Uniaxial Deformation Research Articles

Microstructure and properties of additively-manufactured WC-Co microlattices and WC-Cu composites

Liquid ink-printing followed by sintering is used to fabricate WC-Co microlattices and cutting tools. The microstructure of WC-xCo (x=0.5-20 wt.%) is studied for a range of carbide-to-binder ratios and for various sintering temperatures. For 0.5≤Co≤5 wt.%, struts in microlattices exhibit residual porosity due to incomplete densification, even at the highest sintering temperature of 1650 °C. With 10 wt.% Co, fully dense lattice struts are achieved after sintering at 1450 °C for 1 h. For 1450-1650 °C sintering temperatures, the hardness of WC-xCo struts initially increases (due to increasing densification with increased Co) and then gradually decreases (due to an increase of softer Co phase, at near-full density). Finite-element modeling (FEM) shows that the uniaxial elastic deformation of WC-10Co lattices is mostly supported by stretching-dominated vertical columns formed by stacking filaments in a 0/90˚ cross-ply geometry. WC-10Co lattices are infiltrated with Cu at 1300 °C to obtain dense WC-Cu composites, with an internal architecture consisting of a WC-rich lattice and Cu-rich channels, with a high thermal conductivity of 140±7 W/(m·K). Under compression, WC-Cu infiltrated composites are supported by the ductile Cu phase after compressive failure of the internal WC-rich lattice, unlike the WC-Co open lattices which show brittle failure. A cutting tool with an internal WC-Cu lattice architecture is fabricated by 3D WC-Co ink-extrusion printing, sintering, and Cu infiltration. Laser heating experiments and FEM confirm that the 3D-printed/infiltrated/architectured WC-Cu composite cutting tool maintains lower temperatures than a uniform WC-Co tool, for equal heat input at the corner of the tool.

Mechanical properties and microstructural characteristics of additively manufactured C103 niobium alloy

C103 is a refractory-based niobium alloy for high temperature applications in aerospace. This paper aims to evaluate the compatibility of C103 alloy as a choice of material for laser-based additive manufacturing (AM) technologies. Crack free AM specimens with relative densities of greater than 99% were successfully fabricated via laser powder bed fusion (L-PBF) method with horizontal and vertical build orientations. Static strength levels of the stress-relieved specimens exceeded that of their wrought counterparts. Higher yield and ultimate tensile strength of AM specimens were attributed to high dislocation density and the formation of dislocation cell structures, which was revealed by transmission electron microscopy. Loss of ductility in AM specimens was partially due to high density of dislocations (i.e., shorter mean free path for dislocations), while the presence of defects may also have compromised the total elongation to failure. Electron backscattered diffraction analysis revealed formation of sharp 001||BD rod texture in AM samples, which resulted in anisotropic mechanical behavior. Uniaxial tensile and compressive deformation led to the development of (001) <110> parallel to tensile direction and <111> parallel to build direction, respectively.

Radiation defects in NaCl matrix with reduced lattice symmetry caused by light cation doping and elastic uniaxial deformation

The processes of radiation defect creation and radiative relaxation of electronic excitations under applied local or/and uniaxial elastic deformation have been studied in NaCl crystals by means of optical absorption, luminescence and thermoactivation spectroscopy methods. In NaCl:Li at 80 K, X-ray-induced absorption bands peaked around 3.35 and 4.6 eV have been detected and ascribed to interstitial halide atoms located nearby Li impurity cations, HA(Li) centres. Subsequent thermal annealing of HA(Li) centres leads to the formation of polyhalide centres responsible for the absorption band at 5.35 eV. In an X-irradiated and stressed NaCl:Li crystal (degree of uniaxial elastic deformation of ε = 0.9%), the peak of thermally stimulated luminescence at ~115 K is composed of the ~2.7-eV emission appearing, in our opinion, due to the recombination of the electron, thermally released from an F′ centre, with a hole-type HA(Li) centre. The applied uniaxial elastic stress facilitates the self-trapping of anion excitons in regular regions of a NaCl lattice and impedes the energy transfer by mobile excitons to impurities/defects and, in turn, attenuates the Br-related luminescence peaked at 3.95 eV with respect to the π-emission of self-trapped excitons (~3.35 eV). The 3.95 eV emission has been detected in a natural NaCl crystal containing homologous Br impurity ions.

