Formation Of Microcracks Research Articles

Effect of heat input on the microstructure and impact toughness of submerged arc weld metal for high-performance weathering steel

This study aims to elucidate the appropriate heat input (E j ) range for submerged arc welding (SAW) of high-performance weathering steel. Generally, by increasing E j , the welding efficiency can be improved, but the toughness of the weld metal may be deteriorated. Therefore, SAW was employed to produce the weld microstructure under varying E j from 20 to 50 kJ cm−1. The Charpy V-notch impact tests were conducted at −40 °C, and the weld microstructures were characterized by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD). The results indicate that the weld microstructures consist of polygonal ferrite (PF), acicular ferrite (AF), granular bainitic ferrite (GBF), and martensite/austenite (M/A) constituents under each E j . With the increase in E j , the proportion of PF increases, while AF and GBF are coarsened, and the area fraction (f M/A) and mean size (dM/A) of M/A constituents increase monotonically. Further, the fraction (f MTA>15°) of high-angle grain boundaries (HAGBs) with the misorientation tolerance angles (MTAs) greater than 15° is reduced, while the mean equivalent diameter (MEDMTA≥15°) of ferrite grains with HAGBs increases. Accordingly, with the increase in E j , the impact toughness of weld degrades from 128.4 to 47.6 J. The higher degree of micro-strain concentration caused by the increase in M/A size and area leads to the formation of larger microcracks under small plastic deformation, while the reduced HAGBs have a lower inhibition effect on crack propagation. Finally, the impact toughness decreases with the increase of E j . Overall, the findings suggest that the E j of SAW should not exceed 40 kJ cm−1 in the construction of high-performance weathering steel.

Effect of heat treatment on the structure, fracture toughness and oxidation behavior of a silicon coating by atmospheric plasma spraying

Silicon coatings produced by atmospheric plasma spraying are used as bond coat in environmental barrier coatings which protect silicon carbide fiber reinforced silicon carbide composites against water vapor induced corrosion at high temperatures. Silicon coatings exhibit structural defects such as interlamellar boundaries, gaps, pores and microcracks, some of which become pathways for oxygen penetration, leading to internal oxidation of the silicon coating. This study was conducted to reduce the pathways for oxygen penetration by heat-treatment under argon atmosphere and investigate the effects of heat-treatment on the structure, fracture toughness and oxidation behavior of the silicon coating. The results show that the structural defects could be reduced by a heat-treatment at T = 1300 °C, which resulted in a lower mass gain of the coating in the initial cycle in a cyclic oxidation test. The fracture toughness of the coating was significantly decreased by the heat-treatment, which led to the formation of microcracks in the coating in the cyclic oxidation test. As a result, the heat-treated coating exhibited a higher mass gain rate than the as-sprayed coating, leading to that both coatings exhibited almost the same mass gain at the end of the cyclic oxidation test of n = 10 cycles.

Surface Design Engineering of LiTaO3 Coating on Li-Excess Li2NiO2 Cathode Additive for Lithium-Ion Batteries

The latest research has mainly focused on enhancement in the energy density of lithium-ion batteries (LIBs) to satisfy the rigorous industrial demands. As the most common approach for improving the energy density of LIBs, the utilization of high capacity anode materials, such as silicon suboxide (SiOx), has been highlighted, instead of graphite. SiOx anodes are known to have a high specific capacity and low operating voltage (< 0.5 V vs. Li/Li+). However, the severe initial capacity loss induced by the formation of solid electrolyte interphase (SEI) limits their commercial use in LIBs. To compensate for the initial loss of available Li+, the application of Li-excess cathode additives has been considered as the most practical strategy for supplying surplus Li+ during the initial charge process. In this respect, Li2NiO2 is a suitable cathode additive because it offers a high initial charge capacity (≥ 320 mAh g-1) and a compatible operating voltage with commercial cathode materials. Surface protection of Li2NiO2 is still required, however, due to its vulnerability to moisture (H2O) and carbon dioxide (CO2) in the ambient atmosphere. Moreover, Li2NiO2 becomes more structurally unstable during cycle due to oxygen (O2) gas evolution, leading to the formation of microcracks. To overcome these obstacles, we suggest a functional LiTaO3 coating layer onto the surface of Li2NiO2 for structural stabilization. In practice, the LiTaO3 coating layer can effectively suppress the production of impurities (i.e., LiOH and Li2CO3) at the surface of Li2NiO2 after air exposure. Furthermore, the gas evolution and microcracks can be minimized by enhancing the structural stability of Li2NiO2 during cycling. These synergetic effects of the LiTaO3 coating layer can provide clear insights for the development of Li-excess cathode additives for achieving advanced LIBs.

