The cellular structure is a crucial factor in understanding the mechanical properties of metal additive manufacturing (MAM) processed components. Its systematic monitoring and evaluation are essential for developing new alloys optimized for MAM. This study presents in situ neutron diffraction experiments and dislocation density-based constitutive modeling for the CoCrFeMnNi high entropy alloy (HEA) produced via directed energy deposition (DED). A dislocation density-based constitutive equation, grounded in the Kocks-Mecking-Estrin model, was employed to represent the HEA's cellular structure, combining in situ neutron diffraction data for enhanced accuracy. By parametrically analyzing the constitutive model, we found that the rates of dislocation accumulation and annihilation in the as-built sample were lower than in the heat-treated sample, which lacks a cellular structure. These observed differences can be attributed to the action of a dislocation forest network and local variations in stacking fault energy due to elemental segregation. Furthermore, the study examined dislocation density changes across different regions (cell interiors and cell walls) and compared these observations with dislocation cells formed during plastic deformation.

This focus issue is dedicated to Dr. Ladislas Kubin, an esteemed member of the materials community who passed away in October 2022. Ladislas has been a pioneer in several research areas whose work helped shape the modern day understanding of the physical mechanisms of strength and plasticity of metallic materials. The world of his ideas rested on three whales: in situ transmission electron microscopy (TEM), discrete dislocation dynamics (DDD) simulations, and constitutive modelling of macroscopic plastic deformation

High-pressure torsion (HPT) is widely used as a key method for microstructure control through deformation processing across a broad range of materials. However, certain gaps in process control impact its efficacy. In this study, we investigate the acoustic emission (AE) signals generated during HPT by considering commercially pure molybdenum as an example. By employing the adaptive sequential k-means algorithm, we analyse the AE stream to categorise and identify its sources. By comparing the kinetics of AE signal evolution during HPT processing at pressures of 2 GPa and 5 GPa, two distinct signal types are identified: one linked to plastic deformation and the other to workpiece slippage over HPT anvil surfaces. This research demonstrates the potential of AE tools for operando monitoring of HPT stability and detection of workpiece slippage, thereby enhancing the processing efficiency.

A continuum mechanics approach to cold welding (CW) of metals under shear is considered. The main idea is to treat a weld joint as an extra material—a “third body” in its own right. Its properties stem from plastic co‐deformation of the two contacting alloys. The mechanical characteristics of the weld joint, i.e., its strength and plasticity in the complex stress state, are determined by the deformation history of the “third body.” The proposed approach enables a unified description of the CW process itself, as well as the subsequent variation of shape of the composite material with the weld joint.

An approach for modeling topologically interlocked building blocks that can be assembled in a water‐tight manner (space filling) to design a variety of spatial structures is introduced. This approach takes inspiration from recent methods utilizing Voronoi tessellation of spatial domains using symmetrically arranged Voronoi sites. Attention is focused on building blocks that result from helical stacking of planar 2‐honeycombs (i.e., tessellations of the plane with a single prototile) generated through a combination of wallpaper symmetries and Voronoi tessellation. This unique combination gives rise to structures that are both space‐filling (due to Voronoi tessellation) and interlocking (due to helical trajectories). Algorithms are developed to generate two different varieties of helical building blocks, namely, corrugated and smooth. These varieties result naturally from the method of discretization and shape generation and lead to distinct interlocking behavior. In order to study these varieties, finite‐element analyses (FEA) are conducted on different tiles parametrized by 1) the polygonal unit cell determined by the wallpaper symmetry and 2) the parameters of the helical line generating the Voronoi tessellation. Analyses reveal that the new design of the geometry of the building blocks enables strong variation of the engagement force between the blocks.

The mechanism of structural evolution of a three-layer Cu-Mo-Cu laminate under high-pressure torsion (HPT) was studied using scanning and transmission electron microscopy, atom probe tomography, and nanoindentation, complemented with finite element calculations. The results demonstrate a gradual refinement of the structure of the Mo component; a greater degree of refinement is observed in the peripheral part of the disk-shaped HPT specimen, although some heterogeneity of the structure remains even at a gigantic degree of shear deformation accumulated therein. The elemental distribution calculated from STEM-EDX mapping as well as 3D reconstruction of atom probe tomography results shows a significant degree of mixing of the sample components at the atomic level, the concentration of copper in molybdenum and molybdenum in copper reaching ∼4.3 at. % and ∼6 at. %., respectively. These observations correlate with nanoindentation results showing an increase in the hardness of both phases due to strain hardening and solid solution strengthening, as well as grain refinement. Numerical simulations made it possible to provide a detailed description of the stages of the structure fragmentation, including its self-organizing nature, to show the formation of rupture forerunners in the hard Mo layer, and the deformation of harder fragments in a softer matrix. The experimental results are supported by a model assuming a fractal self-organization of a self-similar structure during HPT processing.