Effects of Al on crack propagation in titanium alloys and the governing toughening mechanism

In this work, molecular dynamics simulation is performed to investigate the effect of Al element on the crack propagation along the {101‾1} twin boundary in Ti alloys with different contents and distribution of Al element during the uniaxial tensile deformation. The results show that increasing Al content obviously promotes the percentage of face-centered cubic (FCC) structure during the deformation process, which hinders the micro-crack propagation and enhances the plasticity and toughness of the Ti–Al alloys. The FCC bands transform from the hexagonal close-packed (HCP) matrix satisfying the basal-type (B-type) orientation relationship (OR) represented by (0001)HCP||(111)FCC and [12‾10]HCP||[11‾0]FCC. The increase of Al element reduces the basal stacking fault (BSF) energy of the HCP structure and thus contributes to the formation of BSFs, which grow into FCC bands with the B-type orientation by the slip of Shockley partial dislocations. The {101‾1} pyramidal SF activated at the crack tip evolves into FCC grains in the Ti–Al alloys following the prismatic-type (P-type) OR with the HCP matrix represented by (101‾0)HCP||(110)FCC and [12‾10]HCP||[11‾0]FCC, during which atomic shear and shuffle is involved.

Effect of Printing Parameters on Mechanical Behaviour of PLA-Flax Printed Structures by Fused Deposition Modelling.

Few studies have reported the performance of Polylactic acid (PLA) flax feedstock composite for additive manufacturing. In this work, we report a set of experiments conducted by fused filament technology on PLA and PLA-flax with the aim of drawing a clear picture of the potential of PLA-flax as a feedstock material. Nozzle and bed temperatures are both combined with the printing angle to investigate their influence on structural and mechanical properties. The study shows a low sensitivity of PLA-flax to process parameters compared to PLA. A varied balance between shearing and uniaxial deformation is found consistent with tensile results where filament crossing at −45/+45° provides the optimal load-bearing capabilities. However, Scanning Electron Microscopy (SEM) and high-speed camera recording shows a limiting reinforcing effect of flax fibre due to the presence of intra-filament porosity and a significant amount of fibre pull-out resulting from the tensile loading. These results suggest that the quality of the bond between PLA matrix and flax fibre, intra-filament porosity, and surface roughness should receive more attention as well as the need for more continuous fibre reinforcement in PLA filaments to optimise the performance of PLA-flax printed materials.

Analysis of a three-dimensional slip field in a hexagonal Ti alloy from in-situ high-energy X-ray diffraction microscopy data

Here we analyze a three-dimensional distribution of crystallographic slip measured in-situ during the uniaxial deformation of hexagonal Ti-7Al. The slip field is reconstructed using a novel methodology that combines spatially resolved lattice orientation fields and grain-averaged stresses measured using high-energy X-ray diffraction microscopy (HEDM) with crystal plasticity. Analysis is performed to explore lattice orientation dependence, stress dependence, and connectivity (network relationships) of grains experiencing elevated amounts of slip. Elevated slip is found to be primarily associated with a single large network of connected grains, and within this network, a clustered group of grains oriented favorably for slip are found to have outsized structural importance. The effect of different slip system strengths and rate sensitivities of families of slip systems on reconstructed slip activity is also discussed.

Regularity Maintenance Properties under Deformation of Kink-Introduced Nano-Mille-Feuille Structures Derived from Interfacial Friction