A comparative study on the dynamic loading response of heat-treated soft and medium hard rocks

The present study assesses the effects of thermal exposure on the strength, stiffness, and fracture propagation in different rocks subjected to impact loading. The dynamic loading response of soft (synthetic die stone) and medium hard (natural marble) rocks are compared in this study. In the experiments, Split Hopkinson Pressure Bar (SHPB) tests are carried out on the heat-treated rock samples. A high-speed camera records the whole failure process of these specimens, and digital image correlation (DIC) is employed to track the evolution of strain during SHPB compression testing. The variation of mechanical properties of intact rocks with heating temperature is characterized by various thermal zones depending on the transition temperature, which identifies a significant shift in the impact loading response due to heating-induced micro-crack formation. Rocks made of synthetic materials and marble have transition temperatures between 100 and 150 °C and 500 – 650 °C, respectively. In the pre-transition regime, the dynamic compressive strength slightly rises or stays almost unchanged, whereas in the transitional regime, compressive strength falls drastically, and it does not show any vital changes with increasing treatment temperatures in the post-transitional regime. For synthetic rocks and marble, the critical heating durations are 10 and 5 h, respectively, and strength changes are relatively minimal after these heating durations. With increasing temperature (from 30 °C to 300 °C), the fracture pattern of synthetic specimens shifts from shear-tensile-dominated to tensile-dominated followed by shear-dominated. For marble specimens, the fracture pattern shifts from shear-tensile-dominated to shear-dominated as the temperature rises from room temperature to 650 °C.

First-Principles Study on the Mechanical Properties of Gd-Doped BCZT Ceramics

Due to their remarkable piezoelectric characteristics, (BaCa)(ZrTi)O3 (BCZT) ceramics exhibit vast potential for being employed in cutting-edge electromechanical apparatus. Extensive experimental studies have been conducted to better meet the practical needs of BCZT-based materials, focusing on their mechanical performance. However, there is a serious lack of research on the theoretical computational aspects. Here, first-principles calculations were utilized to evaluate the mechanical properties of BCZT-xGd ceramics. The structural models were established using the virtual crystal approximation (VCA) method. The investigated compounds demonstrate structural and mechanical strength, as evidenced by their negative formation energies and adherence to the Born stability criteria. Compared to pure BCZT, the substitution of Gd leads to a significant enhancement in the system’s elasticity and stiffness. The BCZT-0.05Gd with B-site doping demonstrates the highest level of Vicker’s hardness (HV), with the noteworthy observation that the inclusion of Gd concomitantly augments its machinability performance. Upon the incorporation of the Gd element, the anisotropic elasticity in the systems gradually transitions into isotropic elasticity, which favors a more uniform stress distribution and consequently reduces sensitivity to the formation and propagation of microcracks. These results indicate that BCZT-xGd exhibits potential for application in electromechanical systems.

Isotropic Microstrain Relaxation in Ni-Rich Cathodes for Long Cycling Lithium Ion Batteries.

Developing isotropic-dominated microstrain relaxation is a vital step toward the enhancement of cyclic performance and thermal stability for high-energy-density Ni-rich cathodes. Here, a microstructure engineering strategy is employed for synthesizing the elongated primary particles radially aligned Ni-rich cathodes only by regulating the precipitation rates of cations and the distributions of flow field. The as-obtained cathode also exhibits an enlarged lattice distance and highly exposed (003) plane. The high aspect ratio and favorable atomic arrangement of primary particles not only enable isotropic strain relaxation for effectively suppressing microcrack formation and propagation, but also facilitate Li-ion diffusion with greatly reduced Li/Ni mixing. Consequently, it shows obvious superiority in the high-rate, long-cycle life, and thermal stability compared with the conventional counterparts. After modification, an exceptionally long life is achieved with a capacity retention of 90.1% at 1C and 84.3% at 5C after 1500 cycles within 3.0-4.3 V in a 1.5-Ah pouch cell. This work offers a universal strategy to achieve isotropic strain distribution for conveniently enhancing the durability of Ni-rich cathodes.

