Boundary Embrittlement Research Articles

Effect of alloy element on hydrogen-induced grain boundary embrittlement in BCC iron

Effect of alloy element on hydrogen-induced grain boundary embrittlement in BCC iron

Correlations between local chemical fluctuations and grain boundary strength in Ti-Zr-Nb-Ta-Mo refractory multi-principal element alloys

Ti-Zr-Nb-Ta-Mo refractory multi-principal element alloys typically exhibit high yield strength while tensile ductility tends to be poor. In this study, we found that even after solid solution treatment of Ti-Zr-Nb-Ta-Mo alloys, significant change in ductility occur due to the local chemical fluctuations. Ta's dramatic chemical fluctuations cause variations in atomic-scale strain fields, leading to reduced grain boundary strength and brittle fracture. First-principles calculations show that (Ti, Zr)-rich at the grain boundaries increases the delocalization of valence electrons, leading to longer bond lengths and reduced crystal orbital bond index values, thereby causing grain boundary embrittlement. Our findings explore the root causes of brittleness in Ti-Zr-Nb-Ta-Mo alloys at the atomic and electronic scales, providing not only a method to analyze grain boundary embrittlement using bond length, electronic localization function and crystal orbital bond index, but also a theoretical guidance for improving the mechanical properties via grain boundary structure optimization.

Comparative study of intergranular acicular nano-precipitate evolution and their effects on creep failure behavior for BM and HAZ in a novel Fe-Ni-based alloy joint

Intergranular acicular nano-precipitate evolution and their effects on creep failure behavior for the base metal (BM) and heat affected zone (HAZ) in a novel cost-effective Fe-Ni-based alloy joint were investigated comparatively. It was found that the BM tended to initiate acicular nano-precipitates perpendicular to grain boundaries, while their formation was suppressed at the HAZ. After creep at 650 °C under 300 MPa, nano-precipitates composed of γ´-Ni3(Al, Ti), M23C6 and σ phase at BM would expand along the grain boundaries (GBs) with significant GB migration. Creep cavities were more prone to initiate at the precipitate-free zone and the acicular nano-precipitate roots due to the precipitation strengthening reduction and local strength mismatch. Furthermore, grain boundary embrittlement induced by acicular precipitates and the presence of recrystallized grains at the crack tip also promoted the crack propagation. In contrast, the welding thermal cycle contributed to the M23C6 redistribution at the GBs, which decreased element diffusion rate and dragging grain boundary movement, thereby exhibiting better creep crack resistance at the HAZ. The finding provided profound implications for the comprehensive understanding of acicular nano-precipitate evolution in the newly designed Fe-Ni-based alloy joint under service condition.

High‐Throughput First‐Principles Calculations and Machine Learning of Grain Boundary Segregation in Metals

The segregation of solute elements to defects in metals plays a fundamental role for microstructure evolution and the material performance. However, the available computational data are scattered and inconsistent due to the use of different simulation parameters and methods. A high‐throughput study is presented on grain boundary and surface segregation together with their effect on grain boundary embrittlement using a consistent first‐principles methodology. The data are evaluated for most technologically relevant metals including Al, Cu, Fe, Mg, Mo, Nb, Ni, Ta, Ti, and W with the majority of the elements from the periodic table treated as segregating elements. Trends among the solute elements are analyzed and explained in terms of phenomenological models and the computed data are compared to the available literature data. The computed first‐principles data are used for a machine learning investigation, showing the capabilities for extrapolation from first‐principles calculation to the whole periodic table of solutes. The present work allows for comprehensive screening of new alloys with improved interface properties.