The regularity-maintaining properties under the deformation of a kink-introduced “Nano-mille-feuille” structure were verified, which might contribute to the development of high-strength, lightweight materials, and evidence of its usefulness was obtained. Simultaneously, a technique for evaluating the mechanical resistance of a highly ordered Nano-mille-feuille structure obtained by alternately layering individual particle films composed of polymer nanospheres and organo-modified inorganic nanoparticles with a common particle diameter of 5 nm was proposed. The Nano-mille-feuille was prepared on a flexible resin substrate that was subjected to uniaxial drawing deformation using a manual drawing machine to monitor changes in the layered regularity of multiparticle layers on its surface. Furthermore, the same estimation as the kink-introduced Nano-mille-feuille, which was created on an underlayer introducing advance nanometer-sized steps on one-half of the substrate, was made and compared to the regularity-maintaining properties of the Nano-mille-feuille. The presence of kink-like nanosteps demonstrated the effect of enhancing the deformation resistance of the layered regularity of the multiparticle layers. It was confirmed that the presence of kinks maintains the layering regularity of nanoparticles even under drawing deformation at a higher magnification. The presence of nanokinks is expected to enhance the interfacial friction of the particle layers and has shown the possibility of structural/functional correlation that can impart high-strength properties to materials derived from light elements and/or lightweight metals in future.

Stress concentration factors around a circular hole in two fiber reinforced materials under large deformations

In this paper, we study the deformation and the stress concentration factor due to a small circular hole in a thin nonlinearly elastic large sheet reinforced by two families of fibers under a state of plane stress undergoing large deformations. We restrict our attention to the class of materials wherein the fibers are mechanically equivalent, rendering the body orthotropically symmetric with respect to the planes whose normals are along the angular bisectors of the angles between the fibers. Furthermore, we limit ourselves to the problems of equibiaxial, uniaxial and pure shear deformations of the body whose fibers are symmetrically oriented with respect to the Cartesian coordinate axes. These deformations indicate strong influence of the fiber angle on the stress concentration factors and a significant difference in the stress concentration factor distribution around the hole when using a constitutive relation based on a partial set of invariants (I4,I6) versus an extended constitutive relation that incorporates the full set of invariants appropriate for a body reinforced with fibers and reduces appropriately to the orthotropic linearized elastic case. We show how two constitutive relations that exhibit a similar biaxial behavior in the absence of discontinuities show noticeable differences under uniaxial and pure shear deformation with the dissimilarities magnified in the presence of discontinuities like a circular hole.

Multifractal wavefunctions of charge carriers in graphene with folded deformations, ripples, or uniaxial flexural modes: Analogies to the quantum Hall effect under random pseudomagnetic fields

The electronic behavior in graphene under arbitrary uniaxial deformations, such as foldings or flexural fields, is studied by including it in the Dirac equation pseudoelectromagnetic fields. General foldings are thus studied by showing that uniaxial deformations can be considered pseudomagnetic fields in the Coulomb gauge norm. This allows one to give an expression for the Fermi (zero) energy mode wavefunctions. For random deformations, contact is made with previous works on the quantum Hall effect under random magnetic fields, showing that the density of states has a power law behavior and that the zero energy mode wavefunctions are multifractal. This hints at an unusual electron velocity distribution. Also, it is shown that a strong Aharonov–Bohm pseudoeffect is produced. For more general nonuniaxial general flexural strain, it is not possible to use the Coulomb gauge. The results presented here helps to tailor-made graphene uniaxial deformations to achieve specific wavefunctions.

Investigating AlSi coating fracture at high temperatures using acoustic emission sensors

In this article, the fracture behavior of AlSi coating at elevated temperatures is investigated. During the heating stage, Fe−Al intermetallics and voids are formed, both of which define the fracture behavior of the AlSi coating layer. After heating, the effects of deformation temperature, strain level and strain rate on the fracture of AlSi coating is investigated, during deformation of the coated press hardening steel. For this purpose, tensile experiments are conducted at elevated temperatures. The experiments include heating the coated steel at 920 °C for 6 minutes, uniaxial tensile deformation at isothermal conditions (400−800 °C) and then quenching to room temperature. Acoustic emission (AE) sensors are incorporated to detect coating fracture at each stage. After quenching, the distribution of coating cracks and its micro-structure are examined via optical and scanning electron microscopy techniques, respectively. The results show that there is a strong correlation between AlSi coating fracture and the deformation temperature, macroscopic strain level and the output AE signals. According to the acoustic and optical measurements, the uniaxial tensile experiments at 400−700 °C show coating fracture: at 400 and 500 °C coating fracture is severe with spallation, while at 600 and 700 °C mode-I coating cracks are generated. However, at 800 °C no coating cracks are observed until 30% macroscopic strain. In conclusion, the experimental results demonstrate that the AlSi coating fracture is strongly dependent on the temperature and strain but not on the strain rate. Furthermore, the agreement between AE signals and optical images confirms that the AE sensors can be reliably used for in-situ detection of AlSi coating fracture during tensile experiments.

