Local Strain Fields Research Articles

Electronic and optical properties of boron-containing GaN alloys: The role of boron atom clustering

Boron (B) containing III-nitride materials, such as wurtzite (wz) (B, Ga)N alloys, have recently attracted significant interest due to their ability to tailor the electronic and optical properties of optoelectronic devices operating in the visible and ultraviolet spectral range. However, the growth of high quality samples is challenging and B atom clustering is often observed in (B, Ga)N alloys. To date, a fundamental understanding of the impact of such clustering on the electronic and optical properties of these alloys is sparse. In this work, we employ density functional theory (DFT) in the framework of the meta-generalized gradient approximation [modified Becke Johnson (mBJ) functional] to provide insight into this question. We use mBJ DFT calculations, benchmarked against state-of-the-art hybrid functional DFT, on (B, Ga)N alloys in the experimentally relevant B content range of up to 7.4%. Our results reveal that B atom clustering can lead to a strong reduction in the bandgap of such an alloy, in contrast to alloy configurations where B atoms are not forming clusters, thus not sharing nitrogen (N) atoms. We find that the reduction in bandgap is linked mainly to carrier localization effects in the valence band, which stem from local strain and polarization field effects. However, our study also reveals that the alloy microstructure of a B atom cluster plays an important role: B atom chains along the wz c axis impact the electronic structure far less strongly when compared to a chain formed within the c-plane. This effect is again linked to local polarization field effects and the orbital character of the involved valence states in wz BN and GaN. Overall, our calculations show that controlling the alloy microstructure of (B, Ga)N alloys is of central importance when it comes to utilizing these systems in future optoelectronic devices with improved efficiencies.

Probing octupolar hidden order via Janus impurities

Quantum materials with non-Kramers doublets are a fascinating venue to realize multipolar hidden orders. Impurity probes which break point group symmetries, such as implanted muons or substitutional impurities, split the non-Kramers degeneracy and exhibit a Janus-faced influence in such systems: they can destroy the very order they seek to probe. Here, we explore this duality in cubic osmate double perovskites which are candidates for exotic d-orbital octupolar order competing with quadrupolar states. Using ab initio computations, Landau theory, and Monte Carlo simulations, we show that Janus impurities induce local strain fields, nucleating quadrupolar puddles and suppressing the octupolar Tc. At the same time, strains mix the non-Kramers doublet with an excited magnetic triplet, creating parasitic dipole moments which directly expose the hidden octupolar order parameter. Our work unravels this Janus duality in recent impurity nuclear magnetic resonance (NMR) experiments, with important implications for uncovering hidden order in diverse multipolar materials.

Thermo-kinetic characteristics on stabilizing hetero-phase interface of metal matrix composites by crystal plasticity finite element method

Using dislocation-based constitutive modeling in three-dimension crystal plasticity finite element (3D CPFE) simulations, co-deformation and instability of hetero-phase interface in different material systems were herein studied for polycrystalline metal matrix composites (MMCs). Local stress and strain fields in two types of 3layer MMCs such as fcc/fcc Cu–Ag and fcc/bcc Cu–Nb have been predicted under simple compressive deformations. Accordingly, more severe strain-induced interface instability can be observed in the fcc/bcc systems than in the fcc/fcc systems upon refining to metallic nanolayered composites (MNCs). By detailed analysis of stress and strain localization, it has been demonstrated that the interface instability is always accompanied by high-stress concentration, i.e., thermodynamic characteristics, or high strain prevention i.e., kinetic characteristics, at the hetero-phase interface. It then follows that the thermodynamic driving force ΔG and the kinetic energy barrier Q during dislocation and shear banding can be adopted to classify the deformation modes, following the so-called thermo-kinetic correlation. Then by inserting a high density of high-energy interfaces into the Cu-Nb composites, such thermo-kinetic integration at the hetero-phase interface allows a successful establishment of MMCs with the high ΔG-high Q deformation mode, which ensures high hardening and uniform strain distribution, thus efficiently suppressing the shear band, stabilizing the hetero-phase interface, and obtaining an exceptional combination in strength and ductility. Such hetero-phase interface chosen by a couple of thermodynamics and kinetics can be defined as breaking the thermo-kinetic correlation and has been proposed for artificially designing MNCs.

