Low Fracture Toughness Research Articles

Fabrication, microstructure, and mechanical properties of novel three‐layer structured bionic ceramic cutting tools

AbstractCeramic cutting tools have low fracture toughness owing to brittleness. To improve the fracture toughness and flexural strength of ceramic cutting tools, the growth model of SiCw during the sintering process was established by the laminated structure and anisotropy of growth of bionic shells. Furthermore, a theoretical model of sintering parameters and total fracture toughness was achieved. Then, the dispersion process of SiCw was improved to enhance the homogeneity of the mixed powders further. Finally, a novel three‐layer structured bionic ceramic cutting tool was fabricated by hot‐pressing. The effects of sintering temperature, holding time, and sintering pressure on the mechanical properties and microstructure of bionic ceramic cutting tools were investigated. The results showed that the flexural strength (three point bending), fracture toughness (indentation method), and Vickers hardness of the three‐layer structured bionic ceramic cutting tool with excellent dispersed were 735 ± 34 MPa, 5.68 ± 0.06 MPa·m1/2, and 19.66 ± 0.12 GPa, respectively. The microstructure of the fracture revealed that the fracture mechanism of the bionic ceramic cutting tool was a mixture of intergranular fracture, transgranular fracture, whisker pull‐out, whisker fracture, forked crack, crack deflection, crack bridging, zig‐zag crack, and crack termination. The accuracy of the total fracture toughness model was reliable.

Research on the influence of rock fracture toughness of layered formations on the hydraulic fracture propagation at the initial stage

Deep underground rocks exhibit significant layered heterogeneity due to geological evolution and sedimentation. Rock fracture toughness, as one of the important indicators of hydraulic crack propagation, also exhibits heterogeneous distribution. In order to investigate the influence of non-uniform fracture toughness of layered rocks on hydraulic crack propagation, this paper establishes a planar three-dimensional hydraulic crack propagation model. The model is numerically solved using the 3D displacement discontinuity method (3D-DDM) and the finite difference method. The calculation results indicate that when the distribution of the fracture toughness of layered rocks changes from uniform to non-uniform, the fracture morphology develops from a standard circular crack to an elliptical crack. When the difference of the rock fracture toughness between adjacent rock layers and the middle rock layer (pay zone) is large enough, the fracture morphology will develop towards a rectangular shape. In addition, when the fracture toughness of rock layers is non-uniformly distributed, the hydraulic crack not only rapidly expand in the softening layer (rock layer with lower fracture toughness), but also slowly propagate in the strong layer (rock layer with higher fracture toughness). However, the propagation speed in the softening layer is much faster than that in the strong layer. The results indicate that the heterogeneity of rock fracture toughness has an important impact on the morphology, propagation speed, and direction of hydraulic fractures.

Investigating the ductile to brittle transition phenomenon in binary Fe-Ni systems using molecular dynamics simulation

Ductile to brittle transition (DBT) phenomenon is commonly observed in BCC Fe-based systems, which significantly deteriorates their low temperature fracture toughness. Ni is known to be an effective alloying element to improve the low temperature toughness by reducing the ductile to brittle transition temperature (DBTT). The present article investigates the role of Ni in tailoring the DBT curve of BCC Fe by employing molecular dynamics (MD) simulation. Here, we performed mode-I loading of different pre-cracked systems (pure Fe, Fe-2 wt% Ni and Fe-4 wt% Ni alloys) over a temperature range of 1–800 K to construct their DBT curves. The DBT curves are subsequently correlated with the temperature sensitivity of fracture stress, which is estimated using the concept of critical strain energy release rate (GIC). The role of Ni in reducing the fracture stress sensitivity is responsible for decreasing the DBTT as well as the slope of the DBT region. The underlying mechanism is discussed in terms of the crack blunting effect of Ni which effectively increases the GIC and hence the fracture stress, particularly at the low temperature regimes. Besides, the significantly large strain rate imposed in MD simulation is mainly responsible for the higher DBTT values when compared with the experimental results. The present paper tackles this strain rate issue by using an appropriate strain rate sensitivity factor to rescale the simulated DBTT values down to the experimental ones, resulting in a good match between the simulated and experimental DBTT.

