Micromechanical Deformation Mechanisms Research Articles

Elastostatics of star-polygon tile-based architectured planar lattices

A panoptic view of architectured planar lattices based on star-polygon tilings was developed. Four star-polygon-based lattice sub-families, formed of systematically arranged triangles, squares, or hexagons, were investigated numerically and experimentally. Finite-element-based homogenization allowed computation of Poisson’s ratio, elastic modulus, shear modulus, and planar bulk modulus. A comprehensive understanding of the range of properties and micromechanical deformation mechanisms was developed. Adjusting the star-polygon angle achieved an over 250-fold range in elastic modulus, over a 10-fold range in density, and a range of -0.919 to +0.988 for Poisson’s ratio. Additively manufactured lattices, achieved by novel printing strategies, showed good agreement in properties. Parametric additive manufacturing procedures for all lattices are available on www.fullcontrol.xyz/#/models/1d3528. Three of the four sub-families exhibited in-plane elastic isotropy. One showed high stiffness with auxeticity at low density and a primarily axial deformation mode as opposed to bending deformation for the other three lattices. The range of achievable properties, demonstrated with property maps, proves the extension of the conventional material-property space. Lattice metamaterials with Triangle-Triangle, Kagome, Hexagonal, Square, Truncated Archimedean, Triangular, and Truncated Hexagonal topologies have been studied in the literature individually. Here, it is shown that these structures belong to the presented overarching lattice family.

Micromechanical aspects of deformation and failure of advanced Iron-Chromium-Aluminum alloys

This work investigates the micromechanical deformation and failure mechanisms of advanced FeCrAl alloys developed for use in nuclear reactors as cladding material. Three different FeCrAl alloys (82-X)Fe-13Cr-5Al-X (X=Nb, TiC, 0 for base alloy) were investigated both in their as-received and welded states. In-situ neutron diffraction with simultaneous digital image correlation was used to determine micromechanical deformation mechanisms not only as a function of elemental composition but also as a condition of state (as-received vs. welded). Ex-situ X-ray computed tomography was used on as-deformed samples to help determine the failure mechanisms and void initiation strains.

Analysis of the Damage Mechanism around the Crack Tip for Two Rubber-Toughened PLA-Based Blends.

The toughening mechanisms of poly(lactic acid; PLA) blended with two different elastomers, namely poly (butylene adipate-co-terephtalate; PBAT) and polyolefin elastomers with grafted glycidyl methacrylate (POE-g-GMA), at 10 and 20 wt.%, were investigated. Tensile and Charpy impact tests showed a general improvement in the performance of the PLA. The morphology of the dispersed phases showed that PBAT is in the form of spheres while POE-g-GMA has a dual sphere/fibre morphology. To correlate the micromechanical deformation mechanism with the macroscopical mechanical behaviour, the analysis of the subcritical crack tip damaged zone of double-notched specimens subjected to a four-point bending test (according to the single-edge double-notch four-point bend (SEDN-4PB) technique) was carried out using several microscopic techniques (SEM, polarized TOM and TEM). The damage was mainly generated by shear yielding deformation although voids associated with dilatational bands were observed.

Plasticity induced by nanoindentation in a CrCoNi medium-entropy alloy studied by accurate electron channeling contrast imaging revealing dislocation-low angle grain boundary interactions

In the present work, interactions of nanoindentation-induced dislocations (NIDs) with a low-angle grain boundary (LAGB) are investigated in a single-crystalline CrCoNi medium-entropy alloy (MEA). Microstructural evolutions before and after nanoindentation were examined using accurate electron channeling contrast imaging (A-ECCI). In the as-grown state, the alloy microstructure consists of subgrains separated by LAGBs. After nanoindentation on the (001) plane far away from LAGBs, the load-displacement curves exhibit the typical behavior of metals and alloys with a pop-in marking the elastic-plastic transition. This pop-in is related to the nucleation of NIDs that are observed to form pile-ups on {111} planes. In contrast, when indents are performed in the vicinity of a LAGB with a low misorientation angle of 0.24° and consisting of dislocations spaced ~60 nm apart, different micromechanical responses and deformation mechanisms are observed depending on the distance between the LAGB and the nanoindenter tip. When the distance between the LAGB and the nanoindenter tip is larger than four times the size of the indent (corresponding ratio: R > 4), the LAGB does not affect the micromechanical response nor interact with NIDs. In contrast, when the indenter comes in direct or indirect contact with the LAGB (R < 1), the load-displacement curve deviates at low loads from the elastic stage, and pop-ins are not observed. In this case, the continuous deformation is accommodated by the movement of the pre-existing LAGB dislocations. For intermediate cases with 1 < R < 4, the load of the initial pop-in is dependent on the local defect density. In this latter case, the pile-ups of NIDs directly impinge on the LAGB. Microstructural analyses reveal that the LAGB accommodates plasticity by blocking the NIDs, activating a dislocation nucleation site in the adjacent subgrain/emission of dislocation from the LAGB, and inducing slight motions of its constituent dislocations.

