Microstructure and Properties in Metals and Alloys, 2nd Volume

The sintering dynamics of nickel nanoparticles (Ni NPs) were investigated through a comprehensive approach that included in situ transmission electron microscopy annealing and molecular dynamics simulations. This study systematically examines the transformation behaviors of Ni NP agglomerates over a temperature spectrum from room temperature to 850 °C. Experimental observations, supported by molecular dynamics simulations, revealed the essential influence of rotational and translational motions of particles, especially at lower temperatures, on sintering outcomes. The effect of the orientation of particles on the sintering process was confirmed, with initial configurations markedly determining sintering efficiency and dynamics. Calculated activation energies from this investigation follow those reported in the literature, confirming surface diffusion as the predominant mechanism driving the sintering of Ni NPs. Full article

In the hydrogen separation membrane, a dense TaTiNbZr amorphous layer was prepared between Pd and Ta to form a Pd/TaTiNbZr/Ta membrane system to prevent the reaction between Pd and Ta at high temperatures. The structural and chemical stability of the Pd/TaTiNbZr/Ta film system at high temperatures were investigated by annealing at 600 °C for 24 h. The high-temperature hydrogen permeation properties of the Pd/TaTiNbZr/Ta film systems were investigated by hydrogen permeation experiments at 600 °C after heat treatment for 6 h. The TaTiNbZr layer was significantly hydrogen-permeable. With the increase in the thickness of the barrier layer, the hydrogen permeability of Pd/TaTiNbZr/Ta decreased, but its hydrogen permeation flux was smaller than that of the highest value of Pd/Ta when it reached the steady state. The presence of the TaTiNbZr layer effectively blocks the interdiffusion between Pd and Ta to form TaPd₃, improving the sustained working ability of the Pd/TaTiNbZr/Ta membrane system. The results show that TaTiNbZr is a candidate material for the intermediate layer to improve the high-temperature stability of metal-composite hydrogen separation membranes. Full article

The primary objective of this paper is to investigate the influence of deformation degree on the microstructure and properties of a Ni-based superalloy. An upsetting experiment was conducted using a free-forging hammer to achieve a deformation degree ranging from 60% to 80%. The impact of the forging deformation degree on the hardness and high-temperature erosion performance was evaluated using the Rockwell hardness tester (HRC) and high-temperature erosion tester, respectively. The experimental results indicate that as the deformation degree increased, the hardness of the forged material progressively increased while the rate of high-temperature erosion gradually decreased. In order to comprehensively study the mechanism behind the variations in forging performance, optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) were employed. The findings reveal that as the deformation degree increased, the presence of small-angle grain boundaries and an increase in grain boundary area contributed to enhanced hardness in the alloy forgings. Furthermore, it was discovered that grain boundaries with twin orientation promoted dynamic recrystallization during deformation, specifically through a discontinuous dynamic recrystallization mechanism. Additionally, the precipitated γ′ phase in the alloy exhibited particle sizes ranging from 40 to 100 nm. This particle size range resulted in a higher critical shear stress value and a more pronounced strengthening effect on the alloy. Full article

The study presented in this paper was undertaken in response to two instances of unexpected blade breakage in the cutting blade used in a Carton Wrap machine (CW). Failure of the Al7075 alloy blade occurred at an indentation during typical operational loading conditions. Subsequent metallographic examinations of the fractured samples confirmed that both cases were attributed to fatigue failure. The main objective of this study is to investigate potential causes of fatigue failure in the CW blade using simplified linear elastic static numerical simulations through Finite Element Analysis (FEA). In this research, we employed the well-established Theory of Critical Distance (TCD), and this case study provided a contextualization at an industrial level. Furthermore, the analysis focused on a second key aspect: proposing a new blade geometry aimed at mitigating the identified issues and eliminating possible causes of failure. In this context, the actual stress concentration at the indentation was determined using the TCD with Line Method (LM). The results from the numerical simulations indicated that the new blade geometry significantly reduced stress concentration, resulting in a risk factor reduction of approximately four compared to the original blade design, even under non-optimal operating conditions. Overall, in conjunction with simple linear static FEA, the proposed numerical approach provided substantial support for designers, especially in fault analysis and when comparing different industrial solutions. Full article

