Mismatch Stress Research Articles

Comprehensive improvement of strength and ductility in CLAM steel subjected to TEU: The role of heterogeneous microstructure

To improve the comprehensive properties of CLAM steel and expecting good performance in service conditions, twist extrusion and upsetting (TEU) based thermo-mechanical treatment (TMT) was introduced into the steel at 600 °C, 670 °C, and 740 °C. Optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) was used to characterize the microstructural evolution during the deformation. Uniaxial tension test was used to evaluate the tensile mechanical property. The results show that the grain size and dislocation density of the processed samples decrease from the as-received condition, but are higher than that in the as-tempered conditions. The particle sizes of M23C6 and MX in the TEUed samples are lower than that in the as-tempered case, which contribute a lot to the yield strength. The yield strength of the TEUed CLAM steel decrease with the increase of deformation temperature and a good match in strength and ductility, relating to heterogeneous microstructure, has been achieved at the deformation temperature of 670 °C (yield stress: 853.59 MPa, ultimate tensile stress: 992.4 MPa, elongation: 15.7%). The K value and exponent in the Hall-Petch relationship should be modified owing to the volume fraction of martensite, which is harder than ferrite and causes interface elastic mismatch stress, is different at different temperatures.

Critical scale grain size for optimal lifetime of TBCs

• The critical scale grain size d c of ∼2.5 μm is identified for optimal TBC lifetime. • When d < d c , the rapid growth of the scale causes delamination failure of TBCs. • When d > d c , the growth stress causes vertical cracking and early failure of TBCs. Thermal barrier coatings (TBCs) suffer from spalling failure resulting from the rapid growth of thermally grown oxides (TGO) on the metallic bond coat. In this study, to achieve the optimal lifetime of TBCs, a lifetime phase diagram was constructed for the entire service of TBCs. Herein, both the thermal mismatch stress during thermal cycling and the TGO growth stress during thermal exposure were considered. First, thickening behavior of the Al 2 O 3 scale related to grain size was investigated. Second, transverse delamination failure due to the thermal mismatch stress was analyzed. Third, vertical cracking failure related to early scale growth was explored. Finally, a critical scale grain size of ∼2.5 μm was identified for achieving the optimal lifetime of TBCs. It was experimentally proved that vertical cracking of the Al 2 O 3 scale occurred during early oxidation as the scale grain size was larger than ∼4 μm after pre-oxidation. The proposed lifetime phase diagram and critical scale grain size effect serve as novel criteria for the design of next-generation durable TBCs.

Langasite microstructure etching and characterization method for high temperature devices

Assessing the health of large industrial components such as aircraft engine blades is essential in high-temperature environments; however, conventional pressure sensors do not function at temperatures above 1100 °C. In this paper, a new Langasite (LGS) fabrication method, which enables the manufacturing of microstructures from high-melting, hard-brittle, and difficult-to-machine materials, is proposed. An HCl and H3PO4 mixed solution is used to achieve micron-level precision, allowing rapid and accurate manufacturing of microstructure equipment and technology. The etched surface was analyzed by X-ray photoradiation spectroscopy, energy-dispersive X-ray spectroscopy, and high-resolution transmission electron microscopy. The results showed that the etching did not affect the crystal's structure and properties. Moreover, a hot-pressing direct bonding method was used to seal the cavity, avoiding the thermal stress mismatch between indirect materials in high-temperature applications. A sealed 1-cm-in-diameter and 200-μm-thick diaphragm pressure sensor was fabricated by wet etching and hot-pressing bonding microstructures. The test results showed that the device can withstand pressures of 1 MPa at 1100 °C with an average sensitivity of 404 Hz/kPa. Therefore, the proposed method provides a simple and low-cost solution for the manufacturing of small and precisely-controlled structures made of high-temperature resistant, hard and brittle materials that are difficult to process.