Phase and formation mechanism of the brittle layer of ASC castable in blast furnace hearth

An Al2O3-SiC C castable BF was dissected and investigated in the present work. The phase composition and microstructure of the brittle layer of castable and the original castable after service were analyzed by XRD, chemical analysis, and SEM-EDS. The results show that a brittle layer of castable with a thickness of about 50–300 mm is found at the bonding surface between the carbon brick and castable on the sidewall of the hearth. Zn enrichment was found in the brittle layer of castable. In the high-temperature environment of the furnace sidewall, the evaporation of water and formation of mullite phase in the process of phase transformation at high-temperature lead to the formation of pores and microcracks in the castable. Stress concentration in the joint area of the castable and carbon brick was produced due to their different expansion coefficient, and the microcracks in the castable gradually develop and expand under the action of thermal stress. In addition, the diffusion of CO, alkali metal steam, and Zinc vapor in the blast furnace hearth and the expansion caused by the reaction product with the original phase of castable accelerated the propagation of microcracks in castable and finally formed the macro-brittle layer.

Evolutions of Microstructure and Impact Toughness of Submerged Arc Weld Metal via Introducing Varied Si for Weathering Bridge Steel

In this paper, the influence of the silicon (Si) content on microstructure and impact property of submerged arc weld metals (WMs) for weathering bridge steel was clarified. Actual submerged arc welding (SAW) was carried out to produce WMs with 0.18 wt.%, 0.36 wt.%, 0.51 wt.%, and 0.60 wt.% of Si. The low temperature impact property of weld metal was detected, and the weld microstructures were characterized by optical microscopy (OM), scanning and transmission electron microscope (SEM and TEM), and electron backscatter diffraction (EBSD). The results indicate that WMs consist of polygon ferrite (PF), acicular ferrite (AF), granular bainitic ferrite (GBF), and martensite/austenite (M/A) constituents in each Si content. With increasing Si, the proportion of PF increased, while AF and GBF coarsened, the area fraction, fM/A, and the mean size, dM/A, of M/A constituents increased, the number of inclusions decreased, but the size increased. Further, the fraction of high-angle grain boundaries (HAGBs) with the misorientation tolerance angles (MTAs) greater than 15° reduced, while the mean equivalent diameter, MEDMTA≥15°, of ferrite grains with HAGBs increased. Accordingly, the impact toughness of WM was degraded from 108.1 J to 39.4 J with the increase in Si. The increase in M/A constituents and inclusions size reduced the critical fracture stress, thereby formation of larger microcracks and cleavage planes occurred. The reduced HAGBs exhibited a low hindering effect on crack propagation, and, consequently, the impact toughness decreased with increasing Si content.

A novel ODS high-entropy composite with improved strength and ductility

Body-centered cubic (BCC) structured high-entropy alloys (HEAs) usually exhibit excellent strength, thermal stability, irradiation tolerance and corrosion resistance but low ductility. In this study, a novel oxide dispersion strengthened (ODS) AlCrFeNi high entropy composite (with BCC-structure reinforced by face-centered cubic (FCC) ODS-CoCrFeNiMn HEA reinforcement was designed and prepared by mechanical alloying and spark plasma sintering. The effects of sintering temperatures (950 °C, 1000 °C, 1050 °C and 1100 °C) and reinforcement amount (5%, 10%,15% and 20%) on microstructure and mechanical properties were investigated. The results showed that the optimal sintering temperature of the ODS AlCrFeNi composites is 1100 °C. The hardness decreases with the increase of the reinforcement content (from 5% to 20%). Compared to the reference alloy (RA) without the addition of reinforcement, the compressive strain of the composite with 15% reinforcement (B–15%F) increases significantly from 8% (RA) to 18%, while the compressive strength increases from 2823 MPa (RA) to 3078 MPa. The B–15%F is composed of matrix, reinforcement and intermediate transition regions. The thickness of transition regions increases with increasing sintering temperature. The matrix consists of fine BCC FeCr phase and B2 NiAl phase, due to spinodal decomposition, together with high-density nanoscale oxides Y4Zr3O12 and a small amount of Y–Al–O. Surprisingly, the crystal structure of reinforcement transforms from FCC into the BCC during SPS. Meanwhile, the mutual diffusion of Al elements in the matrix and Co and Mn elements in the reinforcement also results in similar spinodal decomposition in the reinforcement. The domain size of spinodal decomposition in the reinforcement is coarser than that in the matrix. The reinforcement is mainly distributed on the prior particle boundaries (PPBs), which can effectively reduce/suppress the formation and propagation of micro-cracks along the PPBs, and thus improve both the strength and ductility of the composite.