A porous Ni₅₀Mn₂₈Ga₂₂ alloy was produced using powder metallurgy, with NaCl serving as the pore-forming agent. The phase structure, mechanical properties, and magnetic properties of annealed bulk alloys and porous alloys with different pore sizes were analyzed. Vacuum sintering for mixed green billets in a tube furnace was employed, which facilitated the direct evaporation of NaCl, resulting in the formation of porous alloys characterized by a complete sinter neck, uniform pore distribution, and consistent pore size. The study found that porous alloys within this size range exhibit a recoverable shape memory performance of 3.5%, as well as a notable decrease in the critical stress required for martensitic twin shear when compared to that of bulk alloys. Additionally, porous alloys demonstrated a 2% superelastic strain when exposed to 353 K. Notably, under a 1.5 T magnetic field, the porous Ni₅₀Mn₂₈Ga₂₂ alloy with a pore size ranging from 20 to 30 μm exhibited a peak saturation magnetization of 62.60 emu/g and a maximum magnetic entropy of 1.93 J/kg·K. Full article

In the present work, the structure and properties of Ti_nB_n (n = 2–12) clusters were studied, and the microstructure of a Al-Ti-B system was simulated by molecular dynamics to determine the grain refinement mechanism of an Al-Ti-B master alloy in Al alloy. Based on the density functional theory method, the structural optimization and property calculations of Ti_nB_n (n = 2–12) clusters were carried out. The clusters at the lowest energy levels indicated that the Ti and B atoms were prone to form TiB₂ structures, and the TiB₂ structures tended to be on the surface of the clusters. The Ti₁₀B₁₀ cluster was determined to be the most stable structure in the range of n from 2 to 12 by average binding energy and second-order difference energy. The analysis of HOMOs and LUMOs suggested that TiB₂ was the active center in the cluster; the activity of Ti was high, but the activity of B atoms decreased as the cluster size n increased. Meanwhile, the prediction of reaction sites by Fukui function, condensed Fukui function, and condensed dual descriptor identify that Ti atoms were more active than B atoms. Furthermore, TiB₂ structures were found in the Al-Ti-B system simulated by the ab initio molecular dynamics method, and there were Al atoms growing on the Ti atoms in the TiB₂. Based on the above analysis, this study suggests that TiB₂ may be a heterogeneous nucleation center of α-Al. This work helps to further understand the mechanism of Al-Ti-B induced heterogeneous nucleation in Al alloys, which can provide theoretical guidance for related experiments. Full article

To explore the crystalline arrangement of the alloy and the processes involving iron (Fe) precipitation, we employed molecular dynamics simulation with a cooling rate of 2 × 10¹⁰ for Cu_100-XFe_X (where X represents 1%, 3%, 5%, and 10%) alloy. The results reveal that when the Fe content was 1%, Fe atoms consistently remained uniformly distributed as the temperature of the alloy decreased. Further, there was no Fe atom aggregation phenomenon. The crystal structure was identified as an FCC-based Cu crystal, and Fe atoms existed in the matrix in solid solution form. When the Fe content was 3%, Fe atoms tended to aggregate with the decreasing temperature of the alloy. Moreover, the proportion of BCC crystal structure exhibited no obvious changes, and the crystal structure remained FCC-based Cu crystal. When the Fe content was between 5% and 10%, the Fe atoms exhibited obvious aggregation with the decreasing temperature of the alloy. At the same time, the aggregation phenomenon was found to be more significant with a higher Fe content. Fe atom precipitation behaviour can be delineated into three distinct stages. The initial stage involves the gradual accumulation of Fe clusters, characterised by a progressively stable cluster size. This phenomenon arises due to the interplay between atomic attraction and the thermal motion of Fe-Fe atoms. In the second stage, small Fe clusters undergo amalgamation and growth. This growth is facilitated by non-diffusive local structural rearrangements of atoms within the alloy. The third and final stage represents a phase of equilibrium where both the size and quantity of Fe clusters remain essentially constant following the crystallisation of the alloy. Full article