Modeling and Analysis of Yielding and Strain Hardening in Metastable High‐Entropy Alloys

Metastable dual‐phase high‐entropy alloys (HEAs) have become an attractive paradigm as structural materials due to their outstanding mechanical properties compared with single‐phase HEAs, which originate from the multiple strengthening mechanisms. Herein, a theoretical model is established by integrating the effects of lattice distortion, dislocation, grain boundary, and phase transformation, to study the mechanical responses of metastable HEAs during uniaxial tensile deformation. The results show the contribution of various mechanisms to the strength, and strain hardening can be determined quantitatively. In particular, the face‐centered cubic (FCC) to body‐centered cubic (BCC) phase transformation would dominate the flow stress with plastic strain to exceed 6%, due to the strong phase interface strengthening and the formation of hard BCC phase. This work provides a microstructure‐based constitutive model for predicting the flow stress and strain hardening properties of metastable HEAs.

Generation of 30 kbar hydrostatic pressure in Bi2Te3 thin films by uniaxial deformation and its effect on the band structure

Generation of ultrahigh hydrostatic pressure by application of a strong one-axis deformation is demonstrated in epitaxial ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ films on a mica substrate. This was followed by measurements of transport properties in $n$-type ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ as a function of temperature (100--300 K) and pressure of up to 30 kbar. This pressure allowed studies of the restructuring of the conduction band. The nonmonotonic pressure dependence of the Hall coefficient and the increase of electrical conductivity \ensuremath{\sigma}($P$) can only be explained using two subband structures in the conduction band: subbands of light and heavy electrons with a mobility ratio of 10. The gap between the electron subbands increases with the hydrostatic pressure.

Mechanical response and microstructural evolution of Ni-27 W alloys during uniaxial tension

In face-centered cubic alloys, an outstanding combination of strength and ductility can be achieved through reducing the stacking fault energy. In this paper, the mechanical response and microstructural evolution of Ni-27 W alloys (with 27 wt% W) are studied. The Ni-27 W solid solution alloys have a face-centered cubic structure. A stacking fault energy of ~67 mJ/m2 is evaluated by the thermodynamic method. The yield stress, ultimate tensile strength, and percentage elongation to fracture of the Ni-27 W alloys are 415 MPa, 1285 MPa and 42%, respectively, which shows a high strain hardening capacity. The deformation mechanism of the Ni-27 W alloys has been investigated by means of transmission electron microscopy, electron backscatter diffraction, and high-energy x-ray diffraction at the deformation stages with true strains of 0.02, 0.1, 0.2, and 0.3. A fiber texture with <111> and <001> parallel to the tensile direction is gradually formed under uniaxial deformation. Planar dislocation structure is clearly observed, the spacing of which gets thinner as increasing the deformation strain. Planar slip dominates the plastic deformation, which results in a high strain hardening capacity.

Energy based fracture initiation criterion for strain-crystallizing rubber-like materials with pre-existing cracks