Deep learning interprets failure process of coal reservoir during CO2-desorption by 3D reconstruction techniques

This work aims to investigate the failure and mechanical behaviors of coal rocks during CO2-desorption. The unconfined and confined compression experiments with certain depletion pressure-stress path and desorption of CO2 are conducted, in which the cracking process is captured by X-ray CT imaging. A deep learning-based coal fine-crack segmentation (CFCS) network with 3D reconstruction technique is proposed to study the cracking behaviors. In addition, the CO2 evolution effects on the failure process and mechanical responses are also analyzed. Results show that the proposed CFCS can effectively segment the crack-CO2-soid structure in coal with the errors all less than 10% at the range of 3.5620%–9.9967%. The failure process of coal samples can be divided into the extremely fast CO2-desorption + microcracks formation stage, fast CO2-desorption + stable microcracks propagation stage and slow stable CO2-desorption + microcracks coalescence to form macrocracks stage. The shear failures of coal samples are caused by the reduction of lateral effective stress due to CO2-desorption and drainage, which increases the anisotropy of coal samples with microcracking behaviors. The proposed method provides better understanding of failure mechanism and mechanical responses of coal reservoirs, which are helpful for the CO2 storage-based resource explorations.

Impact damage reduction of woven composites subject to pulse current

3D orthogonal woven composites are receiving increasing attention with the ever-growing market of composites. A current challenge for these materials’ development is how to improve their damage tolerance in orthogonal and layer-to-layer structures under extreme loads. In this paper, a damage reduction strategy is proposed by combining structural and electromagnetic properties. An integrated experimental platform is designed combining a power system, a drop-testing machine, and data acquisition devices to investigate the effects of pulse current and impact force on woven composites. Experimental results demonstrate that pulse current can effectively reduce delamination damage and residual deformation. A multi-field coupled damage model is developed to analyze the evolutions of temperature, current and damage. Parallel current-carrying carbon fibers that cause yarns to be transversely compressed enhance the mechanical properties. Moreover, the microcrack formation and extrusion deformation in yarns cause the redistribution of local current among carbon fibers, and its interaction with the self-field produces an obvious anti-impact effect. The obtained results reveal the mechanism of damage reduction and provide a potential approach for improving damage tolerance of these composites.

Stereomicroscopic examination of dentinal microcracks formation with Protaper Next, Protaper Gold and E-Flex Gold rotary File Systems

Stereomicroscopic examination of dentinal microcracks formation with Protaper Next, Protaper Gold and E-Flex Gold rotary File Systems

Surface and tribology characterization of diamond-like carbon flakes reinforced oxide film by pulse anodizing

Hard anodizing by using direct current (DC) is one of the conventional approaches to fabricate composite oxide film. However, the formation of high porosity and microcracks lead to significant surface defects that limit its usage in load-bearing application. The present study aims to fabricate composite oxide film by applying pulse current (PC) with incorporating Diamond-like carbon (DLC) flakes to improve surface quality. For the first phase, the growth mechanism was studied by different anodizing time at constant (1 g/L DLC) in the electrolyte. Meanwhile, the mechanical and tribological performance of DLC content in the electrolyte were determined at second phase. Oxide film has been successfully fabricated on the surface of Aluminum alloy AA2017-T4 by anodizing in diluted sulphuric acid (20 wt%) containing DLC flakes. Then, surface morphological and tribological properties were evaluated. Results showed the thickness and growth rate of (1 g/L) DLC flakes reinforced oxide film fabricated by pulse current and direct current, approximately measured at 34.41 μm (growth rate: 0.58 μm/min) and 118.08 μm (growth rate: 1.97 μm/min), respectively. By increasing the DLC content (0–20 g/L) in electrolyte, it enhanced the microhardness and durability of the composite oxide film. However, oxide film fabricated with 5 g/L DLC in electrolyte formed enough transfer layer to reduce friction during the sliding wear process.