Effect of metastable precipitate phase transformation on reheat cracking in SS347H welding

Reheat cracking occurred in the welded heat affected zone (HAZ) of the SS347H tube during high temperature applications of approximately 600 °C. The reheat cracks occur when the inside of the grain is strengthened, causing grain boundary embrittlement. In this study, the intra-granular strengthening phase transformation of the HAZ causing the reheat crack was analyzed using TEM.The hardness of SS 347H HAZ with cracks is measured to be 221HV on average, which is about 25HV higher than 196HV of HAZ without cracks. This intra-granular strengthening is due to the formation of intra-granular precipitate phases. The precipitated phase is ultimately NbC, but undergoes an intermediate phase transformation due to diffusion of Nb and C components. The phase transformation consists of the following steps: “(i) SA (solution annealing), (ii) Zone A, (iii) Zone B, and (iv) NbC precipitation.”This is a process in which Nb/C components diffuse and segregate at the reheating temperature from a solid solution state of Nb/C components, and NbC with a size of several tens of nanometers is precipitated. As metastable phases, Zones A and B exist in the intermediate process of precipitation, which is the strengthening stage where intra-granular hardness is maximized. Zone A and B in the pre-precipitation stage means that a new lattice plane is formed as the segregation of component elements forms coherency with the matrix. The relationship between the base and new lattice planes is confirmed by Double Diffraction in the TEM diffraction diagram.The root cause of reheat cracking in SS 347H HAZ is believed to be the intra-granular strengthening phase caused by zone A and zone B as a metastable phase transformation in an intermediate stage before NbC precipitation. In order to suppress reheat cracking, it is desirable to precipitate stable NbC from the metastable phases of Zones A and Zone B through stabilization heat treatment.

Tailoring the microstructure of a non-equiatomic Fe40Mn27Ni26Co5Cr2 high-entropy alloy via friction stir processing

High-entropy alloys (HEAs) with a face-centered cubic (FCC) lattice exhibit remarkable work-hardenability and high fracture toughness at cryogenic temperatures. However, in some non-equiatomic FCC HEAs, grain boundary melting defects may form during the homogenization process, which may lead to embrittlement of the grain boundaries. To address this issue, in this study, we propose the use of friction stir processing (FSP) to tailor the microstructure of a typical non-equiatomic Fe40Mn27Ni26Co5Cr2 HEA. After FSP, the mechanical properties of the non-equiatomic Fe40Mn27Ni26Co5Cr2 HEA were substantially improved. The study revealed that the improved mechanical properties could be attributed to the intense thermal-shear effect of FSP, which simultaneously eliminated grain boundary segregation and induced discontinuous dynamic recrystallization to refine grain size. Our findings not only shed light on understanding the embrittlement mechanism of homogenization treatment in non-equiatomic FCC HEAs, but also provide a useful route to improve the mechanical properties of FCC HEAs.

Effect of alloying elements on zinc-induced liquid metal embrittlement in steels: A first-principles study

Liquid metal embrittlement (LME) induced by Zn melt in galvanized steels during spot-welding poses a significant challenge to the advancement of the automotive industry. A critical strategy to address this challenge is identifying promising alloying elements capable of mitigating LME. High-throughput first-principles calculations were employed to investigate the segregation behavior of 22 common alloying elements at the Σ5(310) grain boundary of iron and their impact on the grain boundary energy. The results indicate that volume distortion upon doping of the alloying elements predominantly influences their segregation behavior, while the weak bonding between Zn and Fe is responsible for the grain boundary embrittlement. Exploration of solute interaction between Zn and the other alloying element in the Fe grain boundary model reveals that the attraction between metallic doping elements and Zn correlates with both the elastic effects and the magnetic properties of the dopant, while the repulsion between non-metallic elements and Zn is primarily governed by the elastic effects. Separating the grain boundary strengthening energy into mechanical and chemical contributions shows that they correlate linearly with variations in volume of the doping site and changes in bond energy for bonds associated with the doping site at the grain boundary, respectively. Accordingly, the strengthening effect of the doping elements can be estimated by evaluating the induced changes in local volume and bond energies on grain boundaries. Furthermore, potential alloying elements to mitigate the Zn-induced LME are screened and discussed, with B and Mo being the most promising candidates.

