Local Strain Development Research Articles

Studying the effect of Cr content on microstructure and deformation behavior of (NiCo)88-xCrxAl10Ta2 medium-entropy alloy through L-DED gradient material

Optimization of Cr content is crucial for γ'-strengthened NiCoCr-based medium entropy alloys, influencing the stacking fault energy, deformation behavior and the precipitation of brittle σ phase. In this study, (NiCo)88-xCrxAl10Ta2 gradient materials were prepared by L-DED, and the influence of Cr content on the phase precipitation and tensile deformation behavior was investigated. As the Cr content decreases from 24.1 at% to 13.9 at%, the aging time for σ phase precipitation at the grain boundaries at 750°C increases from 100 hours to 215 hours. Meanwhile, the coarsening rate of γ' phase at 750°C are found to be 7.84×10−28 m3/s and 13.6×10−28 m3/s for the Cr content of 24.1 at% and 18.7 at%, respectively. It suggests that the reduction in Cr content notably suppresses the precipitation of σ phase, and accelerates the coarsening rate of γ' phase. Tensile testing of the gradient material reveals that the lower Cr content region exhibits fewer and shorter stacking faults during tensile deformation due to its higher stacking fault energy, leading to the rapid development of local strain, subsequent necking, and eventual fracture. Taking into account the evolution of microstructure, hardness, and deformation substructure with varying Cr content, the appropriate Cr content in (NiCo)88-xCrxAl10Ta2 medium entropy alloy is 17–24 at%.

Study on strain localization of frozen sand based on uniaxial compression test and discrete element simulation

Strain localization has always been an important subject in frozen soil mechanics and engineering. To evaluate the development of local strain and the formation of shear bands in frozen soil, uniaxial compression tests have been conducted on frozen sand at various temperatures and particle grades. The strain localization evolution law of the frozen soil is analyzed utilizing the digital image correlation (DIC) method. The test results reveal that the entire process of shear band generation, development, and formation in frozen soil can be well captured. Within the testing temperature and particle grade range in this study, in comparison to particle grade, temperature exerts a more pronounced influence on the shear band angle which increases as temperature decreases. It is discovered that the width of the shear band increases with the decrease in temperature and the increase in mean particle diameter d50. Subsequently, a discrete element method (DEM) model is developed to examine the microscopic mechanical characteristics of frozen sand in uniaxial compression tests. The reliability of the DEM model is verified through comparative analysis with test results. Besides, the development law of the strain localization of the frozen soil obtained by the numerical simulation is revealed based on the quantitative analysis of the rotational angle, bonding state, and displacement of the soil particle during the shearing process.

Micromechanical Modeling for Predicting Residual Stress–Strain State around Nodules in Ductile Cast Irons

In this paper, a micromechanical model was developed to predict the residual stress–strain state that is generated around nodules of a ferritic ductile cast iron during solidification. A finite element analysis was performed on a reference volume element of the material to analyze the local strain development, having modeled both matrix and nodule as deformable bodies in contact. The behavior of the nodule was assumed linear–elastic because of the low stresses to which it is subjected during cooling. On the other hand, elasto-plastic viscous behavior was considered for the matrix, considering both the primary and secondary creep regimes. To make up for the lack of information on the physical–thermomechanical properties of the constituents, the available literature data were integrated with the results obtained from the CALPHAD methodology applied to both cast iron and the steel that constitutes its matrix. The micromechanical model was validated by comparing the resulting residual strains with experimental data available in the literature for a ferritic ductile cast iron. Then, it was used for analyzing the correlation between the solidification history and the mechanical response of cast iron in terms of the uniaxial stress–strain curve.

Physical overloading test for 3D printed caverns: Failure performance and supporting effect

Excavation of a large underground cavern often induces the unstable behavior of the surrounding rock due to intensive stress redistribution; thus, reasonable supporting measurements are necessary. A new 3D print-based physical overloading test method is presented for exposing the failure performance of caverns and the reinforcing effect of supported structures. Taking the Yingliangbao underground caverns as an engineering background, a physical model and supporting elements for the cavern groups are manufactured first by 3D printing (3DP) technology, including a multi-cavern 3DP sand model, 3DP polymer lining and metal bolt. Further overloading tests for the printed physical models are carried out, and the progressive failure performances are exposed, including crack extension, model loading evolution, local strain development, etc., which indicate that shear failure often appears on the mid-wall. Moreover, the overloading test for the supported cavern model also quantitatively reveals the reinforcing role of the 3DP polymer lining and metal bolt, such as increasing the bearing capacity by more than 10%. A corresponding numerical back analysis is further applied to certify the progressive failure process of multiple caverns under overloading conditions, which provides some cures for the supported bolt position and depth in multiple caverns. This method can enrich the physical model test and guide the support design for underground engineering construction.