Oriented preparation of Large-Area uniform Cs2TeI6 perovskite film for high performance X-ray detector

Large-area flexible perovskite films are attracting widespread research interest for applications in wearable solar cells, portable photodetectors, bendable X-ray imaging detectors and other implantable optoelectronic devices. In this work, a facile mobile platform assisted electrospray method is developed to prepare large-area (100 cm2) lead-free Cs2TeI6 film on flexible polyimide substrate. The spraying parameters are coupled with the growth temperature to achieve a dynamic balance. The as-prepared film by optimized process shows high uniformity in grain size, thickness and X-ray response without pinholes and cracks. Moreover, oriented nucleation is more likely to occur on the flexible organic substrates for less growth stress and mismatch stress, leading to preferred (222) plane orientation. X-ray detectors prepared with the films exhibit a resistivity of 1.9 × 1011 Ω·cm, an X-ray sensitivity of 226.8 μC⋅Gyair-1⋅cm−2 and a transient response rise time as fast as 42 ms under 50 kV X-ray at an electrical field of 6.67 × 103 V·mm−1. The modified electrospray method shows great potential applications for large-area devices of radiography, solar cell and other optoelectronic devices.

Lightweight and near-zero thermal expansion ZrW2O8-SiCnw/Al hybrid composites

• We reported a novel hybrid design strategy to achieve near-zero thermal expansion and high strength Al matrix composites. • NTE ZrW 2 O 8 and lowCTE SiC successfully control the high CTE of Al. • The addition of SiC (particle and nanowire) significantly reduces the thermal mismatch stress of ZrW 2 O 8 /Al composites. • Yield stress rises by increasing the SiCnw content, resulting in reduction of the unfavorable high pressure γ-ZrW 2 O 8 and microcracks. Negative thermal expansion particles can compensate metal matrix composites to achieve near-zero thermal expansion materials. However, there are huge thermal residual mismatch stresses in the particles due to the thermal expansion mismatch between matrix and reinforcement. The use of hybrid reinforcement is required to solve the stress concentration problem. In this study, the hybrid Al matrix composites reinforced with ZrW 2 O 8 and SiC reinforcements (SiCp, SiCnw) were fabricated by the pressure infiltration process. The effect of SiC content on microstructure, phase composition, thermal expansion and mechanical properties were examined by a combination of XRD, SEM, thermal dilatometer and bending tests. The results showed that the high-pressure phase (γ-ZrW2O8) content of hybrid metal matrix composites decreased with increasing SiC fraction. Among them, the composites containing 61.7 vol%ZrW 2 O 8 and 3.3vol%SiCnw achieved the best performance with a bending strength of 100–150 MPa and a near-zero thermal expansion (2.0–3.0 ×10 –6 K −1 ) in a wide range of − 50–120 °C, which was attributed to the synergetic hybrid effects of SiCnw and ZrW 2 O 8 . This work opens up a new frontier for the design and preparation of lightweight zero-thermal expansion materials.

High-temperature oxidation behavior and mechanism of the Si-based thermal protective coating for SiCf/SiC composites under static oxidation and H2O/O2/Na2SO4 corrosion oxidation

The hot corrosion and oxidation of SiCf/SiC composites have become pivotal challenges in promoting their application in a heat field environment. This study presents an efficient approach for preparing multilayer oxidation/corrosion resistance coating via the sinter-preoxidation and chemical vapor deposition procedures. The results show that the coated SiCf/SiC composites exhibit stable thermal protective performance with a rather light mass change against static oxidation and H2O/O2/Na2SO4 corrosion oxidation at 1200–1400 °C for 600 h. All tested samples possess a high specific strength of more than 91.2% retention. Moreover, the kinetics and protection/failure mechanisms of the coated SiCf/SiC composites are analyzed that the relevance involves the liquid convection, volatilization of the solid–liquid reaction, H2O/O2/Na2SO4 corrosion, Kirkendall cracks and holes, thermal mismatch stress, and thermally grown oxide (TGO) growth. The heat transfer behavior in different heat fields is also investigated based on the finite element analysis (FEA).