In-situ X-ray computed tomography of high-temperature tensile behavior for laser powder bed fused Invar 36 alloy

The laser powder bed fusion (PBF-LB) process provides great potential for additive manufacturing of Invar 36 alloy, which possesses a unique low coefficient of thermal expansion. However, the high-temperature tensile behavior of PBF-LB processed Invar 36 alloy has not been explored, severely restricting its applications. Hence, herein, in-situ X-ray computed tomography (XCT) tensile tests were conducted at 200 °C and 600 °C for PBF-LB processed Invar 36 alloy, and the microstructure after heat treatment, fracture morphology, post-mortem microstructure, and nano-precipitates were observed. The in-situ XCT analysis of damage evolution reveals that the tensile behavior at elevated temperatures is sensitive to numerous closely spaced lack-of-fusion (LOF) pores with relatively large equivalent diameters, distributed in the adjacent melt pools or deposited layers. These LOF pores promote the stress concentration and facilitate rapid crack propagation, resulting in a significantly diminished strength and ductility. In contrast, the small number of metallurgical and keyhole pores, possessing large spacing and relatively high sphericity, have negligible influence on the tensile behavior. Therefore, Invar 36 alloy with only metallurgical and keyhole pores can be considered defect-free, displaying an excellent yield strength of 360.0 MPa and a considerable elongation of 65.0% at 200 °C. However, as the temperature increases to 600 °C, both the yield strength and elongation show a marked decrease to 150.0 MPa and 5.4%, respectively. This weakening is accompanied by the observation of a brittle fracture and the formation of secondary cracks. The degradation in mechanical properties can be attributed to the decomposition of Cr-containing SiP2O7, which leads to the formation of numerous small-sized SiO2 and P2O5 precipitates at 600 °C. These precipitates induce embrittlement of the grain boundaries and contribute to the formation of secondary cracks. The anomalous brittle fracture observed is attributed to the intergranular fracture mode, which results from the low grain boundary energy as well as the decomposition of nanoprecipitates. Additionally, the perpendicular orientation of flatter columnar grain boundaries to the loading direction plays a role in the formation of secondary cracks. This high-temperature mechanical performance of strength, elongation, failure mode, and corresponding microcosmic mechanism advances the understanding and widespread application of PBF-LB processed Invar 36 alloy.

Atomistic study on the microscopic mechanism of grain boundary embrittlement induced by small dense helium bubbles in iron

Atomistic study on the microscopic mechanism of grain boundary embrittlement induced by small dense helium bubbles in iron

Effect of fission defects on tensile strength of U3Si2 Σ5(210) grain boundary from first-principles calculations.

U3Si2 is regarded as a promising accident tolerant fuel (ATF) to replace the commercial fuel UO2; however, grain boundary (GB) embrittlement of U3Si2 caused by irradiation-induced defect segregation remains to be clarified. In this work, the U3Si2 Σ5(210) symmetrically tilted GB is taken as a representative to elucidate the individual effect of xenon (Xe) and vacancy on the tensile strength and failure of GBs using first-principles calculations. Compared with the predicted segregation energies of defects at the most energetically favourable positions of GBs, Si vacancy (VSi) has a much stronger preference to segregate to GBs than that of Xe substitution on the Si sublattice (XeSi). Moreover, the strengthening/embrittlement potency of GBs with single vacancy/Xe is evaluated using the first-principles-based uniaxial tensile test. Although both VSi and XeSi yield a weakening effect on the strength of the U3Si2 Σ5(210) GB, such defective GBs exhibit significantly stronger interface strengths compared to the corresponding defects segregated to the UO2 Σ3(111) GB. The underlying mechanism of strength change of U3Si2 GBs is discussed in terms of charge analysis. Our results can provide a fundamental understanding of the mechanical behavior of irradiated GBs from an atomic perspective.

The effect of nickel on the carbide precipitation behavior in Cr–Mo–V hot-working die steel

The addition of small amount of nickel to hot-working die steel has been found to be beneficial for hardenability and toughness. In this paper, the addition of nickel to Cr–Mo–V hot-working die steel was studied to improve hardenability, and the inherent mechanism of enhancing toughness was further explored. The research results indicate that nickel as a non-carbide-forming element does not alter the type and total amount of carbide precipitation at equilibrium, but it can dissolve into the matrix and form substitutional lattice distortions with Fe atoms, promoting the dispersion of secondary carbides during the tempering process. This mechanism can reduce intergranular precipitation and inhibit grain boundary embrittlement, thereby improving toughness. However, it can also accelerate the precipitation, aggregation, and growth of secondary carbides during prolonged tempering, which deteriorates the high-temperature anti-softening properties of die steels. The role of Ni has been revealed through thermodynamic and kinetic calculations. The results show that Ni significantly enhances the diffusion coefficient of Cr atoms in the face centered cubic (fcc) Fe matrix during tempering and also improves the phase transition driving force of various precipitates. This work provides new insights into the mechanism of Ni in hot-working die steels, and can provide guidance for designing novel hot-working die steel.