A constitutive model of solid propellants considering aging and confinement pressure

AbstractAging during storage and confinement pressure during launch are the two major loading conditions that affect the integrity of solid rocket motors. In comparison to other component materials, solid propellants, as highly filled composites, have a low modulus and fracture toughness and are therefore common sources of failure. The key to improving the integrity of the solid rocket motor is in assessing the health of the solid propellants during storage or launch. To address this issue, we revised the previous model for the progressive damage viscoelasticity of solid propellants to include the effect of chemical aging during storage and the influence of confinement pressure during launch. Specifically, the increase in relaxation time due to aging and the nonequilibrium volume dilatation characteristics under triaxial tension and compression of solid propellants have been considered. To validate the developed model, standard relaxation tests and uniaxial tensile tests on solid propellants without aging were used to calibrate the model parameters. Furthermore, the model was validated by comparison with uniaxial tensile tests under confined pressure after aging and well predicts the aging temperature/time‐dependent mechanical responses of solid propellants. After validation, the developed model was used to study the influence of confinement pressure on microscopic damage evolution and macroscopic volume expansion. Overall, the developed model can be used for the analysis of the integrity of the solid rocket motor after the aging process.

Toughing epoxy nanocomposites with Graphite-Nanocellulose layered framework

Epoxy nanocomposites hold immense promise for advanced high-performance applications in coatings, adhesives, and fiber-reinforced composite matrices. However, their widespread use is hindered by low fracture toughness, attributed to highly cross-linked network structure. Here, this study presents a synergistic manufacturing strategy combining ultrasonic graphite-nanocellulose assembly and bidirectional freeze casting to fabricate epoxy nanocomposites with remarkable layered architectures. Subsequent infiltration and curing with epoxy prepolymer produce nanocomposites exhibiting a fracture toughness 4.67 times higher than pure epoxy. Both experimental and theoretical analyses demonstrate the nanocomposite's superior crack propagation resistance, attributable to a range of synergistic toughening mechanisms. These include robust interfacial bonding and mechanical interlocking, which arise from fiber pull-out/flake fracture effects, effectively impeding crack growth. Additionally, the distinctive layered architecture promotes tortuous crack pathways, optimizing fracture energy dissipation. This work offers pivotal insights into structure–property relationships, paving the way for the design of next-generation nanocomposites with tailored, superior mechanical performance.

Scratch resistance and in-vitro biocompatibility of plasma-sprayed baghdadite coatings reinforced with carbon nanotubes

Hydroxyapatite (HA, Ca10(PO4)6(OH)2) coatings, commonly used for metallic implants, have limitations such as excessive brittleness, low fracture toughness, inadequate wear resistance, and slow osseointegration properties. In contrast, baghdadite (BAG, Ca3ZrSi2O9), a calcium zirconium silicate-based bioceramic, exhibits outstanding biological properties, to promote cell proliferation and differentiation of human osteoblasts along with adequate mechanical strength. In this study, plasma-sprayed HA, BAG, and carbon nanotubes (CNT, 1 wt%, and 2 wt%) reinforced BAG coatings are deposited on titanium (Ti) substrate to enhance its mechanical as well as biological properties. The microhardness values of CNT-reinforced BAG coatings increase due to a decrease in porosity and the retention of CNT inside the BAG matrix. Progressive loading scratch tests are performed, revealing a reduction in wear volume loss from 15.051 mm3 to 12.574 mm3 and scratch rate from 43.262 mm3N−1 m−1 to 36.172 mm3N−1 m−1 with the addition of 2 wt% CNT in BAG coatings. The scratch hardness test results demonstrated that introducing CNT into BAG coating significantly enhances its performance, showing a remarkable 38.7 % improvement. In-vitro cell culture studies indicate excellent cell adhesion, growth, and proliferation of MC3T3-E1 osteoblast cells on the surfaces of the CNT-reinforced BAG coatings.

Parallel Grooved Microstructure Manufacturing on the Surface of Si3N4 Ceramics by Femtosecond Laser.

Machining special microstructures on the surface of silicon nitride ceramics helps improve their service performance. However, the high brittleness and low fracture toughness of silicon nitride ceramics make it extremely difficult to machine microstructures on their surface. In this study, a femtosecond laser is used to machine parallel grooved microstructures on the surface of silicon nitride ceramics. The effects of the laser polarization angle, laser single pulse energy, scanning line spacing, and laser scan numbers on the surface morphology and geometric characteristics of grooved microstructures are researched. It is found that a greater angle between the direction of the scanning path and laser polarization is helpful to obtain a smoother surface. As the single pulse energy increases, debris and irregular surface structures will emerge. Increasing the laser scan line spacing leads to clearer and more defined parallel grooved microstructures. The groove depth increases with the increase in the scan numbers. However, when a certain number of scans is reached, the depth will not increase further. This study serves as a valuable research foundation for the femtosecond laser processing of silicon nitride ceramic materials.

