Interfacial Microcompartmentalization by Kinetic Control of Selective Interfacial Accumulation

Reported here is a 2D, interfacial microcompartmentalization strategy governed by 3D phase separation. In aqueous polyethylene glycol (PEG) solutions doped with biotinylated polymers, the polymers spontaneously accumulate in the interfacial layer between the oil-surfactant-water interface and the adjacent polymer phase. In aqueous two-phase systems, these polymers first accumulated in the interfacial layer separating two polymer solutions and then selectively migrated to the oil-PEG interfacial layer. By using polymers with varying photopolymerizable groups and crosslinking rates, kinetic control and capture of spatial organisation in a variety of compartmentalized macroscopic structures, without the need of creating barrier layers, was achieved. This selective interfacial accumulation provides an extension of 3D phase separation towards synthetic compartmentalization, and is also relevant for understanding intracellular organisation.

Learning grain boundary segregation energy spectra in polycrystals

The segregation of solute atoms at grain boundaries (GBs) can profoundly impact the structural properties of metallic alloys, and induce effects that range from strengthening to embrittlement. And, though known to be anisotropic, there is a limited understanding of the variation of solute segregation tendencies across the full, multidimensional GB space, which is critically important in polycrystals where much of that space is represented. Here we develop a machine learning framework that can accurately predict the segregation tendency—quantified by the segregation enthalpy spectrum—of solute atoms at GB sites in polycrystals, based solely on the undecorated (pre-segregation) local atomic environment of such sites. We proceed to use the learning framework to scan across the alloy space, and build an extensive database of segregation energy spectra for more than 250 metal-based binary alloys. The resulting machine learning models and segregation database are key to unlocking the full potential of GB segregation as an alloy design tool, and enable the design of microstructures that maximize the useful impacts of segregation.

Predicting segregation energies of alloy systems can be challenging even for a single grain boundary. Here the authors propose a machine-learning framework, which maps the local environments on a distribution of segregation energies, to predict segregation energies of alloy elements in polycrystalline materials.

Improved Hardness and Thermal Stability of Nanocrystalline Nickel Electrodeposited with the Addition of Cysteine

Experiments were conducted for the study of the effect of cysteine addition on the microstructure of nanocrystalline Ni films electrodeposited from a nickel sulfate-based bath. Furthermore, the thermal stability of the nanostructure of Ni layers processed with cysteine addition was also investigated. It was found that with increasing cysteine content in the bath, the grain size decreased, while the dislocation density and the twin fault probability increased. Simultaneously, the hardness increased due to cysteine addition through various effects. Saturation in the microstructure and hardness was achieved at cysteine contents of 0.3-0.4 g/L. Moreover, the texture changed from (220) to (200) with increasing the concentration of cysteine. The hardness of the Ni films processed with the addition of 0.4 g/L cysteine (∼6800 MPa) was higher than the values obtained for other additives in the literature (<6000 MPa). This hardness was further enhanced to ∼8400 MPa when the Ni film was heated up to 500 K. It was revealed that the hardness remained as high as 6000 MPa even after heating up to 750 K, while for other additives, the hardness decreased below 3000 MPa at the same temperature.

Nano-Dual-Phase Metallic Glass Film Enhances Strength and Ductility of a Gradient Nanograined Magnesium Alloy

Magnesium (Mg) alloys are good candidates for applications with requirement of energy saving, taking advantage of their low density. However, the fewer slip systems of the hexagonal-close-packed (hcp) structure restrict ductility of Mg alloys. Here, a hybrid nanostructure concept is presented by combining nano-dual-phase metallic glass (NDP-MG) and gradient nanograin structure in Mg alloys to achieve a higher yield strength (230 MPa, 31% improvement compared with the reference base alloy) and larger ductility (20%, threefold higher than the SMAT-H sample), which breaks the strength-ductility trade-off dilemma. This hybrid nanostructure is realized by surface mechanical attrition treatment (SMAT) on the surface of a crystalline Mg alloy, and followed by physical vapor deposition of a Mg-based NDP-MG. The higher strength is provided by the nanograin layer generated by SMAT. The larger ductility is a synergistic effect of multiple shear bandings and nanocrystallization of the NDP-MG, inhibition of crack propagation from the SMATed nanograined structure by the NDP-MG, and strain-induced grain growth in the SMATed nanograin layer. This hybrid nanostructure design provides a general route to render brittle alloys stronger and ductile, especially in hcp systems.

A coherent set of model equations for various surface and interface energies in systems with liquid and solid metals and alloys

In this paper first a generally valid model is derived from the two fundamental equations of Gibbs for temperature and composition dependences of all types of interfacial energies. This general model is applied here to develop a coherent set of particular model equations for surface tension of liquid metals and alloys, for surface energy of solid metals and alloys, for high-angle grain boundary energy in metals and alloys, for solid/liquid interfacial energy in metals and alloys, for liquid/liquid interfacial energy in alloys and for solid/solid interfacial energy in metals and alloys. The latter case is sub-divided into models on coherent, incoherent and semi-coherent interfaces with the same phases and with different phases on the two sides of the interface. Model parameters are given here as an example for the 111 plane of fcc metals and alloys. For other crystal planes or other crystal structures the model parameters should be adjusted, while the model equations remain the same. The method is demonstrated on various surface and interfacial energies of pure Au, on solid/liquid interfacial energy in the AlCu system, on different types of solid/solid interfacial energies in the AuNi system, on solid/solid, solid/liquid and liquid/liquid interfacial energies in the AlPb system and on the coherent, incoherent and semi-coherent interfacial energies between ordered and disordered fcc phases in the Ni-rich part of the NiAl system. The ability of this method is demonstrated to predict surface and interface transition along free surfaces and grain boundaries and also negative interfacial energies in nano-systems.