A study was undertaken to investigate the effects of small boron additions on the solidification and microstructure of hypo-eutectic alloyed grey cast iron. The characteristic temperatures upon crystallisation of the treated metal melt were recorded, specifically those concerning small boron addition by using thermal analysis with the ATAS system. Additionally, a standardised wedge test was set to observe any changes in chill performance. The microstructures of thermal analysis samples were analysed using a light optical microscope (LOM) and field emission scanning electron microscopy (FE-SEM) equipped with energy dispersive spectroscopy (EDS), which reveal variations in graphite count number with the addition of boron within observed random and undercooled flake graphite. The effect of boron was estimated by the classical analytical and statistical approach. The solidification behaviour under equilibrium conditions was predicted by a thermodynamic approach using Thermo-Calc. Based on all gathered data, a response model was set with boron for given melt quality and melt treatment using the experimentally determined data. The study reveals that boron as a ferrite and carbide-promoting element under the experimental set shows weak nucleation potential in synergy with other heterogenic nuclei at increased solidification rates, but no considerable changes were observed by the TA samples solidified at slower cooling rates, indicating the loss of the overall inoculation effect. The potential presence of boron nitride as an inoculator for graphite precipitation for a given melt composition and melt treatment was not confirmed in this study. It seems that boron at increased solidification rates can contribute to overall inoculation, but at slower cooling rates these effects are gradually lost. In the last solidification range, an increased boron content could have a carbide forming nature, as is usually expected. The study suggests that boron in traces could affect the microstructure and properties of hypo-eutectic alloyed grey cast iron. Full article

With the goal of developing lightweight Al-Ti-containing multicomponent alloys with excellent mechanical strength, an Al–Ti–Cu–Co alloy with a phase-separated microstructure was prepared. The granulometry of metal particles was reduced using planetary ball milling. The particle size of the metal powders decreased as the ball milling time increased from 5, 7, to 15 h (i.e., 6.6 ± 6.4, 5.1 ± 4.3, and 3.2 ± 2.1 μm, respectively). The reduction in particle size and the dispersion of metal powders promoted enhanced diffusion during the spark plasma sintering process. This led to the micro-phase separation of the (Cu, Co)₂AlTi (L2₁) phase, and the formation of a Cu-rich phase with embedded nanoscale Ti-rich (B2) precipitates. The Al–Ti–Cu–Co alloys prepared using powder metallurgy through the spark plasma sintering exhibited different hardnesses of 684, 710, and 791 HV, respectively, while maintaining a relatively low density of 5.8–5.9 g/cm³ (<6 g/cm³). The mechanical properties were improved due to a decrease in particle size achieved through increased ball milling time, leading to a finer grain size. The L2₁ phase, consisting of (Cu, Co)₂AlTi, is the site of basic hardness performance, and the Cu-rich phase is the mechanical buffer layer between the L2₁ and B2 phases. The finer network structure of the Cu-rich phase also suppresses brittle fracture. Full article

Within the drilling, petrochemical, construction, and related industries, coatings are used to recover components that failed during service or to prevent potential failures. Due to high stresses, such as wear and corrosion, which the materials are subjected to, industries require the application of coating between dissimilar materials, such as carbon steels and stainless steels, through arc welding processes. In this work, an austenitic stainless steel (ER308) coating was applied to an H13 tool steel substrate using the gas metal arc welding (GMAW) robotic process. The heat input during the process was calculated to establish a relationship between the geometry obtained in the coating and its dilution percentage. Furthermore, the evolution of the microstructure of the coating, interface, and substrate was evaluated using XRD and SEM techniques. Notably, the presence of martensite at the interface was observed. The mechanical behavior of the welded assembly was analyzed through Vickers microhardness, and a pin-on-disk wear test was employed to assess its wear resistance. It was found that the dilution percentage is around 18% at high heat input (0.813 kJ/mm) but decreases to about 14% with reduced heat input. Microhardness tests revealed that at the interface, the maximum value is reached at about 625 HV due to the presence of quenched martensite. Moreover, increasing the heat input favors wear resistance. Full article

Porous tantalum (Ta) implants have important clinical application prospects due to their appropriate elastic modulus, and their excellent bone growth and bone conduction ability. However, porous Ta microstructure designs generally mimic titanium (Ti) implants commonly used in the clinic, and there is a lack of research on the influence of the microstructure on the mechanical properties and penetration characteristics, which will greatly affect bone integration performance. This study explored the effects of different microstructure parameters, including the fillet radius of the middle plane and top planes, on the mechanics and permeability properties of porous Ta diamond cells through simulation, and put forward an optimization design with a 0.5 mm midplane fillet radius and 0.3 mm top-plane fillet radius in order to significantly decrease the stress concentration effect and improve permeability. On this basis, the porous Ta structures were prepared by Laser Powder Bed Fusion (LPBF) technology and evaluated before and after microstructural optimization. The elastic modulus and the yield strength were increased by 2.31% and 10.39%, respectively. At the same time, the permeability of the optimized structure was also increased by 8.25%. The optimized microstructure design of porous Ta has important medical application value. Full article

