Inhomogeneous Strain Distribution Research Articles

Visualization of Inhomogeneous Strain and Particle Distribution in a Shape Memory Alloy via Composite Reconstruction of 3DXRD Data

Visualization of Inhomogeneous Strain and Particle Distribution in a Shape Memory Alloy via Composite Reconstruction of 3DXRD Data

Deciphering the Atomic-Scale Structural Origin for Photoluminescence Quenching in Tin-Lead Alloyed Perovskite Nanocrystals.

The development of tin-lead alloyed halide perovskite nanocrystals (PNCs) is highly desirable for creating ultrastable, eco-friendly optoelectronic applications. However, the current incorporation of tin into the lead matrix results in severe photoluminescence (PL) quenching. To date, the precise atomic-scale structural origins of this quenching are still unknown, representing a significant barrier to fully realizing the potential of these materials. Here, we uncover the distinctive defect-related microstructures responsible for PL quenching using atomic-resolution scanning transmission electron microscopy and theoretical calculations. Our findings reveal an increase in point defects and Ruddlesden-Popper (RP) planar faults with increasing tin content. Notably, the point defects include a spectrum of vacancies and previously overlooked antisite defects with bromide vacancies and cation antisite defects emerging as the primary contributors to deep-level defects. Furthermore, the RP planar faults exhibit not only the typical rock-salt stacking pattern found in pure Pb-based PNCs but also previously undocumented microstructures rich in bromide vacancies and deep-level cation antisite defects. Direct strain imaging uncovers severe lattice distortion and significant inhomogeneous strain distributions caused by point defect aggregation, potentially breaking the local force balance and driving RP planar fault formation via lattice slippage. Our work illuminates the nature and evolution of defects in tin-lead alloyed halide perovskite nanocrystals and their profound impact on PL quenching, providing insights that support future material strategies in the development of less toxic tin-lead alloyed perovskite nanocrystals.

Assessment of phenomenological models for Indentation Size Effect (ISE) through physically based dislocation density evolution

The depth dependent hardness, referred to as indentation size effect (ISE) has gained considerable attention. ISE is attributed to the strain gradient caused by the inhomogeneous strain distribution beneath the indenter tip. The depth dependence of hardness is phenomenologically accounted using an appropriate length scale parameter. The Unified model is one such model that is built on Nix–Gao’s model and focuses on integrating various length scale parameters resulting from both microstructural and geometrical effects. One aspect of the current work is to assess the applicability of the Unified model to a material like bulk Nickel with a history of cold work. Also, the physics of ISE in metals can be related to the dislocation mean free path influenced by strengthening mechanisms. Past attempts are scarce to quantify the ISE through the microstructure variables. In the present work, an attempt has been made to simulate the nanoindentation test using a dislocation density based constitutive model. The objective of this study is to establish a correlation between the equivalent plastic strain resulting from deep depth indentation (which does not exhibit a size effect) and an equivalent dislocation density. The dislocation density here serves as a state variable that describes the microstructure-dependent mechanical behaviour. The modified KME, a dislocation density-based model, is incorporated as a user subroutine in a commercial finite element solver. It has been proven to accurately predict the indentation behaviour at substantially large depths, which aligns well with the predictions of the Unified model or experimental observations. The good correlation between the value of effective dislocation density and equivalent plastic strain in the plastic zone underneath the indenter assures to extend the use of dislocation density as a state variable to predict ISE.

Elucidating Piezoelectricity and Strain in Monolayer MoS2 at the Nanoscale Using Kelvin Probe Force Microscopy.

Strain engineering modifies the optical and electronic properties of atomically thin transition metal dichalcogenides. Highly inhomogeneous strain distributions in two-dimensional materials can be easily realized, enabling control of properties on the nanoscale; however, methods for probing strain on the nanoscale remain challenging. In this work, we characterize inhomogeneously strained monolayer MoS2 via Kelvin probe force microscopy and electrostatic gating, isolating the contributions of strain from other electrostatic effects and enabling the measurement of all components of the two-dimensional strain tensor on length scales less than 100 nm. The combination of these methods is used to calculate the spatial distribution of the electrostatic potential resulting from piezoelectricity, presenting a powerful way to characterize inhomogeneous strain and piezoelectricity that can be extended toward a variety of 2D materials.