Ultrahigh-strength and ductile superlattice alloys with nanoscale disordered interfaces

Alloys that have high strengths at high temperatures are crucial for a variety of important industries including aerospace. Alloys with ordered superlattice structures are attractive for this purpose but generally suffer from poor ductility and rapid grain coarsening. We discovered that nanoscale disordered interfaces can effectively overcome these problems. Interfacial disordering is driven by multielement cosegregation that creates a distinctive nanolayer between adjacent micrometer-scale superlattice grains. This nanolayer acts as a sustainable ductilizing source, which prevents brittle intergranular fractures by enhancing dislocation mobilities. Our superlattice materials have ultrahigh strengths of 1.6 gigapascals with tensile ductilities of 25% at ambient temperature. Simultaneously, we achieved negligible grain coarsening with exceptional softening resistance at elevated temperatures. Designing similar nanolayers may open a pathway for further optimization of alloy properties.

A model for grain boundary thermodynamics

Systematic microstructure design requires reliable thermodynamic descriptions of each and all microstructure elements. While such descriptions are well established for most bulk phases, thermodynamic assessment of microstructure defects is challenging because of their individualistic nature. In this paper, a model is devised for assessing grain boundary thermodynamics based on available bulk thermodynamic data. We propose a continuous relative atomic density field and its spatial gradients to describe the grain boundary region with reference to the homogeneous bulk and derive the grain boundary Gibbs free energy functional. The grain boundary segregation isotherm and phase diagram are computed for a regular binary solid solution, and qualitatively benchmarked for the Pt-Au system. The relationships between the grain boundary's atomic density, excess free volume, and misorientation angle are discussed. Combining the current density-based model with available bulk thermodynamic databases enables constructing databases, phase diagrams, and segregation isotherms for grain boundaries, opening possibilities for studying and designing heterogeneous microstructures.

Power laws in pressure-induced structural change of glasses

Many glasses exhibit fractional power law (FPL) between the mean atomic volume v_a and the first diffraction peak position q₁, i.e. \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$v_{\mathrm{a}} \propto q_1^{ - d}$$\end{document}va∝q1−d with d ≃ 2.5 deviating from the space dimension D = 3, under compression or composition change. What structural change causes such FPL and whether the FPL and d are universal remain controversial. Here our simulations show that the FPL holds in both two- and three-dimensional glasses under compression when the particle interaction has two length scales which can induce nonuniform local deformations. The exponent d is not universal, but varies linearly with the deformable part of soft particles. In particular, we reveal an unexpected crossover regime with d > D from crystal behavior (d = D) to glass behavior (d < D). The results are explained by two types of bond deformation. We further discover FPLs in real space from the radial distribution functions, which correspond to the FPLs in reciprocal space.

A puzzle in metallic glass research is the existence of the fractional power law in reciprocal space, whilst its origin remains controversial. Zhang et al. show that nonuniform local deformations under compression induce this phenomenon and quantify the power law exponent at both two and three dimensions.

Rapid heating induced ultrahigh stability of nanograined copper

Inherent thermal and mechanical instability of nanograined materials bottlenecks their processing and technological applications. In addition to the traditional stabilization strategy, which is based on alloying, grain boundary relaxation was recently found to be effective in stabilizing nanograined pure metals. Grain boundary relaxation can be induced by deforming very fine nanograins below a critical size, typically several tens of nanometers. Here, we found that rapid heating may trigger intensive boundary relaxation of pure Cu nanograins with sizes up to submicrometers, a length scale with notable instability in metals. The rapidly heated Cu nanograins remain stable at temperatures as high as 0.6 T _m (melting point), even higher than the recrystallization temperature of deformed coarse-grained Cu. The thermally induced grain boundary relaxation originating from the generation of high-density nanotwins offers an alternative approach to stabilizing nanostructured materials.

Ultimate Strength of Metals

We present a theoretical model that predicts the peak strength of polycrystalline metals based on the activation energy (or stress) required to cause deformation via amorphization. Building on extensive earlier work, this model is based purely on materials properties, requires no adjustable parameters, and is shown to accurately predict the strength of four exemplar metals (fcc, bcc, and hcp, and an alloy). This framework reveals new routes for design of more complex high-strength materials systems, such as compositionally complex alloys, multiphase systems, nonmetals, and composite structures.

In-Situ Helium Implantation and TEM Investigation of Radiation Tolerance to Helium Bubble Damage in Equiaxed Nanocrystalline Tungsten and Ultrafine Tungsten-TiC Alloy

The use of ultrafine and nanocrystalline materials is a proposed pathway to mitigate irradiation damage in nuclear fusion components. Here, we examine the radiation tolerance of helium bubble formation in 85 nm (average grain size) nanocrystalline-equiaxed-grained tungsten and an ultrafine tungsten-TiC alloy under extreme low energy helium implantation at 1223 K via in-situ transmission electron microscope (TEM). Helium bubble damage evolution in terms of number density, size, and total volume contribution to grain matrices has been determined as a function of He⁺ implantation fluence. The outputs were compared to previously published results on severe plastically deformed (SPD) tungsten implanted under the same conditions. Large helium bubbles were formed on the grain boundaries and helium bubble damage evolution profiles are shown to differ among the different materials with less overall damage in the nanocrystalline tungsten. Compared to previous works, the results in this work indicate that the nanocrystalline tungsten should possess a fuzz formation threshold more than one order of magnitude higher than coarse-grained tungsten.

