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.

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.

Hydrogen embrittlement (HE), degradation of the mechanical properties of metals due to the presence of hydrogen, is a persistent problem that has been attracting the attention of the material science community for about fifteen decades. Extensive experimental observations indicate the presence of nanovoids and the increase of free volume at the grain boundaries in hydrogen contaminated metals. This rate-dependent phenomenon motivates theoretical investigations of the underlying mechanisms. Here, a hydrogen enhanced cross-slip (HECS) mechanism in the close vicinity of the grain boundaries is demonstrated by direct molecular dynamics simulations and theoretical calculations. To this end, the interaction of screw dislocations with a variety of symmetric tilt grain boundaries in H-charged and H-free bicrystalline nickel is examined. The presence of segregated H atoms at the grain boundaries induces a stress field in their vicinity, and thus,- the barrier for cross-slip of screw dislocations considerably decreases. The enhanced cross-slip of dislocations facilitates the formation of jogs on bowedout dislocations. These jogs can form vacancies during the glide process. This mechanism of defect production shows nanoscale evidence of enhanced vacancy formation and subsequent increase in the free volume along the grain boundaries in the presence of H.

Fe-Mn-Al-C steels are of interest because of their low weight and the wide tunability of their strength-ductility combinations. Despite their high mechanical strength, these advanced steels, particularly when equipped with a multiphase microstructure, are often prone to hydrogen embrittlement (HE), a phenomenon characterized by the abrupt loss of the materials' load-bearing capacity resulting from H ingress. This motivates us to develop strategies to make these alloys HE resistant. Here, we investigate and quantify the influence of interfacial B segregation on the HE resistance in a typical high-Mn, high-Al lightweight steel with a two-phase austenite-ferrite microstructure. Unlike the commonly reported beneficial role of B segregation in suppressing H-induced damage in single-phase alloys, we reveal that B segregation in the studied steel can lead to both detrimental and beneficial effects. On the one hand, it suppresses H-induced cracking along austenite-ferrite phase boundaries, primarily due to segregation-mediated interface strengthening. On the other hand, B segregation appears to promote H diffusion, increasing H contamination across wider microstructure regions and facilitating HE by H-induced cleavage fracture within ferrite-an alternative damage mode to interface cracking. The ingress and penetration of microstructures by H are closely connected with the kinetics and trapping features of H transport along grain boundaries (GBs, gamma/gamma and alpha/alpha) and phase boundaries (alpha/gamma). We observe that interfacial H diffusion can be enhanced in B-doped steels, due to the repulsion and competition between segregated B and H atoms, as exemplified by ab initio calculations. The interplay between these two effects, resulting from interfacial B segregation, leads to an overall diminished HE resistance in the studied material compared to its B-free counterpart. These findings underscore the mechanistic complexity of B segregation in influencing HE resistance in multiphase steels, highlighting its dual role that must be accounted for in the development of future H-tolerant alloys.

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.

Thermodynamic bulk phase diagrams have become the roadmap used by researchers to identify alloy compositions and process conditions that result in novel materials with tailored properties. Recent experimental studies show that changes in the alloy composition can drive not only transitions in the bulk phases present in a material, but also in the concentration and type of defects they contain. Defect phase diagrams in combination with density functional theory provide a natural route to study these chemically driven defects. Our results reveal, however, that direct application of equilibrium bulk thermodynamics can fail to reproduce experimentally observed defect formation. Therefore, we extend the concept to metastable defect phase diagrams to account for kinetic limitations that prevent the system from reaching equilibrium. We apply this concept to successfully explain the formation of large concentrations of planar defects in supersaturated Fe-Nb solid solutions. We then utilize it to design suitable conditions for synthesis, which we subsequently realized experimentally, successfully validating the formation of the predicted defects in Mg-Al-Ca alloys. The concept offers new avenues for the design of materials performance by tailoring defect structures.

Hydrogen embrittlement in Zr alloy fuel cladding is a primary safety concern for water based nuclear reactors. Here we investigated the stabilisation of planar defects within the forming hydrides by Sn, the primary alloying element of Zircaloy-4 used in the cladding. In order to explain formation of hydrides and planar defects observed in our experiments, we performed atomic-scale ab initio calculations focusing on the solute interactions with generalized stacking faults in hcp $α$-Zr and fcc zirconium hydrides. Our calculations showed that an increase in Sn concentration leads to a stabilisation of stacking faults in both $α$-Zr and hydride phases. However, the solution enthalpy of Sn is lower in the $α$-Zr as compared to the other hydride phases indicative of two competing processes of Sn depletion/enrichment at the Zr hydride/matrix interface. This is corroborated by experimental findings, where Sn is repelled by hydrides and is mostly found trapped at interfaces and planar defects indicative of stacking faults inside the hydride phases. Our systematic investigation enables us to understand the presence and distribution of solutes in the hydride phases, which provides a deeper insight into the microstructural evolution of such alloy's properties during its service lifetime.