We posited that the grain size dependence of the tensile necking stress, as determined by the Considère criterion for plastic instability, is a more meaningful characteristic of the Hall–Petch (H–P) effect than that of the yield stress or the 0.2% proof stress. An inverse square-root dependence of the necking stress on the grain size was derived from a dislocation dynamics-based constitutive model. In this model, the grain size effect enters the stress indirectly via the evolution of the dislocation density. Model predictions were confirmed by the experimental data for nickel and titanium.

The article presents a theoretical study of the regimes of high-pressure torsion (HPT) for which slippage of the deforming material on the interfaces with anvils is possible. The approach taken is a generalisation of the currently accepted view of the HPT process. It enables a rational explanation of its salient features and the effects observed experimentally. These include a lag in the rotation angle of the specimen behind that of the anvils, an outflow of the material from the deformation zone, enhancement in gripping the specimen with anvils with increasing axial pressure, etc. A generalised condition for gripping the specimen with anvils, providing a basis for an analytical investigation of the HPT deformation at a qualitative level, is established. The results of the analytical modelling are supported by finite-element calculations. It is shown that for friction stress below the shear stress of the specimen material (i.e., for the friction factor m < 1), plastic deformation is furnished by non-shear flows, which expands the range of possible process regimes. The potential of these flow modes is impressive, which is reflected in the second meaning of the word “gripping” in the title of the article. Non-shear flows manifest themselves in the spreading of the material over the anvil surfaces whose cessation signifies the end of deformation and the beginning of slippage of the specimen as a whole. The model shows that for m < 1 such a finale is inevitable at any axial pressure. It predicts, however, that the highest achievable strain is increased when the axial pressure is raised in the course of the HPT process. Unlimited deformation of the specimen is only possible for m = 1, when slippage of the deforming material relative to the anvils is suppressed.

Through the current work, the usefulness of the concept of architectured rod lattices based on unit cell motifs designed at mesoscale is demonstrated. Specifically, 2D triangular lattices with unit cells containing different numbers of rods are considered. Combinations of rods of two different types provide the lattices explored with a greater complexity and versatility. For mesocells with a large number of variable parameters, it is virtually impossible to calculate the entire set of the points mapping the material onto its property space, as the volume of calculations would be gigantic. The number of possible motifs increases exponentially with the number of rods. Herein, the lattice metamaterials with mesoscale motifs are investigated with the focus on their elastic properties by combining machine learning techniques (specifically, Bayesian optimization) with finite element computations. The proposed approach made it possible to construct property charts illustrating the evolution of the boundary of the elastic compliance tensor of lattice metamaterials with an increase in the number of rods of the mesocell when a full‐factor experiment would not be possible.

Crumpled metallic thin foils are very promising as weight‐saving and energy‐absorption materials that can easily be fabricated. To achieve practical industrial applications, it is necessary to improve the understanding of the process of crumpling and the mechanical behavior of crumpled materials considering their complex internal structures. Herein, two possible strategies for computational simulations of crumpled material under closed‐die compression are presented for the first time. The first one entails computations performed at the scale of the foil (direct method), while the second one considers the structure as a continuum with porosity. The analysis performed shows that the continuum‐based approach is more suitable for representing the macroscopic mechanical behavior of crumpled materials with the relative densities ranging from 2% to 40%. An additional benefit is the low computational cost and high efficacy of the porous continuum approach. However, the direct method is shown to be the preferable computational tool when the internal structural patterns changes need to be adequately reproduced, e.g., for a better prediction of the mechanical response under complex loading conditions.

Magnesium and its alloys are the most investigated materials for solid-state hydrogen storage in the form of metal hydrides, but there are still unresolved problems with the kinetics and thermodynamics of hydrogenation and dehydrogenation of this group of materials. Severe plastic deformation (SPD) methods, such as equal-channel angular pressing (ECAP), high-pressure torsion (HPT), intensive rolling, and fast forging, have been widely used to enhance the activation, air resistance, and hydrogenation/dehydrogenation kinetics of Mg-based hydrogen storage materials by introducing ultrafine/nanoscale grains and crystal lattice defects. These severely deformed materials, particularly in the presence of alloying additives or second-phase nanoparticles, can show not only fast hydrogen absorption/desorption kinetics but also good cycling stability. It was shown that some materials that are apparently inert to hydrogen can absorb hydrogen after SPD processing. Moreover, the SPD methods were effectively used for hydrogen binding-energy engineering and synthesizing new magnesium alloys with low thermodynamic stability for reversible low/room-temperature hydrogen storage, such as nanoglasses, high-entropy alloys, and metastable phases including the high-pressure γ-MgH2 polymorph. This work reviews recent advances in the development of Mg-based hydrogen storage materials by SPD processing and discusses their potential in future applications.