Thick grain boundary induced strengthening in nanocrystalline Ni alloy

Grain refinement has been extensively used to strengthen metallic materials for decades. Grain boundaries act as effective barriers to the transmission of dislocations, consequently leading to strengthening. Conventional grain boundaries have a thickness of 1-2 atomic layers, typically ∼0.5 nm for most metallic materials. Here, we report, however, the formation of ∼3 nm thick grain boundaries in a nanocrystalline Ni alloy. In situ micropillar compression studies coupled with molecular dynamics simulations suggest that the thick grain boundaries are stronger barriers than conventional grain boundaries to the transmission of dislocations. This study provides a fresh perspective for the design of high strength, deformable nanostructured metallic materials.

Alloying Element Segregation and Grain Boundary Reconstruction, Atomistic Modeling

Grain boundary (GB) segregation is an important phenomenon that affects many physical properties, as well as microstructure of polycrystals. The segregation of solute atoms on GBs and its effect on GB structure in Al were investigated using two approaches: First principles total energy calculations and the finite temperature large-scale atomistic modeling within hybrid MD/MC approach comprising molecular dynamics and Monte Carlo simulations. We show that the character of chemical bonding is essential in the solute–GB interaction, and that formation of directed quasi-covalent bonds between Si and Zn solutes and neighboring Al atoms causes a significant reconstruction of the GB structure involving a GB shear-migration coupling. For the solutes that are acceptors of electrons in the Al matrix and have a bigger atomic size (such as Mg), the preferred position is determined by the presence of extra volume at the GB and/or reduced number of the nearest neighbors; in this case, the symmetric GB keeps its structure. By using MD/MC approach, we found that GBs undergo significant structural reconstruction during segregation, which can involve the formation of single- or double-layer segregations, GB splitting, and coupled shear-migration, depending on the details of interatomic interactions.

Hierarchical nanostructured W-Cu composite with outstanding hardness and wear resistance

Hierarchical nanostructured W-Cu composite with an average W size below 200 nm and nanocrystalline structure inside the W phase was obtained by refining the inner structure of the initial ultrafine powders combined with high-pressure spark plasma sintering. It revealed that an atomic scale combination can be formed at both the W grain boundaries and W/Cu interfaces. Accordingly, the nanostructured W-Cu composite exhibits twofold hardness and greatly improved wear resistance with satisfactory electrical conductivity, as compared to those of their fine grain structured counterparts. The upgraded wear resistance is attributed to the restricted micro-plowing and the mechanically mixed layer, induced by a refined microstructure, intrinsic high hardness, and the composition modulation on the wear surface.

Microstructures and Hardness Prediction of an Ultrafine-Grained Al-2024 Alloy

High-pressure torsion (HPT) is a high efficiency processing method for fabricating bulk ultrafine-grained metallic materials. This work investigates microstructures and evaluates the corresponding strengthening components in the center of HPT disks, where effective shear strains are very low. An Al-4.63Cu-1.51Mg (wt. %) alloy was processed by HPT for 5 rotations. Non-equilibrium grain and sub-grain boundaries were observed using scanning transmission electron microscopy in the center area of HPT disks. Solute co-cluster segregation at grain boundaries was found by energy dispersive spectrometry. Quantitative analysis of X-ray diffraction patterns showed that the average microstrain, crystalline size, and dislocation density were (1.32 ± 0.07) × 10−3, 61.9 ± 1.4 nm, and (2.58 ± 0.07) × 1014 m−2, respectively. The ultra-high average hardness increment was predicted on multiple mechanisms due to ultra-high dislocation densities, grain refinement, and co-cluster–defect complexes.

Theoretical Assessment of Thermodynamic Stability in Nanocrystalline Metallic Alloys

Thermal stability in nanocrystalline alloys has been extensively explored while using both experimental and theoretical approaches. From the theoretical point of view, the vast majority of the models proposed in the literature have been implicitly limited to immiscible or dilute systems and thus lack the necessary generality to make predictions for different alloying interactions and in the case of intermetallic compounds formation. In this work, a general theoretical description for the case of binary W-based alloys is presented. It is shown that a critical value Ω ∗ of the interaction energy in the grain boundary Ω ( g b ) exists, such that the condition Ω ( g b ) < Ω ∗ can be regarded as a criterion for thermodynamic stability assessment. A procedure for calculating the value of Ω ∗ for each specific alloy is illustrated. A preliminary qualitative comparison between the model predictions and properly selected experimental findings taken from the literature and related to the W-Cr system is also provided.

Influence of Bath Additives on the Thermal Stability of the Nanostructure and Hardness of Ni Films Processed by Electrodeposition

The effect of bath additives on the thermal stability of the microstructure and hardness of nanocrystalline Ni foils processed by electrodeposition was studied. Three samples with a thickness of 20 μ m were prepared: one without any additive and two others with saccharin or trisodium citrate additives. Then, the specimens were heat-treated at different temperatures up to 1000 K. It was found that for the additive-free sample the recovery of the microstructure and the reduction of the hardness started only at temperatures higher than 500 K. At the same time, a decrease of the defect density and hardness was observed even at 400 K for the additive-containing films. This was explained by the higher defect density, which increased the thermodynamic driving force for recovery during annealing. At the highest applied temperature (1000 K), this larger thermodynamic driving force resulted in a recrystallization in the sulfur-containing sample, leading to a very low hardness of about 1000 MPa as compared to the additive-free sample (1300 MPa). On the other hand, the sample deposited with trisodium citrate additive showed a better thermal stability at 1000 K than the additive-free sample: the hardness remained as high as 2000 MPa even at 1000 K.