Effect of Cr coating on the mechanical integrity of Accident Tolerant Fuel cladding under ring compression test

The mechanical behavior of Cr-coated Zr-Nb alloy cladding was investigated with Ring Compression Test (RCT) and Finite Element Analysis (FEA). The post-RCT specimens were examined by SEM analysis with a focus on crack characterization. The experimental result shows that Cr-coating less than 20 μm does not introduce notable changes in the load-displacement behavior of the cladding structure until the point at which the Cr-coated cladding undergoes premature failure. FEA results demonstrate that the premature failure of the Cr-coated cladding cannot be explained by local stress and strain fields as they are not affected by the presence of the coating if the crack of the coating layer is not considered. Under RCT, a crack is initially formed across the coating thickness and acts as a pre-existing flaw that concentrates stresses, growing towards the underlying Zr-Nb alloy. Once the crack depth attains the critical size, it unstably grows, resulting in cladding failure which is shown as an earlier load drop compared to the uncoated cladding. FEA results using eXtended Finite Element Method (XFEM) demonstrate the stress concentration by the coating crack which results in the premature failure of the Cr-coated cladding in RCT. Increasing coating thickness decreases crack density, thereby reducing the uncovered area of the underlying cladding in case where the coating experiences extended cracking. On the other hand, the thicker coating, when cracked, can induce extended damage to the underlying cladding by introducing larger cracks. The net benefit of these two competing effects needs to be considered to optimize the thickness of Cr-coating.

Control of mechanical and shape memory characteristics in martensitic NiTi by setting L-PBF parameters and build orientation

The current work encompasses a study of as-fabricated martensitic NiTi samples printed via Laser Powder Bed Fusion (L-PBF) in the horizontal and vertical build orientations, with systematic variations in the L-PBF process parameters. The sample densities, compositions, thermal characteristics, and mechanical responses were analysed and correlated with L-PBF parameters. Mechanical responses measured and analysed included microhardness, impact energies, and elastic modulus. The process parameters were found to have a significant effect on the resulting properties. The variation in physical properties and mechanical responses between each sample of the vertical build were less compared to those measured for the horizontal built samples. The L-PBF samples were compressed to 6% engineering strain (within elastic limit), and their shape memory strains were thermally recovered. Digital Image Correlation (DIC) based full-field strain analysis was performed during compression to measure local strain fields which revealed in detail the macroscopic effects during detwinning. The shape memory capability of the L-PBF samples was quantised based on strain recoveries attained after thermal recovery. Microstructural variations in submicron scale were also analysed in as-fabricated, post-compression and post-recovery stages, showing the sample response during different deformation mechanisms.

Multi‐Material Strain Mapping with Scanning Reflectance Anisotropy Microscopy

AbstractStrain‐engineering of materials encompasses significant elastic deformation and leads to breaking of the lattice symmetry and as a consequence to the emergence of optical anisotropy. However, the capability to image and map local strain fields by optical microscopy is currently limited to specific materials. Here, a broadband scanning reflectance anisotropy microscope as a phase‐sensitive multi‐material optical platform for strain mapping is introduced. The microscope produces hyperspectral images with diffraction‐limited sub‐micron resolution of the near‐normal incidence ellipsometric response of the sample, which is related to elastic strain by means of the elasto‐optic effect. Cutting edge strain sensitivity is demonstrated using a variety of materials, such as metasurfaces, semiconductors, and metals. The versatility of the method to study the breaking of the lattice symmetry by simple reflectance measurements opens up the possibility to carry out non‐destructive mechanical characterization of multi‐material components, such as wearable electronics and optical semiconductor devices.

Engineering Dynamic Structural Color Pixels at Microscales by Inhomogeneous Strain-Induced Localized Topographic Change.