Mechanical properties and [formula omitted] interfacial misfit network evolution: A study towards the creep behavior of Ni-based single crystal superalloys

The aim of this study is to investigate the role of the temperature, stress, and rhenium (Re) on the γ/γ' interfacial misfit dislocation network and mechanical response of Ni-based single crystal superalloys. After aging at elevated temperatures, mismatch between the two phases results in an interfacial dislocation network to relieve the coherency stress. Molecular dynamics (MD) simulations have been performed to study the properties of the (100), (110), and (111) phase interface crystallographic directions. Increasing temperature disperses the atomic potential energy at the interface diminishing the strength and stability of the networks. In the case of loading, when a constant strain rate of 2×108(s−1) is applied at 0 K, the (100) and (111) phase interface models lose their co-coordinating role of maintaining the dynamic equilibrium. Dislocation propagation in the γ phase is the dominant deformation mechanism in these two interfacial models, resulting in dislocations pile-up in the damaged area, and the network is no longer able to fortify the interface. For the (110) phase interface model, the dominant deformational mechanism is precipitate shearing. As temperature increases, the elastic modulus, initial mismatch stress, and yield strength decrease. Also, the pinning effect of Re atoms is evaluated in the γ phase at 1600 K. The dislocation hampering property of Re is more perceptible when enough dislocations in the γ phase are moving at elevated temperatures. In addition, Re manages to relieve the interfacial stress field and does not affect the network morphology. Finally, an investigation of the creep behavior of the superalloy is provided. It is observed that the escalated damage to the interfacial network due to the increased temperature leads to the domination of the softening mechanisms (cross-slip and dislocation climb) on the deformation and shortens the steady-state creep. Moreover, Re atoms act as an extra hardening factor to improve the tertiary creep.

Delamination model of an epoxy-impregnated REBCO superconducting pancake winding

Rare-earth barium copper oxide (REBCO) coated conductor (CC) tapes are promising for high-energy and high-field applications. In epoxy-impregnated REBCO superconducting windings, during the cooling process, the delamination induced by thermal mismatch stress significantly threatens stable operation due to the weak c-axial strength of REBCO CC tapes. In this study, a two-dimensional axisymmetric multilayer delamination finite element model with main layers of REBCO CC tapes and insulation materials was developed based on the bilinear cohesive zone model. Based on the proposed model, the stress distribution and delamination properties of an epoxy-impregnated REBCO winding induced during the cooling process were investigated. Furthermore, the effects on the structural configuration on the delamination and optimisation schemes of fabrication are discussed. The model results indicate that during the cooling process of the REBCO winding, when the radial tensile stress induced by the thermal mismatch stress is greater than the delamination strength, interface cracking will occur. Interface failure occurs in regions containing several turns of coils and not just a single-turn coil. Our analyses indicate that structural failure caused by delamination depends on the radius ratio of the outer to the inner winding, where a small radius ratio is preferred to reduce the risk of delamination. An excessive radius ratio can result in multiple delamination failures during the cooling process. The optimisation scheme, which divides the winding into several sub-windings with a consistently small radius ratio, is an effective method for mitigating the risk of delamination failure, which is consistent with available experimental results. Additionally, by reducing the thermal expansion coefficient of the epoxy resin, the risk of delamination failure during cooling can be significantly reduced. For the winding structure considered in this study, if the coefficient of thermal expansion of the epoxy resin is reduced to 5 ( K−1), then delamination failure caused by thermal mismatch will be eliminated. Our model results are consistent with those of several valuable experimental phenomena and numerical calculated in the literature.

Control of deviatoric stress in the diamond anvil cell through thermal expansion mismatch stress in thin films

Control of deviatoric stress in the diamond anvil cell through thermal expansion mismatch stress in thin films

Pre-Buried Additive for Cross-Layer Modification in Flexible Perovskite Solar Cells with Efficiency Exceeding 22.

Halide perovskites have shown superior potentials in flexible photovoltaics due to their soft and high power-to-weight nature. However, interfacial residual stress and lattice mismatch due to the large deformation of flexible substrates have greatly limited the performance of flexible perovskite solar cells (F-PSCs). Here, ammonium formate (HCOONH4 ) is used as a pre-buried additive in electron transport layer (ETL) to realize a bottom-up infiltration process for an in situ, integral modification of ETL, perovskite layer, and their interface. The HCOONH4 treatment leads to an enhanced electron extraction in ETL, relaxed residual strain and micro-strain in perovskite film, along with reduced defect densities within these layers. As a result, a top power conversion efficiency of 22.37% and a certified 21.9% on F-PSCs are achieved, representing the highest performance reported so far. This work links the critical connection between multilayer mechanics/defect profiles of ETL-perovskite structure and device performance, thus providing meaningful scientific direction to further narrowing the efficiency gap between F-PSCs and rigid-substrate counterparts.