Effect of low-vacuum environment on the strength and permeability of cement-based materials

The exposure of cement-based materials to low-vacuum environments will result in fast moisture loss, which will cause phase transformation and pore structure changes, as well as variations in mechanical and permeability performances. In this study, a series of experiments were conducted to investigate the effects of various drying conditions (i.e., standard drying, low-vacuum drying, and oven drying conditions) on the properties of cement-based materials, including paste, mortar, and concrete. The tested properties included the mass loss during the exposure to drying conditions, as well as the compressive strength and permeability after the exposure. Results show that the drying rate of cement-based materials was higher under low-vacuum conditions than oven drying for the first two hours, but slower thereafter. Vacuum drying led to a greater enhancement in compressive strength than oven drying, with the most significant effect identified in cement paste. Additionally, oven drying showed more significant deterioration effects on permeability of cement-based materials compared to vacuum drying. The presence of aggregates in concrete has two contrasting effects on the medium permeability: intricated pathways would decrease permeability, but the microcrack formation at the interface during drying would increase permeability. Conventional two-phase material models based on the results of cement paste permeability tests cannot accurately predict the permeability of mortar or concrete after exposure to drying conditions in this study. The paper is expected to provide a basis for the investigation of the mechanical properties and durability evolution of cement-based materials under a low-vacuum environment.

&lt;b&gt;Influence of a Decarburized Layer on the Formation of Microcracks in Railway Rails: On-site Investigation&lt;/b&gt;

&lt;b&gt;Influence of a Decarburized Layer on the Formation of Microcracks in Railway Rails: On-site Investigation&lt;/b&gt;

The influence of different shaped osteocyte lacunae on microcrack initiation and propagation

BackgroundThe morphology of osteocyte lacunae varies in bones of different ages and bone pathologies. Osteocyte lacunae can cause stress concentration and initiate microcracks. However, the influence of changes in osteocyte lacunar shape on microcrack is unknown. Therefore, the aim of this study was to determine the effects of osteocyte lacunae with different shapes on microcrack initiation and propagation. MethodsOsteon models containing osteocyte lacunae with different shapes were established. The progressive damage analysis method, based on computer simulations, was used to study the evolution of microdamage within the osteon, including the processes of intralaminar and interlaminar microdamage. FindingsModels with larger DoE values can effectively delay or prevent the formation of linear microcracks, which ensures high fracture toughness of cortical bone. It is subjected to stronger mechanical stimulation, making it more sensitive to loads. Models with smaller DoE values increase the load threshold for microdamage generation and reduces its impact on bone mechanical performance, making it less susceptible to microdamage than models with larger DoE values. InterpretationThese findings enhance the limited knowledge of the influence of the lacunar shape on microdamage and contribute to a better understanding of bone biomechanics.

Damage Evolution and Failure Mechanism of Red-Bed Rock under Drying–Wetting Cycles

The rock mass on the bank slope of the Three Gorges Reservoir (TGR) area often suffers from a drying–wetting cycle (DWC). How the DWCs significantly affect the mechanical properties and the stability of the rock mass is worth comprehensively investigating. In this study, the influence of the DWC on the mechanical properties of red-bed rock, mainly purplish red argillaceous siltstone, is explored in detail. Triaxial compression tests were conducted on siltstones that were initially subjected to different DWCs. The results show that DWCs lead to a decrease in mechanical properties such as peak stress, residual stress, and elastic modulus, while an increase in confining pressure (CP) levels leads to an increase in these mechanical properties. Significant correlations are found between the energy parameters and the DWC or the CP. Notably, the total absorption energy (TAE) demonstrates a positive correlation with the CP, but the capability of siltstones to absorb energy shows a negative correlation with DWC. Moreover, the study also examines the damage evolution laws of rocks under different DWCs by proposing a damage variable (DV). Results demonstrate that the effect of the CP on the DV is more pronounced than that of DWCs. A novel brittleness index (BI) was also proposed for estimating rock brittleness through damage strain rate analysis. The effectiveness of the proposed BI is validated by evaluating the effects of DWCs and CP on rock brittleness. Finally, the failure mechanism of the rocks under water–rock interaction is revealed. The weakening of mechanical properties occurs due to the formation of microcracks in response to DWCs. These findings provide valuable guidance for the long-term stability assessment of bank slope engineering projects under DWCs.

