Excess vacancies introduced by rapid quenching or irradiation strongly influence phase transformations and solute diffusion in aluminum alloys. The supersaturation of vacancies leads to vacancy clustering, which in turn causes the formation of Frank loops. By correlating transmission electron microscopy (TEM) observations of Frank loop distributions with simulations of vacancy generation and annihilation, we investigate the evolution and spatial distribution of excess vacancies during quenching and natural aging. TEM analysis shows that up to similar to 10-4 site fraction of vacancies are condensed into Frank loops in the grain interior after quenching. A local equilibrium between these loops and the matrix sustains the residual vacancy supersaturation. The interplay between loop nucleation and growth produces distinct spatial variations in loop characteristics: the loop size does not always vary monotonically with distance from the grain boundary, leading to the formation of an "abnormal-size zone," where the Frank-loop density continues to increase while their sizes fall below 20 nm. This zone begins at similar to 1.4 mu m from the grain boundary. These microstructure-dependent features arise from the coupled effects of vacancy diffusion, vacancy absorption at grain boundaries, and the kinetics of loop nucleation and growth, and is therefore highly dependent on the quenching rate. The results demonstrate that the supersaturated vacancy concentration in aluminum is governed by a local equilibrium between the density and size of Frank loops and the surrounding matrix. This study highlights the critical role of localized vacancy clustering in controlling vacancy-concentration evolution and provides fundamental insights into vacancy-mediated processes in aluminum alloys.

To achieve high strength and ductility in steels without relying on high-purity demands or the addition of expensive alloying elements, the present study explored a strategy that combines warm rolling with the formation of nanoscale Cu-rich precipitates. Suppressing phosphorus (P) segregation at prior-austenite grain boundaries (PAGBs) improved strength and ductility, with retained P contributing to strengthening through solid-solution and deformation-induced dislocation interactions. Atom probe tomography results revealed that P and B co-segregated to PAGBs in the hot-rolled steel, whereas an intermediate heat treatment followed by warm rolling substantially reduced P enrichment at PAGBs. This suppression was attributed to the increased dislocation density, modified substructure, and possible interactions associated with Cu-rich clusters. These results indicate that concurrent dislocation multiplication with Cu clustering by warm deformation can alter solute partitioning behavior and suppress P segregation to prior grain boundaries, providing a mechanistic pathway for impurity tolerance in low-purity martensitic steels.

Lightweight, high-strength steels and parts made from them for low-temperature hydrogen storage and transport must have tolerance to hydrogen-rich and cryogenic conditions, as well as resistance to the associated microstructural damage mechanism. Here, we explore boron doping as a strategy to mitigate intergranular failure in austenitic high-Mn, high-Al lightweight steel under hydrogen exposure and at cryogenic temperatures down to -196°C. A tailored heat treatment promotes boron segregation to grain boundaries in the solute state (i.e., without boride formation), as verified by site-specific atom probe tomography (APT). The effects of boron on mechanical performance were evaluated through tensile testing with and without in-situ hydrogen charging, Charpy impact testing at room temperature and down to -196°C, and thermal desorption spectroscopy (TDS). We reveal that boron doping markedly suppresses intergranular cracking, reducing hydrogen-induced ductility loss from 38% to 8% at room temperature and shifts the ductile-to-brittle transition temperature from -62°C to -155°C. DFT calculations indicate that boron enhances GB cohesive strength by 0.84 eV per atom, while hydrogen weakens GB by 0.40 eV per atom. Co-segregation of a B-H pair still results in a net increase of cohesion of 0.47 eV/pair. While enhanced GB cohesion emerges as the dominant mechanism, additional contributions to the improved mechanical performance, including reduced hydrogen solubility and diffusivity at GBs, as well as improved slip transmission across GBs, are also discussed. These findings establish boron doping as a key mechanism-informed approach for enhancing GB cohesion and resistance to hydrogen embrittlement in FCC alloys.