Effects of sealing and prior shot peening treatments on fatigue behavior of sulphuric acid anodic oxidation coated 6082-T6 aluminium alloy

In this study, the effects of sealing and prior shot peening treatments on the fatigue behavior of the sulphuric acid anodic oxidation coated 6082-T6 aluminium alloy were investigated. Experimental results indicate that the preferential fracture of anodic oxidation coatings due to its higher brittleness and lower fracture toughness as well as the stress concentration of near-interface substrate caused by the interface cyclic dislocation accumulation could cause the fatigue failure of anodic oxidation coated specimens. Consequently, the fatigue property of anodic oxidation coated specimens notably decreases compared to as-machined specimens, with a decrease of 26.7 % in the fatigue limit corresponding to 2 × 107 fatigue cycles. Additionally, the sealing treatment introduces numerous cracks that penetrate the coating entirely, diminishing the coating's resistance to cracking under fatigue loading; this alteration changes the fatigue failure mechanism of anodic oxidation coated specimens, ultimately resulting in a decrease of 15.5 % in the fatigue limit of anodic oxidation coated specimens. Despite exacerbating the surface integrity and increasing the surface roughness of anodic oxidation coated specimens, the prior shot peening treatment increases the fatigue limit of anodic oxidation coated specimens by 82.8 % since the affecting layer beneath the interface plays a crucial role in inhibiting the further propagation of coating cracks into the substrate as well as the crack propagation in the substrate beneath the interface.

Biosafety and chemical solubility studies of multiscale crystal-reinforced lithium disilicate glass-ceramics.

Lithium disilicate (Li2 Si2 O5 ) glass-ceramics are currently a more widely used all-ceramic restorative material due to their good mechanical properties and excellent aesthetic properties. However, they have a series of problems such as high brittleness and low fracture toughness, which has become the main bottleneck restricting its development. Therefore, in order to compensate for these shortcomings, we propose to prepare a reinforced glass-ceramics with better mechanical properties and to test the biosafety and chemical solubility of the material. Li2 Si2 O5 whiskers were synthesized by a one-step hydrothermal method, and multi-scale crystal-enhanced Li2 Si2 O5 glass-ceramics were prepared by reaction sintering. The biosafety of multi-scale crystal-reinforced Li2 Si2 O5 glass-ceramics was investigated by in vitro cytotoxicity test, rabbit pyrogen test, mice bone marrow micronucleus test, skin sensitization test, sub-chronic systemic toxicity test, and chronic systemic toxicity test. Additionally, the chemical solubility of multi-scale crystal-reinforced Li2 Si2 O5 glass-ceramics was investigated. The test results showed that the material was non-cytotoxic, non-thermogenic, non-mutagenic, non-sensitizing, and non-systemic. The chemical solubility, determined to be 377 ± 245 μg/cm2 , complied with the ISO 6872 standard for the maximum solubility of ceramic materials. Multi-scale crystal-reinforced Li2 Si2 O5 glass-ceramics' biosafety and chemical solubility met current normative criteria, and they can move on to mechanical property measurements (such as flexural strength test, fatigue life test, friction and wear property study, etc.) and bonding property optimization, which shows promise for future clinical applications.

Hybrid HfC‐SiCN matrix for improved oxidation resistance of carbon fiber–reinforced mini‐composites