Electron microscopy and atom probe tomography of nanoindentation deformation in oxide dispersion strengthened steels

Oxide Dispersion Strengthened (ODS) steels are candidates for fuel cladding materials in sodium-cooled fast reactors and for structural materials in nuclear fusion power reactors. The effect yttrium-titanium-oxygen (Y-Ti-O) nano-oxide precipitates within ODS steels have on the micromechanical deformation mechanisms has been investigated. The aim is to assess the extent of any direct link between the Y-Ti-O dispersion and nanoindentation hardness, using electron backscatter diffraction, transmission electron microscopy (TEM), and atom probe tomography (APT) studies of a Fe-14Cr-3W-0.2Ti-0.25Y2O3 (wt%) ODS steel at room temperature. Y-Ti-O nanoclusters had a non-uniform distribution and average number density that ranged from 5.8 ± 0.1 × 1023 to 1.3 ± 0.1 × 1024 m−3 and Guinier radii ranging from 1.8 ± 0.2 nm to 2.1 ± 0.2 nm. Surprisingly, the local Y-Ti-O distribution did not correlate strongly with the nanohardness, indicating that the dominant hardening mechanisms was at best only weakly related to the Y-Ti-O distribution. Instead, scanning TEM and APT confirmed that the dominant hardening mechanism was due to the grain boundary refinement, and was further enhanced by tungsten enrichment to ~3.5 at.% at grain boundaries.

Investigation of Strain‐Rate Effects in Ni/PU Hybrid Foams under Low‐Impact Velocities

Metal foams are often used as energy absorbers and lightweight materials. Inspired by a natural blueprint, open‐cell metal foams can significantly reduce the mass of a structure. The innovative manufacturing process of electrodeposition provides the possibility to customize the coating layer thickness of nickel (Ni) on a polyurethane (PU) precursor foam. Consequently, the mechanical properties can be adjusted according to the requirements of the expected application. Herein, quasistatic compression tests and low‐velocity impact tests are conducted on open‐cell Ni/PU hybrid foams to investigate the strain‐rate effects for strain rates in the range of 10−3 to 550 s−1. Furthermore, digital image correlation is performed with the intention of comparing the micromechanical deformation mechanisms under quasistatic loading with those under dynamic loading. For the first time, the heat evolution at different impact velocities of metal foams has been investigated with an infrared camera.

Study of elasto-plastic deformation in a cast AlCu7 alloy

The need for efficient and clean solutions, due to the increasing current environmental regulations puts extra pressure on new combustion engine development, to compete in a market with alternative driving concepts. Downsizing and weight reduction can reduce the engine emission and efficiency, but require light alloys with superior thermo-mechanical properties for high temperature exposure to maintain the same engine performance. Cast Al-Cu could be alternative to standard Al-Si alloys for new engine generations due to their higher temperature strength, creep-resistance and long term stability of engine components. In Al-Si and Al-Cu cast alloys with heterogeneous microstructures a composite-like deformation behavior is responsible for superior high temperature properties. Stiff Si or Al2Cu particles, respectively reinforce a ductile α-Al matrix to a composite with improved thermo-mechanical strength. However, different Young’s moduli and coefficients of thermal expansion are responsible for micro stress gradients and unpredictable micro crack formation under operation. These micro-mechanical deformation mechanisms in Al-Si and Al-Cu systems, responsible for crack initiation and growth, have been scarcely investigated so far.This manuscript describes an example of elasto-plastic deformation mechanisms in an AlCu7 alloy. Tensile testing shows anomalous macroscopic deformation behavior indicating unknown internal micro-mechanical processes. External loading until yield strength and beyond are applied under laboratory conditions and during in-situ neutron diffraction. The results of macroscopic deformation and micro strain evolution are compared and correlated with the heterogeneous micro structure. High resolution synchrotron computed tomography reveals conclusions on the micro-mechanic deformation mechanisms and their effects on the macroscopic damage initiation and material’s service performance.