The interaction between boron and hydrogen at grain boundaries has been investigated experimentally and numerically in boron-doped and boron-free martensitic steels using thermal desorption spectrometry (TDS) and ab initio calculations. The calculations show that boron and hydrogen are attracted to grain boundaries but boron can repel hydrogen. This behavior has also been observed using TDS measurements, with the disappearance of one peak when boron is incorporated into the microstructure. Additionally, the microstructure of both steels has been studied through electron backscattered diffraction, electron channeling contrast imaging, synchrotron X-ray measurements, and atom probe tomography. While they have a similar grain size, grain boundary distribution, and dislocation densities, a pronounced boron segregation into PAGBs is observed for boron-doped steels. Then, the equilibrium hydrogen concentration in different trapping sites has been evaluated using the Langmuir-McLean approximation. This thermodynamic model shows that the distribution of hydrogen is identical for all traps when the total hydrogen concentration is low for boron-free steel. However, when it increases, traps of the lowest segregation energies (mostly PAGBs) are firstly saturated, which promotes failure initiation at this defect type. This finding partially explains why PAGBs are the weakest microstructure feature when martensitic steels are exposed to hydrogen-containing environments.

Material sustainability requires energy-efficient and rapid strengthening processes. In alloys, strengthening through diffusion-driven precipitation is limited by the low vacancy concentration, with fewer than one vacancy per 100 billion lattice sites at room temperature in metals such as aluminum and iron under thermodynamic equilibrium. Artificially increasing vacancy concentrations by 1 to 7 orders of magnitude above equilibrium levels through quenching, irradiation, or deformation can significantly accelerate material strengthening. However, measuring vacancy concentrations below 10-7 in alloys and achieving spatial mapping remain challenging. Here, a vacancy-mediated gradient microstructure near grain boundaries is reported and analyzed to investigate diffusion enhancement and the local vacancy population in an Al-Zn system. This method uses cryogenic processes to preserve excess vacancies and halt microstructure evolution, enabling intermittent measurement of compositional fluctuations during ultrafast spinodal decomposition. It allows for the assessment of diffusion enhancement and determination of vacancy supersaturation in sub-micrometer regions. Liquid nitrogen-quenched Al-12.5 at.% Zn alloy shows a vacancy concentration of ≈10-7 at room temperature, dropping to 10-9 after 3 h, with significant spatial variation near grain boundaries. This work addresses gaps in understanding the evolution and distribution of vacancies across various measurement scales, advancing the control of vacancies to enhance the strengthening of engineering alloys.

Reversion kinetics during austenite reversion treatment (ART) significantly influence the microstructure and mechanical properties of medium Mn steels. Unfortunately, commercial diffusion simulation software typically utilizes ferrite phase databases, limiting the precision of reversion kinetics predictions due to the differing dislocation densities between ferrite and martensite phases. This study developed a model to predict reversion kinetics at various temperatures for Fe-6 Mn wt.% steel, incorporating a dislocation pipe diffusion mechanism and temperature-dependent changes in the diffusivity of Mn in martensite. By introducing the diffusivity enhancement parameter (DEP) associated with the high dislocation density of martensite, the model overcomes the limitations of previous DICTRA-based predictions. Predictions incorporating DEP accurately reproduce reversion kinetics at temperatures above 600 ℃, where austenite nucleation minimally influences transformation, demonstrating strong agreement with measured dilatometer data. This model reduces experimental effort, time, and cost, offering practical guidelines for optimizing the reversion process in medium Mn steel.

The accumulation of Fe impurities throughout the lifecycle of secondary scrap-based Al-Si alloys can degrade their mechanical properties. This often necessitates neutralizing the detrimental Fe contamination with Mn addition. However, some reports suggest that specific ranges of Fe concentrations have minimal deleterious effects, and Mn introduction can yield only minor or even adverse impacts. Our working hypothesis is that the tolerances for Fe and Mn can be rationalized based on the underlying microstructure-property relationships. Calculations of non-equilibrium solidification paths, evaluations of precipitation driving force, and estimations of interfacial energy were employed to predict the resulting microstructure characteristics. The findings indicate that in moderately-contaminated alloys with 0.8 wt.% Fe, near-eutectic compositions demonstrate greater Fe tolerance than far-from-eutectic alloys. This is because near-eutectic alloys require less undercooling to promote the preferred formation of the metastable Chinese-script alpha phase and inhibit the undesired platelet-shaped beta and delta phases. Conversely, in severely-contaminated alloys with 2.0 wt.% Fe, far-from-eutectic compositions display better Fe tolerance. The enhanced tolerance is attributed to the higher nucleation rate of the platelet beta phase in far-from-eutectic alloys, leading to a more refined size distribution. Sufficient undercooling of the platelet phases facilitates the formation of the Chinese-script alpha phase in moderately-contaminated alloys and the polyhedral gamma phase in severely-contaminated alloys. Consequently, Mn addition for neutralizing Fe contamination becomes redundant. This study investigates how Si, Fe, Mn, and cooling rate affect the formation of Fe-rich intermetallic phases. Moreover, it provides general recommendations for designing sustainable Al-Si alloys with less compromised properties.