The investigation of interface residual stress and interface elastic moduli tensors plays a crucial role in enhancing the conventional theory of elasticity. These considerations allow us to account for size effects and eliminate nonphysical singularities near defects. The primary objective of this study is to comprehensively calculate interface energy, residual stress components, and interface elastic moduli tensors for various interfaces between non-magnetic fcc metals using density functional theory (DFT). We achieve this by performing ab initio DFT calculations to determine the interface energies for different coherent interfaces among these metals. Subsequently, these energies are integrated into the corresponding formulations within surface/interface elasticity theory, enabling us to derive the residual stress and interface elastic constants specific to the target interface. Additionally, we identify and analyze unstable interfaces that exhibit phase transformations, cleavage, and mixing instabilities. Validation is also made with the available laboratory experimental results.

The electrochemical corrosion mechanisms of Mg alloys were extensively studied in previous investigations of different chemical compositions, modified surface states and various electrolyte conditions. However, recent research focused on the active state of Mg dissolution, leading to unresolved effects of secondary phases adjacent to a stable α-solid solution passive layer. The present study investigates the fundamental electrochemical corrosion mechanisms of three different Laves phases with varying phase morphologies and phase fractions in the passive state of Mg-Al-Ca alloys. The microstructure was characterized by (transmission-) electron microscopy and synchrotron-based transmission X-ray microscopy. The electrochemical corrosion resistance was determined with a standard three-electrode setup and advanced in-situ flow cell measurements. A new electrochemical activity sequence (C15>C36>α-Mg>C14) was obtained, as a result of a stable passive layer formation on the α-solid solution. Furthermore, nm-scale Mg-rich precipitates were identified within the Laves phases, which tend to inhibit the corrosion kinetics.

Thermodynamic bulk phase diagrams have become the roadmap used by researchers to identify alloy compositions and process conditions that result in novel materials with tailored microstructures. Recent experimental studies show that changes in the alloy composition can drive not only transitions in the bulk phases present in a material, but also in the concentration and type of defects they contain. Defect phase diagrams in combination with density functional theory provide a natural route to study these chemically driven defects. Our results show, however, that direct application of thermodynamic approaches can fail to reproduce the experimentally observed defect formation. Therefore, we extend the concept to metastable defect phase diagrams to account for kinetic limitations that prevent the system from reaching equilibrium. We successfully applied this concept to explain the formation of large concentrations of planar defects in supersaturated Fe-Nb solid solutions and to identify in a joint study with experiments conditions in Mg-Al-Ca alloys for defect phase occurrence. The concept offers new avenues for designing materials with tailored defect structures.

This paper examines the problem of the fully coupled magneto-electro-elastic (MEE) scattering of SH-waves incident upon a heterogeneous MEE scatterer which is embedded in an unbounded medium. The scatterer consists of a circular core and a circular encapsulator with eccentricity. All three regions: the core, encapsulator, and the surrounding matrix have distinct MEE properties and fully coupled constitutive relations. The generated coupled MEE fields coexist simultaneously in all these regions without resort to any simplifying assumptions. The precise description of the multifunctionality involves the solution of three fully coupled partial differential equations in three different regions. The associated Green’s function equations involve 9 independent components of Green’s functions. The behaviors of the regions are described by the generalized constitutive equations suitable for transversely isotropic MEE properties. Conventionally, wave function approach has been used to study the elastodynamic fields associated with the purely elastic axisymmetric problems; such a treatment encounters serious difficulties in the presence of eccentricity. As a rigorous analytical remedy the dynamic magneto-electro-mechanical equivalent inclusion method (DMEMEIM) will be developed in this work. To this end, the notions of eigenstress, eigenbody-force, eigenelectric, and eigenmagnetic fields will be introduced. As it will be shown, the employment of these notions in conjunction with the eigenfunction space of the pertinent coupled field equations provides a meticulous mathematical framework for the treatment of the proposed problem. The exact analytical formulation for the fully coupled total MEE scattering cross-section is derived. The ramifications of the MEE couplings as well as the wavenumber on the induced scattered fields are considered. As it will be seen, the magnetic field has a substantial effect on the total scattering cross-section. The interfacial stresses are remarkably affected not only by the eccentricity, but also by the magnetic parameters. Moreover, the dynamic electric displacement concentration factor (DEDCF), the dynamic stress concentration factor (DSCF), the electric potential, and the magnetic potential will be examined for different wavenumbers.