Precipitation behaviors of Ni-Cr-Co-based superalloys with different Ti/Al ratios aged at 750, 800, and 850 °C for up to 10,000 h were investigated using scanning and transmission electron microscopy. The Ti/Al ratio did not significantly affect the diameter of the γ′ phase. However, the volume fraction of the γ′ phase increased with increasing Ti/Al ratios. The η phase was not observed in alloys with a small Ti/Al ratio, whereas it was precipitated after aging at 850 °C for 1000 h in alloys with a Ti/Al ratio greater than 0.80. Higher aging temperatures and higher Ti/Al ratios led to faster η formation kinetics and accelerated the degradation of alloys. It is thought that the increase in hardness with an increase in the Ti/Al ratio is attributed to the effective inhibition of the γ′ phase on dislocation movement due to the increase in the volume fraction of the γ′ phase and an increase in the antiphase boundary (APB) energy. Full article

Austenitic stainless steels are widely used in cryogenic pressure vessels, liquefied natural gas pipelines, and offshore transportation liquefied petroleum gas storage tanks due to their excellent mechanical properties at cryogenic temperatures. To meet the lightweight and economical requirements, pre-strain of austenitic stainless steels was conducted to improve the strength at cryogenic temperatures. The essence of being strengthened by strain (strain strengthening) and the phase-transformation mechanism of austenitic stainless steels at cryogenic temperatures are reviewed in this work. The mechanical properties and microstructure evolution of austenitic stainless steels under different temperatures, types, and strain rates are compared. The phase-transformation mechanism of austenitic stainless steels during strain at cryogenic temperatures and its influence on strength and microstructure evolution are summarized. The constitutive models of strain strengthening at cryogenic temperatures were set to calculate the volume fraction of strain-induced martensite and to predict the mechanical properties of austenitic stainless steels. Full article

An elastoplastic phase field model was employed for simulations to investigate the influence of external loading on the martensitic phase transformation kinetics in steel. The phase field model incorporates external loading and plastic deformation. During the simulation process, the authenticity of the phase field model is ensured by introducing the relevant physical parameters and comparing them with experimental data. During the calculations, loads of various magnitudes and loading conditions were considered. An analysis and discussion were conducted concerning the volume fraction and phase transition temperature during the phase transformation process. The simulation results prominently illustrate the preferential orientation of variants under different loading conditions. This model can be applied to the qualitative phase transition evolution of Fe-Ni alloys, and the crystallographic parameters adhere to the volume expansion effect. It is concluded that uniaxial loading promotes martensitic phase transformation, while triaxial compressive loading inhibits it. From a dynamic perspective, it is demonstrated that external uniaxial loading accelerates the kinetics of martensitic phase transformation, with uniaxial compression being more effective in accelerating the phase transformation process than uniaxial tension. When compared to experimental data, the simulation results provide evidence that under the influence of external loading, the martensitic phase transformation is significantly influenced by the applied load, with the impact of external loading being more significant than that of plastic effects. Full article

In this work, Al-10Sr alloy and Na₂CO₃ were added to LM6 (reference alloy) as hybrid modifiers through ladle metallurgy. The microstructure enhancement was analyzed using an optical microscope (OM). The results were further confirmed with Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDX) spectroscopy. The results showed that Na₂CO₃ and Al-10Sr alloy successfully hybrid modified the sharp needle-like eutectic Si into fibrous eutectic Si. Soft primary Al dendrites were also discovered after the hybrid modification. The formation of β-Fe flakes was suppressed, and α-Fe sludge was transformed into Chinese script morphology. A 2.13% density reduction was recorded. A hardness test was also performed to investigate the mechanical improvement of the hybrid-modified LM6. 2.3% of hardness reduction was recorded in the hybrid-modified LM6 through ladle metallurgy. Brittle cracks were not observed, while ductile pile-ups were the main features that appeared on the indentations of hybrid-modified LM6, indicating a brittle to ductile transformation after hybrid modification of LM6 by Na₂CO₃ and Al-10Sr alloy through ladle metallurgy. Full article