Thermal resistance of steels with increased strength properties for pressure vessels of advanced VVER reactors of various designs

The paper considers the results of structural studies and mechanical tests after a long-term thermal exposure of laboratory heats of the metallurgically improved 15Kh2NMFA steel and steel with an increased content of nickel considered as materials for the pressure vessels of advanced VVER-type reactors of various designs. It has been shown that, both for the improved 15Kh2NMFA steel and the high-nickel steel, there are no signs of grain boundary embrittlement after an segregation provoking embrittlement heat treatment. This is explained by the extremely low grain boundary segregation of phosphorus in the initial state caused by a high degree of the structure dispersity as well as by rather a low content of impurities. Besides, no changes have been found in the yield strength value for the improved 15Kh2NMFA steel, which agrees with the structure investigation results. For the high-nickel steel, a tendency towards a minor yield strength decrease by 5 to 10% and a regular reduction of the critical brittleness temperature has been revealed. A decrease in the mechanical properties has been caused by a relatively low temperature of tempering for the high-nickel steel and, accordingly, by the potential occurrence of the structure recovery during long-term thermal exposure, as evidenced by the results of an X-ray diffraction analysis. Despite the structure recovery in the high-nickel steel under the long-term thermal exposure, the main strengthening carbide phases remain stable. Due to this, the yield strength value remains at a relatively high level that exceeds the values for the modern VVER-type vessel steels, even in the case of a thermal exposure much in excess of the expected operating conditions for advanced VVER reactors. The observed decrease of critical brittleness temperature during the long-term thermal exposure contributes to an increase in the steel resistance to brittle fracture.

Segregation-enhanced grain boundary embrittlement of recrystallised tungsten evidenced by site-specific microcantilever fracture

Tungsten stands a prime candidate for plasma-facing applications in fusion reactors, attributed to its capacity to withstand high temperatures and intensive particle fluxes. The operational heat flux, however, can induce recrystallisation of the initial microstructure, increasing the brittle-to-ductile transition temperature. Although such a phenomenon is thought to result from impurity segregation to grain boundaries, direct evidence of impurity-induced grain boundary embrittlement has not yet been reported. Addressing this, our study employs microcantilever testing, coupled with local chemical analysis via atom probe tomography, to unveil the impact of impurity segregation on the fracture toughness of recrystallised tungsten with a purity of 99.98 at.%. The in situ fracture toughness measurements were performed with the notch placed directly at random high-angle grain boundaries, revealing brittle failure regardless of grain boundary misorientation or grain orientation. Notably, both single-crystalline microcantilevers and the as-received material exhibited significant plasticity before failure, with instances without crack propagation. In contrast, recrystallised grain boundaries displayed a fracture toughness of 4.7 ± 0.4 MPa·√m, determined using a linear elastic approach - notably lower than for cleavage plane fracture in tungsten microcantilevers. Local atom probe analysis of the high-angle grain boundaries exposed phosphorous segregation exceeding 2 at.% at the recrystallised interfaces, stemming from recrystallisation. Atomistic simulations confirmed the role of phosphorous in embrittling high-angle grain boundaries in tungsten, while additionally revealing mechanisms of crack-grain boundary interactions and their dependence on phosphorous segregation.