Analysis of strain in ion implanted 4H-SiC by fringes observed in synchrotron X-ray topography

A novel high energy implantation system has been successfully developed to fabricate 4H-SiC superjunction devices for medium and high voltage via implantation of dopant atoms with multi-energies ranging from 13 to 66 MeV. The significantly higher levels of energy used compared to conventional implantation processes, necessitates detailed characterization of the lattice damage caused by implantation. To achieve this by employing the novel high energy system, 4H-SiC wafer with 12 μm epilayers were blanket implanted by 13.8–65.7 MeV Al atoms. The lattice damages induced by the implantation were primarily characterized by Synchrotron X-ray Plane Wave Topography (SXPWT) and Reciprocal Space Mapping (RSM). Topographs reveal fringe contrast akin to multiple asymmetric diffraction peaks with an angular separation of only 2″ (arcseconds) observed on rocking curves, indicating inhomogeneous strain distribution across the implanted layer. The strain profile of the implanted layer was extracted from the fringe contrast by applying Rocking-curve Analysis by Dynamical Simulation (RADS). The maximum strain value is similar to that measured on the RSM.

A multi-angular extrusion process for fine-grain magnesium alloy plate

The precise forming process with fine-grain strengthening method of magnesium alloy is a common technical problem that needs to be urgently addressed for lightweight application. In this paper, a multi-angular extrusion process integrating severe plastic deformation (SPD) technique and hot forming process was proposed to manufacture the fine-grain plate of AZ80 magnesium alloy. Shear deformation path was introduced via the variable section and secondary angle of mold cavity. The unified internal state variable (ISV) based finite element (FE) model was established to simulate the multi-angular extrusion process. The interaction of viscoplastic flow and microstructure evolution was embedded into the user defined subroutine of DEFORM-3D software. The inhomogeneous strain distribution and microstructure characteristics during the extrusion process were investigated. The improvement of fine-grain homogeneity via the two-pass angular shear deformation was revealed. A fine-grain magnesium alloy plate with good forming performance and average grain size of 9 μm was manufactured. The yield strength is increased to 259 MPa by 23.3 %, ultimate tensile strength is increased to 338 MPa by 27.5 %, and elongation is increased to 12.9 % by 122.4 %. The developed multi-angular extrusion process can be applied to the integrated manufacturing of shape and performance of fine-grain Mg components.

Investigation of the Abnormally Large Grains of a Super304H Heat-Resistant Steel Tube over Long-Term, High-Temperature Service

Abstract Abnormally large grains (ALGs) were detected in the outer walls of Super304H superheater steel tubes of an ultra-supercritical unit in long-term service. In this study, the microstructure and mechanical properties of steel tube samples were investigated to reveal the formation mechanism of the ALGs. The results showed that the room-temperature and high-temperature strength and plasticity of the coarse-grain zone were much lower than those of the fine-grain zone. Inhomogeneous residual strain distribution in the outer wall led to high distortion energy differences in the as-supplied steel tube. The low-distortion austenite grains annex the neighboring high-distortion grains via the strain-induced grain boundary migration mechanism. Moreover, the two-dimensional step mechanism of the faceted grain boundaries also causes the ALGs to form.