Local electronic descriptors for solute-defect interactions in bcc refractory metals

The interactions between solute atoms and crystalline defects such as vacancies, dislocations, and grain boundaries are essential in determining alloy properties. Here we present a general linear correlation between two descriptors of local electronic structures and the solute-defect interaction energies in binary alloys of body-centered-cubic (bcc) refractory metals (such as W and Ta) with transition-metal substitutional solutes. One electronic descriptor is the bimodality of the d-orbital local density of states for a matrix atom at the substitutional site, and the other is related to the hybridization strength between the valance sp- and d-bands for the same matrix atom. For a particular pair of solute-matrix elements, this linear correlation is valid independent of types of defects and the locations of substitutional sites. These results provide the possibility to apply local electronic descriptors for quantitative and efficient predictions on the solute-defect interactions and defect properties in alloys.

The effect of space-charge formation on the grain-boundary energy of an ionic solid

Taking the model system of an oxide containing acceptor dopant cations and charge-compensating oxygen vacancies, we calculate at the continuum level the change in the excess grain-boundary energy of an ionic solid upon space-charge formation. Two different cases are considered for the space-charge layers: (i) only vacancies attain electrochemical equilibrium and (ii) both dopants and vacancies attain electrochemical equilibrium. The changes calculated for a specific set of grain boundaries indicate that, depending on dopant concentration, space-charge formation can decrease the excess free energy by up to 15% in the first case and by up to 45% in the second case. The possibility of the excess grain-boundary energy going to zero and the possible effects of an external electric field on the excess grain-boundary energy are also discussed. This article is part of a discussion meeting issue 'Energy materials for a low carbon future'.

Bulk ultrafine grained/nanocrystalline metals via slow cooling

Cooling, nucleation, and phase growth are ubiquitous processes in nature. Effective control of nucleation and phase growth is of significance to yield refined microstructures with enhanced performance for materials. Recent studies reveal that ultrafine grained (UFG)/nanocrystalline metals exhibit extraordinary properties. However, conventional microstructure refinement methods, such as fast cooling and inoculation, have reached certain fundamental limits. It has been considered impossible to fabricate bulk UFG/nanocrystalline metals via slow cooling. Here, we report a new discovery that nanoparticles can refine metal grains to ultrafine/nanoscale by instilling a continuous nucleation and growth control mechanism during slow cooling. The bulk UFG/nanocrystalline metal with nanoparticles also reveals an unprecedented thermal stability. This method overcomes the grain refinement limits and may be extended to any other processes that involve cooling, nucleation, and phase growth for widespread applications.

Strain-stress relationship and dislocation evolution of W-Cu bilayers from a constructed n-body W-Cu potential

An n-body W-Cu potential is constructed under the framework of the embedded-atom method by means of a proposed function of the cross potential. This W-Cu potential is realistic to reproduce mechanical property and structural stability of WCu solid solutions within the entire composition range, and has better performances than the three W-Cu potentials already published in the literature. Based on this W-Cu potential, molecular dynamics simulation is conducted to reveal the mechanical property and dislocation evolution of the bilayer structure between pure W and W0.7Cu0.3 solid solution. It is found that the formation of the interface improves the strength of the W0.7Cu0.3 solid solutions along tensile loading perpendicular to the interface, as the interface impedes the evolution of the dislocation lines from the W0.7Cu0.3 solid solutions to the W part. Simulation also reveals that the interface has an important effect to significantly reduce the tensile strength and critical strain of W along the tensile loading parallel to the interface, which is intrinsically due to the slip of the edge or screw dislocations at low strains as a result of the lattice mismatch.

Thermal stability and irradiation response of nanocrystalline CoCrCuFeNi high-entropy alloy

Grain growth and phase stability of a nanocrystalline face-centered cubic (fcc) Ni_0.2Fe_0.2Co_0.2Cr_0.2Cu_0.2 high-entropy alloy (HEA), either thermally- or irradiation-induced, are investigated through in situ and post-irradiation transmission electron microscopy (TEM) characterization. Synchrotron and lab x-ray diffraction measurements are carried out to determine the microstructural evolution and phase stability with improved statistics. Under in situ TEM observation, the fcc structure is stable at 300 °C with a small amount of grain growth from 15.8 to ∼20 nm being observed after 1800 s. At 500 °C, however, some abnormal growth activities are observed after 1400 s, and secondary phases are formed. Under 3 MeV Ni room temperature ion irradiation up to an extreme dose of nearly 600 displacements per atom, the fcc phase is stable and the average grain size increases from 15.6 to 25.2 nm. Grain growth mechanisms driven by grain rotation, grain boundary curvature, and disorder are discussed.