AbstractHafnium carbide (HfC) is an ultrahigh‐temperature ceramic with high melting point, chemical stability, hardness, and wear resistance. However, its low fracture toughness and poor thermal shock resistance limit its structural applications in extreme environments. In this study, co‐curing of liquid precursors was carried out prior to complete pyrolysis of individual polymeric precursors. First, HfC preceramic polymer precursor was cured, followed by silicon carbonitride (SiCN) precursor curing on a 2D carbon fiber (CF) cloth using the drop‐coating process. The infiltrated CFs were pyrolyzed at 800°C to achieve CF/HfC‐SiCN ceramic mini‐composites. The cross‐linked precursor‐to‐ceramic yield was observed to be as high as 65% when the procedure was carried out in an inert environment. Although stable up to 1200°C, CF/HfC‐SiCN samples demonstrated susceptibility to oxidation at 1500°C in ambient air. The oxidation of HfC in the presence of SiC leads to the formation of a hafnium‐containing silicate (HfxSiyOz) along with hafnia (HfO2). This compound of silicate and hafnia limits oxygen diffusion better than SiO2 and HfO2 individually. The incorporation of SiCN in HfC ceramic led to improved phase stability compared to a neat HfC system. The results of this study also show that the use of liquid‐phase precursors for HfC and SiCN in the polymer‐infiltrated pyrolysis method is a promising approach to fabricating high‐temperature structural ceramic matrix composites with good oxidation resistance.

Tribological behavior under 1200°C elevated temperature of spark plasma sintered h-BN bulk

h-BN with a layered crystal structure is one type of effective solid lubricants, especially at elevated temperatures. In present work, h-BN with relative density of about 91% was prepared by SPS technology, and its tribological behavior under 1200 °C was investigated. The results exhibited that the friction coefficient of h-BN was relatively stable when paired with Si3N4, which stayed at the range of 0.215–0.305. Low friction coefficient under 1200 °C was mainly thought to be the effect of weaker interlaminar binding of h-BN and easier slip of h-BN plates under higher temperature. Because of low hardness and fracture toughness, wear mechanism of h-BN was mainly brittle fracture, while coexistence of B2O3 and Al5BO9 was beneficial for reducing wear rate.

The Effect of Sintering Temperature on Phase-Related Peculiarities of the Microstructure, Flexural Strength, and Fracture Toughness of Fine-Grained ZrO2–Y2O3–Al2O3–CoO–CeO2–Fe2O3 Ceramics

The lifetime of products made of ceramic materials is related to their mechanical characteristics such as strength, hardness, wear resistance, and fracture toughness. The purpose of this work was to study the effect of sintering temperature on the phase-related peculiarities of the microstructures, causing changes in the flexural strength and fracture toughness of fine-grained ZrO2–Y2O3–Al2O3–CoO–CeO2–Fe2O3 ceramics. Flexural strength and fracture toughness tests were carried out using ceramics sintered in three modes (2 h at 1550 °C, 1580 °C, and 1620 °C in argon), and thorough phase, microstructure, and fractographic analyses were performed. For the ceramic sintered at 1550 °C, a mixed mechanism of intergranular fracture of the t-ZrO2 phase particles and cleavage fracture of the Ce–Al–O phase particles was found, which is reflected in its comparatively low fracture toughness. For the ceramic sintered at 1580 °C, a fracture developed along the boundaries of the aggregates, made of completely recrystallized fine ZrO2 grains with a high bond strength between adjacent t-ZrO2 grains; this corresponds to the highest fracture toughness (5.61 ± 0.24 MPa·m1/2) of this ceramic. For the ceramic sintered at 1620 °C, a transgranular fracture of the t-ZrO2 phase and Ce–Al–O phase particles and crack propagation along the t-ZrO2/Ce–Al–O interface were revealed; this caused a decrease in fracture toughness.

Influence of oxygen functional groups on the crack growth behavior of graphene with edge and middle crack: A molecular dynamics study

In the last decade, graphene has found a wide role and application in nanocomposites and nanodevices for its extraordinary properties. However, due to its low fracture toughness and brittleness, graphene is brittle, and crack growth occurs rapidly. As a result, crack growth behavior and control of its progression are important. In this study, molecular dynamics modeling was used to investigate the crack growth behavior and mechanical properties in functionalized graphene (with middle and edge crack) with different oxide groups (hydroxyl, epoxide, and carboxyl) at different percentages of graphene oxide (0–3 wt. %). The results show that functionalized graphene with oxide groups changes the properties of graphene, effectively reducing the crack growth in crack length (4.9 Å), crack opening displacement (1.4 Å), tensile strength for edge crack (74 to 54 Gpa), and middle crack (128 to 63 Gpa). Also, carboxyl and epoxide groups have a greater reduction in crack growth rate, opening displacement growth rate, and stress concentration. In addition, increase in the number of atoms in oxide groups around the initial crack (with radius 13 Å) reduces crack growth rate, opening displacement growth rate, and stress concentration. This study provides a broader insight into crack growth behavior in functionalized graphene with oxide groups.

