Search

In this study, a Mg-0.3at.%Y alloy was provided for a severe plastic deformation by high pressure torsion (HPT) and subsequent annealing. After the HPT by 5 rotations, nanocrystalline structures with a mean grain size of 0.23 μm having deformed characteristics were obtained. Fully recrystallized microstructures with mean grain sizes ranging from 0.66 μm to 32.7 μm were obtained by subsequent annealing at various temperatures. Room temperature tensile tests revealed that ultrafine grained (UFG; grain sizes smaller than 1 μm) specimen exhibited very high yield strength over 250 MPa but limited ductility. In contrast, good balance of strength and ductility was realized in fine grained specimens with grain sizes around 2~5 μm. Particularly, the yield strength and total tensile elongation of a specimen with a mean grain size of 2.13 μm were 184 MPa and 37.1%, respectively, which were much higher than those of pure Mg having a similar grain size. The significant effects of grain size and Y addition on the mechanical properties were discussed.

International audience; Using a recent linear scaling method that fully accounts for anharmonic thermal vibrations, we calculated the activation free energy for {101¯2} twin boundary migration in magnesium up to 450 K, under both resolved shear stresses and non-glide stresses resulting from c-axis tension. Comparing to direct molecular dynamics data, we show that the harmonic transition state theory unexpectedly overestimates the activation entropy above temperatures as low as 100 K, leading to underestimates of the nucleation time by many orders of magnitude. Whilst a specific interface is studied, anharmonic and non-glide effects are expected to be generally significant in thermally activated interface migration.Anharmonic vibrational effects are shown to affect the kinetics of thermally activated processes even at low temperatures, which in the present case leads to an unexpected decrease in the nucleation rate.

The application of existing semigrand canonical ensemble Monte Carlo algorithms to alloys requires the chemical potential difference values between pairs of atomic species in the alloys as inputs. However, finding the appropriate values for a target system at a desired temperature and bulk composition is a time-consuming task consisting of multiple test runs to determine the chemical potential differences. This problem becomes more serious when dealing with systems containing three or more atomic species, such as medium- and high-entropy alloys, due to the increase of the number of chemical potential differences that need to be calculated. Here we propose a method for sampling from the semigrand canonical ensemble that relies on energy databases acting as an external atomic reservoir at the desired temperature and composition. Given these energy databases, the desired bulk composition and corresponding chemical potential differences can be satisfied in a ``single'' Monte Carlo simulation. Moreover, the energy databases shed light on the underlying energetics of alloys, reflecting their local chemical ordering. We demonstrate the validity of this method using analyses of segregation isotherms at grain boundaries and dislocations in two alloy systems: Fe--1-at.-$%$-Si and NiCoCr medium-entropy alloy. We also discuss the possibly relevant information contained in such energy databases.

Solid-state precipitation is a key heat-treatment strategy for strengthening engineering alloys. Therefore, predicting the precipitation process of localized solute-rich clusters, such as Guinier–Preston (GP) zones, is necessary. We quantitatively evaluated the critical nucleus size and nucleation barrier of GP zones in Al–Cu alloys, illustrating the precipitation preferences of single-layer (GP1) and double-layer (GP2) GP zones. Based on classical nucleation theory using an effective multi-body potential for dilute Al–Cu systems, our model predicted GP1 and GP2 precipitation sequences at various temperatures and Cu concentrations in a manner consistent with experimental observations. The crossover between formation enthalpy curves of GP1 and GP2 with increasing cluster size determines the critical conditions under which GP2 zones can nucleate without prior formation of GP1 zones. This relationship reflects competing interactions within and between clusters. The results illustrate the underlying mechanisms of competing nucleation between zones, and provide guidance for tailoring aging conditions to achieve desired mechanical properties for specific applications.

The s'' precipitate is a primary strengthening precipitate in Mg-Y alloys. It nucleates as localized zigzag- and hexagonal-shaped clusters. Studies on the nucleation kinetics of s'' precipitate are scarce. In this study, we applied the kinetic Monte Carlo (KMC) approach to explore the nucleation kinetics of the s'' precipitates in the Mg-3.0 at.% Y system using a density functional theory-based interatomic potential. The time evolution of nucleation of the s'' precipitates was characterized based on the KMC results. Using these results, we predicted the existence of an optimum temperature for the formation of the s'' precipitates to be 550 K, at which the time necessary for nucleation is the shortest. Moreover, an upper temperature limit, above which the s'' precipitates cannot nucleate, was computed as 700 K. This study explains precipitate nucleation in Mg-Y alloys at an atomic level and provides the theory for obtaining an optimal age-hardening response.