Enabling a hydrogen economy requires the development of materials resistant to hydrogen embrittlement (HE). More than 100 years of research have led to several mechanisms and models describing how hydrogen interacts with lattice defects and leads to mechanical property degradation. However, solutions to protect materials from hydrogen are still scarce. Here, we investigate the role of interstitial solutes in protecting critical crystalline defects sensitive to hydrogen. Ab initio calculations show that boron and carbon in solid solutions at grain boundaries can efficiently prevent hydrogen segregation. We then realized this interface protection concept on martensitic steel, a material strongly prone to HE, by doping the most sensitive interfaces with different concentrations of boron and carbon. These segregations, in addition to stress relaxations, critically reduce the hydrogen ingress by half, leading to an unprecedented resistance against HE. This tailored interstitial segregation strategy can be extended to other metallic materials susceptible to hydrogen-induced interfacial failure.

The microstructure of a two-phase medium manganese steel is decorated by interfaces whose character is defined by crystallography and the misorientation between adjacent grains, which in turn influences elemental segregation and shapes the resulting decorations. This study investigates how adjacent grain and phase boundaries impact a boundary's segregation behavior, with a focus on the competition for carbon (C) enrichment in a laminated ferrite (alpha)- austenite (gamma) microstructure subjected to a series of heat treatments. It was found that semicoherent alpha-gamma Kurdjumov-Sachs (KS) phase boundaries show less carbon segregation than general gamma grain boundaries. Furthermore, when a gamma grain boundary is present at a junction with the phase boundaries, it acts as an extracting agent for C. DFT calculations support these observations, demonstrating that carbon segregation is energetically more favorable at the gamma grain boundary compared to the alpha/gamma phase boundary, due to the more negative segregation energy at the former.

In the current work, we study the role of grain boundary (GB) misorientation-dependent segregation on austenite nucleation in a 50 % cold rolled intercritically annealed 10Mn-0.05C-1.5Al (wt. %) medium Mn steel. During intercritical annealing at 500 degrees C, austenite nucleates predominantly at high-angle GBs. At 600 degrees C, austenite nucleates additionally at low-angle GBs, exhibiting a temperature dependance. Correlative transmission Kikuchi diffraction /atom probe tomography reveals a misorientation-dependent segregation. While GB segregation has been reported to assist austenite nucleation in medium manganese steels (3-12 wt. % Mn), an understanding of the temperature and misorientation dependance is lacking, which is the aim of current work. Since artifacts of the atom probe can cause a broadening of the segregation width, we combined experiments with results from density functional theory (DFT) calculations that reveal that the Mn segregation is not limited to the GB plane but confined to a region in the range of approximately 1 nm. Consequently, GB segregation alters both the GB interface energy and the free energy per unit volume corresponding to the transformation. We estimate the local driving force for austenite nucleation accounting for the segregation width. Based on classical nucleation theory, we clarify the effect of GB segregation on the critical radius and activation energy barrier for confined austenite nucleation at the GB.