Analysis of Strain in Ion Implanted 4H-SiC by Fringes Observed in Synchrotron X-Ray Topography

A novel high energy implantation system has been successfully developed to fabricate 4H-SiC superjunction devices for medium and high voltages via implantation of dopant atoms with multi-energy ranging from 13 to 66 MeV to depths up to 12um. Since the level of energies used is significantly higher than those employed for conventional implantation, lattice damage caused by such implantation must be characterized in detail to enhance the understanding of the nature of the damage. In regard to this, by employing the novel high energy system, 4H-SiC wafers with 12μm thick epilayers were blanket implanted by Al atoms at energies ranging from 13.8MeV to 65.7MeV and N atoms at energies ranging up to 42.99MeV. The lattice damages induced by the implantation were primarily characterized by Synchrotron X-ray Plane Wave Topography (SXPWT). 0008 topographs recorded from the samples are characterized by an intensity profile consisting of multiple asymmetric diffraction peaks with an angular separation of only 2” (arcseconds). Using Rocking-curve Analysis by Dynamical Simulation (RADS) program, diffracted intensity profile was used to extract the corresponding strain profile indicating an inhomogeneous strain distribution across the depth of the implanted layer.

Profile defects caused by inhomogeneous longitudinal strain distribution in roll forming

Roll forming is a sheet metal forming operation that incrementally forms flat sheets into a desired profile geometry. The process is characterized by a high material utilization and a high output quantity. Concomitant with these advantages, profile defects such as bow and twist of the profile can occur. In the literature, an inhomogeneous longitudinal strain distribution across the profile cross-section is considered to be the cause of these defects. However, a quantitative cause and effect analysis is missing up to now. This paper presents an analytical model that shows a quantitative relationship between profile defects and the underlying longitudinal strain distributions. The model can be used to calculate the longitudinal strain distribution of a roll-formed profile across its cross-section based on given values for bow and twist or vice versa. It is compared with results from simulations and experiments and clearly reveals the cause for twist and bow in roll forming.Graphical abstract

Off-Axis Tension Behaviour of Unidirectional PEEK/AS4 Thermoplastic Composites

An experimental method for non-standard off-axis tension tests of unidirectional composites is developed. A new oblique end-tab is designed to eliminate stress concentration and in-plane bending moment induced by off-axis tension loading. Finite element analysis and experiments on Polyetheretherketone (PEEK)/AS4 unidirectional thermoplastic composites (CFRTP) were conducted to evaluate the effectiveness of the proposed testing method. Simulation and test results demonstrate that the use of oblique end-tabs eradicates stress concentration and bending movements. The digital image correlation (DIC) method was used to help investigate the full-field tension/shear coupling deformation response of the off-axis specimen. Test results show significant nonlinear behaviour and inhomogeneous strain distribution under tension/shear combined stresses. A fractographic study was carried out to study the damage mechanisms under a tension/shear combined stress state. Specimens with 30°, 45° and 60° off-axis angles, fail in tension/shear mixed failure mode. Fracture surface morphology indicates that matrix plastic deformation and ductile drawing under tension/shear coupled stress state induced the nonlinear stress-strain response.

Effect of Pulsed Current-Assisted Tension on the Mechanical Behavior and Local Strain of Nickel-Based Superalloy Sheet

Electrically assisted (EA) forming is a plastic forming technique under the coupling action of multiple energy fields, such as force field, temperature field, and electric field. It is suitable for the forming of difficult-to-deform materials such as nickel-based superalloys. In this paper, uniaxial tensile tests on nickel-based superalloy sheets were carried out using the pulsed current assisted with different parameters. The experimental results show that the flow stress of the material decreased with the increase in the current density under a high-frequency pulsed current, and the Joule heating effect explains the flow stress drop. In the pulsed current application process, the different types of Portevin-Le Chatelier phenomena appeared with the increase in the current density. The decrease in elongation assisted by the pulsed current was explained by analyzing the inhomogeneity of the maximum Joule heating temperature distribution. In addition, the digital image correlation (DIC) analysis was used to analyze the local strain behavior of the pulsed current-assisted tensile process. Under the application of a high-frequency pulse current, the specimen exerted an inhomogeneous temperature increase and local hot pressing stress, which resulted in the inhomogeneous distribution of the local strain.