In-situ and ex-situ microtensile testing of individual struts of Al foams and Ni/Al hybrid foams

Metal foams are cellular materials, offering a strong structure-property relationship. Their global properties are strongly connected to the local micromechanical properties of the struts and to the microstructure of the foams. The mechanical properties of individual struts differ widely from the bulk matrix material as a result of differences in solidification conditions and the surface-to-volume ratio. A robust micromechanical understanding and knowledge of micromaterial parameters is essential to design components made of foams e.g. supported by numerical simulations. However, up to now, micromechanical charaterisation of individual struts is very challenging but an emerging field of research.The present contribution deals with the microstructural and micromechanical characterisation of individual struts of open-cell aluminium foams and Ni/Al hybrid foams. The crystallographic texture is investigated by electron backscatter diffraction. Microtensile testing is performed on individual struts to determine real micromechanical material properties by ex-situ testing using a specially developed device and a digital image correlation method. Furthermore, in-situ microtensile tests using scanning electron microscopy are conducted for high-resolution investigation of the micromechanical deformation and damage mechanism occuring under loading of individual struts. The experimentally determined Young's modulus of the nickel coating is compared with theoretical calculations based on the grain orientation data.

Study of the scratch resistance criteria and their relationship with mechanical properties and microstructure in a ternary thermoplastic blend

Polymeric materials generally suffer from a low resistance to various types of surface mechanical damage, among which scratch is a prominent example. While several attempts have been made to assess the scratch resistance and its relationship with the bulk mechanical properties in simple polymeric systems, the available research on the polymeric blends is scarce. This study attempts to fill this gap by studying the link between the scratch resistance and mechanical properties in a ternary polymeric nanocomposite, which has proved to feature superior and tunable mechanical properties. For this aim, blends of polypropylene and polyamide 6 were prepared using melt blending with two different types and various amounts of compatibilizers. Their microstructure was studied using scanning electron microscopy and atomic force microscopy. It was observed that the type of morphology depends upon both compatibilizer type and content. Bulk mechanical properties were investigated at different loading conditions. The microstructure was seen to have a noticeable effect on ductility of the blends. Different scratch groove geometry parameters were seen to be affected by different mechanical properties. It was shown that relying exclusively on scratch groove geometry might lead to contradicting conclusions regarding the effect of bulk mechanical properties on scratch resistance. Instead, a heavier emphasis was placed upon the respective contribution of each parameter to the overall scratch visibility. In addition, it was shown that micro-mechanical scratch deformation mechanisms have a crucial impact on scratch visibility, and they are affected by a complex interplay among various mechanical properties. An analysis of these relationships is provided with the aim of moving towards a comprehensive understanding of scratch visibility and routes to its minimization.

On the mechanistic basis of deformation at the microscale in hexagonal close-packed metals

This is an overview of micromechanical deformation mechanisms in hexagonal close-packed metals. We start with an in-depth discussion of single-crystal behaviour concerning crystallographic slip, plastic anisotropy and deformation twinning. We move on to discuss some complexities involved in polycrystalline deformation and modelling approaches, focusing on rate effects in titanium alloys that are thought to play a significant role in dwell fatigue. We finish our review with a brief commentary on current understanding and state-of-the-art techniques, and outline some key areas where further study is recommended.

In Situ Neutron Diffraction Studies of Large Monotonic Deformations of Superelastic Nitinol

Superelastic Nitinol micromechanics are studied well into plastic deformation regimes using neutron diffraction. Insights are made into the nature of initial transformation, bulk transformation, plastic deformation, and unloading. Schmid factor predictions based on habit plane variants are found to best describe the very first grains that transform, prior to the transformation plateaus. However, the bulk transformation behavior that gives rise to transformation plateaus violates single crystal Schmid factor analyses, indicating that in bulk polycrystals, it is the effect of grain neighborhoods, not the orientations of individual grains, that drives transformation behaviors. Beyond the plateaus, a sudden shift in micromechanical deformation mechanisms is observed at ~8.50 %/4.75 % tension/compression engineering strain. This mechanism results in reverse-phase transformation in both cases, indicating a strong relaxation in internal stresses of the samples. It is inferred that this mechanism is most likely initial bulk plastic flow, and postulated that it is the reason for a transition from fatigue life enhancement to detriment when pre-straining superelastic Nitinol. The data presented in this work provide critical datasets for development and verification of both phenomenological internal variable-driven and micromechanical theories of transformation-plasticity coupling in shape memory alloys.