Dimensional Stability of Mirror Substrates Made of Silicon Particle Reinforced Aluminum.

In the present study, the thermal cycling stability of mirrors made of silicon particle reinforced aluminum compounds, containing an amount of 42 ± 2 wt.% silicon particles, is investigated with respect to thermal loading. The compound is processed by single-point diamond turning to optical mirrors that were subsequently thermally cycled in a temperature range between 40 °C to −60 °C and between 20 °C and −196 °C, respectively. The residual shape change of the optical surface was analyzed using Fizeau interferometry at room temperature. The change of shape deviation of the mirrors is compared with dilatometric studies of cylinders using the same temperature regime. Due to different coefficients of thermal expansion of the two constituents of the compound, thermal mismatch stresses in the ductile aluminum matrix and the brittle silicon particles are induced by the investigated thermal loads. The plasticity that occurs causes the formation of dislocations and stacking faults as substantiated by Transmission Electron Microscopy. It could be shown that the silicon particles lead to the cold working process of the reinforced aluminum matrix upon thermal cycling. By using interferometry, a higher dimensional stability of mirrors made of silicon particle reinforced aluminum due to thermal loads is demonstrated.

Interplay between Thermal Stress and Interface Binding on Fracture of WS2 Monolayer with Triangular Voids.

The defect engineering of two-dimensional (2D) materials has become a pivotal strategy for tuning the electrical and optical properties of the material. However, the reliable application of these atomically thin materials in practical devices require careful control of structural defects to avoid premature failure. Herein, a systematic investigation is presented to delineate the complex interactions among structural defects, the role of thermal mismatch between WS2 monolayer and different substrates, and their consequent effect on the fracture behavior of the monolayer. Detailed microscopic and Raman/PL spectroscopic observations enabled a direct correlation between thermal mismatch stress and crack patterns originating from the corner of faceted voids in the WS2 monolayer. Aberration-corrected STEM-HAADF imaging reveals the tensile strain localization around the faceted void corners. Density functional theory (DFT) simulations on interfacial interaction between the substrate (Silicon and sapphire -Al2O3) and monolayer WS2 revealed a binding energy between WS2 and Si substrate is 20 times higher than that with a sapphire substrate. This increased interfacial interaction in WS2 and substrate-aided thermal mismatch stress arising due to difference in thermal expansion coefficient to a maximum extent leading to fracture in monolayer WS2. Finite element simulations revealed the stress distribution near the void in the WS2 monolayer, where the maximum stress was concentrated at the void tip.

3D printing auxetic draft-angle structures towards tunable buckling complexity

With the development of 3D printing technology, auxetic structures have attracted extensive attention due to their unusual mechanical properties. In this study, we design a 3D printed auxetic structure using 2D draft angles to achieve a tunable out-of-plane double hyperbolic buckling behavior by effectively continuously varying stiffness across thickness. The influences of radii and draft angles on the buckling behaviors of the 3D printed draft-angle auxetic structures are studied by finite element method. The constitutive relationships between stress, strain, radius, and draft angle have been formulated and discussed to identify the working principle behind the mechanical performance of draft-angle auxetic structures. Finally, the buckling behavior is modelled by a laminate structure, and the accuracy of these analytical results has then been verified by experiment. This study is expected to provide a design guideline for achieving tunable buckling behavior of auxetic structures via the novel stress mismatch of draft angles and thus continuously varying stiffness along the thickness direction. The current work constitutes an initial attempt to realize the tunability of the 3D out of plane deformation of 2D plane structures under in-plane compression.