Бейгельзімер Я. Ю., Кулагін Р. Ю., Естрін Ю. З., Давиденко О. А., Дмитренко В. Ю. Литоподібні архітектури, сформовані методами інтенсивної пластичної деформації Одна з найбільш ефективних концепцій сучасного матеріалознавства - матеріали з внутрішньою архітектурою (architectured materials), полягає в максимально можливому використанні тих резервів, які надає структура матеріалу по формуванню його властивостей. У статті описаний новий підхід до створення таких матеріалів, заснований на тому, що композиції з різних металів піддають великому зсуву під високим тиском. Для цього використовують добре розвинені в даний час методи інтенсивної пластичної деформації (ІПД): кручення під високим тиском, рівноканальне кутове пресування, гвинтова екструзія тощо. Дослідниками показано, що ІПД-обробка призводить до міцного з'єднання компонентів композиції між собою та контрольованого формування в ній мультімасштабних структур. На нижньому масштабному рівні створюються наноструктури, головним елементом яких є нерівноважні висококутові границі зерен, товщиною близько 1 нм. На проміжних масштабних рівнях, з характерним розміром елементів порядку 1-100 мкм, формуються мезоструктури, подібні до тих, що спостерігаються в літосфері землі: складки, будини, вихрові структури, кінк-бенди, смуги зсуву та ін. Звідси і назва нового підходу - літоміметіка. Є підстави вважати, що на цьому шляху можуть бути створені нові матеріали з високою в'язкістю руйнування і з властивостями, які зазвичай разом непоєднувані, наприклад: високою міцністю, високою пластичністю, низьким модулем Юнга, малою густиною, гарною біосумісністю та ін.

The term 'solid-state turbulence' may sound like an oxymoron, but in fact it is not. In this article, we demonstrate that 'solid-state turbulence' may emerge owing to a defining property of the solid state: the ability of a solid to retain its shape. We consider shear flow under high-pressure torsion of layers of solids with different flow stress and show that the stiffer ones may spontaneously decompose into a set of blocks. This effect is fundamental for the occurrence of 'solid-state turbulence' (SST). To visualize SST, we use a heuristic model based on discretization of a continuum into interacting 'particles'. The outcomes of the numerical experiments conducted support the occurrence of pulsations of velocity and pressure in plastically deforming solids and the emergence of vortices characteristic of classical turbulence. This phenomenon may have important practical implications for solid-state mixing as an ecologically beneficial alternative to conventional metallurgical processing routes.

This overview highlights some salient features of one of the most popular severe plastic deformation techniques: high-pressure torsion (HPT). It focuses on the unresolved challenging problems of HPT. The problems selected touch upon some fundamental questions of mechanics of plasticity, fracture, and friction that are at the core of the HPT process. The scientific significance of these problems and the proposed pathways to resolving them are discussed. The article is meant to promote the use of HPT as a potent tool for studying plasticity at large strains theoretically and also as a practical method enabling novel micromanufacturing routes.

A physically based model is employed to trace the strain-hardening behavior of a metastable medium-entropy alloy (MEA) Fe 61 (CoNi) 29 Cr 10 governed by a deformation-induced martensitic transformation. The approach is based on calculating the local quantities of the individual phases by means of constitutive modeling. An acceptable agreement between the experimental and the calculated stress–strain curves is verified. The model also demonstrates the variation of the phase composition of the MEA with strain due to the phase transformation.

It is almost commonplace to say that physics-based constitutive models developed to characterize the mechanical behavior of materials are to be preferred over phenomenological models. However, the constitutive relations offered by physics-based approaches are oftentimes too involved to be handled in finite element (FE) simulations for practical applications. There is a demand for physics-based, yet robust and user-friendly models, and one such model will be highlighted in this article. A simple constitutive model developed recently by Bouaziz to extend the classical physics-based Kocks-Mecking model provides a viable tool for modelling a broad range of materials – beyond the single-phase coarse-grained materials it was initially devised for. The efficacy of the model was put to the test by investigating its applicability for different materials. A broad interval of the true stress vs. true strain curve was studied by the measurement-in-neck-section method in the uniaxial tensile mode for six types of metallic materials, and simulations using the finite element method emulating the experimental conditions were developed. In this way, the engineering stress-strain curves were obtained corresponding to the true stress-strain curves for these materials. A comparison of the numerical simulations of the tensile behaviour of all six materials with the experimental results for a broad range of strains showed that among the models trialled, the Bouaziz model was the best-performing one. The proposed model can be recommended for use in FE simulations of the mechanical behaviour of engineering structures as a viable alternative to complex physics-based or simplistic phenomenological constitutive models.

A new kind of composite were manufactured by densification of co-crumpled aluminium and tantalum thin foils using close die compression. It was shown by optical micrography that its microstructure is highly interlocked. The morphology was analysed quantitatively in terms of the following three parameters: the area of the foil interface per unit volume, the interface tortuosity, and a characteristic of the local orientation of the foil surface. Based on these parameters, co-crumpled material studied has been compared with conventional laminates. A fractal nature of its self-similar structure was revealed.

In this article, we present our take on modeling the Bauschinger effect. The main goal is to correlate the microstructure-based modeling developed for uniaxial tension/compression deformation and the tensorial modeling approach of the continuum mechanics. After a brief historial review, we present a microstructure-related model that was proven to provide an adequate description of the Bauschinger effect in terms of kinematic and isotropic strain hardening. Its generalization to the case of multiaxial loading is then formulated in terms of a continuum mechanics model. The full tensorial model developed is now being offered to the solid mechanics and physical metallurgy communities as an advanced modeling tool.