Laves phases often form as secondary phases in metallic alloys and have a significant effect on their structural properties. Thus, phase stability studies for these chemically and structurally complex phases in addition to mechanical behavior studies are of great interest. In this work, we use the concept of metastable bulk phase and defect phase diagrams to augment the understanding of the bulk phase and defect phase stability in Laves phases in Mg-based alloys. In this way, we resolve the discrepancy between bulk phase diagrams and experimental observations regarding the formation of Mg-rich C14 and Al-rich C15 Laves phases in MgAlCa alloys at moderate temperatures. Moreover, the effect of the thermodynamic state of alloys on the competition between solute-rich hcp-like planar defects and stoichiometric basal stacking faults is clarified, which determines the brittleness of these alloys. \end{abstract}

Climate change motivates the search for non-carbon-emitting energy generation and storage solutions. Metal hydrides show promising characteristics for this purpose. They can be further stabilized by tailoring the negative pressure of microstructural and structural defects. Using systematic ab initio and atomistic simulations, we demonstrate that an enhancement in the formation of hydrides at the negatively pressurized crack tip region is feasible by increasing the mechanical tensile load on the specimen. The theoretical predictions have been used to reassess and interpret atom probe tomography experiments for a high-strength 7XXX-aluminium alloy that show a substantial enhancement of hydrogen concentration at structural defects near a stress-corrosion crack tip. These results contain important implications for enhancing the capability of metals as H-storage materials.

Advanced lightweight high-strength steels are often compositionally and microstructurally complex. While this complex feature enables the activation of multiple strengthening and strain-hardening mech-anisms, it also leads to a complicated damage behavior, especially in the presence of hydrogen (H). The mechanisms of hydrogen embrittlement (HE) in these steels need to be properly understood for their successful application. Here we focus on a high-Mn (-20 wt.%), high-Al (-9 wt.%) lightweight steel with an austenite (-74 vol.%) and ferrite (-26 vol.%) two-phase microstructure and unravel the interplay of H-related decohesion and localized plasticity and their effects on failure. We find that HE in this alloy is driven by both, H-induced intergranular cracking along austenite-ferrite phase boundaries and H-induced transgranular cracking inside the ferrite. The former phenomenon is attributed to the mechanism of H -enhanced decohesion. For the latter damage behavior, systematic scanning electron microscopy-based characterization reveals that only parts of the transgranular cracks inside ferrite are straight (-52% pro-portion) and along the cleavage plane. Other portions of these transgranular cracks show a distinct devi-ation from the {100} planes at certain stages of crack propagation, which is associated with a mechanism transition from the H-enhanced transgranular decohesion of the ferrite by cleavage to the H-associated localized plasticity occurring near the propagating crack tip. These mechanisms are further discussed based on a detailed comparison to the damage behavior at cryogenic temperatures and on the nanoin-dentation results performed with in-situ H-charging. The findings provide new insights into the under-standing of the interplay between different HE mechanisms operating in high-strength alloys and their synergistic effects on damage evolution.(c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Stacking faults (SFs), similar to other planar defects such as grain boundaries, twin boundaries and heterogeneous interfaces, can be tailored to simultaneously increase strength and ductility of metal matrix composites. However, SFs are rarely observed in Al alloy matrix composites, resulting from the high stacking fault energy (SFE) (166 mJ/m(2)) of pure Al and its weak dependence on most alloying elements. Therefore, strengthening Al alloy matrix composites with SFs has been a longstanding challenge. In the current work, SFs with a density of 2.8 x 10(15) m(-2) are introduced into an ultrafine-grained Al(Mg)-Al3Mg2 composite fabricated by extruding compacted mechanically alloyed and nanostructured Al-11 at.% Mg alloy powder at 703 K. These SFs, together with grain boundaries and solute Mg, increase the yield strength of the as-extruded composite to 700 MPa at a tensile elongation of similar to 4%. We show that the Al(Mg) alloy matrix-Al3Mg2 interfaces enable the formation of the densely arranged SFs. This finding offers a pathway to tailoring high strength Al alloy matrix composites through manipulating SFs by interface engineering.