In this study, a chemical order related concept “history-dependent multi-layer generalized stacking fault energy” (HDML-GSFE) was proposed, and it was then demonstrated by employing the recent, very interesting multi-principal element alloy (CoCrNi medium-entropy alloys; MEA) with different chemical short-range order (CSRO) levels using a density functional theory (DFT)-based neural network interatomic potential. To demonstrate the impacts of the history dependency and interlayer (atomic interlayers of the slip system) coupling effect on the GSFE of CSRO MEAs, HDML-GSFEs were computed for different shear deformation pathways of the MEAs with different CSRO levels, such as interlayer multiple-time slipping, twin growth, and γ - e (FCC- HCP) phase transformation. It was demonstrated that multiple-time slipping induces CSRO collapse, leading to local shear softening due to the history dependency of GSFE. In addition, it was found that the slipping of neighboring atomic interlayers is affected by the slipping resulting from the induced CSRO collapse of present interlayers because of the interlayer coupling effect of GSFE. Eventually, by employing a novel kinetic Monte Carlo (kMC) simulation method based on dislocation/disconnection loop nucleation events and using the HDML-GSFE with the history dependency and interlayer coupling effect, we proposed a laminated micro-substructure evolution that involves twinning and γ - e phase transformations subject to a finite shear strain rate and finite temperature.

Segregation of rare earth alloying elements are known to segregate to grain boundaries in Mg and suppress grain boundary sliding via strong chemical bonds. Segregation of Mn, however, has recently been found to enhance grain boundary sliding in Mg and thereby boosting its ductility. Taking the Mg (-2114) twin boundary as an example, we have performed a first-principles comparative study on the segregation and chemical bonding of Y, Zn, and Mn at this boundary. We find that both Y-4d and Mn-3d states hybridize with the Mg-3sp states, while Zn-Mg bonding is characterized by charge transfer only. Strong spin-polarization of Mn pushes the up-spin 3d states down, leading to less anisotropic Mn-Mg bonds with more delocalized charge distribution at the twin boundary, and thus promotes grain boundary plasticity, e.g., grain boundary sliding.

Power laws are omnipresent and actively studied in many scientific fields, including plasticity of materials. Here, we report the power-law statistics in the second and subsequent pop-in magnitudes during load-controlled nanoindentation testing, whereas the first pop-in is characterized by Gaussian-like statistics with a well-defined average value. The transition from Gaussian-like to power-law is due to the change in the deformation mechanism from dislocation nucleation to dislocation network evolution in the sharp-indenter induced abruptly decaying stress and dislocation density fields. Based on nanoindentation testing on the (100) and (111) surfaces of body-centered cubic (BCC) iron and the (100) surface of face-centered cubic (FCC) copper, the scaling exponents of the power laws were determined to be 5.6, 3.9, and 6.4, respectively. These power-law exponents are much higher than those typically observed in micro-pillar plasticity (1.0–1.8), suggesting that the nanoindentation plasticity belongs to a different universality class than the micro-pillar plasticity., Although power laws are observed during nanoindentation and the power-law exponents are estimated to be approximately 1.5-1.6 for face-centered cubic metals, the origin of the exponent remains unclear. In this paper, we show the power-law statistics in pop-in magnitudes and unveil the nature of the exponent.

Vacancy diffusion is fundamental to materials science. Hydrogen atoms bind strongly to vacancies and are often believed to retard vacancy diffusion. Here, we use a potential-of-mean-force method to study the diffusion of vacancies in Cu and Pd. We find H atoms, instead of dragging, enhance the diffusivity of vacancies due to a positive hydrogen Gibbs excess at the saddle-point: that is, the migration saddle attracts more H than the vacancy ground state, characterized by an activation excess ΓHm ≈ 1 H, together with also-positive migration activation volume Ωm and activation entropy Sm. Thus, according to the Gibbs adsorption isotherm generalized to the activation path, a higher μH significantly lowers the migration free-energy barrier. This is verified by ab initio grand canonical Monte Carlo simulations and direct molecular dynamics simulations. This trend is believed to be generic for migrating dislocations, grain boundaries, and so on that also have a higher capacity for attracting H atoms due to a positive activation volume at the migration saddles.