Achieving Ultrahigh Hardness in Electrodeposited Nanograined Ni-Based Binary Alloys

Annealing hardening has recently been found in nanograined (ng) metals and alloys, which is ascribed to the promotion of grain boundary (GB) stability through GB relaxation and solute atom GB segregation. Annealing hardening is of great significance in extremely fine ng metals since it allows the hardness to keep increasing with a decreasing grain size which would otherwise be softened. Consequently, to synthesize extremely fine ng metals with a stable structure is crucial in achieving an ultrahigh hardness in ng metals. In the present work, direct current electrodeposition was employed to synthesize extremely fine ng Ni-Mo and Ni-P alloys with a grain size of down to a few nanometers. It is demonstrated that the grain size of the as-synthesized extremely fine ng Ni-Mo and Ni-P alloys can be as small as about 3 nm with a homogeneous structure and chemical composition. Grain size strongly depends upon the content of solute atoms (Mo and P). Most importantly, appropriate annealing induces significant hardening as high as 11 GPa in both ng Ni-Mo and Ni-P alloys, while the peak hardening temperature achieved in ng Ni-Mo is much higher than that in ng Ni-P. Electrodeposition is efficient in the synthesis of ultrahard bulk metals or coatings.

An Automated Computational Approach for Complete In-Plane Compositional Interface Analysis by Atom Probe Tomography

We introduce an efficient, automated computational approach for analyzing interfaces within atom probe tomography datasets, enabling quantitative mapping of their thickness, composition, as well as the Gibbsian interfacial excess of each solute. Detailed evaluation of an experimental dataset indicates that compared with the composition map, the interfacial excess map is more robust and exhibits a relatively higher resolution to reveal compositional variations. By field evaporation simulations with a predefined emitter mimicking the experimental dataset, the impact of trajectory aberrations on the measurement of the thickness, composition, and interfacial excess of the decorated interface are systematically analyzed and discussed.

Charge-State Field Evaporation Behavior in Cu(V) Nanocrystalline Alloys

Atom probe tomography (APT) of a nanocrystalline Cu-7 at.% V thin film annealed at 400°C for 1 h revealed chemical partitioning in the form of solute segregation. The vanadium precipitated along high angle grain boundaries and at triple junctions, determined by cross-correlative precession electron diffraction of the APT specimen. Upon field evaporation, the V2+/(V1+ + VH1+) ratio from the decomposed ions was ~3 within the matrix grains and ~16 within the vanadium precipitates. It was found that the VH1+ complex was prevalent in the matrix, with its presence explained in terms of hydrogen's ability to assist in field evaporation. The change in the V2+/(V1+ + VH1+) charge-state ratio (CSR) was studied as a function of base temperature (25-90 K), laser pulse energy (50-200 pJ), and grain orientation. The strongest influence on changing the CSR was with the varied pulse laser, which made the CSR between the precipitates and the matrix equivalent at the higher laser pulse energies. However, at these conditions, the precipitates began to coarsen. The collective results of the CSRs are discussed in terms of field strengths related to the chemical coordination.

Solute segregation and thermal stability of nanocrystalline solid solution systems

A model coupling first principles and thermodynamics was developed to describe the thermal stability of a nanograin structure in solid solution alloys. The thermodynamic functions of solute segregation and conditions for thermal stabilization were demonstrated for both strongly and weakly solute-segregating systems. The dependence of segregation behavior on the grain size, solute concentration and temperature was quantified, where the parameters to control destabilization of the nanograin structure at a given temperature were predicted. For the first time it was found that there exists a transformation from the single-extreme to dual-extreme rule of the total Gibbs free energies of the solid solution systems with the decrease of solute concentration or increase of temperature. The model calculations were confirmed quantitatively by the experimental results, and a nanocrystalline W-10 at%Sc solid solution with a highly stable grain structure in a broad range from room temperature to 1600 K was prepared. The universal mechanism disclosed in this study will facilitate the design of nanocrystalline alloys with high thermal stability through matching of the doping element and the initial grain size.

Characterization of a Nanocrystalline Structure Formed by Crystal Lattice Transformation in a Bulk Steel Material

The formation of nanocrystalline structures in bulk metal materials is of great significance for both investigating the structural features of nanocrystalline materials and enhancing the value of bulk metal materials in engineering applications. Herein, we report a nanocrystalline structure formed by lattice transformation in a three-dimensional bulk metal material. We characterized its phase composition, three-dimensional features, and boundary structure. This nanocrystalline structure had microscale length and height and nanoscale width, which gave it a “nanoplate” structure in three-dimensional space. We observed edge dislocations in the interior of the nanocrystalline structure. A unique transitional boundary that contributed to maintaining its nanoscale size was found at the border between the parent phase and the nanocrystalline structure.

Ultrastrong nanocrystalline steel with exceptional thermal stability and radiation tolerance

Nanocrystalline (NC) metals are stronger and more radiation-tolerant than their coarse-grained (CG) counterparts, but they often suffer from poor thermal stability as nanograins coarsen significantly when heated to 0.3 to 0.5 of their melting temperature (T_m). Here, we report an NC austenitic stainless steel (NC-SS) containing 1 at% lanthanum with an average grain size of 45 nm and an ultrahigh yield strength of ~2.5 GPa that exhibits exceptional thermal stability up to 1000 °C (0.75 T_m). In-situ irradiation to 40 dpa at 450 °C and ex-situ irradiation to 108 dpa at 600 °C produce neither significant grain growth nor void swelling, in contrast to significant void swelling of CG-SS at similar doses. This thermal stability is due to segregation of elemental lanthanum and (La, O, Si)-rich nanoprecipitates at grain boundaries. Microstructure dependent cluster dynamics show grain boundary sinks effectively reduce steady-state vacancy concentrations to suppress void swelling upon irradiation.

Weaker ferritic/matensitic steels rather than stronger austenitic steels are usually candidates for nuclear reactors since they do not easily swell under irradiation. Here, the authors make an ultrastrong lanthanum-doped nanocrystalline austenitic steel that is thermally stable and radiation-tolerant.