Investigation of Relationships between Grain Structure and Inhomogeneous Deformation by Means of Laboratory-Based Multimodal X-Ray Tomography: Strain Accuracy Analysis

The internal strain distribution developing during plastic deformation is important for understanding the mechanical properties of polycrystalline materials. Such distributions may be determined by the microstructural feature tracking method. However, the strain accuracy and the sources of uncertainty of this method are not well quantified yet. Evaluate the accuracy of local strain determination based on laboratory-based multimodal X-ray tomography measurements. The plastic deformation behavior of a particle-contained fully recrystallized Al-4mass%Cu alloy was characterized during in-situ tensile testing using absorption contrast tomography to reveal marker particles and diffraction contrast tomography to reveal the grain structure. It was found that the accuracy in strain measurement is inversely proportional to the particle distance, whereas it is independent of the particle size, and the accuracy is not biased by grain boundaries. As an illustrative example, the preliminary analysis of the microstructure-strain relationships reveals significant strain differences both between and within individual grains. The results document the validity and limitations of the microstructural feature tracking method for local strain measurements using a laboratory X-ray source, are of importance for local strain analysis in general and may be considered as an alternative to other methods, e.g. digital image correlation. The measurements allow evaluation of effects of the grain microstructure on the local strain development during plastic deformation. By a preliminary analysis of the strain difference between two selected grains, it is suggested that the grain shape and crystallographic relationships between neighboring grains are of importance for the development of local strains.

Identifying the weak spots in\xa0packaging paper: local variations in grammage, fiber orientation and density and the resulting local strain and failure under load

Measured local paper structure—i.e. local basis weight, local thickness, local density and local fiber orientation—has been linked to local strain and local material failure (local temperature increase due to energy dissipation upon fiber–fiber bond failure) measured during tensile testing. The data has been spatially linked through data map registration delivering several thousand 1times 1,hbox{mm}^2 paper regions, each containing all measured properties. The relation between local paper structure and resulting local deformation and failure is studied with regression models. Multiple linear regression modeling was used to identify the paper structure related drivers for local concentrations of strain under load and local concentrations of material failure, which are both starting to occur considerably before rupture of the paper. Analyzing the development of local strain in paper we found that regions with higher basis weight and higher fiber orientation in load direction tend to exhibit considerably lower strain during tensile testing. Furthermore, the relation between local strain and local grammage can be predicted with the statistical theory of elasticity. Also regions with higher density have lower local strain, but not as pronounced. The findings for local fiber–fiber bond failure of paper are similar but not equivalent. The strongest correlation exists with local grammage. Local density and local fiber orientation show in turn weaker correlation with local bond failure. Local variations in paper thickness were not relevant in any case. These findings are highlighting the relevance of local fiber orientation and local density variations as structural mechanisms governing paper failure. In the past the focus has been mostly on paper formation. Together with local grammage (formation) they are responsible for the weak spots in paper, and thus cause local concentrations of paper strain and the initiation of failure under tensile load.

Measurement of local plastic strain during uniaxial reversed loading of nickel alloy 625

The reverse loading behaviour of polycrystalline alloys is challenging to predict, as it depends on complex multi-scale interactions that are not well understood and difficult to study. We have used high resolution digital image correlation (HRDIC) to measure the local strain development during reversed loading of a nickel based superalloy with sub-micron spatial resolution. A specially designed rig ensured that deformation remained uni-axial during reversal, and measurements were made in-situ, with the sample under load. The deformation data was correlated with the underlying crystallographic microstructure using orientation maps from electron back-scatter diffraction (EBSD), which was transformed using an affine transformation, to account for measurement related distortion. The strain was found to be localised into crystallographic slip bands separated by regions with much lower, mostly elastic, strain. On unloading, no localised deformation reversal could be measured in slip bands or elsewhere, implying that most of the strain reversal is elastic and that any plasticity during unloading is small, evenly distributed, and could not be detected using HRDIC. These results imply that HRDIC studies on unloaded samples can provide representative measurements of the deformed state of the material during reversal. On reversal, deformation is accommodated primarily by slip on slip bands formed during forward loading. In some grains, however, the slip band patterns appear to change location and sharpen with further deformation. This suggests that slip extent of slip reversal at the scale of slip bands is limited and can be quantified, opening up a new way to study reverse loading in advanced alloys.