High entropy alloy has attracted extensive attention in nuclear energy due to outstanding irradiation resistance, partially due to sluggish diffusion. The mechanism from a defect-generation perspective, however, has received much less attention. In this paper, the formation of dislocation loops, and migration of interstitials and vacancies in CoNiCrFeMn high entropy alloy under consecutive bombardments were studied by molecular dynamics simulations. Compared to pure Ni, less defects were produced in the CoNiCrFeMn. Only a few small dislocation loops were observed, and the length of dislocation was small. The dislocation loops in Ni matrix were obviously longer and so was the length of dislocation. The interstitial clusters had much smaller mean free path during migration in the CoNiCrFeMn. The mean free path of 10-interstitial clusters in CoNiCrFeMn was reduced over 40 times compared to that in pure Ni. In addition, CoNiCrFeMn had a smaller difference of migration energy between interstitial and vacancy, which increased the opportunity of recombination of defects, therefore, led to less defects and much fewer dislocation loops. Our results provide insights into the mechanism of irradiation resistance in the high entropy alloy and could be useful in material design for irradiation tolerance and accident tolerance materials in nuclear energy.

In this study, a chemical order related concept “history-dependent multi-layer generalized stacking fault energy” (HDML-GSFE) was proposed, and it was then demonstrated by employing the recent, very interesting multi-principal element alloy (CoCrNi medium-entropy alloys; MEA) with different chemical short-range order (CSRO) levels using a density functional theory (DFT)-based neural network interatomic potential. To demonstrate the impacts of the history dependency and interlayer (atomic interlayers of the slip system) coupling effect on the GSFE of CSRO MEAs, HDML-GSFEs were computed for different shear deformation pathways of the MEAs with different CSRO levels, such as interlayer multiple-time slipping, twin growth, and γ − e (FCC-HCP) phase transformation. It was demonstrated that multiple-time slipping induces CSRO collapse, leading to local shear softening due to the history dependency of GSFE. In addition, it was found that the slipping of neighboring atomic interlayers is affected by the slipping resulting from the induced CSRO collapse of present interlayers because of the interlayer coupling effect of GSFE. Eventually, by employing a novel kinetic Monte Carlo (kMC) simulation method based on dislocation/disconnection loop nucleation events and using the HDML-GSFE with the history dependency and interlayer coupling effect, we proposed a laminated micro-substructure evolution that involves twinning and γ − e phase transformations subject to a finite shear strain rate and finite temperature.

The experimental determination of critical stresses for yielding and plastic flow is of upmost importance for the understanding of the atomic-scale mechanical behaviors of nanoscale metals, which is limited in computational molecular dynamics due to their inherent high strain rates and empirical interatomic potentials. Here, we propose an in situ atomic-scale experimental mechanics, the so-called experimental molecular dynamics, which is capable of studying the stress-strain relations with respect to the individual atomic-scale plastic events, including full dislocation slip, deformation twinning and shear band in nanoscale metals with different crystal structures. The local stress, strain and their relationships were obtained based on the analyses of the change in lattice strain gauge, interplanar spacing and gauge length. Using this method, drastic stress drops and strain bursts, the characteristics of individual plastic events, are investigated. The critical stresses for activating the nucleation and growth of atomic-scale defects are obtained. The newly developed experimental molecular dynamics with in situ mechanics approach has the advantage of quasi-static strain rate and no requirement of interatomic potentials over the computational one, which may provide new clues to establish the stress-based criteria for atomic-scale yielding and plastic flow.

The nature of collective diffusion in amorphous solids is in strong contrast with diffusion in crystals. However, the atomic-scale mechanism and kinetics of such collective diffusion remains elusive. Here the free energy landscape of collective diffusion triggered by single atom hopping in a prototypical Cu50Zr50 metallic glass is explored with well-tempered metadynamics which significantly expands the observation timescale of diffusion at atomic-scale. We clarify an experimentally suggested collective atomic diffusion mechanism in the deep glassy state. The collective nature is strongly temperature-dependent. It evolves from string-like motion with only several atoms to be large size collective diffusion at high temperature, which can promote the atomic transport upon glass transition temperature. We also clarify the apparent diffusivity is dominated by the highest free energy barrier of atomic diffusion among widely distributed free energy barriers due to the dynamic heterogeneity of metallic glass, which suggests the sequential nature of diffusion is a proper assumption to the metallic glasses with dynamic heterogeneity. The temperature and pressure dependence of diffusion free energy landscape are further quantified with activation entropy, (19.6 ± 2.5)kB, and activation volume, (7.9 ± 3.4) A3, which agree quantitatively with experiments. Laboratory timescale simulations of atomic diffusion brings physical insights into the unique atomic motion mechanism in non-crystalline materials.