New nanoscale toughening mechanisms mitigate embrittlement in binary nanocrystalline alloys

Nanocrystalline metals offer significant improvements in structural performance over conventional alloys. However, their performance is limited by grain boundary instability and limited ductility. Solute segregation has been proposed as a stabilization mechanism, however the solute atoms can embrittle grain boundaries and further degrade the toughness. In the present study, we confirm the embrittling effect of solute segregation in Pt-Au alloys. However, more importantly, we show that inhomogeneous chemical segregation to the grain boundary can lead to a new toughening mechanism termed compositional crack arrest. Energy dissipation is facilitated by the formation of nanocrack networks formed when cracks arrested at regions of the grain boundaries that were starved in the embrittling element. This mechanism, in concert with triple junction crack arrest, provides pathways to optimize both thermal stability and energy dissipation. A combination of in situ tensile deformation experiments and molecular dynamics simulations elucidate both the embrittling and toughening processes that can occur as a function of solute content.

Higher Temperatures Yield Smaller Grains in a Thermally Stable Phase-Transforming Nanocrystalline Alloy

Grains in crystalline materials usually grow with increased thermal exposure. Classical phenomena such as recrystallization may lead to a purely temporary decrease in the grain size, while recent advances in alloy design can yield thermally stable nanocrystalline materials in which grain growth stagnates. But grains never shrink, since there is a lack of interface-generating mechanisms at high temperatures, which are required to decrease the grain size if such was the system's thermodynamic tendency. Here we sidestep this paradigm by designing a nanocrystalline alloy having an allotropic phase transformation-an interface-generating mechanism-such that only the high-temperature phase is stabilized against grain growth. We demonstrate that for an Fe-Au alloy cycled through the α↔γ transformation, the high-temperature phase (γ-Fe) has a stable fine grain size, smaller than its low-temperature counterpart (α-Fe). The result is an unusual material in which an increase in temperature leads to finer grains that are stable in size.

Simultaneous Enhancement of Mechanical and Magnetic Properties in Extremely-Fine Nanograined Ni-P Alloys

Exploring structural effects that influence both the mechanics and magnetism in nanocrystalline materials, particularly extremely-fine nanograined ones with grain sizes down to several nanometers, is of high interest for developing multifunctional materials combining superior mechanical and magnetic performances. We found in this work that electrodeposited extremely-fine nanograined Ni-P alloys exhibit a significant enhancement of magnetization, simultaneously along with an increase in hardness, after low-temperature annealing. The relaxation of non-equilibrium structures, precipitation of the second phase and the segregation of P atoms to grain boundaries (GBs) during annealing have then been sequentially evidenced. By systematically comparing the variations in macroscopic and microstructural investigation results among several Ni-P alloys with different P contents, we suggest that the second phase has little effect on magnetization enhancement, and essentially both the structural relaxation and GB segregation can play important roles in hardening by governing GB stability, and in the improvement of magnetization by enhancing Ni–Ni atom exchange interactions.

Achieving Ultralow Wear with Stable Nanocrystalline Metals

Recent work suggests that thermally stable nanocrystallinity in metals is achievable in several binary alloys by modifying grain boundary energies via solute segregation. The remarkable thermal stability of these alloys has been demonstrated in recent reports, with many alloys exhibiting negligible grain growth during prolonged exposure to near-melting temperatures. Pt-Au, a proposed stable alloy consisting of two noble metals, is shown to exhibit extraordinary resistance to wear. Ultralow wear rates, less than a monolayer of material removed per sliding pass, are measured for Pt-Au thin films at a maximum Hertz contact stress of up to 1.1 GPa. This is the first instance of an all-metallic material exhibiting a specific wear rate on the order of 10^-9 mm³ N^-1 m^-1 , comparable to diamond-like carbon (DLC) and sapphire. Remarkably, the wear rate of sapphire and silicon nitride probes used in wear experiments are either higher or comparable to that of the Pt-Au alloy, despite the substantially higher hardness of the ceramic probe materials. High-resolution microscopy shows negligible surface microstructural evolution in the wear tracks after 100k sliding passes. Mitigation of fatigue-driven delamination enables a transition to wear by atomic attrition, a regime previously limited to highly wear-resistant materials such as DLC.

Ultrastrong Al0.1CoCrFeNi high-entropy alloys at small scales: effects of stacking faults vs. nanotwins

Metastability engineering opens a new avenue to design high-entropy alloys (HEAs) originally proposed to benefit from phase stabilization. Meanwhile, boundary engineering via embedding planar defects such as stacking faults and nanotwins into the matrix of metals provides them with unique mechanical properties. In this work, for the first time, we combine the above two strategies to prepare Al0.1CoCrFeNi HEA pillars populated with a high density of stacking faults and nanotwins. It is uncovered that the stacking faulted (SF) Al0.1CoCrFeNi HEA pillars manifest ultrahigh strength exceeding 4.0 GPa and considerable compressive plasticity over 15%, much superior to their nanotwinned (NT) counterparts. Compared with the nanotwins undergoing detwinning during plastic deformation, the stacking faults in Al0.1CoCrFeNi high-entropy alloy thin films (HEAFs) are quite stable to hinder dislocation motion. Our findings not only endow the Al0.1CoCrFeNi HEAs with a predominant combination of strength and compression deformability, but also shed light on a new perspective for overcoming the strength and ductility trade-off in structural materials.