Role of Persistent Slip Bands and Persistent Slip Markings in Fatigue Crack Initiation in Polycrystals

The cyclic plastic deformation of polycrystals leads to the inhomogeneous distribution of the cyclic plastic strain. The cyclic plastic strain is concentrated in thin bands, called persistent slip bands (PSBs). The dislocation structure of these bands generally differs from the matrix structure and is characterized by alternating dislocation-rich and dislocation-poor regions. The mechanisms of the dislocation motion in the PSBs and the formation of the point defects and their migration are quantitatively described. It is shown that, due to localized cyclic plastic straining in the PSBs, persistent slip markings (PSMs) are produced where the PSBs emerge on the surface. They typically consist of a central extrusion accompanied by one or two parallel intrusions. The deep intrusion is equivalent to the crack-like surface defect. The concentration of the cyclic strain in the tip of an intrusion leads to intragranular fatigue crack initiation. The mechanism of the early crack growth in the primary slip plane is proposed and discussed. Numerous PSMs are produced on the surface of the cyclically loaded materials. PSMs contribute to the formation of the surface relief, as well as the relief on the grain boundary. PSMs from one grain impinging the grain boundary are sufficient to create sharp relief on the grain boundary. Void-like defects weaken the grain boundary cohesion and extra material push both grains locally apart. The conditions necessary for the weakening of the grain boundary are enumerated and examples of grain boundary crack initiations are shown. The relevant parameters affecting grain boundary initiation are identified and discussed. The collected experimental evidence and analysis is mostly based on the papers published by the author and his colleagues in the Institute of Physics of Materials in Brno.

Advanced tensile testing as a new tool to quantify properties of food

Mechanical properties of food products are regularly analysed by tensile tests. The aim of this study was to demonstrate the potential of using advanced tensile testing techniques to better understand the mechanical properties of anisotropic food products, such as meat analogues and certain dairy products. The effects of various tensile testing parameters, including tensile gauge length and deformation rate, on the interpretation of mechanical properties of meat analogues was studied. Additionally, digital image correlation, an image analysis technique, was used for true distance recording and analysis of fracturing behaviour of the products. An isotropic product was prepared from solely soy protein isolate, and an anisotropic product was prepared from soy protein isolate and pectin using the shear cell technology. The tensile properties of the products were studied with four different moulds with varying gauge lengths of 17.5, 15, 11.5, and 8.5 mm, and at three deformation rates of 46.2, 23.1, and 11.6 mm/min. A smaller gauge length and slower deformation rate improved visualization and interpretation of the multi-stage descending branch in force – distance curves of anisotropic products. Additionally, tensile parameters, specifically toughness, proved to be more accurate at small gauge length and slow deformation rate, because overestimation due to rapid crack propagation was prevented. True distance data obtained with digital image correlation further improved the interpretation of the fracturing behaviour of the products. Inhomogeneous strain distribution in anisotropic products was shown with digital image correlation, in contrast to the homogeneous strain distribution observed in isotropic products. Furthermore, the Poisson's ratio, obtained through digital image correlation, explained inherent differences in structure and plasticity between isotropic and anisotropic meat analogues. This study shows the importance of careful selection of testing parameters and techniques. Moreover, it advises the use of digital image correlation for better measurement of fracture mechanics and strain distribution.

Photo-induced spatiotemporal bending of shape memory polymer beams

In response to external stimuli, such as heat, light, or magnetic fields, stimuli-responsive soft materials can change their current configuration to a new equilibrium state through non-equilibrium kinetic processes, including reaction, diffusion, and viscoelastic relaxation, which generates novel spatiotemporal shape-morphing behavior. Using a photothermal shape memory polymer (SMP) cantilever beam as a model system, this work analytically, numerically, and experimentally studies its non-equilibrium kinetic processes and spatiotemporal bending under light illumination. We establish a thermomechanical model for SMPs capturing the concurrent non-equilibrium processes of heat transfer and viscoelastic relaxation, which induces inhomogeneous temperature and strain distributions through the thickness of the beam, resulting in its bending and unbending. By varying the key dimensionless parameters, we theoretically and experimentally observe different types of bending dynamics. Moreover, our theory takes into consideration changes in the angles of incidence caused by extensive beam bending, and demonstrates that this effect can dramatically delay the bending due to reduction of the effective light intensity, which is further validated experimentally. This work demonstrates programmable and predictable spatiotemporal morphing of SMPs, and provides design guidelines for SMP morphing structures and robots.