Structural colors in homogeneous elastomeric materials predominantly exhibit uniform color changes under applied strains. However, juxtaposing mechanochromic pixels that exhibit distinct responses to applied strain remains challenging, especially on the microscale where the demand for miscellaneous spectral information increases. Here, we present a method to engineer microscale switchable color pixels by creating localized inhomogeneous strain fields at the level of individual microlines. Trenches produced by transfer casting from 2.5D structures into elastomers exhibit a uniform structural color in the unstretched state due to interference and scattering effects, while they show different colors under an applied uniaxial strain. This programmable topographic change resulting in color variation arises from strain mismatch between layers and trench width. We utilized this effect to achieve the encryption of text strings with Morse code. The effective and facile design principle is promising for diverse optical devices based on dynamic structures and topographic changes.

Ab initio study of effects of Al on the defect behaviors of AlxCoCrFeNi high entropy alloys

The influence of Al on the defect behaviors of AlxCoCrFeNi high-entropy alloys (HEAs) is studied by ab initio calculations. The lattice distortion, defect formation energies, and defect migration energies were calculated for different Al concentrations ranging from 0 to 6.25 at. %. First, the increase in the lattice distortion of AlxCoCrFeNi by the addition of Al is demonstrated, but the increment is smaller than that predicted by the atomic size difference of constituent elements. Second, although formation energies of Al vacancies and dumbbells containing Al are higher, the addition of Al decreases the average formation energies of vacancies and interstitials, which is probably due to the Al-induced local strain field and lattice distortion. The migration energies of interstitials and vacancies exhibit opposite variations, the migration energies of interstitials are increased while that of vacancies are decreased with the Al content. Moreover, the distributions of formation and migration energies of both defects are significantly broadening with increasing Al content, which could further enhance the characteristic effects of HEAs and play a more important role in the irradiation resistances of HEAs. Based on the calculation results, the effects of Al on the irradiation responses of AlxCoCrFeNi are discussed.

Integrated experimental–numerical investigation of strain partitioning and damage initiation in a low-carbon lath martensitic steel

Lath martensite is a key constituent of advanced high strength steels (AHSS). Its deformation behavior is intrinsically related to its hierarchical structure composed of packets, blocks and laths exhibiting specific topological and orientation relationships. To improve the ductility of lath martensite, it is therefore important not only to clarify the deformation mechanisms but to also gain a better understanding of the damage initiation mechanisms and their connections with local micro-mechanical features. To this end, an integrated experimental–numerical investigation of a low-carbon lath martensitic steel combining large field-of-view high-resolution digital image correlation (HR-DIC) analyses and crystal plasticity (CP) simulations with different modeling assumptions is proposed. The global and local plastic strain field distributions are first comparatively and quantitatively analyzed to clarify the predictive capabilities and limitations of the CP models within the continuum mechanics approximation. The results confirm that plasticity is predominantly carried out by interface sliding so that a model discriminating bulk and boundary elements with respect to their initial in-lath and out-of-lath slip strengths can reasonably reproduce the plastic deformation in terms of spatial distribution and intensity. Severe strain localization is observed along block boundaries with high in-lath Schmid factor, at packet interfaces but also at prior austenite grain boundaries and probably at an annealing twin boundary. The relationship between the strain localization sites and local micro-mechanical fields, in particular the stress triaxiality is discussed.

Dynamic compression behavior of TiZrNbV refractory high-entropy alloys upon ultrahigh strain rate loading

In this study, the dynamic compressive response behavior of a body-centered cubic (BCC) single-phase TiZrNbV refractory high-entropy alloy (RHEA) was investigated under impact at speeds of 313–1584 m s–1 using two-stage, gas-gun-driven, high-speed plate-impact experiments; recovery sample analysis; and theoretical calculations. The strain rate and pressure were approximately 107 s−1 and 5.07–29.37 GPa, respectively. The results showed that the TiZrNbV RHEA had a Hugoniot elastic limit of 4.12–5.86 GPa and a spall strength of 1.84–2.03 GPa. The initial yield strength of the alloy showed a strong strain-rate dependence and could be described by the modified Zerilli–Armstrong model, while the phonon-damping effect was the main reason for its high strain-rate sensitivity. Microstructural analysis showed that the dynamic deformation of the TiZrNbV RHEA was controlled by the dislocation slip, dislocation proliferation, intersection of the deformation bands, and grain refinement. The analysis also showed that the intergranular, transgranular, and mixed-type cracks dominated the spall failure of the material. The dynamic Hall–Petch effect and pinning from the lattice distortion led to high dynamic yield strength. The critical strain rate for the phonon drag effect was positively related to the relative atomic mass and local strain field of the metals. Within the experimental loading range, the RHEA showed good structural stability, and simultaneously, the theoretical calculation method for the equation of state based on a cold-energy mixture could accurately predict its shock-response behavior. The valence-electron concentration (VEC) had a direct effect on the shock-compression properties of the HEAs; higher VEC implied more difficulty in compressing the HEAs. The findings of this study provide insights into understanding the mechanical response characteristics of RHEAs under extreme conditions such as high-speed impact and ultrahigh strain-rate loading.