The urgent need for clean energy coupled with the exceptional promise of hydrogen (H) as a clean fuel is driving development of new metals resistant to hydrogen embrittlement. Experiments on new fcc high entropy alloys present a paradox: these alloys absorb more H than Ni or SS304 (austenitic 304 stainless steel) while being more resistant to embrittlement. Here, a new theory of embrittlement in fcc metals is presented based on the role of H in driving an intrinsic ductile-to-brittle transition at a crack tip. The theory quantitatively predicts the H concentration at which a transition to embrittlement occurs in good agreement with experiments for SS304, SS316L, CoCrNi, CoNiV, CoCrFeNi, and CoCrFeMnNi. The theory rationalizes why CoNiV is the alloy most resistant to embrittlement and why SS316L is more resistant than the high entropy alloys CoCrFeNi and CoCrFeMnNi, which opens a path for the computationally guided discovery of new embrittlement-resistant alloys.

Hydrogen embrittlement (HE) is a ubiquitous and catastrophic mode of fracture in metals. Here, embrittlement is considered as an intrinsic ductile-brittle transition at the crack tip, where H at the crack tip can reduce the stress intensity K-Ic for cleavage below the value K-Ie required for ductile dislocation emission and blunting. Specifically, cleavage fracture along (111) planes in Ni occurs due to the formation of just 3 planar layers of H interstitial occupation at a sharp crack tip. During the cleavage process, the sub-surface H in the upper and lower layers can rapidly diffuse to the fracture surface, lowering the net fracture free energy to K-Ic < K-Ie and enabling brittle fracture. Details of the process are demonstrated using both first-principles density functional theory and a new interatomic potential for Ni-H. Thermodynamic and kinetic models show that the 3 layers of H can form at the crack tip in equilibrium at room temperature with bulk H concentrations and loading rates where H embrittlement in Ni is observed. The kinetic model also predicts the slow crack growth rate in agreement with experiments. The energetics of the mechanism is then shown to apply to cleavage along grain boundaries. All together, these results show that a version of "Hydrogen enhanced decohesion" is the operative embrittlement mechanism in Ni. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Atomistic simulations are a powerful complement to experimental probes for understanding the nanoscale processes associated with the effects of hydrogen (H) on plasticity and fracture that are the underlying causes of hydrogen embrittlement (HE). Current experimental techniques provide quantitative measures of the macroscopic effects of H on plastic flow and fracture but are unable to determine the nanoscale distribution of H atoms in equilibrium nor, more importantly, as a function of time. Conversely, atomistic simulations can provide information on the nanoscale distribution of H around important lattice defects (vacancies, dislocations, grain boundaries, cracks) and probe the mechanical behavior of these defects in the presence and absence of H. Thus, in principle, atomistic simulations can test fundamental theories and conjectures that arise in attempting to rationalize experimental features of HE. However, atomistic simulations have a range of limitations that must be well-recognized. Accurate ab initio simulations are limited to small numbers of atoms and cannot capture necessary time evolution. Molecular simulations using semi-empirical interatomic potentials can handle more atoms and longer time scales, but are limited by accuracy of the potentials and time scales that remain far smaller than experimental time scales. The value of atomistic simulations thus lies primarily in creating targeted simulations to assess the energetics of specific configurations or specific mechanisms of deformation or fracture, along with theoretical models to estimate realistic time scales that remain inaccessible in simulations. Because of their limitations, atomistic simulations may not be definitive, but they nonetheless provide considerable insight by supporting or contradicting conjectures and concepts proposed to rationalize experiments. Here, the above issues are discussed in more detail and several examples, mainly from the work of the current authors, and including previously-unpublished studies on the effects of H on the bowout of the edge dislocations in alpha-Iron and predictions of solute-drag by H in nickel, serve to demonstrate how atomistic simulations can be used to reveal important features of the behavior of H in metals.

A key issue in understanding and effectively managing hydrogen embrittlement in complex alloys is identifying and exploiting the critical role of the various defects involved. A chemo-mechanical model for hydrogen diffusion is developed taking into account stress gradients in the material, as well as microstructural trapping sites such as grain boundaries and dislocations. In particular, the energetic parameters used in this coupled approach are determined from ab initio calculations. Complementary experimental investigations that are presented show that a numerical approach capable of massive scale-bridging up to the macroscale is required. Due to the wide range of length scales accounted for, we apply homogenisation schemes for the hydrogen concentration to reach simulation dimensions comparable to metallurgical process scales. Via a representative volume element approach, an ab initio based scale bridging description of dislocation-induced hydrogen aggregation is easily accessible. When we extend the representative volume approach to also include an analytical approximation for the ab initio based description of grain boundaries, we find conceptual limitations that hinder a quantitative comparison to experimental data in the current stage. Based on this understanding, the development of improved strategies for further efficient scale bridging approaches is foreseen.