Microstructure and radiation tolerance of molybdenum-rich glass composite nuclear waste forms

Non-active molybdenum-rich Ca/Zn glass composite materials (GCM) were prepared on the Vitrification Test Rig (VTR) in the UK. The GCM samples were subjected to Ni and Au ion irradiations to simulate the effects of alpha recoil damage and to determine how the crystallinity characteristics might affect the overall radiation tolerance of high-level wastes (HLW) generated during post-operational clean-out (POCO) operations at the Sellafield nuclear site. The typical crystal phases identified in the GCM were: powellite, ruthenium dioxide, zincochromite, zircon and cerianite. Gadolinium (a proxy for radioactive elements) accumulated mainly in powellite, zircon and cerianite crystals. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and grazing incidence X-ray diffraction (GIXRD) analyses showed that cerianite is a highly radiation-tolerant phase, whereas powellite and zircon became amorphous and swelled considerably after the Ni and Au ion irradiations. This research shows the first evidence of powellite amorphisation under heavy-ion irradiation and suggests that powellite is susceptible to amorphisation by alpha recoils in HLW materials, in contrast to previous findings. The evolution of cooling-related and irradiation-induced microcracks is also described. In HLW glass composite materials, the formation of microcracks is expected in the middle of the canister, where relatively large powellite and zircon crystals appear.

Precipitates evolution and fracture mechanism of the isothermally solidified TLP bonding joints between 316LN stainless steel and IN718 Ni-based alloy

In this study, the dissimilar TLP bonding joints of Inconel 718 nickel-base alloy and AISI 316LN stainless steel free of brittle athermal solidification zone (ASZ) were obtained. It was recognized that a good metallurgical bonding is achieved at the interface between the isothermal solidification zone (ISZ) and diffusion affected zone on the side of IN718 alloy (hereafter referred to as DAZ-IN718), while the interface of ISZ and DAZ-316LN is more sensitive to crack initiation. A lot of precipitates were found in DAZ on both sides near the base metal, which were identified as Cr2B, Cr5B3, Nb3B2, and BN. Moreover, two different sizes of voids form at the ISZ/DAZ-316LN interface. The bigger voids are caused by BN phases, while the smaller voids were identified as Kirkendall voids. The tensile test results indicate that the tensile strength and elongation decrease with increasing bonding time. In-situ tensile tests and digital image correlation (DIC) analysis indicate that there are two factors that cause the fracture of dissimilar TLP bonding joints at the ISZ/DAZ-316LN interface. The first factor is the brittle BN phases and Kirkendall voids at the ISZ/DAZ-316LN interface. During tensile tests, dislocations slide along the slip surface until they are blocked by precipitates in the DAZ, resulting in extremely high-stress concentrations at these locations. Moreover, the second factor is the poor ability of DAZ-316LN to resist plastic deformation. The high degree of strain concentration in DAZ-316LN also promotes the formation of microcracks at the ISZ/DAZ-316LN interface.

Effects of Cu Content on Inclusions and Pitting Corrosion in S32205 Duplex Stainless Steel

Cu is an austenite‐forming element in duplex stainless steel, and the precipitation of Cu‐rich phase can improve the strength of the alloy. However, the role of Cu element in the corrosion resistance of stainless steel is widely controversial. Herein, the effects of different Cu content on the formation of inclusions and the pitting resistance of duplex stainless steel are studied by using potentiodynamic anodic polarization test, immersion corrosion, scanning electron microscope, and electron probe microanalysis. The results show that the pitting potential decreases significantly with the increase of Cu content, which greatly reduces its ability to resist Cl− corrosion. The reason is that Cu increases the driving force for the formation of Al‐containing oxides, which increases the number and size of Al‐containing inclusions formed in the process of melting and solidification. At the same time, Cu promotes the formation of Cr‐depleted zone, which promotes the formation of microcracks and pitting corrosion between steel matrix and inclusions in duplex stainless steel.