The chemical compatibility of metallic materials with thermal transfer/storage media, which often involves aggressive working conditions (i.e., high-temperature, corrosive environments), challenges the safe operations advanced and sustainable energy-related infrastructures. Here, we report the corrosion-oxidation behaviors three multi-component alloys (MCAs) when exposed to a corrosive heavy-liquid metal condition (i.e., molten at 650 degrees C with 10-6 wt% oxygen dissolved). The two compositions, Al0.36Cr0.67FeNi0.98 (HAl11) Al0.27Cr0.71FeNi1.16Nb0.17 (HAl8Nb), show excellent corrosion-resistance via passivating a protective oxide on the alloy surface. Further characterizations of the oxide layers differentiate their corrosion-oxidation mechanisms: a protective Al2O3 oxide layer (with Cr and Fe segregation outmost) formed on HAl11 and a duplex oxide layer (outward growth of FeCr2O4/Cr2O3 layer plus inward growth of an Al2O3 layer) with internal oxidation HAl8Nb. Adding Nb improved the corrosion-oxidation resistance ("Nb-doping effect") by enhancing the outward diffusion of metallic elements and promoting the rapid establishment of an alumina scale. Besides, the presence of AlNbO4, which was predicted by thermodynamics calculation, lying between the spinel and Al2O3 formation, was also confirmed by experimental observations. Our findings advance the mechanistic understanding of MCAs' performances in extreme conditions and provide novel strategies for designing corrosion-resistant alloys geting aggressive application environments.

Infrastructure parts for a hydrogen (H) economy need alloys that are mechanically strong and at the same time resistant to the most dangerous and abrupt type of failure mode, namely, H embrittlement. These two properties are in fundamental conflict, as increasing strength typically amplifies susceptibility to H-related failure. Here, we introduce a new approach to make alloys resistant to H embrittlement, by creating a topological passivation layer (up to a few hundred micrometers thick) near the material surface, the region that is most vulnerable to H ingress and attack. It features instead a layer of ultrafine laminated grains with tens of times higher dislocation density than conventional materials, altering H diffusion, trapping and crack evolution. We tested the concept on a face-centered cubic (FCC) CoCrNi medium entropy model alloy which undergoes severe H-induced intergranular cracking. Two key mechanisms create the topological passivation: First, the high density (up to 1.3e15 m-2) of H-trapping dislocations within the passivating grain layer decelerates H migration by up to about an order of magnitude, delaying H-induced crack initiation at grain boundaries. More importantly, once unavoidable micro-sized H-induced intergranular cracks emerge in the topmost surface region, they become completely arrested by the laminated grains, due to a transition in the embrittlement mechanism from H-enhanced grain boundary decohesion to highly energy-dissipative dislocation-associated cracking. These effects almost completely eliminate H embrittlement, at even doubled yield strength, when exposing the so architected material to harsh H attack. Our approach leverages surface mechanical treatments to tailor metallic microstructures in surface regions most susceptible to H attack, providing a scalable solution to protect alloys from H-induced damage.

In this work, we find that the engineering grain boundary boron (B) segregation can effectively enhance the resistance to hydrogen embrittlement (HE) of a low-carbon lath martensitic steel. The B segregation in prior austenite grain boundaries (PAGBs) strongly suppresses H-induced intergranular cracking. Such a beneficial role, associated with B segregation, is interpreted as the suppression of H-trapping at PAGBs and the enhancement of the atomic bonding strength of grain boundaries. Despite the improvement in HE resistance, an H-induced quasi-cleavage fracture still occurs in the B-doped samples. Further detailed fractography analysis, conducted using electron backscatter diffraction and transmission electron microscopy, reveals that the primary H-induced damage mode in the B-doped samples is {110} plane cracking, resulting from a combined effect of H-enhanced decohesion and H-enhanced localized plasticity. The obtained results advance the understanding of HE in lath martensitic steel with solute segregation and provide a strategy for mitigating such an issue.