Effect of Cu and Mg co-segregation on the strength of the Al grain boundaries: A molecular dynamics simulation

Molecular dynamics simulations employing machine learning potential functions of Al-Cu-Mg ternary alloys, were performed to elucidate the segregation behavior of Cu and Mg solutes and study their effects on the mechanical properties of Al grain boundaries. The calculation results show that Cu segregation enhances the strength of the Al Σ5 (210)[001] grain boundary, while the Mg instead weakens the Al Σ5 (210)[001] grain boundary. Cu and Mg solutes lead to the Al Σ41 (540)[001] grain boundary embrittlement. Dislocation plays a crucial role in the analysis of plastic deformation. We found that the reinforcing effect of the Cu solutes lies in the dislocation pile-up in plastic deformation, and the stair-rod dislocations inhibited the movement of the Shockley dislocations. The embrittlement cause of Mg solutes is that Mg solutes inhibit dislocation emission and shorten the dislocation nucleation cycle. This work illustrates the relationship between solute strengthening mechanism and dislocation evolution at the atomic scale.

Mechanical Properties Enhancement of Biomedical Au–Cu–Al Shape Memory Alloys by Phase Manipulation

β-type Au-Cu-Al alloy, which is known as Spangold, has been developed for biomedical applications due to its high biocompatibility and X-ray contrast. Despite that, the workability of the polycrystalline functional phases (i.e., L21 parent and martensite phases) is limited by grain boundary embrittlement. To enhance the mechanical properties of the martensite phase (M-phase) alloy, this study aims to introduce a ductile α-fcc phase with various fractions into the M-phase alloy by manipulating the alloy chemical composition. Fundamental analysis such as phase identifications, thermal analysis, microstructure observations, and tensile tests were performed. As a result, the dual-phase M+α-fcc alloys with varying α-fcc phase fractions were successfully practiced. Among all alloys, their lattice parameters and phase transformation temperatures showed merely slight alteration, suggesting that the chemical compositions of the functional M-phases are similar for all alloys. Hence, the variations in the mechanical properties mainly originate from the different α-fcc phase fractions. The tensile test results indicate that both the ultimate tensile strength and fracture strain are promoted as the α-fcc phase fraction increases. The 50Au-38Cu-12Al alloy with the highest α-fcc phase fraction performs the most optimized mechanical properties.

Creep anisotropy reduction and improvement via post-heat treatment in yttrium-added Hastelloy-X fabricated by laser powder bed fusion

The development of laser powder bed fusion (LPBF) has made it possible to produce complex three-dimensional components for high-temperature applications. The LPBF process needs to be refined to address several key factors, such as high-temperature elongation, microstructure heterogeneity, and mechanical anisotropy. Hastelloy-X Ni-based superalloy was used to illustrate these issues in this study. First, 0.046 wt.% of yttrium (Y) was added to Hastelloy-X (HX-y) to prevent grain boundary embrittlement. The second step involves two kinds of post-heat treatments; i) at 850 °C for 2 h (DA) for carbides and ii) solution heat treatment at 1240 °C for 8 h upon aging at 850 °C for 2 h (HTA). The creep properties of the samples were compared at 900 °C/ 80 MPa to understand the effect of Y addition and post heat treatments. The HX-y specimen was strengthened by solid solution and dispersion of Y-rich oxides, together with stabilization of oxygen-based contamination at the grain boundaries. The DA and HTA HX-y specimens had better creep properties than the HX specimens. The HX-y specimen showed superior creep properties to the HX specimen due to the presence of carbides M6C and Cr23C6 inside grains and at grain boundaries. However, carbides remained stable even at high temperatures within grains and at grain boundaries. Nevertheless, the HTA HX-y specimen exhibited superior isotropic creep properties. As a result of grain boundary pinning, serrated grain boundaries prevented grain boundary sliding. In contrast, HX specimens exhibited poor creep properties. This study confirmed that the optimal addition of Y together with the optimization of post-heat treatment drastically enhances the creep properties of materials fabricated by the LPBF process.

