Molecular dynamics simulations of severe plastic deformation of monocrystalline and polycrystalline samples by multiaxial compression of aluminum are carried out. The structure evolution is tracked, with special attention paid to the evolution of crystal orientation, flow stress, dislocation density, and dislocation structure. The results show the formation of dislocation cells and subgrain boundaries in the early stage of deformation of the monocrystal, with a gradual increase in misorientation of the subgrain structure until grain refinement is reached. A similar stable grain size distribution is reached after severe plastic deformation of either a monocrystal or a nanocrystalline sample. The subgrains produced in the simulations have a size of approximately similar to 20 nm, and this size agrees with trends observed in real experiments. This study demonstrates for the first time that the principle of similitude holds across 12 orders of magnitude of strain rate. It is necessary, though, to take into consideration that the high strain rates (109 s-1) produce higher dislocation densities ((2-4) & times; 1013 cm-2) and higher stresses (similar to 1600 MPa) in the simulations compared to real experiments.

Molecular dynamics simulations of compression tests were performed for aluminum considering different polycrystalline systems and different average grain sizes. The simulations spanned a grain size range between 5 and 100 nm and the mean flow stress for each sample was determined. The simulations show there is a transition from grain refinement hardening for grain sizes larger than 20 nm to grain refinement softening for smaller grain sizes. The grain size for the maximum flow stress depends on the testing strain rate. The results allowed an estimation of the grain refinement hardening coefficient, equivalent to the Hall–Petch coefficient, and the results are in good agreement with predictions from the model of conventional grain boundary sliding. The stress state in each atom was tracked and used to calculate the local effective stress distribution. It is shown that stress concentrations up to 5 times the applied external stress can develop near grain boundaries and these high stresses are associated with the homogeneous nucleation of dislocations. These dislocations glide across the grains and are absorbed at the opposite grain boundaries without the formation of any dislocation substructures within the grain interiors.

The present work investigates the range of critical cooling rates required to form icosahedral quasicrystals during the solidification of an Al90Cu4Fe2Cr4 (

Metal particles can be welded without significant heating due to severe plastic deformation caused by high pressure torsion. This process has been used to consolidate metallic particles and incorporate ceramic materials into a metallic matrix. The present study demonstrates for the first time that this technique can also be used to incorporate pharmaceuticals into a metallic matrix. In addition, it shows that embedding pharmaceuticals in a biodegradable metal matrix enables the creation of composites with drug delivery capabilities. The results obtained here indicate the successful incorporation of ofloxacin, amphotericin B, and diclofenac sodium into a continuous magnesium matrix. The antifungal and antibacterial activities of the different composites are evaluated along with their drug release kinetics. A new class of biomaterials, biodegradable metals with integrated drug delivery functionality, is introduced in this study.

Recovery plays distinct roles in nanostructured and coarse-grained metallic materials. While static and dynamic recovery usually soften work-hardened, coarse-grained materials, static recovery has been shown to strengthen nanostructured metals. This study extends this understanding by demonstrating that dynamic recovery can also strengthen nanostructured metals under deformation. Tensile, creep, and plane strain compression tests on nanostructured aluminum reveal a trend of increasing strain-hardening with decreasing strain rate and increasing temperature. Molecular dynamics simulations further indicate that sudden strain rate reductions lead to an initial drop in flow stress, followed by strain hardening. These findings suggest that dynamic recovery could serve as an effective strengthening mechanism for nanostructured metals, offering improvements in uniform elongation.

The effect of high-pressure torsion (HPT) processing on mechanical properties in a (3 Ti-42Nb alloy micrometric powder consolidated in a nanostructured bulk disc through severe plastic deformation for biomedical applications is investigated. XRD, SEM and TEM/ASTAR characterization are used to characterize microstructure of the original micrometric powder and the consolidated disc of (3 Ti-42Nb alloy after HPT. The results show that severe plastic deformation processing improves Vickers hardness of (3 Ti-42Nb alloy, and leads to a low modulus bulk material with E = 63 GPa after HPT, comparable with that of as-cast alloy. The obtaining of nanostructured grain size around 55 nm bulk (3 Ti-42Nb alloy consolidated through HPT from micrometric atomized powder opens perspectives of applications as biomedical implants.