Interface segregation affects the microstructure evolution and mechanical properties of alloys, including strength, ductility and damage tolerance. This is particularly true for multiphase high-strength steels containing multiple types of interfaces whose characteristics are key factors influencing the steels' mechanical performance. The different tendencies of solute segregation to different types of interfaces can lead to complex segregation behavior, which needs to be understood. Here, we focus on the segregation behavior of B in a high-Mn, high-Al lightweight steel with a two-phase austenite-ferrite microstructure. We find distinct B segregation at both austenite and ferrite grain boundaries as well as at austenite-ferrite phase boundaries after high temperature annealing (1100 degrees C) and fast quenching. The segregation process is governed by local equilibrium between bulk and interfaces as discussed in terms of thermodynamic and ab initio calculations. Our findings reveal a dependence of B segregation on the interface structure regardless of the adjacent phases, which can be explained in terms of respective interfacial energy in accord with the Gibbs adsorption isotherm. In addition, co-segregation of B and C is observed at both high-angle and low-angle ferrite grain boundaries due to the attractive interaction between the two solutes in the bulk ferrite phase. In contrast, for austenite grain boundaries, C depletion is observed owing to its site competition effect and repulsive interaction with B in austenite. These observations help to guide interface segregation engineering in complex multiphase lightweight steels to improve their mechanical performance.

The mechanical properties of metallic materials often degrade under harsh cryogenic conditions, posing challenges for low-temperature infrastructures1. Here we introduce a dual-scale atomic-ordering nanostructure, characterized by an exceptionally high number density of co-existing subnanoscale short-range ordering (approximately 2.4 × 1026 m-3) and nanoscale long-range ordering (approximately 4.5 × 1025 m-3) domains, within a metallic solid-solution matrix in a CoNiV-based alloy to improve the synergy of strength and ductility at low temperatures. We observe an ordering-induced increase in dislocation shear stress as well as a more rapid dislocation multiplication owing to the dislocation blocking effect of nanoscale long-range ordering and the associated generation of new dislocations. The latter effect also releases stress concentrations at nanoscale long-range-ordered obstacles that otherwise would promote damage initiation and failure. Consequently, the alloy shows a strength-elongation product of 76 GPa % with a yield strength of approximately 1.2 GPa at 87 K, outperforming materials devoid of such ordering hierarchy, containing only short-range ordered or coherent precipitates of a few tens of nanometres. Our results highlight the impact of dual co-existing chemical ordering on the mechanical properties of complex alloys and offer guidelines to control these ordering states to enhance their mechanical performance for cryogenic applications.

Steel production accounts for approximately 8% of all global CO2 emissions, with the primary steelmaking route using iron ores contributing approximately 80% of those emissions, mainly due to the use of fossil-based reductants and fuel. Hydrogen-based reduction of iron oxide is an alternative for primary synthesis. However, to counteract global warming, decarbonization of the steel sector must proceed much faster than the ongoing transition kinetics in primary steelmaking. Insufficient supply of green hydrogen is a particular bottleneck. Realizing a higher fraction of secondary steelmaking is thus gaining momentum as a sustainable alternative to primary production. Steel production from scrap is well established for long products (rails, bars, wire), but there are two main challenges. First, there is not sufficient scrap available to satisfy market needs. Today, only one-third of global steel demand can be met by secondary metallurgy using scrap since many steel products have a lifetime of several decades. However, scrap availability will increase to about two-thirds of total demand by 2050 such that this sector will grow massively in the next decades. Second, scrap is often too contaminated to produce high-performance sheet steels. This is a serious obstacle because advanced products demand explicit low-tolerance specifications for safety-critical and high-strength steels, such as for electric vehicles, energy conversion and grids, high-speed trains, sustainable buildings, and infrastructure. Therefore, we review the metallurgical and microstructural challenges and opportunities for producing high-performance sheet steels via secondary synthesis. Focus is placed on the thermodynamic, kinetic, chemical, and microstructural fundamentals as well as the effects of scrap-related impurities on steel properties.