Significance of Mn concentration on aging behavior, microstructure evolution and mechanical properties of Fe–Ni–Mn alloys

Fe–Ni–Mn alloys, are ultra-high-strength steels, with suitable ductility and age-hardenability in the solution annealed (SA) condition. However, they suffer from grain boundary embrittlement after aging at ∼480OC, when Mn content goes beyond a specific value. This study was conducted to investigate systematically the effects of the Mn concentration on the microstructural evolution, aging behavior, and the resulting mechanical properties of Fe–10Ni–xMn alloys containing 3.5, 6 and 9Mn wt.%. Samples containing 3.5 and 6 wt.% Mn showed age-hardenability during aging and higher Mn content resulted in a higher hardness value which in turn showed higher strength. The Mn content of about 9 wt.% led to the stability of the austenite phase at room temperature (RT). The results showed that although age-hardening did not occur in this microstructure, a good combination of strength and ductility was achieved. Studying the effects of Mn content in a wide range, resulted in different properties by introducing the proper chemical composition and the preferred heat treatments in this type of alloys, which could be age-hardenable, easy to fabricate, and low-cost.

Effects of trace elements on mechanical properties of the TiZrHfNb high-entropy alloy

Refractory high-entropy alloys have great potential to be utilized as high-temperature materials, and the repeatability and reproducibility of their mechanical properties are critical for practical applications. In this work, nevertheless, we found that the mechanical properties of the TiZrHfNb HEA greatly varied with the content of impurities in the samples even using high-purity raw materials. Specifically, the oxygen impurity is mainly responsible for the increment of the yield stress due to the strong interstitial hardening effect, whilst the ductility deterioration closely associates with the content of metalloid elements B, C, and Si. Our analysis reveals that the metalloid elements not only tend to segregate at grain boundaries but also enhance the aggregation of Zr and Ti. Such co-segregation induced the formation of strong (Zr, Ti)-metalloid bonds, resulting in grain boundary embrittlement and brittle fracture. Our current work demonstrates that the impurity contents in refractory HEAs need to be strictly controlled during production in order to improve their stability of mechanical performance.

Embrittlement mechanism of 460 MPa-Grade nuclear power steel deposited after heat treatment

The effects of post-welded heat treatment on microstructure evolution and embrittlement mechanism of 460 MPa-grade nuclear power steel deposited metal were studied using optical microscope, scanning electron microscope, transmission electron microscope and electron backscattered diffraction (EBSD). The results showed that there was no significant change in the type of deposited metal microstructure before and after heat treatment, except for some acicular ferrite turning into block ferrite. Further studies showed that the dispersive martensite-austenite (M-A) constituent did not worsen the toughness of the deposited metal. However, after heat treatment, the M-A constituent decomposed into ferrite and carbides, where the carbides were precipitated on the grain boundary, resulting in grain boundary embrittlement. The precipitated carbides also increased the dislocation tangle on the grain boundary, which was prone to causing stress concentration and induced crack initiation, resulting in the cleavage fracture. The analysis of the orientation characteristics of the impact fracture profile via EBSD showed that the ferrite laths were coarsened and the high-angle grain boundaries were reduced after heat treatment. Meanwhile, the plastic crack propagation was terminated earlier resulting in the early initiation of cleavage fracture. Moreover, the crack propagation path was smooth and secondary cracks formed during the propagation process. Finally, after heat treatment the inclusions easily fell off and formed micro-voids, which provided favorable channels for crack propagation and accelerated the deterioration of impact toughness. All of these were factors for the significant decline in the toughness of the heat-treated deposited metal.

Helium in Cu-Ag-Fe triphase immiscible nanocomposites: An in situ sequential dual beam TEM study

Helium (He) generated during irradiation is notorious for inducing volumetric swelling and grain boundary embrittlement. It is imperative to design advanced materials with effective capability to manage He. Here, we report a largelydifferent bubble swelling behavior in Cu50Ag50, Cu45Ag45Fe10 and Cu33Ag33Fe34 nanocomposites with similar grain sizes. In situ He pre-injection leads to similar He bubble size and density in all three alloy systems. After subsequent Kr irradiation, the average bubble size and swelling in Cu33Ag33Fe34 is much less than Cu50Ag50. Additionally, while bubble size is much greater along triple junctions and grain boundaries than in matrix in Cu50Ag50, the bubble size at the triple junctions in Cu33Ag33Fe34 is much less, suggesting the vital role of Cu-Ag-Fe triphase triple junctions and phase boundaries in managing He. The present study provides new perspective for improving the radiation tolerance of nanocrystalline alloys by microstructure design.