Operando time- and space-resolved high-energy X-ray diffraction measurement to understand hydrogen-microstructure interactions in duplex stainless steel

Local strain development in the microstructure of a commercial 25Cr-7Ni super duplex stainless steel was mapped using high-energy x-ray diffraction during cathodic hydrogen charging under constant uniaxial load. The infusion of hydrogen resulted in tensile strains in austenite grains, one order of magnitude larger than those in the ferrite. Most strain evolution occurred at the near-surface, with compensating compressive forces developed in underlying regions, with up to two-times more compression occurring in the ferrite than the austenite. The strains along the loading axis were more pronounced than in the transverse direction, in which mostly compressive strains developed in the ferrite.

Macroscopic Geometry-Dominated Orientation of Symmetric Microwrinkle Patterns.

Orientated wrinkle patterns with controlled microarchitectures are highly attractive because of their potential and broad application in technologies ranging from flexible electronic devices to smart windows. Here, we demonstrate a macroscopic, geometry-dominated strategy to fabricate symmetric microwrinkles with precisely controllable pattern dimensions and orientations through a dynamic interfacial release process. The release-induced approach is based on the release of multilayer elastomer composites from polymeric sacrificial layers in solutions combined with crosslinking-induced contraction of the elastomer substrates. Crosslinking-induced contraction provides the driving force for developing and stabilizing surface wrinkle formation, whereas the polymeric sacrificial layer provides a mild and simultaneous release process to form orientated wrinkles through kinetic control of local strain development. The macroscopic shape of the composite controls release kinetics, hence strain history, leading to the generation of photonic reflective surfaces. Moreover, stable wrinkles fabricated from various materials including metals, ceramics, and carbons can be achieved. This versatile, mold-free, and cost-effective platform technology demonstrates how global strain distributions can be harnessed through kinetics to drive local pattern development.

Experimental Investigations on the Progressive Failure Characteristics of a Sandwiched Coal-Rock System Under Uniaxial Compression

Overlapped residual coal pillars, together with the surrounding rock strata, play a combined bearing role in ultra-close multiple seam mining. Global stability of the whole bearing system is significant for the mining design, construction, and operation. Laboratory uniaxial compressive experiments for different kinds of sandwiched coal-rock specimens are carried out to investigate the progressive failure characteristics and mechanisms. Results show that: (1) The mechanical behavior of the sandwiched coal-rock specimen is mainly divided into four stages during the failure process. The response of the electrical resistivity and the evolution of acoustic emission (AE) energy are in good agreement with the mechanical behaviors at different stages, which are a reflection of the global failure characteristics of sandwiched specimens. (2) The distribution of AE events and the development of local strain can provide further insight into the local failure characteristics of coal elements or rock elements in sandwiched specimens. AE events are more likely to be generated in coal elements, which can propagate across coal-rock interfaces and induce damage to rock elements in a certain area. Similarly, the unbalanced deformation characteristics of coal elements and rock elements are apparently revealed in the progressive failure process. (3) Progressive failure of a sandwiched coal-rock specimen is closely associated with the interactions between the coal elements and rock elements. Initial failure usually appears in the coal elements. At this process, the recovery of elastic deformation and the output of strain energy are observed in the rock elements, which can accelerate the rupture of coal elements. In turn, the dynamic fracture energy generated in the rupture process of coal elements can propagate into rock elements and induce damage to rock elements a certain area. (4) The experimental results are helpful for maintaining the long-term stability of a sandwiched coal-rock system in ultra-close multiple seam mining.

In Situ Transmission Electron Microscopy to Uncover Electrochemical Reactions

This talk gives an overview of the PI’s research program on the opportunities and challenges of in situ transmission electron microscopy (TEM) to study novel two dimensional materials and metal oxides for Li-ion and Na-ion battery applications. Various anode materials including SnO2, ZnSb, MnO2 , phosphorene were subjected to lithiation process and the transport of Li ions was visualized within their atomic structure. For SnO2 nanowires, it was observed that the Li ion transport results in local strain development preferably along (200) or (020) plans and [001] crystallographic directions. Extremely fast ionic transport was also observed along [100] directions of phosphorene and in ZnSb crystalline materials indicating that atomic structure engineering of electrode materials can provide an efficient path for the movement of larger ions such as sodium ions overcoming their sluggish kinetics. In one-dimensional materials the structural engineering of tunnels and the tunnel stabilizers can be very effective in allowing the ions to be transported at fast rates. Overall a summary will be given on new science obtained from the in situ TEM studies and how this new information can help to further grow the opportunities in materials designed for electrochemistry.