The W alloying effect on thermal stability and hardening of nanostructured Cu-W alloyed thin films

In order to achieve desired mechanical properties of alloys by manipulating grain boundaries (GBs) via solute decoration, it is of great significance to understand the underlying mechanisms of microstructural evolution and plastic deformation. In this work, nanocrystalline (NC) Cu-W alloyed films with W concentrations spanning from 0 to 40 at% were prepared by using magnetron sputtering. Thermal stability (within the temperature range of 200 °C-600 °C) and hardness of the films were investigated by using the x-ray diffraction, transmission electron microscope (TEM) and nanoindentation, respectively. The NC pure Cu film exhibited substantial grain growth upon all annealing temperatures. The Cu-W alloyed films, however, displayed distinct microstructural evolution that depended not only on the W concentration but also on the annealing temperature. At a low temperature of 200 °C, all the Cu-W alloyed films were highly stable, with unconspicuous change in grain sizes. At high temperatures of 400 °C and 600 °C, the microstructural evolution was greatly controlled by the W concentrations. The Cu-W films with low W concentration manifested abnormal grain growth (AGG), while the ones with high W concentrations showed phase separation. TEM observations unveiled that the AGG in the Cu-W alloyed thin films was rationalized by GB migration. Nanoindentation results showed that, although the hardness of both the as-deposited and annealed Cu-W alloyed thin films monotonically increased with W concentrations, a transition from annealing hardening to annealing softening was interestingly observed at the critical W addition of ∼25 at%. It was further revealed that an enhanced GB segregation associated with detwinning was responsible for the annealing hardening, while a reduced solid solution hardening for the annealing softening.

A Scalable Synthesis Pathway to Nanoporous Metal Structures

A variety of nanoporous transition metals, Fe, Co, Au, Cu, and others, have been readily formed by a scalable, room-temperature synthesis process. Metal halide compounds are reacted with organolithium reductants in a nonpolar solvent to form metal/lithium halide nanocomposites. The lithium halide is then dissolved out of the nanocomposite with a common organic solvent, leaving behind a continuous, three-dimensional network of metal filaments that form a nanoporous structure. This approach is applicable to both noble metals (Cu, Au, Ag) and less-noble transition metals (Co, Fe, Ni). The microstructures of these nanoporous transition metals are tunable, as controlling the formation of the metal structure in the nanocomposite dictates the final metal structure. Microscopy studies and nitrogen adsorption analysis show these materials form pores ranging from 2 to 50 nm with specific surface areas from 1.0 m²/g to 160 m²/g. Our analysis also shows that pore size, pore volume, and filament size of the nanoporous metal networks depend on the mobility of target metal and the amount of lithium halide produced by the conversion reaction. Further, it has been demonstrated that hybrid nanoporous structures of two or more metals could be synthesized by performing the same process on mixtures of precursor compounds. Metals (e.g., Co and Cu) have been found to stabilize each other in nanoporous forms, resulting in smaller pore sizes and higher surface areas than each element in their pure forms. This scalable and versatile synthesis pathway greatly expands our access to additional compositions and microstructures of nanoporous metals.

Stabilizing Nanocrystalline Oxide Nanofibers at Elevated Temperatures by Coating Nanoscale Surface Amorphous Films

Nanocrystalline materials often exhibit extraordinary mechanical and physical properties but their applications at elevated temperatures are impaired by the rapid grain growth. Moreover, the grain growth in nanocrystalline oxide nanofibers at high temperatures can occur at hundreds of degrees lower than that would occur in corresponding bulk nanocrystalline materials, which would eventually break the fibers. Herein, by characterizing a model system of scandia-stabilized zirconia using hot-stage in situ scanning transmission electron microscopy, we discover that the enhanced grain growth in nanofibers is initiated at the surface. Subsequently, we demonstrate that coating the fibers with nanometer-thick amorphous alumina layer can enhance their temperature stability by nearly 400 °C via suppressing the surface-initiated grain growth. Such a strategy can be effectively applied to other oxide nanofibers, such as samarium-doped ceria, yttrium-stabilized zirconia, and lanthanum molybdate. The nanocoatings also increase the flexibility of the oxide nanofibers and stabilize the high-temperature phases that have 10 times higher ionic conductivity. This study provides new insights into the surface-initiated grain growth in nanocrystalline oxide nanofibers and develops a facile yet innovative strategy to improve the high-temperature stability of nanofibers for a broad range of applications.

Novel phase diagram behavior and materials design in heterostructural semiconductor alloys

Structure and composition control the behavior of materials. Isostructural alloying is historically an extremely successful approach for tuning materials properties, but it is often limited by binodal and spinodal decomposition, which correspond to the thermodynamic solubility limit and the stability against composition fluctuations, respectively. We show that heterostructural alloys can exhibit a markedly increased range of metastable alloy compositions between the binodal and spinodal lines, thereby opening up a vast phase space for novel homogeneous single-phase alloys. We distinguish two types of heterostructural alloys, that is, those between commensurate and incommensurate phases. Because of the structural transition around the critical composition, the properties change in a highly nonlinear or even discontinuous fashion, providing a mechanism for materials design that does not exist in conventional isostructural alloys. The novel phase diagram behavior follows from standard alloy models using mixing enthalpies from first-principles calculations. Thin-film deposition demonstrates the viability of the synthesis of these metastable single-phase domains and validates the computationally predicted phase separation mechanism above the upper temperature bound of the nonequilibrium single-phase region.