Mass transport driven by temperature gradient is commonly seen in fluids. However, here we demonstrate that when drawing a cold nano-tip off a hot solid substrate, thermomigration can be so rampant that it can be exploited for producing single-crystalline aluminum, copper, silver and tin nanowires. This demonstrates that in nanoscale objects, solids can mimic liquids in rapid morphological changes, by virtue of fast surface diffusion across short distances. During uniform growth, a thin neck-shaped ligament containing a grain boundary (GB) usually forms between the hot and the cold ends, sustaining an extremely high temperature gradient that should have driven even larger mass flux, if not counteracted by the relative sluggishness of plating into the GB and the resulting back stress. This GB-containing ligament is quite robust and can adapt to varying drawing directions and velocities, imparting good controllability to the nanowire growth in a manner akin to Czochralski crystal growth., Inspired by the traditional Czochralski method of growing single crystal from liquid, the authors demonstrated that at nanoscale, metallic nanowires including Al, Ag, Cu and Sn can be controllably grown from the surface of a hot solid, by simply drawing a cold tip back after touching.

Our first-principles calculations demonstrate that hydrogen is more stable in carbon supersaturated martensite than in α‑iron, due to the carbon-induced tetragonality in martensite lattice. The trapped hydrogen leads to remarkable decohesion between (110) planes both inside the martensite and along the martensite/ferrite interface, with the former being more significant than the latter. This decohesion can explain recent precise observations that in martensite/ferrite dual-phase steels the hydrogen-promoted crack was initiated in the martensite region and that in lath martensite steel it propagated not on lath boundaries but showed quasi-cleavage feature along (110) planes at very high hydrogen concentration.

The formation of hydrides challenges the integrity of zirconium (Zr) fuel cladding in nuclear reactors. The dynamics of hydride precipitation are complex. Especially, the formation of the butterfly or bird-nest configurations of dislocation structures around hydride is rather intriguing. By in-situ transmission electron microscopy experiments and density functional theory simulations, it is discovered that hydride growth is a hybrid displacive-diffusive process, which is regulated by intermittent dislocation emissions. A strong tensile stress field around the hydride tip increases the solubility of hydrogen in Zr matrix, which prevents hydride growth. Punching-out dislocations reduces the tensile stress surrounding the hydride, decreases hydrogen solubility, reboots the hydride precipitation and accelerates the growth of the hydride. The emission of dislocations mediates hydride growth, and finally, the consecutively emitted dislocations evolve into a butterfly or bird-nest configuration around the hydride.

Multi-principal-element alloys—so-called high-entropy alloys (HEAs)—contain multiple equiatomic or nearly equiatomic elements and are attracting increasing attention in basic and applied research because of their superior mechanical properties. Recently, the existence of chemical short-range order (CSRO)/local chemical ordering in HEAs has been experimentally confirmed and its effects on the mechanical properties of HEAs have been studied. However, the formation process and kinetics of CSRO have not yet been fully clarified. In the present study, we propose a simulation framework to study CSRO formation kinetics based on Monte Carlo and kinetic Monte Carlo simulation methods. Applying the simulation framework to quinary face-centered-cubic multi-principal-element alloys described by Lennard–Jones interatomic model potentials, we obtained the temperature-dependent CSRO formation kinetics via vacancy diffusion and constructed a time–temperature–CSRO degree diagram, which enables the CSRO of HEAs to be tailored via thermal processing.

FeN alloy film is a promising spintronic material with the theoretically ultra-strong magnetism (saturation magnetization MS and magnetic anisotropy Keff) and high spin polarization, which relies on the degree of N ordering interstice occupancy (S). However, due to the high activation energy for N ordering, the S value of an actual FeN film is mostly lower than 35% and this restricts the achievable magnetism and transportation property. Thus, the construction of a FeN alloy film with well-controlled magnetism and efficient electronic transportation remains a long-standing challenge. Here, we tackle the problem by strain engineering. Using an Fe/Cr underlayer, we introduced a considerable epitaxial strain in the FeN lattice. The strain is proven to effectively promote the S value to over 60%, resulting in remarkable enhancement of MS value from 2.18T to 2.81T (30% increment) and effective tunability of Keff value ranging 1.3∼2.2 × 106 J/m3. Besides, the matched energy band symmetry (Δ5) between Cr and Fe16N2 facilitates the efficient electronic transportation for spintronic applications. By simulating interstice distribution with the first-principles calculations, the lattice strain is found to decrease the activation energy for N interstitial migration, which serves as a thermodynamic driving force for the magnetism tunability.

We report a first-principles study on how H2 molecules emerge in a nanovoid in α-iron. In a 9-vacancy void, after the walls are decorated with 24 H atoms, only H-dimers are allowed to be adsorbed on the walls; whereas in a spherical 27-vacancy void, H2 molecules start to appear in the center of the void after the walls are saturated by 54 H atoms. The bubble pressure can reach 3.5 GPa, comparable to the measured H2 pressure in blisters at the micrometer scale. The H-saturated nanovoid attracts vacancy more strongly than does the pristine nanovoid through strong H-vacancy interaction.