Aluminum alloys play an important role in circular metallurgy due to their good recyclability and 95% energy gain when made from scrap. Their low density and high strength translate linearly to lower greenhouse gas emissions in transportation, and their excellent corrosion resistance enhances product longevity. The durability of Al alloys stems from the dense barrier oxide film strongly bonded to the surface, preventing further degradation. However, despite decades of research, the individual elemental reactions and their influence on the nanoscale characteristics of the oxide film during corrosion in multicomponent Al alloys remain unresolved questions. Here, we build up a direct correlation between the near-atomistic picture of the corrosion oxide film and the solute reactivity in the aqueous corrosion of a high-strength Al-Zn-Mg-Cu alloy. We reveal the formation of nanocrystalline Al oxide and highlight the solute partitioning between the oxide and the matrix and segregation to the internal interface. The sharp decrease in partitioning content of Mg in the peak-aged alloy emphasizes the impact of heat treatment on the oxide stability and corrosion kinetics. Through H isotopic labelling with deuterium, we provide direct evidence that the oxide acts as a trap for this element, pointing at the essential role of the Al oxide might act as a kinetic barrier in preventing H embrittlement. Our findings advance the mechanistic understanding of further improving the stability of Al oxide, guiding the design of corrosion-resistant alloys for potential applications.

Entropy-related phase stabilization can allow compositionally complex solid solutions of multiple principal elements. The massive mixing approach was originally introduced for metals and has recently been extended to ionic, semiconductor, polymer and low-dimensional materials. Multielement mixing can leverage new types of random, weakly ordered clustering and precipitation states in bulk materials as well as at interfaces and dislocations. The many possible atomic configurations offer opportunities to discover and exploit new functionalities, as well as to create new local symmetry features, ordering phenomena and interstitial configurations. This opens up a huge chemical and structural space in which uncharted phase states, defect chemistries, mechanisms and properties, some previously thought to be mutually exclusive, can be reconciled in one material. Earlier research concentrated on mechanical properties such as strength, toughness, fatigue and ductility. This Review shifts the focus towards multifunctional property profiles, including electronic, electrochemical, mechanical, magnetic, catalytic, hydrogen-related, Invar and caloric characteristics. Disruptive design opportunities lie in combining several of these features, rendering high-entropy materials multifunctional without sacrificing their unique mechanical properties. High-entropy materials leverage phase stabilization through mixing several elements and are primarily known for their mechanical strength and high toughness. This Review explores their use as a platform for multifunctional material design, in which several, even conflicting, properties can be reconciled because of the compositional tolerance inherent in the high-entropy concept, including electronic, magnetic, mechanical, catalytic, thermal expansion and hydrogen storage properties.

The precipitation of grain boundary (GB) κ-carbides critically influences the damage-tolerance ability of high-Mn high-Al lightweight steels, particularly in harsh environments (cryogenic and H environments). The formation and growth behavior of these carbides thus need to be understood. In this work, we use atom probe tomography and four-dimensional scanning transmission electron microscopy to study the formation mechanisms of GB κ-carbides in a Fe-28Mn-8Al-1.3C (wt.%) steel that is aged at 550°C, a temperature at which GB κ-carbide formation is often believed to be delayed or avoided. We observe that the formation of GB κ-carbides results from spinodal decomposition of grain interior (GI) κ-carbides and their further interaction with GB planes rather than from heterogeneous GB nucleation, as has been formerly proposed. However, the subsequent growth of GB κ-carbides shows different kinetics in comparison to GI κ-carbides. The underlying reason and its implications for future microstructure design of such steels are discussed.

Atom probe tomography (APT) is extensively used to measure the local chemistry of materials. Site-specific preparation via a focused ion beam (FIB) is routinely implemented to fabricate needle-shaped specimens with an end radius in the range of 50 nm. This preparation route is sometimes supplemented by transmission Kikuchi diffraction (TKD) to facilitate the positioning of a region of interest sufficiently close to the apex. Irradiating the specimen with energetic electrons and ions can lead to the generation of vacancies and even amorphization of the specimen. These extrinsically created vacancies become crucial for probing the hydrogen or deuterium distribution since they act as a strong trap. Here, we investigated the feasibility of site-specific preparation of a two-phase medium-Mn steel containing austenite (fcc) and ferrite (bcc). Following gaseous charging of APT specimens in deuterium (D2), clusters enriched by up to 35 at.% D, are imaged after Pt deposition, conventional Ga-FIB preparation, and TKD conducted separately. These D-rich clusters are assumed to arise from the agglomeration of vacancies acting as strong traps. By systematically eliminating these preparation-induced damages, we finally introduce a workflow allowing for studying intrinsic traps for H/D inherent to the material.(c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.