Microstresses and crack formation in AlSi7MgCu and AlSi17Cu4 alloys for engine components

The increasing demand for light weight and efficiency of modern combustion engines requires sophisticated light alloys with improved high-temperature strength and creep resistance. Nowadays AlSi alloys are used for structural parts as these materials offer increased wear resistance and long-term stability under operating conditions. The heterogeneous microstructure of AlSi combines the thermal properties of stiff Si particles with ductile α-Al into a composite with sophisticated thermomechanical strength. However, the different Young’s moduli and coefficients of thermal expansion cause large microstress gradients and microcrack formation in service. The micromechanical deformation mechanisms in AlSi systems, responsible for crack initiation and growth, are unknown so far. The current work describes an experimental approach that combines non-destructive diffraction and imaging techniques to investigate the elastoplastic deformation behavior of two engineering alloys, i.e. the hypoeutectic AlSi7MgCu standard alloy for castings and the hypereutectic AlSi17Cu4 piston alloy for improved surface wear resistance. Neutron diffraction is applied for microstress measurements under external load and during thermal cycling. Complementary synchrotron tomography is performed for imaging of microcrack formation and damage within the microstructure. Stress-induced micromechanical deformation mechanisms could be revealed and correlated to damage mechanisms critical for engine components under operating conditions.

An approach for nonlinear modeling of polymer matrix composites

In the present work, a mechanistic modeling approach is pursued for material characterization and for modeling inelastic deformation of polymer matrix composite components. The model attempts to capture the dominant micromechanical deformation mechanisms in laminated composites caused by matrix inelasticity at elevated temperatures. Given material characteristics of the constituent materials, the model can be used in predicting stress, time and temperature-dependent response of a composite under a broad range of thermal and mechanical load conditions. This article describes the modeling approach and examples of its use in a finite element analysis framework. Examples include analyses of simple test specimen coupons, stress concentration at holes and a structural element configuration of a polymer matrix composite. In each case, the model predictions are compared with the experimental measurements.

Modeling of stress concentration in ceramic matrix composites

This paper describes a mechanistic modeling approach that attempts to capture the dominant micromechanical deformation and damage mechanisms in woven composites over the temperature and stress ranges relevant to gas turbine engine components. The model is incorporated into a finite-element framework and applied to analyze stress concentration at holes. The materials considered in this study are two-dimensional woven SiC/SiC and SiC/MAS composites. The results include the strain field near circular holes and global load–displacement behavior of the structure. In each case, the model predictions are compared with experimental measurements. Based on these comparisons, conclusions are drawn regarding the efficacy of the modeling approach and needs for further investigations are identified.

Deformation mechanisms in polylactic acid/natural rubber/organoclay bionanocomposites as revealed by synchrotron X-ray scattering

The micromechanical deformation mechanisms of a polylactic acid (PLA)/natural rubber (NR) blend (PLA/NR 90/10 wt%) and its organoclay filled bionanocomposites have been investigated by small and wide angle X-ray scattering (SAXS–WAXS) under tensile conditions. The addition of NR to a PLA matrix changed the brittle fracture of PLA to a ductile deformation through the debonding of the rubber droplets. Otherwise, the formation of cavities between PLA and NR was hampered by the nanoclays since they were mainly located at the polymer blend interface. In this case, the nanoclays acted as craze nucleation sites. At 1 wt% of filler concentration, the crazes were able to fully develop in the blend and to evolve into stable microvoids, which kept growing and orienting in the tensile direction. These mechanisms also explained the progressive plastic deformation of the polymer chains and the preferential orientation of the nanoclay platelets.