The role of the lipid environment in the activity of G protein coupled receptors

G protein coupled receptors (GPCRs) are a class of membrane proteins that sense extracellular signals ranging from light to odorants and small molecules and activate intracellular signaling pathways that control important physiological responses. Being composed of 7 transmembrane helices linked by extracellular and intracellular loops, the great majority of the sequence of these receptors is embedded in the lipid membrane. Therefore, it is expected GPCR structure and function to be impacted by the surrounding lipid environment and the lipid membrane physico-chemical and mechanical properties. A large number of examples from the literature is provided to highlight the role of the lipid nature (lipid headgroup, membrane polyunsaturation and cholesterol) and membrane physical and mechanical properties (curvature elastic stress, membrane thickness and hydrophobic mismatch, fluidity) in the activity of different GPCRs. In addition, lipids are important co-factors being identified in very specific locations in several GPCR structures. GPCRs and G proteins can also be lipid post-translationally modified and such events can significantly impact membrane binding, trafficking and signaling. These aspects are all treated in this review. Understanding how the lipid can modulate GPCR activity is important not only from a fundamental point of view but also due to the fact that certain pathologies, where GPCRs are central targets, have been associated with important lipid imbalance. Establishing a link between the lipid pathological imbalance and the receptor functioning in such environment is thus essential as it can open avenues to potentially innovative therapeutic strategies.

Fracture toughness of VO2 microcrystals across metal-insulator transition

In this work, we report the fracture behavior of plate-shaped VO2 microcrystals across metal–insulator transition with the aid of in-situ nanoindentation at 25 °C and 95 °C. The fracture toughness of VO2 is found to be influenced by the thermal mismatch stress and transformation stress. The measured crack length generated from indentations is 40% lower after transition. The determined fracture toughness value for the monoclinic phase is 2.02 ± 0.07 MPa.m1/2, while it is 2.67 ± 0.02 MPa.m1/2 for the tetragonal phase.

Effects of blister formation on the degradation of organic light emitting devices

This paper presents the results of a combined experimental and computational study of the mechanisms of blister formation, and their effects on the degradation of organic light emitting devices (OLEDs). Blister formation is attributed to the effects of thermally induced mismatch stresses associated with applied bias. These result in interfacial cracking phenomena that are affected by the solvents that are used in OLED fabrication. The OLEDs are first fabricated using an electron transport layer of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) deposited on an active layer made from solutions of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] dissolved in different solvents (toluene, chloroform, and chlorobenzene). The formation of blisters and degradation is then studied under applied bias for devices fabricated using different solvents (toluene, chloroform, and chlorobenzene) and emissive layer thicknesses. The underlying layer mechanical properties are then incorporated into interfacial fracture mechanic models that explain the formation of blisters that degrade the performance of OLED structures.

Electromagnetic-thermal-structure multi-layer nonlinear elastoplastic modelling study on epoxy-impregnated REBCO pancake coils in high-field magnets

• Multi-physics elastoplastic nonlinear FE model with main constituent materials of epoxy impregnated REBCO pancake winding is developed. • Discrete stresses appear on all the constituent materials. • Stress of each constituent material induced by thermal mismatch cannot be ignored. • Plastic failure mainly induced by hoop stress propagates from inner to outer turns with transport current increasing. • Failure of the superconducting layer occurs before yield in Hastelloy due to the direct action of Lorentz force. The elastoplastic mechanical behaviour of an epoxy-impregnated REBCO pancake winding under cryogenics and high magnetic field is investigated in the frame of finite element (FE) modelling. A two-dimensional axisymmetric electromagnetic-thermal-structure multi-physics multi-layer FE model with main layers of the coated conductor and insulation materials is developed. The radial stress and hoop stress on each constituent layer induced by thermal mismatch stress during cooling are investigated. The mechanical behaviour of each constituent material is also analysed by considering the thermal mismatch stress and electromagnetic force under 20 T background field. The results show that discrete stresses appear in all the constituent materials indicating that the multi-layer winding model containing the main constituent materials is necessary for the accurate stress analysis in an epoxy-impregnated REBCO winding. The stress of each constituent material induced by thermal mismatch during cooling process are too high to be ignored in the subsequent electromagnetic structure analysis. The mechanical-magnetic coupling analyses show that the stresses of all the constituent materials increase with the transport current. The plastic failure mainly induced by hoop stress successively occurs on copper stabilizer and Hastelloy substrate of the innermost turn, and the plastic region propagates from the inner turns to the outer turns with the increase of transport current. The failure of the superconducting layer occurs before the yield in Hastelloy due to the direct action of Lorentz force on the superconducting layers. The transverse tensile stress increases with the increasing transport current, indicating that the risk of transverse delamination failure increases with the increase of transport current. The mechanical failure modes including delamination within the conductor, plastic deformation in substrate and crack in superconducting layer should be seriously considered.