In this first-principles study, we investigate the effect of many kinds of substitutional solute species on screw dislocation motion in bcc-Fe, dominating the strength of dilute Fe alloys. Most of the solute species show a significant interaction with the dislocation core, while only several solute species among them, such as Si, P, and Cu, significantly lower the Peierls potential of the screw dislocation motion. A first-principles interaction energy with the “Easy-core” structure excellently correlates with the change in the γ -surface caused by solute atoms (i.e., chemical misfit). Based on the interaction energy, we predicted the effect of each species on macroscopic critical resolved shear stress (CRSS) of the dilute Fe alloy. The CRSS at low and high temperature for various alloys basically agree with experiment CRSS. These results provide a novel understanding of the interaction between a screw dislocation and solute species from the first-principles.

A recent experiment (Scripta Mater., 65:990, 2011) shows that the Σ7 {132}/{132} grain boundary in Al can migrate under external stress but produces no strain. Here, based on a bi-crystallographic analysis, an atomic shuffling path was identified as the feasible mechanism for this grain boundary migration. By a density functional theory calculation, it reveals that the enthalpy barrier of this atomic shuffling path increases by external shear stress applied with shear of the grain boundary along the tilt axis 〈111〉, which is in good agreement with experimentally measured shear-direction-dependence of activation enthalpy for this grain boundary migration.

The physical origins of the mechanical properties of Fe-rich Si alloys are investigated by combining electronic structure calculations with statistical mechanics means such as the cluster variation method, molecular dynamics simulation, etc, applied to homogeneous and heterogeneous systems. Firstly, we examined the elastic properties based on electronic structure calculations in a homogeneous system and attributed the physical origin of the loss of ductility with increasing Si content to the combined effects of magneto-volume and D03 ordering. As a typical example of a heterogeneity forming a microstructure, we focus on grain boundaries, and segregation behavior of Si atoms is studied through high-precision electronic structure calculations. Two kinds of segregation sites are identified: looser and tighter sites. Depending on the site, different segregation mechanisms are revealed. Finally, the dislocation behavior in the Fe–Si alloy is investigated mainly by molecular dynamics simulations combined with electronic structure calculations. The solid-solution hardening and softening are interpreted in terms of two kinds of energy barriers for kink nucleation and migration on a screw dislocation line. Furthermore, the clue to the peculiar work hardening behavior is discussed based on kinetic Monte Carlo simulations by focusing on the preferential selection of slip planes triggered by kink nucleation.

Structural rejuvenation in metallic glasses by a thermal process (i.e. through recovery annealing) was investigated experimentally and theoretically for various alloy compositions. An increase in the potential energy, a decrease in the density, and a change in the local structure as well as mechanical softening were observed after thermal rejuvenation. Two parameters, one related to the annealing temperature, T a/T g, and the other related to the cooling rate during the recovery annealing process, V c/V i, were proposed to evaluate the rejuvenation phenomena. A rejuvenation map was constructed using these two parameters. Since the thermal history of metallic glasses is reset above 1.2T g, accompanied by a change in the local structure, it is essential that the condition of T a/T g ≥ 1.2 is satisfied during annealing. The glassy structure transforms into a more disordered state with the decomposition of icosahedral short-range order within this temperature range. Therefore, a new glassy structure (rejuvenation) depending on the subsequent quenching rate is generated. Partial rejuvenation also occurs in a Zr55Al10Ni5Cu30 bulk metallic glass when annealing is performed at a low temperature (T a/T g ~ 1.07) followed by rapid cooling. This behavior probably originates from disordering in the weakly bonded (loosely packed) region. This study provides a novel approach to improving the mechanical properties of metallic glasses by controlling their glassy structure.

The behavior of H isotopes in crystals is a fundamental and recurrent theme in materials physics. Especially, the information on H diffusion over a wide temperature range provides a critical insight into the quantum mechanical nature of the subject; however, this is not yet fully explored. From state-of-the-art ab initio calculations to treat both electrons and nuclei quantum mechanically, we found that the temperature dependence of H isotope diffusivities in face-centered-cubic (fcc) Pd has an unconventional ``reversed S'' shape on Arrhenius plots. Such irregular behavior is ascribed to the competition between different nuclear quantum effects with different temperature and mass dependencies, which leads to a peculiar situation, where the heavier tritium ($^{3}\mathrm{H}$) diffuses faster than the lighter protium ($^{1}\mathrm{H}$) in the limited temperature range of 80--400 K. This unveils the mechanism of anomalous crossovers between the normal and reversed isotope effects observed in the experiments at high and low temperatures.