Experimental and numerical investigation of strain inhomogeneity in zirconia during a Brazilian test

Zirconia-based ceramics is a material with a wide variety of applications. Problems of strength of such ceramics, being prepared by different methods, under different loading conditions, and with different porosity, are of practical importance. Inhomogeneous strain distributions in different-porosity zirconia ceramics during a Brazilian test are studied using the digital image correlation and numerical simulation techniques. Spatiotemporal patterns of strain localization across the compression axis are obtained. It is found that strain components are nonuniformly distributed over the specimen surface, and zirconia experiences macroscopically localized deformation. The fracture patterns reveal the triple-cleft mode. Numerical simulations performed both for a homogeneous material and with consideration for the material inhomogeneity show that different models yield different patterns of inhomogeneous strain distribution under diametral compression. Account for internal friction and dilatancy provides eight-shaped cracks. Random distributions of strength and elastic properties over the computational domain built with the use of pseudo-random number generators and/or Gaussian random fields allow one to control the size of strain inhomogeneities.

Microstructure evolution and constitutive modeling for mesoscaled tension of pure titanium thin sheet

The design and fabrication of micro components of pure titanium thin sheets face a challenging problem with size effects. Size effects will affect the plastic deformation mechanism of thin sheets in addition to causing unpredictable flow stresses. Although the impact of size on strength is obvious, further research is still needed, particularly for metal thin sheet with HCP lattice, to thoroughly understand the impact on deformation mechanisms. To assess the size effects on flow stress, uniaxial tensile experiments were carried out on specimens with varying thicknesses and grain sizes in this work. Additionally, Electron Backscattered Diffraction (EBSD) technology was used to analyze the dislocation evolution and twinning behavior in surface and inner grains for specimens with various thicknesses and grain sizes employing electrolytically polished specimens under different strain conditions. The results demonstrate that the feature size (t/d) has a substantial impact on the flow stress of pure titanium sheet, as seen by a rapid reduction in flow stress when t/d < 7. Microstructure analysis showed that the feature size effect has a significant impact on the deformation mechanism. Inhomogeneous strain distribution and geometrically necessary dislocation (GND) density distribution are produced by the reduction in thickness and the increase in grain size. There is a change from slip dominated to slip-twin deformation mechanism as the t/d decrease. Decreased twin nucleation stress and worsened inhomogeneous deformation are the inherent causes of twin activation as t/d decreases. When the t/d is less than 1, the dislocation starving mechanism causes a paradoxical increase in the flow stress. Furthermore, there are similarities and variances between the surface and the inner grains. Regarding GND density, there is not much of a difference between the two. However, there is a significant difference between the two in terms of twin volume fraction. The twinning volume fraction in the surface grains is much lower than that in the inner grains. To evaluate the contribution of size effect to flow stress, the variation Equation of twinning volume fraction and dislocations in surface and inner grains are quantified. These results were incorporated into a novel mesoscaled constitutive model that includes lattice friction and size-dependent grain boundaries, dislocations and twinning strengthening. The reduction of flow stress in relation to t/d is correctly predicted by the novel model, which also agrees with the experimental results. This study deepens the understanding of deformation mechanism of pure titanium thin sheets in microforming and provides numerical model for accurate forming.