Specimen geometry design for plasticity and fracture characterization of sheet metal under high testing speed and various stress states

To model a vehicle crash event using the finite element (FE) method, accurate material properties of automotive sheet metals at high strain rates and various stress states need to be known. In this paper, a series of specimen geometries were designed for dynamic tensile tests in a servo-hydraulic tensile machine to get rid of the system ringing effect. These specimen geometries, which consist of different Stress triaxialities and Lode angle parameters, were optimized according to a similar physical principle as that used for the Gen. III specimen of Fang and Grams using FE simulations. Oscillation-free forces were obtained during the high-speed tests in the strain rate range of up to 103 s−1 using specimens that have a novel design. The deformation and local strain fields of the specimens were measured using digital image correlation (DIC) techniques. Furthermore, the deformation and fracture behavior of a micro-alloyed steel (H340) were characterized and analyzed. The determination of the model parameters for plasticity, damage/fracture modeling, and the material’s strain-rate dependency could be conducted more accurately based on the improved oscillation-free force measurements, which resulted in an improved prediction of the plasticity and fracture behavior of the investigated steel.

In situ determination of high-temperature oxidation evolution using micro-pillar array and scanning probe microscopy

Understanding the chemo-mechanics during oxidation is of great importance for designing novel countermeasures for materials protections. However, it is yet challenging to accurately measure the coupled mechanical stress/strain components at micro-/nanoscale. In this work, we propose a novel method to combine surface markers (SiO2 micro-pillars) and high temperature scanning probe microscopy (HTSPM) for quantifying the growing oxide thickness, while mapping the local displacement and strain fields at the surface of a nickel-based alloy during high temperature oxidation. The proposed method displays great potential in quantitatively mapping the non-uniform oxide distribution, providing a novel approach to investigate the high-temperature oxidation process.

Microscale experimental results and their implications for mesoscale modeling of damage initiation in cross-ply fiber reinforced composites

The results from a combined experimental and computational study to evaluate local in-plane variations of elastic properties (at the microscale fiber/matrix level) in mesoscale progressive failure analysis models of composites due to local fiber volume fraction variations are reported. A rectangular cross-ply specimen subjected to tensile loading is used as a case study in this investigation. Experimental results in support of understanding the correlation between local fiber volume fraction and in-plane strain field variations, by using specimens with small length scales is reported. A micromechanics computational model is used to understand and explain the experimental findings. The microscale finite element results suggest that the strain variations on the surface of the outer 90 plies prior to damage initiation are caused by local geometrical changes caused by the fibers non-uniformly packed and lying closest to the surface and not by the through-ply thickness local fiber volume fraction variations. Implications of this finding for homogenized mesoscale models for damage and failure are discussed.

Sensitivity of local cyclic deformation in lath martensite to flow rule and slip system in crystal plasticity