Tailoring the Mechanical Properties of X–C–Si (X = Al, Mg) Alloys for Aerospace and Automotive Industries: An ab Initio Study

Silicon carbide (SiC) is a strong and adaptable material that has a good number of uses, including in the automotive industry for power electronics and electric vehicle components; abrasive material used in grinding and cutting; and in the production of refractory materials. These applications are due to the superior properties of SiC, which include high hardness, chemical inertness, and mechanical strength. However, its brittleness, low fracture toughness, as well as relatively high density hinder it from being applied in other areas such as in making bodies of airplanes as well as automobiles. Through careful alloying with lighter materials, this shortcoming can be addressed. Using an ab Initio approach, this study examined the mechanics of magnesium (Mg) and aluminum (Al) alloys with SiC as a potential candidate for the automotive and aerospace industries. Al or Mg atoms were substituted for some of the carbon atoms in the SiC to complete the alloying process. The results showed that the alloys had lower bulk moduli, shear moduli, Youngs moduli, and density compared to those of SiC. However, their ductility and fracture toughness increased. Although the mechanical properties reduced, they were found to be still much better than those of the common alloys for the aerospace. The alloys were thus found to be suitable for the construction of the shell of the airplanes, owing to their superior ductility and fracture toughness.

Effects of carbon/glass nonwoven interleaving veils and their areal density on opening and shearing mode interlaminar fracture toughness of glass epoxy composites

The study explores the impact of Carbon and Glass nonwoven interleaving veils with varying areal density on delamination resistance in unidirectional (UD) Glass epoxy composites under Opening (Mode-I) and Shearing (Mode-II) mode loadings. Commercially available non-woven Carbon and Glass interleaves with areal densities of 15, 20, 25, and 30 g/m2 were used to manufacture the interleaved Glass epoxy laminates. Double Cantilever Beam (DCB) and End Notch Flexural (ENF) specimens were used to determine the mode-I and mode-II fracture energies. The results reveal that, during mode-I initiation, interleaved laminates exhibit a lower interlaminar fracture toughness (IFT) except for the Carbon fiber interleaved laminates of areal density 30 g/m2. However, during propagation, all veils interleaved laminates presented significant improvement in IFT ranging from 5 to 25 % accompanied by substantial fiber bridging. In case of mode-II test, the laminates with Carbon and Glass veils exhibit enhanced IFT (60–182 %) with increasing areal density compared with the non-interleaved Glass laminates. Optical and scanning electron micrographs (SEM) provide evidence of fracture mechanisms involved in the interleaved and non-interleaved composite laminates.

High entropy carbide (ZrNbTiCr)C ceramic composite coating with fine grains fabricated by plasma spraying

Despite the high hardness of high-entropy carbides, their applications are limited by their low fracture toughness. This study reports the successful preparation of two high-entropy carbide (ZrNbTiCr)C composite coatings by reactive plasma spraying. A comparison between the Zr-Nb-Ti-Cr-SiC and ZrO2-Nb2O5-TiO2-Cr2O3-Al-SiC system coatings reveals that the latter displays a lower porosity of 3.04 %, a higher microhardness of 1082 HV0.1 and an improved toughness of 4.16 MPa·m1/2. The presence of fine-grained (ZrNbTiCr)C, α-Al2O3 located between (ZrNbTiCr)C particles and metastable t-ZrO2 phases in the coating, can strengthen and toughen the ZrO2-Nb2O5-TiO2-Cr2O3-Al-SiC composite coating through synergistic effects. The research also outlines the reaction process of the two systems composite powders during the plasma spraying process. Overall, this work introduces a new approach to toughening high-entropy carbides by utilizing their compositional flexibility.

Effects of Grain Boundaries and Surfaces on Electronic and Mechanical Properties of Solid Electrolytes

AbstractExtended defects, including exposed surfaces and grain boundaries (GBs), are critical to the properties of polycrystalline solid electrolytes in all‐solid‐state batteries (ASSBs). These defects can alter the mechanical and electronic properties of solid electrolytes, with direct manifestations in the performance of ASSBs. Here, by building a library of 590 surfaces and grain boundaries of 11 relevant solid electrolytes—including halides, oxides, and sulfides— their electronic, mechanical, and thermodynamic characteristics are linked to the functional properties of polycrystalline solid electrolytes. It is found that the energy required to mechanically “separate” grain boundaries can be significantly lower than in the bulk region of materials, which can trigger preferential cracking of solid electrolyte particles in the grain boundary regions. The brittleness of ceramic solid electrolytes, inferred from the predicted low fracture toughness at the grain boundaries, contributes to their cracking under local pressure imparted by lithium (sodium) penetration in the grain boundaries. Extended defects of solid electrolytes introduce new electronic interfacial states within bandgaps of solid electrolytes. These states alter and possibly increase locally the availability of free electrons and holes in solid electrolytes. Factoring effects arising from extended defects appear crucial to explain electrochemical and mechanical observations in ASSBs.