Modeling texture evolution during uni-axial deformation of Zircaloy-2

Texture development in zirconium reactor components greatly affects their in-service performance, in particular influencing dimensional stability, hydride induced cracking and fracture. Understanding texture evolution in terms of micromechanical deformation mechanisms contributes to improved engineering design of the components. The texture evolution during uni-axial deformation of Zircaloy-2 obtained using neutron diffraction was interpreted with a visco-plastic self-consistent model, based on an extensive data set achieved through in situ and ex situ neutron diffraction measurements. In the model, the influences of prism, basal, pyramidal〈a〉, pyramidal〈c + a〉 slip, tensile twinning, and hardening due to dislocation interactions were considered. Model parameters were adjusted to obtain a simultaneous ‘best-fit’ to the stress–strain curves, Lankford coefficients, development of peak intensities in three dimensions and post-deformation textures, for specimens compressed or pulled in three principal directions relative to the slab texture. The texture formation was found to be directly related to the activation of different deformation mechanisms. Pyramidal〈a〉 slip was unnecessary in the modeling. Accounting for interactions between deformation modes was critical to correctly predicting texture development and stress–strain curves. This study provides further understanding on the plastic deformation of Zircaloy-2.

Energy Approach Towards Stress-Dilatancy Formulation based on Double Slip-Rotation Rate Concept

This paper proposes a new approach for the analysis of shear-induced volume change of granular materials. Following a revisit to the micro-mechanical deformation mechanism and the findings from discrete element method simulations, it is concluded that the double slip-rotation rate mechanism is appropriate for the description of granular materials subjected to shear. The implementation of energy principle with the double slip-rotation rate mechanism provides a new method for the derivation of stress-dilatancy relation under general stress conditions without adopting Rowe's hypothesis of minimum energy ratio. This new approach can be easily extended to take into account the non-coaxility of granular material deformation. The dilatancy formulations proposed by Taylor, Rowe and Matsuoka-Nakai can all be recovered as special cases of the proposed approach. Comparisons with experimental data show that the new stress-dilatancy relation correctly captures the dilatancy behaviour of sand observed in both general 3D stress conditions and simple shear tests.

Temperature and direction dependence of internal strain and texture evolution during deformation of uranium

Depleted uranium is of current programmatic interest at Los Alamos National Lab due to its high density and nuclear applications. At room temperature, depleted uranium displays an orthorhombic crystal structure with highly anisotropic mechanical and thermal properties. For instance, the coefficient of thermal expansion is roughly 20 × 10 −6 °C −1 in the a and c directions, but near zero or slightly negative in the b direction. The innate anisotropy combined with thermo-mechanical processing during manufacture results in spatially varying residual stresses and crystallographic texture, which can cause distortion, and failure in completed parts, effectively wasting resources. This paper focuses on the development of residual stresses and textures during deformation at room and elevated temperatures with an eye on the future development of computational polycrystalline plasticity models based on the known micro-mechanical deformation mechanisms of the material.

A micro-mechanically based quadratic yield condition for textured polycrystals

In the present paper a two-scale approach for the description of anisotropies in sheet metals is introduced, which combines the advantages of a macroscopic and a microscopic modeling. While the elastic law, the flow rule, and the hardening rule are formulated on the macroscale, the anisotropy is taken into account in terms of a micro-mechanically defined 4th-order texture coefficient. The texture coefficient specifies the anisotropic part of the elasticity tensor and the quadratic yield condition. The evolution of the texture coefficient is described by a rigid-viscoplastic Taylor type model. The advantage of the suggested model compared to the classical v. Mises-Hill model is first that macroscopic anisotropy parameters can be identified based on a texture measurement, and second that the anisotropy of the elastic and the plastic behavior is generally pathdependent, and that this path-dependence is related to a micro-mechanical deformation mechanism. An explicit modeling of the plastic spin is circumvented by the aforementioned micro-mechanical approach. The model is implemented into the FE code ABAQUS and applied to the simulation of the deep drawing process of aluminum.

On the free volume evolution in a deformed epoxy composite. A positron annihilation study

After fabrication of an epoxy system filled with aluminum powder, followed by inelastic deformations under compression of the specimens, Positron Annihilation Lifetime Spectroscopy (PALS) was used to follow the evolution of the free-volume holes in the matrix. In order to describe the micromechanical deformation mechanism that takes place in the matrix around the inclusions, the experimental free-volume holes data were analyzed in terms of a model specifically developed. This model involves a hydrostatic internal stress resulting from the fabrication process of the composites and the deviatoric part of the applied stress during inelastic deformation. The influence of both kinds of stresses on the modification of the free-volume sizes in the matrix is discussed.