The prediction of the cyclic deformation behaviour in lath martensite-based high-strength steels requires constitutive models that reflect the local stress and strain fields as accurately as possible. At the same time, the constitutive models should act as efficiently as possible in order to achieve the required high number of cycles in a finite time. Only few research works have studied the sensitivity of the local cyclic deformation in lath martensite to the power law-based flow rule (Hutchinson or Chaboche–Cailletaud) and the active body-centred cubic (bcc) slip systems ({110}〈111〉 and {112}〈111〉) in the crystal plasticity finite element method (CPFEM). This paper, therefore, aims to provide some guidance in the selection of suitable flow rule and slip systems. Based on full-field micromechanical modelling of a medium-carbon steel under symmetric strain-controlled cyclic loading, it can be shown that the two most commonly used flow rules according to Hutchinson and Chaboche–Cailletaud are equally capable of predicting the local stress and strain distributions within the hierarchical martensitic microstructure. However, using the Hutchinson flow rule increases the computational performance for the quasi-rate-independent problem considered here. The local distributions found differ strongly from those in the parent austenitic microstructure. If plastic deformation is assumed not only on the slip systems {110}〈111〉, as often done, but also on the {112}〈111〉 type, a redistribution of the bimodal distributed local stresses occurs at a significantly lower stress level. The unimodal distributed local strains are less affected by this. In addition, it is found that slightly different critical resolved shear stress (CRSS) values for both slip system types influence the local stress and strain distributions less severely than the additional plastic slip activation in the material.

Nanoscale Thermal Strain Engineering-Driven Ferroelastic Domain Evolution in CH3NH3PbI3 Perovskites.

A local thermal strain engineering approach via an ac-heated thermal probe was incorporated into methylammonium lead triiodide (MAPbI3) crystals and acts as a driving force for ferroic twin domain dynamics, local ion migration, and property tailoring. Periodically, striped ferroic twin domains and their dynamic evolutions were successfully induced by local thermal strain and high-resolution thermal imaging, giving decisive evidence of the ferroelastic nature in MAPbI3 perovskites at room temperature. Local thermal ionic imaging and chemical mappings demonstrate that domain contrasts are from local methylammonium (MA+) redistribution into the stripes of chemical segregation in response to the local thermal strain fields. The present results reveal an inherent coupling among local thermal strains, ferroelastic twin domains, local chemical-ion segregations, and physical properties and offer a potential path to improve the functionality of metal halide perovskite-based solar cells.

DIC analysis of gears in bending condition

Thin rim gears present a complex strain and stress field due to its particular geometry, above all in bending problems. From this point of view, the position of the most stressed point and the corresponding equivalent stress value are useful for an accurate design of thin rim gears. More in detail, in case of bending failure, both crack initiation point and corresponding propagation path are strictly related to the gear’s geometry.In this work, an experimental analysis was performed to evaluate how light weight gears geometry may influence the strain field close to the tooth root.To this aim, an experimental activity was carried on by a dedicated equipment for bending tests and the gear deformation was monitored by using the 3D Digital Image Correlation (DIC) technique.The local strain field in both tooth and web portions was measured for two types of gears (standard gear and thin-rim gear) in order to identify the stress condition due to the bending loading. A particular attention was devoted to point out the most stressed point for both gears. Results were also compared with XFEM model available in literature.

CALPHAD-aided design for superior thermal stability and mechanical behavior in a TiZrHfNb refractory high-entropy alloy

TiZrHfNbTa refractory high-entropy alloys (RHEAs) are now at the research frontier of advanced metallic materials due to their exceptional mechanical performance, particularly at high temperatures. However, the TiZrHfNbTa RHEAs exhibit poor phase stability at intermediate temperatures (600 – 1,000 °C). The present study aimed to tailor their phase stability and mechanical properties via the calculation of phase diagrams (CALPHAD) approach. We found that Ta and Hf were detrimental to the phase stability of the TiZrHfNbTa RHEAs. Accordingly, a Ta-free and Hf-depleted Ti30Zr30Hf16Nb24 RHEA with outstanding phase stability was designed, which could remain a single-phase body-centered cubic (BCC) structure after annealing at 600 °C for 200 h. Furthermore, numerous (Ti, Zr)-rich nano-precipitates were dispersedly formed in the cold-rolled plus recrystallization-annealed (CR+A) Ti30Zr30Hf16Nb24 RHEA. The nano-precipitates controlled by the spinodal decomposition mechanism had an identical BCC lattice. The lattice fringes with (11¯0) Miller indices bent from the matrix phase to the nano-precipitates, causing a strong local strain field near the phase boundaries. The CR+A alloy possessed a yield strength of ∼ 800 MPa and tensile fracture elongation of ∼ 34.0%, showing a superior strength-ductility combination. The strain measurement by a digital image correlation indicated that the CR+A alloy exhibited a more substantial plastic stability than the as-cast alloy. Detailed observations of deformation microstructures through a transmission electron microscope and electron back-scattered diffraction revealed the origin of strength and ductility. Dislocation cross-slip and kink bands tended to form in the CR+A alloy during deformation and were capable of accommodating dislocation slip against stress concentration. Labusch's model uncovered that solid-solution strengthening contributed the most to yield strength. The present study provides a paradigm for the superior thermostability and controllable nanophase-precipitation behavior in RHEAs.