The development of laser cutting and drilling processes for thick Lithium Niobate wafers for electronic sensor applications

Lithium Niobate (LiNbO3) is a CVD grown synthetic salt having a trigonal crystal structure consisting of niobium, lithium, and oxygen. Due to its variety of mechanical and optical properties, single crystal LiNbO3 finds end use applications in optical waveguides, piezoelectric sensors, optical modulators, and electronic oscillators.LiNbO3 wafers are traditionally mechanically processed using abrasive disks and wire saws, and as a result of the inherently low fracture toughness cut-edge chipping and fracture occur lowering process yields. This study aimed to develop a laser process for the cutting, profiling and drilling of thick whole LiNbO3 wafers, using a femtosecond pulsed 515 nm wavelength source. The stresses induced during processing were controlled through a combination of wafer annealing, crystallographic (012) and (010) cleavage plane orientation. Process generated thermal gradients were controlled using beam manipulation and a water spray liquid flow process. The developed processes substantially reduced material fracture and significantly enhanced cut-edge quality, elevating yields above 80%.

Improving the mechanical properties and oxidation resistance of Nb-16Si-20Ti alloys with B4C addition

Nb-Si-based alloys, as one of the candidate materials which may replace nickel-based alloys for the next generation aero-engines, its low fracture toughness (KQ) and poor oxidation resistance severely limited their application and development. Research has shown that Nb-Si-based alloys with a refined microstructure usually have good fracture toughness, whereas B and C elements are good microstructure refinement elements. So, the effects of B4C on the microstructure evolution and properties of Nb-16Si-20Ti alloys were studied. The results showed that the microstructure of Nb-16Si-20Ti was composed of Nb-based solid solution (Nbss) and Nb3Si phases. With the addition of B4C, the coarse Nb3Si decomposed and transformed into (α, γ)-Nb5Si3 + Nbss eutectic structure, refining the microstructure of the alloys. The room temperature fracture toughness and compressive strength of Nb-16Si-20Ti alloy were improved by adding B4C, among which the fracture toughness of 0.50B4C alloy was up to 11.5 MPa·m1/2, which increased by 98.3% compared with that of the 0B4C alloy. The improvement of the fracture toughness was mainly due to the increment of the ductile Nbss content in the alloy and the refinement of the microstructure. In addition, the addition of an appropriate amount of B4C (≤0.5 at.%) also improved the oxidation resistance of Nb-Si-based alloys.

Fracture behavior of S355J2W steel and weld at low temperature

AbstractThe weather‐resistant steel S355J2W shows potential for use in the bogies of high‐speed trains operating in extremely cold regions. In this study, the mechanical properties and fracture behavior of S355J2W steel and its MAG welding joints were investigated using low‐temperature uniaxial tensile tests, Charpy impact tests, and three‐point bending tests (TPB), which play an important role in the safety and reliability of train operation. The ductility and toughness indices decreased as the temperature decreased from room temperature to −114°C, and the weld is more dangerous and prone to fracture than the base material area. The бs/бb values of WM and HAZ samples slightly increased by 3.7% and 3.8% from 20 to −114°C, respectively. And the ductile–brittle transition temperature (DBTT) of BM, WM, and HAZ were −75.6, −46.1, and −56.6°C, respectively. The fracture micro‐mechanisms were evaluated by analyzing the grain misorientation along the residual cracks using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The high‐angle grain boundaries (GBs) in the base metal were observed to greatly contribute to stopping or deflecting the crack propagation, thereby improving the low‐temperature toughness. In contrast, the cracks easily penetrate the low‐angle GBs of the weld (which made up 38% of the grain boundaries of the welds), thereby decreasing the lower temperature fracture toughness of the weld. The dislocations of S355J2W steel and the weld metal were investigated by TEM analysis.