We used ReaxFF reactive molecular dynamics simulations to systematically investigate the effects of a pre-oxide shell layer on the mechanical properties of [001]-oriented nickel (Ni) nanowires (NWs) under the uniaxial compressive loading at room temperature. The pristine Ni NWs are considered as references to compare the mechanical properties of the oxide-coated NWs. We found that the mechanical properties of pristine Ni NWs under uniaxial compression are sensitive to both the diameter of the NWs and the pre-oxide shell layer, and their combined effect determines the overall stress and strain behaviors. The compressive strength of the pristine NWs decreases significantly with the decreasing diameter. We observe that the native defected amorphous pre-oxide shell layer with similar to 1.0 nm thickness leads to a lowering of the mechanical compressive resistivity of NWs and causes additional softening. Oxide-coated NWs exhibit a lesser size-dependent unique properties and a lower overall yield strength compared to their pristine counterparts. The reduction of the mechanical compressive yield stress and strain with the decreasing diameter is due to the substantial changes in plastic flow as well as correlated with the existence of the pre-oxide shell layer as compared to its pristine counterpart. Particularly, pre-oxide shell layers have pronounced effects on the initiation of initial dislocations to onset plastic deformation and consequently on the overall plastic response. Published under license by AIP Publishing.

Developing steels with high strength and ductility is needed in order to improve the mechanical reliability and environmental performance of engineering products. The addition of Mn is a key technology for developing next-generation high-strength steels. However, the addition of Mn leads to a serious side effect, grain boundary (GB) embrittlement, which decreases the mechanical toughness of steels. Understanding the mechanism of GB embrittlement due to Mn is an essential process for improving the toughness of steels containing Mn. In this work, in order to reveal the fundamental mechanism of GB embrittlement by Mn, the effect of Mn on the cleavage fracture of bcc-Fe GBs, especially the influence of the difference in the magnetic coupling state between Mn and Fe, is investigated using uniaxial tensile simulations of the bcc-Fe $\mathrm{\ensuremath{\Sigma}}3(111)$ GB with and without Mn segregation using the first-principles density functional theory (DFT). The uniaxial tensile simulations demonstrate that Mn decreases the cleavage-fracture energy of the GB. In particular, the ferromagnetically coupled Mn substantially decreases the cleavage-fracture energy of the GB, promoting cleavage fracture. When ferromagnetically coupled Mn is present in the bcc-Fe GBs, the electrons contributing to the bonds between Mn and the surrounding Fe atoms easily localize to the Mn atom with increasing stress, and the bonding between Mn and the surrounding Fe atoms rapidly weakens, leading to a cleavage fracture of the GBs at a lower stress and strain. This unusual behavior is derived from the stability of the nonbonding Mn as a result of its half-filled d shell. These results show that the local magnetic state in GBs is one of the factors determining the macroscopic mechanical properties of steels containing Mn.

The FeNiCoCr-based high entropy alloys (HEAs) exhibit excellent mechanical properties, such as twin-induced plasticity (TWIP) and phase transformation plasticity (TRIP) that can reach a remarkable combination of strength and ductility. In this work, the face-centered-cubic (FCC) single-crystal FeNiCoCrAl0.36 HEAs were studied, using the density functional theory (DFT) combined with the phonon calculation to estimate the stacking fault energies, temperature-dependent phase stabilities of different structures. And the kinetic Monte Carlo (kMC) is used to predict the substructures evolution based on the transition state energies obtained from DFT calculations. We proposed two different formation paths of nano-twins in this Al-composited HEA and found that short-range hexagonal-close-packed (HCP)-stacking could occur in this HEA. DFT calculations suggest that this HEA has negative stacking fault energy, HCP formation energy, and twin-formation energy at 0 K. Phonon calculations represent that at the finite temperature, the competing FCC/HCP phase stability and propensity for twinning makes it possible to form HCP-like twin boundaries. The kMC simulations suggest that under deformation, HCP substructures could form followed by twins which differs to the study of others. With the great agreement of results from kMC simulations and experiments, this twin-formation path offers a new concept of designing TWIP HEAs containing tunable twin structures with HCP and TWIN lamellae structures, which results in better mechanical properties of HEAs.