Multiple deformed substructures including dislocation cells, nanotwins (NTs) and martensite were introduced in super austenitic stainless steels (SASSs) by cryogenic rolling (Cryo-R, 77 K/22.1 mJ·m⁻²). With the reduction increasing, a low stacking fault energy (SFE) and increased flow stress led to the activation of secondary slip and the occurrence of NTs and martensite nano-laths, while only dislocation tangles were observed under a heavy reduction by cold-rolling (Cold-R, 293 K/49.2 mJ·m⁻²). The multiple precursors not only possess variable deformation stored energy, but also experience competition between recrystallization and reverse transformation during subsequent annealing, thus contributing to the formation of a heterogeneous structure (HS). The HS, which consists of bimodal-grained austenite and retained martensite simultaneously, showed a higher yield strength (~1032 MPa) and a larger tensile elongation (~9.1%) than the annealed coarse-grained Cold-R sample. The superior strength–ductility and strain hardening originate from the synergistic effects of grain refinement, dislocation and hetero-deformation-induced hardening. Full article

The evolution of the microstructure, the precipitation behavior, and the mechanical performances of Nb-V-Ti micro-alloyed steel prepared under different tempering time were studied using transmission electron microscopy (TEM), X-ray diffraction (XRD), and mechanical tests. It was found that the width of the martensite laths increases with the increasing tempering time. Several kinds of carbides, including M₃C, M₂C, M₂₃C₆, M₇C₃, and MC particles, were identified after tempering. The MC carbides remain stable during tempering, but the transformation behavior of other carbides was identified. The transformation sequence can be summarized as: M₃C → M₂C → M₇C₃ → M₂₃C₆. The strength decreases and the Charpy impact toughness increases gradually with the increase in the tempering time. The ultimate strength (UTS) decreases from 1231 to 896 MPa, and the yield strength (YS) decreases from 1138 to 835 MPa. The −40 °C Charpy impact toughness increases from 20 to 61 J as the tempering time increases from 10 min to 100 h. The evolution of carbides plays an important role in their mechanical performances. Full article

Additive manufacturing (AM) is an attractive set of processes that are being employed lately to process specific materials used in the fabrication of electrical machine components. This is because AM allows for the preservation or enhancement of their magnetic properties, which may be degraded or limited when manufactured using other traditional processes. Soft magnetic materials (SMMs), such as Fe–Si, Fe–Ni, Fe–Co, and soft magnetic composites (SMCs), are suitable materials for electrical machine additive manufacturing components due to their magnetic, thermal, mechanical, and electrical properties. In addition to these, it has been observed in the literature that other alloys, such as soft ferrites, are difficult to process due to their low magnetization and brittleness. However, thanks to additive manufacturing, it is possible to leverage their high electrical resistivity to make them alternative candidates for applications in electrical machine components. It is important to highlight the significant progress in the field of materials science, which has enabled the development of novel materials such as high-entropy alloys (HEAs). These alloys, due to their complex chemical composition, can exhibit soft magnetic properties. The aim of the present work is to provide a critical review of the state-of-the-art SMMs manufactured through different AM technologies. This review covers the influence of these technologies on microstructural changes, mechanical strengths, post-processing, and magnetic parameters such as saturation magnetization (M_S), coercivity (H_C), remanence (B_r), relative permeability (M_r), electrical resistivity (r), and thermal conductivity (k). Full article

This work was focused on studying the possibility of increasing the strength of non-heat-treatable sheet alloy Al2Cu1.5Mn (wt.%) by the joint addition of 1% Mg and 1% Zn. The effect of these elements on the structure and mechanical properties of the new sheet Al2Cu1.5Mn alloy designed for Al₂₀Cu₂Mn₃ dispersoids has been studied by calculations and experimental methods. The obtained data on the phase composition, microstructure, and physical and mechanical properties of the new alloy for different processing routes (including hot rolling, cold rolling, and annealing) have been compared with those for the ternary Mg- and Zn-free alloy. It has been shown that the formation of nanosized Al₂₀Cu₂Mn₃ dispersoids (~7 vol.%) provides for the preservation of the non-recrystallized grain structure after annealing at up to 400 °C (3 h), while Mg and Zn have a positive effect on the strength due to the formation of alloyed aluminum solid solution. As a result, cold-rolled sheets of the Al2Cu1.5Mn1Mg1Zn model alloy showed a substantially higher strength performance after annealing at 400 °C in comparison with the ternary reference alloy. In particular, the UTS is ~360 vs. ~300 MPa, and the YS is 280 vs. 230 MPa. For the example of the Al2Cu1.5Mn1Mg1Zn model alloy, it has been shown that the system is promising for designing new heat-resistant alloys as a sustainable alternative to the 2xxx alloys. The new alloy has an advantage over the commercial alloys (particularly, 2219, 2024, 2014), not only in manufacturability but also in thermal stability. The sheet production cycle for the model alloy is much shorter because the stages of homogenization, solution treatment, and water quenching are excluded. Full article

