.

Structure, property, and function of sheepshead ( Archosargus probatocephalus ) teeth

OBJECTIVES This paper studies A. probatocephalus teeth and investigates the mechanical properties and chemical composition of the enameloid and dentin. DESIGN Nanoindentation tests with a max load of 1000 μN and X-ray Energy Dispersive Spectroscopy (EDS) were performed along the diameter of the polished sample. Microstructural analysis of the dentin tubules was performed from SEM images. RESULTS From nanoindentation testing, the dentin of the sheepshead teeth has a nanoindentation hardness of 0.89 ± 0.21 (mean ± S.D.) GPa and a reduced Youngs modulus of 23.29 ± 5.30 GPa. The enameloid of A. probatocephalus has a hardness of 4.36 ± 0.44 GPa and a mean reduced Youngs modulus of 98.14 ± 6.91 GPa. Additionally, nanoindentation tests showed that the enameloids hardness and modulus increased closer to the surface of the tooth. X-ray Energy Dispersive Spectroscopy (EDS) data further suggests that the gradient may be a result of the wt% fluoride within the enameloid, where an increase in fluoride results in an increase in reduced Youngs modulus and hardness. CONCLUSION The microstructural characterization of the number density and area of the dentin tubules were used to address the porosity effect in the dentin to achieve the experimentally validated microhardness. The mechanical properties of the sheepshead teeth were also compared with previous nanoindentation tests from other aquatic species. The sheepshead teeth exhibit a greater reduced Youngs modulus and hardness compared to shark and piranha teeth.

The Effect of \{ 10\bar{1}2\} Twin Boundary on the Evolution of Defect Substructure

Pure Mg single crystals were deformed at room temperature along two orientations in sequence, in order to activate a specific dislocation slip mode followed by \( \left\{ {10\bar{1}2} \right\} \) twinning. The defects in both the matrix and twin crystals were analyzed with a transmission electron microscope (TEM). This study reveals the collective evolution of the defect substructure when a dislocated crystal is “invaded” by a moving twin boundary. When primarily \( \left[ c \right] \)-containing defects in the matrix were incorporated by a moving twin boundary, including \( \langle c + a\rangle \), pure \( \left[ c \right] \) dislocations and \( I_{1} \) stacking faults, the twin contains homogeneously distributed \( I_{1} \) stacking faults, which in some instances appear to be connected on twin boundary to the faults in the matrix.

Robust Intermediate Strain Rate Experimentation Using the Serpentine Transmitted Bar

The stress-strain behavior of a material at intermediate strain rates (between 5/s and 500/s) is important for characterizing dynamic deformation events. A material’s mechanical behavior can be strain rate dependent; calibrating constitutive models at actual strain rates of interest are essential for high fidelity simulations. Strain rates below 5/s are easily accomplished with conventional electro-mechanical or servo-hydraulic load frames. Strain rates above 500/s are typically performed with the split Kolsky/Hopkinson pressure bar (SHPB) and other devices depending upon the strain rate. The intermediate strain rate regime is a difficult test regime in which researchers have tried to extend the use of specially instrumented servo-hydraulic load frames or very long Hopkinson bars. We describe a novel design of a serpentine Hopkinson transmitted bar that allows for accurate and robust load acquisition in the intermediate strain rate regime. This design produces repeatable stress-strain results without the stress oscillations typical of a specially instrumented servo-hydraulic load frame and produces data for a longer test time than a conventional Kolsky/Hopkinson bar of the same length.

Effects of Homogenization on Microstructure and Mechanical Properties of a ZE20 Mg Alloy Processed by Indirect Extrusion

The effects of billet homogenization on final grain size, texture, and mechanical properties after indirect extrusion of a ZE20 magnesium (Mg) alloy were studied. Using a laboratory scale indirect extrusion process, both as-cast, and homogenized ZE20 billets were extruded into solid rods with an extrusion ratio of 25, at 454°C and at several ram speeds. Electron backscatter diffraction (EBSD) was used to characterize texture and grain size of the extruded material. Energy-dispersive X-ray spectroscopy (EDS) was used to characterize the effect of homogenization on elemental segregation. To determine the effects of homogenization and extrusion on mechanical properties, tension and compression tests were performed at room temperature and quasi-static test conditions. In tension, the homogenized extruded material exhibited a 29% elongation to failure, a 5% increase over the as-cast extruded material that obtained a 24% elongation to failure. The texture of the homogenized extruded material was also significantly weaker than that of the as-cast extruded materials.