Fracture prediction is indispensable for polymers, like rubbers, which have a broad range of applications mainly due to their high extensibility. The phenomenon known as strain-induced crystallization further contributes to the fracture toughness of certain rubbers. In this study, a criterion based on internal bond energy, incorporating the effects of crystallization, is proposed to predict fracture initiation in rubber-like materials with pre-existing cracks. First, a multi-scale mechanical model is developed for characterizing the behavior of rubber when subjected to both uniaxial and biaxial deformation states. At the microscale, both the amorphous and crystalline chain segments are modeled as elastic in order to consider the energy contribution by the molecular bond distortions. This internal energy is considered along with the entropic and crystalline free energy for each chain. In the chain model, the effects of loading condition and the relative orientation of a chain on its crystallinity are taken into account. At the macroscopic scale, an existing crystallinity distribution function is adapted and a mixed finite element formulation with an augmented Lagrangian multiplier is utilized to impose the incompressibility constraint. A non-affine maximal advance path constraint based homogenization model is utilized for bridging the two scales. Its potential to account for anisotropy in the stretched network compels the model to be preferable due to its physical significance, for the purpose of fracture modeling. The rigidity of the crystallites is accounted for by proposing a crystallite distortion energy in addition to the critical bond dissociation energy, for fracture initiation to occur. The model is validated by comparison with existing experimental results for both crystallizing and non-crystallizing rubbers. In addition to its potential to predict the material behavior when subjected to uniaxial and biaxial loading, the capability of the model to quantitatively estimate the effect of crystallization on fracture initiation is also verified.

Fabrication of Ti-Alloy Powder/Solid Composite with Uniaxial Anisotropy by Introducing Unidirectional Honeycomb Structure via Electron Beam Powder Bed Fusion

In this study, a Ti–6Al–4V alloy composite with uniaxial anisotropy and a hierarchical structure is fabricated using electron beam powder bed fusion, one of the additive manufacturing techniques that enable arbitrary fabrication, and subsequent heat treatment. The uniaxial anisotropic deformation behavior and mechanical properties such as Young’s modulus are obtained by introducing a unidirectional honeycomb structure. The main feature of this structure is that the unmelted powder retained in the pores of the honeycomb structure. After appropriate heat treatment at 1020 °C, necks are formed between the powder particles and between the powder particles and the honeycomb wall, enabling a stress transmission through the necks when the composite is loaded. This means that the powder part has been mechanically functionalized by the neck formation. As a result, a plateau region appears in the stress–strain curve. The stress transfer among the powder particles leads to the cooperative deformation of the composites, contributing to the excellent energy absorption capacity. Therefore, it is expected that the composite can be applied to bone plates on uniaxially oriented microstructures such as long bones owing to its excellent energy absorption capacity and low elasticity to unidirectionally suppress stress shielding.

Molecular Dynamics Study on Uniaxial Compression Transformation of Magnesium Alloy

As a new method of calculating materials, molecular dynamics simulation can effectively reproduce the mechanical behavior of materials at the atomic level. In this paper, through the construction of the AZ31 magnesium alloy model, the uniaxial compression deformation of magnesium alloy at different temperatures and strain rate is simulated by molecular dynamics method, the mechanical properties and microstructure changes of magnesium alloy are analyzed, the phase transformation mechanism of magnesium alloy under uniaxial compression is revealed, and the effects of temperature and strain rate on the phase transformation of magnesium alloy are explored at the nanometer scale. It provides a theoretical basis and necessary basic knowledge for the design and development of Mg-based nanostructured alloys with excellent mechanical properties.

General Molecular Dynamics Approach to Understand the Mechanical Anisotropy of Monocrystalline Silicon under the Nanoscale Effects of Point Defect

Mechanical anisotropy and point defects would greatly affect the product quality while producing silicon wafers via diamond-wire cutting. For three major orientations concerned in wafer production, their mechanical performances under the nanoscale effects of a point defect were systematically investigated through molecular dynamics methods. The results indicated anisotropic mechanical performance with fracture phenomena in the uniaxial deformation process of monocrystalline silicon. Exponential reduction caused by the point defect has been demonstrated for some properties like yield strength and elastic strain energy release. Dislocation analysis suggested that the slip of dislocations appeared and created hexagonal diamond structures with stacking faults in the [100] orientation. Meanwhile, no dislocation was observed in [110] and [111] orientations. Visualization of atomic stress proved that the extreme stress regions of the simulation models exhibited different geometric and numerical characteristics due to the mechanical anisotropy. Moreover, the regional evolution of stress concentration and crystal fracture were interrelated and mutually promoted. This article contributes to the research towards the mechanical and fracture anisotropy of monocrystalline silicon.

The Effect of Hydrogen on the Stress-Strain Response in Fe3Al: An ab initio Molecular-Dynamics Study.