Microstructural Development Due to Laser Treatment and Its Effect on Machinability of Ti6Al4V Alloy

Application of the laser in machining has been demonstrated to improve the machinability in several metals and alloys. A very high heating and cooling rate during laser treatment tends to modify the microstructure significantly. In some materials, the change in the microstructural features affects the machinability of the materials. In this work, microstructure evolution due to laser treatment and its effect on the machinability of Ti6Al4V was studied using advanced characterization techniques. The microstructure of the surface and subsurface of the cylindrical Ti6Al4V rod was modified using a high-power laser source with varying laser scanning speeds. The laser treatment resulted in three distinctly different microstructures along the radial direction of the rod; these were classified as the lath dominant zone, a mixture of laths with equiaxed grains and equiaxed grains surrounded by bands. Rapid heating and cooling during laser scanning lead to the formation of the martensite phase and local strain development. Further, at the boundaries of laths, compressive twins (57 deg $$ \left\langle {\bar{1}2\bar{1}0} \right\rangle $$ ) were formed because of laser heating. These twins are different from tensile twins (94.8 deg $$ \left\langle {\bar{1}2\bar{1}0} \right\rangle $$ ), which are formed at the machined subsurface during deformation. The formation of a martensite phase and local strain development due to laser treatment resist the formation of localized shear bands along the thickness of machined chips during machining. This in turn also repels the crack propagation along the shear band developed during machining and enhances the machinability of the material.

Strain Modulation by van der Waals Coupling in Bilayer Transition Metal Dichalcogenide.

Manipulation of lattice strain is emerging as a powerful means to modify the properties of low-dimensional materials. Most approaches rely on external forces to induce strain, and the role of interlayer van der Waals (vdW) coupling in generating strain profiles in homobilayer transition metal dichalcogenide (TMDC) films is rarely considered. Here, by applying atomic-resolution electron microscopy and density functional theory calculations, we observed that a mirror twin boundary (MTB) modifies the interlayer vdW coupling in bilayer TMDC films, leading to the development of local strain for a few nanometers in the vicinity of the MTB. Interestingly, when a single MTB in one layer is "paired" with another MTB in an adjacent layer, interlayer-induced strain is reduced when the MTBs approach each other. Therefore, MTBs are not just 1D discontinuities; they can exert localized 2D strain on the adjacent lattices.

Real-Time Observations of Electrochemical Reactions in Rechargeable Energy Storage Systems

Electrodes in rechargeable batteries undergo complex electrochemically-driven phase transformations upon driving Li ions into their structure. Such phase transitions in turn affect the reversibility and stability of the battery. This presentation gives an overview of the PI’s research program on in-situ transmission electron microscopy (TEM) of battery materials. In-situ TEM has been shown to be a very powerful technique in shedding light to some of the mysteries in electrochemical performance of new materials. Various anode materials including SnO2, MnO2, ZnSb were subjected to lithiation process and the transport of Li ions was visualized within their atomic structure. For SnO2 nanowires, it was observed that the Li ion transport results in local strain development preferably along (200) or (020) plans and [001] crystallographic directions. The lithiation behavior in the presence of twin boundary defects was completely different compared to pristine state with no twin boundary defect. We showed that twin boundaries in general provide a more accessible pathway for Li ion transport. Anisotropic plastic deformation was also observed along [010] directions of MnO2 nanowires. Sb-based intermetallics, such as Zn-Sb system, which have been proved to be promising anode materials for Li-ion batteries, are also capable of storing of sodium ions. We investigated the microstructural changes and phase evolution of the Zn-Sb intermetallic nanowires using in-situ TEM. Zn-Sb alloys also exhibit a new cubic alloying phase Li2ZnSb that form by intermixing of the ABAB atomic ordering in hexagonal LiZnSb due to Li inclusion in their lattices. Our results indicate that the reaction between Zn-Sb and sodium proceeds through a different pathway during the first compared to the subsequent cycles. Atomic resolution imaging shows that NaZnSb has a layered structure, which provides channels for fast Na+ diffusion.