Effects of anisotropic surface drift diffusion on the strained heteroepitaxial nanoislands subjected to electromigration stressing

A systematic study based on self-consistent dynamical simulations is presented for the morphological evolutionary behavior of an isolated thin Ge/Si nanoisland (quantum dot) on a rigid substrate exposed to electromigration forces. This morphological evolution is basically induced by the anisotropic surface drift diffusion, driven by the capillary forces, the lattice mismatch stresses, and the wetting potential. In this study, we have mainly focused on the size and shape development kinetics of quantum dots, known as the “Stranski–Krastanov” (SK) morphology, influenced by applied electromigration stresses. Emphasis is given to the effects of rotational symmetry associated with the anisotropic diffusivity in 2D space (i.e., quantum wires in 3D). The pointed bullet-shaped “Stranski–Krastanov” islands with high aspect ratios, ξ = 0.77, are formed at the cathode edge, while the whole nanoisland slightly creeps out of the initial computational domain. The favorable configuration of the Ge20/Si80 alloy test module, which resulted in ζ = 0.37 enhancement in the contour surface area, has a dome shape attached to the [010] top surface of the Si substrate with a zone axis of {010}/⟨001⟩. The anisotropic surface diffusion dyadic has a fourfold rotational symmetry axis [001] lying on the (001) plane of the Si substrate, and its major axis is tilted at about ϕ = 45° from the applied electrostatic field extended along the longitudinal axis [100] of the substrate. This particular experiment resulted in a SK singlet peak with a small satellite with a very small aspect ratio of ≅0.2 that may be appropriate for the conception of quantum optoelectronic devices or inter-band structures to generate photoelectrons having large energy spectra, thereby increasing the efficiency of photovoltaics exposed to solar radiations.

Enhanced Plasticity and Corrosion Resistance in Mg-Zn-Ca-Cu Amorphous Alloy Composite via Plasma Electrolytic Oxidation Treatment

In this study, a dendrite-reinforced Mg-based amorphous alloy composite was prepared through an in situ precipitation strategy. After plasma electrolytic oxidation (PEO) treatment, the Mg85.1Zn12.7Ca2Cu0.2 amorphous alloy composite exhibited enhanced plasticity and corrosion resistance in a simulated body fluid solution (SBF). The PEO-treated composite showed a significant plastic strain of 10.5 ± 1.1%, as well as outstanding strain-hardening behavior. The enhancement of plasticity may be attributed to the in-situ formed coating, which can not only serve as a propagation barrier for shear bands but can also introduce nucleation sites in the bands as a result of stress mismatch and compositional heterogeneity. The corrosion density in the SBF decreased by three orders compared with the composite substrate. The spontaneous formation of apatite on the porous layer demonstrated that the prepared PEO coating has high bioactivity. The current work may provide a fundamental basis for developing biomedical Mg-based alloys with excellent comprehensive mechanical properties and corrosion resistance.

High temperature bending properties and failure mechanism of 3D needled C/SiC composites up to 2000 ℃

Three-dimensional (3D) needled C/SiC composites were prepared and subjected to three-point bending tests from room temperature (RT) to 2000 ℃ under vacuum. The results show that the flexural strength and modulus increase in the range of RT to 800 °C due to the release of thermal residual stress (TRS). At 800–1700 °C, the modulus further increases for the further release of TRS, while the destruction of the pyrolytic carbon (PyC) coating reduces the flexural strength. Up to 2000 ℃, the thermal mismatch stress in the composites cause fiber slippage and matrix crack deflection to be zigzag, which increase the fracture strength. The change of components properties mediated by high temperature and the release of TRS play a leading role in the flexural strength and fracture mode. The results provide important support for the mechanical behavior of 3D needled C/SiC composites at ultra-high temperature.