Additive Manufacturing of Metal Structures at the Micrometer Scale

Currently, the focus of additive manufacturing (AM) is shifting from simple prototyping to actual production. One driving factor of this process is the ability of AM to build geometries that are not accessible by subtractive fabrication techniques. While these techniques often call for a geometry that is easiest to manufacture, AM enables the geometry required for best performance to be built by freeing the design process from restrictions imposed by traditional machining. At the micrometer scale, the design limitations of standard fabrication techniques are even more severe. Microscale AM thus holds great potential, as confirmed by the rapid success of commercial micro-stereolithography tools as an enabling technology for a broad range of scientific applications. For metals, however, there is still no established AM solution at small scales. To tackle the limited resolution of standard metal AM methods (a few tens of micrometers at best), various new techniques aimed at the micrometer scale and below are presently under development. Here, we review these recent efforts. Specifically, we feature the techniques of direct ink writing, electrohydrodynamic printing, laser-assisted electrophoretic deposition, laser-induced forward transfer, local electroplating methods, laser-induced photoreduction and focused electron or ion beam induced deposition. Although these methods have proven to facilitate the AM of metals with feature sizes in the range of 0.1-10 µm, they are still in a prototype stage and their potential is not fully explored yet. For instance, comprehensive studies of material availability and material properties are often lacking, yet compulsory for actual applications. We address these items while critically discussing and comparing the potential of current microscale metal AM techniques.

Nanocrystalline High-Entropy Alloys: A New Paradigm in High-Temperature Strength and Stability

Metals with nanometer-scale grains or nanocrystalline metals exhibit high strengths at ambient conditions, yet their strengths substantially decrease with increasing temperature, rendering them unsuitable for usage at high temperatures. Here, we show that a nanocrystalline high-entropy alloy (HEA) retains an extraordinarily high yield strength over 5 GPa up to 600 °C, 1 order of magnitude higher than that of its coarse-grained form and 5 times higher than that of its single-crystalline equivalent. As a result, such nanostructured HEAs reveal strengthening figures of merit-normalized strength by the shear modulus above 1/50 and strength-to-density ratios above 0.4 MJ/kg, which are substantially higher than any previously reported values for nanocrystalline metals in the same homologous temperature range, as well as low strain-rate sensitivity of ∼0.005. Nanocrystalline HEAs with these properties represent a new class of nanomaterials for high-stress and high-temperature applications in aerospace, civilian infrastructure, and energy sectors.

The thermal stability of the nanograin structure in a weak solute segregation system

A hybrid model that combines first principles calculations and thermodynamic evaluation was developed to describe the thermal stability of a nanocrystalline solid solution with weak segregation. The dependence of the solute segregation behavior on the electronic structure, solute concentration, grain size and temperature was demonstrated, using the nanocrystalline Cu-Zn system as an example. The modeling results show that the segregation energy changes with the solute concentration in a form of nonmonotonic function. The change in the total Gibbs free energy indicates that at a constant solute concentration and a given temperature, a nanocrystalline structure can remain stable when the initial grain size is controlled in a critical range. In experiments, dense nanocrystalline Cu-Zn alloy bulk was prepared, and a series of annealing experiments were performed to examine the thermal stability of the nanograins. The experimental measurements confirmed the model predictions that with a certain solute concentration, a state of steady nanograin growth can be achieved at high temperatures when the initial grain size is controlled in a critical range. The present work proposes that in weak solute segregation systems, the nanograin structure can be kept thermally stable by adjusting the solute concentration and initial grain size.

Strong, ductile, and thermally stable Cu-based metal-intermetallic nanostructured composites

Bulk metallic glasses (BMGs) and nanocrystalline metals (NMs) have been extensively investigated due to their superior strengths and elastic limits. Despite these excellent mechanical properties, low ductility at room temperature and poor microstructural stability at elevated temperatures often limit their practical applications. Thus, there is a need for a metallic material system that can overcome these performance limits of BMGs and NMs. Here, we present novel Cu-based metal-intermetallic nanostructured composites (MINCs), which exhibit high ultimate compressive strengths (over 2 GPa), high compressive failure strain (over 20%), and superior microstructural stability even at temperatures above the glass transition temperature of Cu-based BMGs. Rapid solidification produces a unique ultra-fine microstructure that contains a large volume fraction of Cu₅Zr superlattice intermetallic compound; this contributes to the high strength and superior thermal stability. Mechanical and microstructural characterizations reveal that substantial accumulation of phase boundary sliding at metal/intermetallic interfaces accounts for the extensive ductility observed.

Grain Boundary Specific Segregation in Nanocrystalline Fe(Cr)

A cross-correlative precession electron diffraction - atom probe tomography investigation of Cr segregation in a Fe(Cr) nanocrystalline alloy was undertaken. Solute segregation was found to be dependent on grain boundary type. The results of which were compared to a hybrid Molecular Dynamics and Monte Carlo simulation that predicted the segregation for special character, low angle, and high angle grain boundaries, as well as the angle of inclination of the grain boundary. It was found that the highest segregation concentration was for the high angle grain boundaries and is explained in terms of clustering driven by the onset of phase separation. For special character boundaries, the highest Gibbsain interfacial excess was predicted at the incoherent ∑3 followed by ∑9 and ∑11 boundaries with negligible segregation to the twin and ∑5 boundaries. In addition, the low angle grain boundaries predicted negligible segregation. All of these trends matched well with the experiment. This solute-boundary segregation dependency for the special character grain boundaries is explained in terms of excess volume and the energetic distribution of the solute in the boundary.