The magnetization and magnetoresistance of Ni46Mn23Ga22Co5Cu4 shape memory microwires after mechanical training

Magnetization and magnetoresistance are significant for the application of Ni-Mn-Ga shape memory alloys. The Ni46Mn23Ga22Co5Cu4 microwires with excellent mechanical properties were selected to investigate the influence of mechanical training on the microstructure and magnetic properties. The largest change of magnetization (ΔM) and magnetoresistance fluctuations were found after training 10k cycles. Furthermore, the microstructure and magnetic configuration were studied by TEM, GPA, and MFM to investigate the mechanical training effect and the internal mechanism. Nano-precipitated phases were found embedded in the matrix and the dislocation stacking around the precipitate induced a local strain field which could be released during mechanical training. The magnetic domain was transformed from indistinct into strip-type after training, which affects the magnetic properties of microwires. This work provides an idea for improving the magnetic and magnetoresistance performance and building a link between microstructures and electric signals of shape memory alloys.

Shear loading dominated damage mechanisms and strain localization studied by in situ 3D laminography imaging and Digital Volume Correlation for AA2198-T8

Damage and strain changes during shear dominated loading were assessed in situ via synchrotron laminography. In combination with digital volume correlation (DVC), local strain fields inside the material were determined thanks to the contrast provided by intermetallic particles. Testing was performed on a thin sheet sample with an optimized geometry that induced shear stress states in two ligaments opposite to each other with respect to the loading axis. Examining one of the ligaments, the strain fields in the material showed the expected localization in the shear plane with a central peak in shear strain. The Lode parameter fields associated with incremental strain measurements were determined for the analyzed zone and found to be close to zero. The total Hencky von Mises strain close to failure was measured in the material bulk and reached values between 0.6 and 0.7 for a DVC element length of 20μm. In the shear zone, damage onset in the form of grain-related elongated cracks and of voids nucleating on intermetallic particles was detected from about half of the maximum value of the equivalent strain to failure onward. Voids between particle clusters grew in shear as the particles were rigid and mostly brittle. Damage was mainly concentrated in the gage section subjected to shear and away from free surfaces, in particular away from the notches. It was characterized and quantified with respect to the maximum equivalent strain found inside the band.

線維走行と直交するスリット状損傷部に形成されるマウス膝蓋腱治癒組織の巨視的・微視的力学解析

A lack of the application of appropriate mechanical stimuli to cells within healing tissue in tendon defects has been proposed as a possible mechanism of incomplete defect healing with scar formation. This suggested that different mechanical environment in a tendon defect with a different geometry may result in different healing outcome. Therefore, the present study adopted a slit injury model in a mouse patellar tendon model and performed macroscopic and microscopic mechanical analysis on tissue regenerated in the central slit defect. Tensile strength and tangent modulus of the tissue in the defect at 3 weeks of healing were lower significantly than that of normal patellar tendon, which recovered to a level still lower than the normal level but without statistical significance at 6 weeks of healing. Micromechanical analysis on local strain field revealed that microscopic tensile strain was not different between normal patellar tendon and healing tissue at 6 weeks. However, microscopic transverse and shear strain was significantly lower in healing tissue than normal tendon tissue. The former may indicate that collagen fibers in healing tissue exhibit no crimp morphology, and the latter demonstrates that collagen fibers were not aligned to the long axis of the tendon. Although healing tissue in the slit defect possessed better strength that that in a rectangular defect examined in our previous study, microscopic strain field was similar between two models, possibly due to that healing tissue in both models was still immature structurally at 6 weeks of healing when compared to the normal patellar tendon.