Compressive deformation behavior and microstructure evolution of AA2099 Al–Li alloy containing Al3Li precipitates based on experimental characterization and crystal plasticity finite element simulation

Deformation behavior of AA2099 Al–Li alloy containing Al3Li (δ′) precipitates at room temperature has been systematically investigated via uniaxial compression experiments, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) measurements as well as crystal plasticity finite element (CPFE) simulation. By applying an appropriate heat treatment, these nano-sized δ′ precipitates with the diameter of 10–20 nm are obtained in AA2099 Al–Li alloy and the corresponding area fraction is about 1.12%. Microstructure characterizations confirm that these pre-existing δ′ precipitates survive during plastic deformation, meanwhile they would contribute to the formation of tangled and dense dislocations in Al matrix. An ingenious strategy is then proposed and coupled into the CPFE modeling to take account of the effect of δ′ phase on deformation behavior, namely δ′ precipitates contribute to the activity of co-planner slip and they are adverse to the activity of co-directional slip. These predicted mechanical behavior and texture evolution agree well with those experimentally measured ones, which verifies the validity of the conducted CPFE simulation. Moreover, CPFE modeling successfully predicts the occurrence of large and inhomogeneous Von Mises stress and dislocation density distribution in the case with δ′ phase. Besides, the statistical analysis on the coefficient variant about equivalent plastic strain strongly illustrates that these pre-existing δ′ precipitates contribute to the inhomogeneous distribution of plastic strain during plastic deformation.

Intergranular Cracking in Mg-Gd-Y Alloy during Tension Test

The intergranular cracking in the Mg-Gd-Y alloy was investigated by an in situ tension test combined with an electron backscattered diffraction (EBSD) measurement and digital image correlation (DIC). During the tension test, the crack was found at the triangle point of the grain boundary area with profuse slip traces. DIC results show that inhomogeneous strain distribution can be found at the triangle point area, leads to serious deformation incompatibility at this point. This also leads to a weak point for crack initiation, and as tension strain increases, the crack would extend from the weak point along the grain boundary.

Numerical Simulation of Microstructure Evolution of Large GCr15 Bar during Multi-Pass Rough Rolling

Severe temperature gradients and inhomogeneous strain distribution exist in the large cross-section of GCr15 bearing steel during the hot bar rolling process, resulting in a complex microstructure evolution in the bar. To promote the performance of the bar, a thermal-mechanical coupled finite element (FE) model was developed to capture the variations in temperature and deformation strain. A subroutine, considering the dynamic recrystallization (DRX), meta-dynamic recrystallization (MDRX), static recrystallization (SRX), and grain growth (GG) of austenite grains of GCr15 steel, was developed and coupled to the FE model to predict the microstructure’s evolution during rough rolling. The simulation implies that the inner part of the bloom is deformed at high temperatures due to the heat generated by plastic deformation and slow heat conduction, while the surface temperature decreases along with the passes. The heavy reduction design with 11 passes was found to introduce higher strains at the center regions than those of the same rough rolling reduction divided into 13 passes. The higher strains at the center regions refined the grain size and promoted microstructure homogeneity. The observation of the microstructures after hot bar rolling confirmed the refinement of the heavy reduction design for rough rolling. Furthermore, the heavy rough rolling reduction was found to be beneficial for alleviating the macrosegregation of the casting bloom.

Interactions between martensitic NiTi shape memory alloy and Nb nanowires in composite wire during tensile deformation

This study investigated the interactions between martensitic NiTi shape memory alloy and Nb nanowires in composite wire during tensile loading by means of in-situ high-energy X-ray diffraction. Nb nanowires have significant influence on the transformation behavior, annealed texture, and martensite variant reorientation behavior including texture evolution and load transfer among martensite variants of NiTi matrix. Meanwhile, the existence of densely distributed Nb nanowires increases the internal stress in NiTi matrix generated during martensite reorientation process, and causes the negative axial lattice stresses of all unfavored NiTi martensite variants after reorientation when the composite wire is under axial tensile stress. In addition, an exceptionally large elastic strain of 4.7% was produced over Nb nanowires after tensile deformation . Meanwhile, heterogeneous strain distribution exists in Nb nanowires, and it is associated with heterogeneous martensite reorientation strain of NiTi matrix. • Interactions between martensitic NiTi alloy and Nb nanowires is investigated. • Nanowires inhibit NiTi martensitic transformation and deformation twinning. • Nanowires induce the negative lattice stresses of all unfavored NiTi variants. • Inhomogeneous strain distribution exists in nanowires.