Non-destructive and three-dimensional measurement of local strain development during tensile deformation in an aluminium alloy

Anisotropy of mechanical responses depending on crystallographic orientation causes inhomogeneous deformation on the mesoscopic scale (grain size scale). Investigation of the local plastic strain development is important for discussing recrystallization mechanisms, because the sites with higher local plastic strain may act as potential nucleation sites for recrystallization. Recently, high-resolution X-ray tomography, which is non-destructive inspection method, has been utilized for observation of the materials structure. In synchrotron radiation X-ray tomography, more than 10,000 microstructural features, like precipitates, dispersions, compounds and hydrogen pores, can be observed in aluminium alloys. We have proposed employing these microstructural features as marker gauges to measure local strains, and then have developed a method to calculate the three-dimensional strain distribution by tracking the microstructural features. In this study, we report the development of local plastic strain as a function of the grain microstructure in an aluminium alloy by means of this three-dimensional strain measurement technique. Strongly heterogeneous strain development was observed during tensile loading to 30%. In other words, some parts of the sample deform little whereas another deforms a lot. However, strain in the whole specimen was keeping harmony. Comparing the microstructure with the strain concentration that is obtained by this method has a potential to reveal potential nucleation sites of recrystallization.

Mechanistic Understanding of Li Transport in Rechargeable Batteries

Electrodes in rechargeable batteries undergo complex electrochemically-driven phase transformations upon driving Li ions into their structure. Such phase transitions in turn affect the reversibility and stability of the battery. It is of prime importance to better understand how Li ions transport within the host electrodes and what phase transitions are triggered during such interaction. This presentation gives an overview of the PI’s research program on in situ transmission electron microscopy (TEM) of battery materials. Various anode materials including SnO2, Zn-Sb were subjected to lithiation process and the transport of Li ions was visualized within their atomic structure. For SnO2 nanowires, it was observed that the Li ion transport results in local strain development preferably along (200) or (020) plans and [001] crystallographic directions. Anisotropic plastic deformation was also observed along [010] directions of MnO2 nanowires. Zn-Sb alloys also exhibit a new cubic alloying phase Li2ZnSb that form by intermixing of the ABAB atomic ordering in hexagonal LiZnSb due to Li inclusion in their lattices.

Fibre–matrix interfaces in carbon fibre reinforced PPS composites: damage initiation and propagation in tensile tests

This paper investigates the effect of fibre-matrix adhesion on the local strain development, as measured by the Digital Image Correlation technique, and on damage initiation and propagation in ±45° and 0° tensile tests of 5HS carbon fibre reinforced Polyphenylene sulphide. The sides of both ±45° and 0° samples have been monitored by a high-speed camera during testing. Scanning electron microscopy (SEM) analysis of cross-sections of samples tested up to different strain levels has been performed to help the identification of the main failure mechanisms. In the case of 0° tensile tests, the fractured samples’ sides have been also observed by optical microscopy and the fracture areas have been analysed by SEM. In the ±45° tensile tests, strain concentration is observed in the middle of the samples with low fibre-matrix adhesion, where the interface is not able to transfer the load to the fibres in the outer part of the sample, contrary to what happens in samples with good bonding. A fragile failure is observed in samples with good interfacial bonding, whereas a very extensive delamination failure is obtained in samples with poor interface. In the 0° tensile tests, local strain concentrations develop at each weave cross-over points in composites with poor fibre-matrix adhesion. This leads to weft fibre bundle debonding from the resin and eventually to extensive delamination. A more uniform strain field is observed in samples with good fibre-matrix adhesion, where a fragile type of failure is obtained.

Portevin–LeChatelier effect in strain and stress controlled tensile tests

The nucleation and propagation of Portevin–LeChatelier (PLC) deformation bands in Cu–15at.%Al have been studied by means of a novel multi-zone scanning laser extensometer providing information on the development of local strain along the main part of the specimen, in addition to the conventional measurement of stress serrations. By means of tensile tests with imposed constant strain rate independent data have been determined on propagation velocity, concentrated strain and width of the bands appearing in three different types of PLC-deformation. The tensile test machine has been softened to permit measurements with imposed constant stress rate. First results will be presented and compared to the strain-controlled ones on Cu–15at%Al.

Determination of the plastic characteristics of materials with a tendency to local strain development

Determination of the plastic characteristics of materials with a tendency to local strain development