Tailoring nanoscale defect structures for desirable deformation behaviors is crucial to designing and optimizing the mechanical properties of alloys. Distinguishing from the predominant toughening mechanisms (e.g., mechanical twinning and deformation-induced phase transformation), here we report an unusual stacking-fault-mediated deformation in equiatomic FeNiCoCr high-entropy alloy (HEA) by controllably introducing helium nanobubbles with high pressures of ~2.5-4.7 gigapascals. Using in situ transmission electron microscopy nanomechanical testing, we demonstrate that highly pressurized helium nanobubbles can not only increase the strength by serving as dislocation obstacles but also enhance the strain hardening capacity and accommodate considerable plasticity via facilitating the multiplication and interaction of interwoven stacking faults. Through atomistic simulations, we reveal that high helium pressures contribute to reducing the nucleation energy of partial dislocations at the nanobubbles surface, which enhances dislocation nucleation rates and offers sustainable stacking fault sources for retaining ductility. Our results provide a novel design strategy for tuning deformation mechanisms of HEAs via introducing highly pressurized helium nanobubbles, which may open up avenues towards the facile tailoring of mechanical responses in micro/nanoscale HEA components.

Body-centered cubic (bcc) refractory multicomponent alloys are of great interest due to their remarkable strength at high temperatures. Optimizing the chemical compositions of these alloys to achieve a combination of high strength and room-temperature ductility remains challenging. Systematic predictions of these correlated properties across a vast compositional space would speed the alloy discover process. In the present work, we performed first-principles calculations with the special quasi-random structure (SQS) method to predict the unstable stacking fault energy ( γ usf ) of the ( 1 1 ¯ 0 ) [ 111 ] slip system and the ( 1 1 ¯ 0 ) -plane surface energy ( γ surf ) for 106 individual binary, ternary and quaternary bcc solid-solution alloys with constituent elements among Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re and Ru. Moreover, with the first-principles data and a set of physics-informed descriptors, we developed surrogate models based on statistical regression to accurately and efficiently predict γ usf and γ surf for refractory multicomponent alloys in the 10-element compositional space. Building upon binary and ternary data, the surrogate models show outstanding predictive capability in the high-order multicomponent systems. The ratio between γ surf and γ usf can be used to populate a model of intrinsic ductility based on the Rice model of crack-tip deformation. Therefore, using the surrogate models, we performed a systematic screening of γ usf , γ surf and their ratio over 112,378 alloy compositions to search for alloy candidates that may have enhanced strength-ductility synergies. Search results were also validated by additional first-principles calculations.

One-dimensional carbon nanotubes (CNT), which are mechanically strong and flexible, enhance strength of the host metal matrix. However, the reduction of ductility is often a serious drawback. Here, we report significantly enhanced plastic flow strength, while preventing tensile ductility reduction, by uniformly dispersing CNTs in Al matrix. Nanoscale plasticity and rupturing processes near CNTs were observed by in-situ mechanical tests inside Transmission Electron Microscope (TEM). CNTs act like forest dislocations and have comparable density (∼1014/m2), and such 1D nano-dispersion hardening is studied in detail by in situ TEM and molecular dynamics simulations. Rupture-front blunting and branching are seen with in situ TEM, which corroborates the result from macro-scale tension tests that our Al+CNT nanocomposite is quite damage- and fault-tolerant. We propose a modified shear-lag model called “Taylor-dispersion” hardening model to highlight the dual roles of CNTs as load-bearing fillers and “forest dislocations” equivalent that harden the metal matrix, for the plastic strength of metal+CNT nanocomposite.

The nature of the reaction and transient behavior of I1-type stacking faults (SFs) under shear stress was investigated to understand the role of I1 SFs in non-basal plastic deformation of Mg. This was because it has been suggested that I1 SFs formed via solute (such as Y) additions to Mg may act as the nucleation source of non-basal dislocations, which serve as important deformation modes to improve the workability. Crystal models of pure Mg containing an I1 SF were deformed by simple shear along the basal plane using molecular dynamics simulations with many-body interatomic potentials. The characteristic dissociation reaction was observed at the boundaries of the I1 SF, where 〈 c + a 〉 dislocations on the pyramidal plane, Shockley partial dislocations on the basal plane, and stair-rod dislocations were nucleated. Interestingly, instead of activation of the non-basal dislocations, { 1 1 2 ¯ 1 } twins were nucleated in the early stage of the reaction and grew steadily as the applied stress was increased. It was suggested that the I1 SF was likely to act as a “reactive” defect to assist and accommodate the c-axis deformation. Further, the deformation-induced twin boundaries were found to act as moderate obstacles to dislocation movement on the basal planes in the Mg matrix, without significantly sacrificing its plasticity.

The atomistic pathways of deformation twinning can be computed ab initio, and quantified by two variables: strain which describes shape change of a periodic supercell, and shuffling which describes non-affine displacements of the internal degrees of freedom. The minimum energy path involves juxta-position of both. But if one can obtain the same saddle point by continuously increasing the strain and relaxing the internal degrees of freedom by steepest descent, we call the path strain-controlled, and vice versa. Surprisingly, we find the { 10 1 ¯ 2 } 〈 10 1 ¯ 1 ¯ 〉 twinning of Mg is shuffling-controlled at the smallest lengthscale of the irreducible lattice correspondence pattern, that is, the reaction coordinate at the level of 4 atoms is dominated by non-affine displacements, instead of strain. Shuffling-controlled deformation twinning is expected to have different temperature and strain-rate sensitivities from strain-controlled deformation twinning due to relatively weaker strength of long-range elastic interactions, in particular at the twin nucleation stage. As the twin grows large enough, however, elastic interactions and displacive character of the transformation should always turn dominant.