We performed a quantum-mechanical molecular-dynamics (MD) study of FeAl with and without hydrogen atoms under conditions of uniaxial deformation up to the point of fracture. Addressing a long-lasting problem of hydrogen-induced brittleness of iron-aluminides under ambient conditions, we performed our density-functional-theory (DFT) MD simulations for T = 300 K (room temperature). Our MD calculations include a series of H concentrations ranging from 0.23 to 4 at.% of H and show a clear preference of H atoms for tetrahedral-like interstitial positions within the D0 lattice of FeAl. In order to shed more light on these findings, we performed a series of static lattice-simulations with the H atoms located in different interstitial sites. The H atoms in two different types of octahedral sites (coordinated by either one Al and five Fe atoms or two Al and four Fe atoms) represent energy maxima. Our structural relaxation of the H atoms in the octahedral sites lead to minimization of the energy when the H atom moved away from this interstitial site into a tetrahedral-like position with four nearest neighbors representing an energy minimum. Our ab initio MD simulations of uniaxial deformation along the 〈001〉 crystallographic direction up to the point of fracture reveal that the hydrogen atoms are located at the newly-formed surfaces of fracture planes even for the lowest computed H concentrations. The maximum strain associated with the fracture is then lower than that of H-free FeAl. We thus show that the hydrogen-related fracture initiation in FeAl in the case of an elastic type of deformation as an intrinsic property which is active even if all other plasticity mechanism are absent. The newly created fracture surfaces are partly non-planar (not atomically flat) due to thermal motion and, in particular, the H atoms creating locally different environments.

The formation mechanisms of amorphous bands of boron carbide nanopillars under uniaxial compressions and their effects on mechanical properties from molecular dynamics simulation

Boron carbide is considered as an ideal armor material, but its abnormal failure under high speed impact hinders its application. In order to explore the compression deformation behavior and mechanism of boron carbide, molecular dynamics simulation of reactive force-field (ReaxFF) interatomic potential is adopted to simulate the uniaxial compression deformation of boron carbide nanopillars in different directions at room temperature. The compression deformation behaviors of B11CpCBC nanopillars show significant anisotropy. No twin formation is observed during any compression. The B11CpCBC nanopillars have similar uniaxial compressive strength and corresponding strain along c-axis and b-axis, which are 59.21GPa and 0.185, 54.9GPa and 0.187, respectively, while along d-axis are higher, which are 82.06GPa and 0.234. During the compression along axes of b, c and d, the nanopillars undergo the same deformation processes in sequence as: symmetry reducing region is formed, extends aslant through the model and forms amorphous band. The formation mechanisms of amorphous bands of B11CpCBC nanopillars are as follows. During the compression along axis c, the three-atom chains constantly bend and the bonds among the three polar site B atoms of the icosahedron fracture at large strain leading to icosahedron collapse. During the compression along axes b and d, new bonds are formed between vertical adjacent icosahedrons at large strain leading to icosahedron collapse. In all three compression cases, the collapse extends aslant through the nanopillars under shear stress, during which polar site B atoms of icosahedrons bond with C and B atoms of triatomic chains, and finally forms amorphous bands. The formation of amorphous bands results in the decrease of compressive strength of B11CpCBC.

Influence of uniaxial deformation on surface morphology and corrosion performance of chromium-based coatings for packaging steel

Chromium-based coatings on steel, cathodically electroplated from Cr(VI) and Cr(III) electrolytes, were studied to compare their corrosion resistance following application of uniaxial strain. An organic overcoat (PVB) was applied to strained samples and an in-situ scanning Kelvin probe technique was used to determine rates of PVB cathodic disbondment. The Cr(VI) derived coatings were fully resistant to cathodic disbondment post-deformation. The Cr(III) derived coatings exhibited increased rates of cathodic disbondment with increasing uniaxial strain. The Kinetics of cathodic disbondment are explained on the basis of coating morphology produced by deformation, and the exposure of underlying iron. The electrochemical availability of iron is estimated by measuring hydrogen evolution kinetics on cathodically polarised samples.