The present study investigates the creep behavior and deformation mechanism of a third-generation single-crystal Ni-based superalloy at 1170 °C under a range of stress levels. Scanning electron microscopes (SEM) and transmission electron microscopes (TEM) were employed to observe the formation of a rafted γ′ phase, which exhibits a topologically close-packed (TCP) structure. The orientation relationship and elemental composition of the TCP phase and matrix were analyzed to discern their impact on the creep properties of the alloy. The primary deformation mechanism of the examined alloy was identified as dislocation slipping within the γ matrix, accompanied by the climbing of dislocations over the rafted γ′ phase during the initial stage of creep. In the later stages of creep, super-dislocations with Burgers vectors of a<010> and a/2<110> were observed to shear into the γ′ phase, originating from interfacial dislocation networks. Up to the fracture, the sequential activation of dislocation shearing in the primary and secondary slipping systems of the γ′ phase occurs. As a consequence of this alternating dislocation shearing, a twist deformation of the rafted γ′ phase ensued, ultimately contributing to the fracture mechanism observed in the alloy during creep. Full article

In this study, Ti-48Al-3Nb-1.5Ta powders were manufactured from cast bars by the supreme-speed plasma rotating electrode process (SS-PREP) and used to prepare hot isostatically pressed (HIPed) material at 1050–1260 °C with 150 MPa for 4 h. The phase, microstructure and mechanical performance were analyzed by XRD, SEM, electrical universal material testing machine and other methods. The results revealed that the phase constitution changed from γ phase to α₂ phase and then to γ phase with the material changing from as-cast to powders and then to as-HIPed. Compared with the as-cast material, the grain size and element segregation were significantly reduced for both powders and as-HIPed. When the hot isostatic pressing (HIP) temperature was low, the genetic characteristics of the powder microstructure were evident. With the HIP temperature increasing, the homogeneity of the composition and microstructure increased, and the prior particle boundaries (PPBs) gradually disappeared. The elastic moduli of powder and as-HIPed were superior to those of as-cast, which increased with the HIP temperature increasing. The hardness of as-HIPed was lower than that of the powder. The compressive strength, compressive strain, bending strength, and tensile strength of as-HIPed were higher than those of as-cast. With an increase in the HIP temperature, the compressive strength decreased gradually, and the compressive strain first decreased and then increased. Full article

The development of high-efficiency multi-wire submerged arc welding technology in bridge engineering has been limited due to the high mechanical performance standards required. In this paper, weld metal was obtained by welding at three different high heat inputs with the laboratory-developed high-efficiency submerged arc welding wire for bridges. The effect of changing different high heat inputs on the microstructure and impact toughness of high efficiency submerged arc weld metal was systematically investigated by cutting and Charpy V-notch impact tests at −40 °C, using optical microscopy, scanning electron microscopy, energy-dispersive electron spectroscopy, electron backscatter diffraction, and transmission electron microscopy to characterize and analyze. With the increase in heat input from 50 kJ/cm to 100 kJ/cm, the impact absorption energy decreased significantly from 130 J to 38 J. The number of inclusions in the weld metal significantly decreased and the size increased, which led to a significant decrease in the number of inclusions that effectively promote acicular ferrite nucleation, further leading to a decrease in the proportion of acicular ferrite in the weld metal. At the same time, the microstructure of the weld metal was significantly coarsened, the percentage of high-angle grain boundaries was decreased, and the size of martensite/austenite constituents was significantly increased monotonically. The crack initiation energy was reduced by the coarsened martensite/austenite constituents and inclusions, which produced larger local stress concentrations, and the crack propagation was easier due to the coarsened microstructure and lower critical stress for crack instability propagation. The martensite/austenite constituents and inclusions in large sizes worked together to cause premature cleavage fracture of the impact specimen, which significantly deteriorated the impact toughness. The heat input should not exceed 75 kJ/cm for high-efficiency submerged arc welding wires for bridges. Full article