The workability and ductility of metals usually degrade with exposure to irradiation, hence the phrase "radiation damage". Here, we found that helium (He) radiation can actually enhance the room-temperature deformability of submicron-sized copper. In particular, Cu single crystals with diameter of 100-300 nm and containing numerous pressurized sub-10 nm He bubbles become stronger, more stable in plastic flow and ductile in tension, compared to fully dense samples of the same dimensions that tend to display plastic instability (strain bursts). The sub-10 nm He bubbles are seen to be dislocation sources as well as shearable obstacles, which promote dislocation storage and reduce dislocation mean free path, thus contributing to more homogeneous and stable plasticity. Failure happens abruptly only after significant bubble coalescence. The current findings can be explained in light of Weibull statistics of failure and the beneficial effects of bubbles on plasticity. These results shed light on plasticity and damage developments in metals and could open new avenues for making mechanically robust nano- and microstructures by ion beam processing and He bubble engineering.

The β ′ precipitate phases formed in Mg–Y alloys, which exhibit characteristic periodic arrays of Y-rich zigzag-shaped atomic clusters at regular intervals of 1.1 nm, have attracted significant attention owing to their precipitation-hardening effect on the matrix. To investigate the formation mechanism of such cluster arrays, the interaction energies between Y clusters at various distances were quantitatively evaluated using first-principles calculations based on density functional theory. We found a weak but distinct interaction between the clusters caused by the interplay between attractive chemical interactions and repulsive relaxation energies. This interplay determines the energetically favorable structure of the cluster arrangements, and these structures are consistent with the experimental observations. We suggest that the long-range intercluster interactions dominate the alignment of Y clusters, which leads to the formation of β ′ precipitates in the Mg matrix, followed by the short-range clustering of Y atoms.

In steel, P and S cause serious grain boundary (GB) embrittlement, which is associated with high segregation energies. To investigate the origins of such high segregation energies of P and S, we applied the combination of ab initio local energy analysis and crystal orbital Hamiltonian population (COHP) analysis for the GB segregation of Al, Si, P, and S in bcc-Fe, which can provide local energetic and bonding views of segregation behavior of each solute, associated with the replacement between solute–Fe and Fe–Fe bonding at GB and bulk sites. The local energy analysis revealed that GB segregation of such solutes is mainly caused by the difference between local energy changes of Fe atoms adjacent to a solute atom in the GB and bulk sites, and that the local energy change of each Fe atom depends on the solute–Fe interatomic distance with a unique functional form for each solute species. The COHP analysis showed that such distance dependency of the Fe-atom local energy change is caused by that of solute–Fe bonding interactions, relative to the Fe–Fe ones, governed by the valence atomic-orbital characters of each solute species. P and S have smaller extents of atomic orbitals and larger numbers of valence electrons; thus, they greatly lower the local energies of Fe atoms at interatomic distances shorter than the bulk first-neighbor one, and they greatly increase those of Fe atoms at longer interatomic distances around the bulk second-neighbor one. Thus, high segregation energies of P and S occur at GB sites with short first-neighbor distances and reduced coordination numbers within the bulk second-neighbor distance. The GB embrittlement by P and S was also discussed by this local-bonding viewpoint. The combination of local energy and COHP analyses can provide novel insights into the behavior of solute elements in various materials.

Single phase Fe16N2 is a potential material in building high-performance magnetic writing heads and permanent magnets due to its ultra-high saturation magnetization (MS) and magnetic anisotropy (Keff). However, the poor controllability of phase constituent and low thermostability (decomposed at 200 °C) are big obstacles to its practical applications. In this work, we have devised a novel Fe/Cr/FeN:Co heterostructure to introduce both lattice strain and Co doping for tuning the FeN constituent and enhancing the phase stability synergistically. With effective regulation, the FeN layer can possesse both superior MS (2.4–2.8 T) and high thermostability with standing 450 °C annealing. Furthermore, by first-principles calculations, we reveal that the synergistic regulation on the thermostability is closely related to the solution heat tunability of Fe-N phases. The Fe/Cr/FeN:Co heterostructure may serve as a promising material for constructing high-efficient writing heads, permanent magnets, and other magnetic devices.