Advanced structural materials are expected to display significantly improved mechanical properties and this may be achieved, at least in part, by refining the grain size to the submicrometer or the nanocrystalline range. This report provides a detailed summary of the role of grain size in the mechanical properties of metals. The effect of grain size on the high temperature behavior and the development of superplasticity is illustrated using deformation mechanism maps and the development of exceptional strength through grain refinement hardening at low temperatures is also discussed. It is shown that the deformation mechanism of grain boundary sliding, as developed theoretically, can be used to effectively predict both the high and low temperature behavior of metals having different grain sizes. This analysis explains the increase in strain rate sensitivity in ultrafine-grained metals with low and moderate melting points and the ability to increase both the strength and ductility of these materials to thereby overcome the strength-ductility paradox. The recent development of hybrid materials is also reviewed and it is demonstrated that, although these hybrids have received only limited attention to date, they provide a potential for making significant advances in the production of new structural materials.

The ability of a material to stretch in tension is strongly influenced by the strain rate sensitivity and this parameter plays an even bigger role during deformation of ultrafine and nanocrystalline materials. It was recently shown that a deformation mechanism based on grain boundary sliding can predict the strain rate sensitivity of these materials and the conditions for superplastic elongations. However, other strengthening mechanisms must be taken into account when evaluating the low temperature deformation behavior. The present study advances in this topic by considering two mechanisms to estimate the relationship between the flow stress and the strain rate. The model of grain boundary sliding is used to estimate the grain size strengthening and a general thermally activated mechanism is used to estimate the other strengthening mechanisms. The procedure is validated by hundreds of data points from the literature for different materials with different grain sizes and tested at different temperatures and strain rates. By considering this model, strain rate sensitivity maps are designed and predict the deformation conditions for high ductilities. These maps are further validated by comparing the elongations reported in the literature to the predicted strain rate sensitivities.

The cold angular rolling process (CARP) is being developed as a continuous severe plastic deformation technique, which can process metal sheets without any length limitations at room temperature. CARP contains cold rolling and equal‐channel angular process components. The sheet thickness is kept consistent before and after CARP, allowing multiple passes of the sheet. The desired microstructure and mechanical properties can be achieved in the processed metallic sheets. The current study is aimed to evaluate the capability of CARP by processing copper sheets with different sheet widths for repetitive passes. The CARP‐treated sheets are examined by lab‐scale X‐ray and high‐energy synchrotron X‐ray diffraction to investigate the evolution in dislocation density, texture, and strain anisotropy, and by tensile testing to identify the bulk mechanical properties. The digital image correlation method is applied to tensile testing so that strain localization within the sample gauge is visualized and deformation behavior is evaluated after yielding till postnecking by estimating the hardening exponent and strain hardening rate of the CARP‐treated sheet. Comparing the reported continuous and multiple‐step processes on Cu and its alloys, the present study confirms that the CARP is potentially a useful sheet process for strengthening ductile metals.

In the materials science domain, the accurate prediction of the yield strength of metallic compositions has often resulted in extensive experimental endeavors, leading to inefficiencies in both time and resources. Here, we introduce an innovative approach to predict yield strength, which can be applied to a variety of metallic substances ranging from the simplest pure metals to the most intricate alloys under varying temperatures and strain rates. The fusion of grain boundary sliding mechanism and cutting-edge machine-learning algorithm forges an expansive framework, which can help realize the critical factors influencing yield strength. The validity and wide applicability of the proposed framework were rigorously confirmed through experimental evaluations conducted on selected Fe-based alloys, such as Fe60Ni25Cr15, Fe60Ni30Cr10, and Fe64Ni15Co8Mn8Cu5. This breakthrough study significantly streamlines experimental design processes, optimizes resource utilization, and marks a significant leap forward in creating a reliable predictive framework for realizing material properties.

Solid-state welding of Al 1043 sheets is achieved via high-pressure torsion (HPT) processing to produce bulk nanostructured Al disks. A homogeneous nanostructure without segregation is observed, with grain sizes of approximate to 430-470 nm. Miniature tensile testing, coupled with the digital image correlation (DIC) technique, is employed to determine the room-temperature tensile deformation behavior, particularly the nonuniform behavior with necking, of the HPT-bonded ultrafine-grained (UFG) aluminum, comparing it with annealed coarse-grained counterpart. The HPT-bonded UFG Al exhibits a large fraction of post-necking strain, which is supported by the estimated high strain rate sensitivity value of m = 0.085, suggesting the delay of local necking leading to tensile fracture. Detailed DIC analysis reveals prolonged diffuse necking, thus delaying local necking, in the HPT-bonded UFG Al, while the annealed samples show high fractions of local necking during the nonuniform deformation. Moreover, the DIC data illustrate that local necking predominantly occurred at a limited neck zone, maintaining a plateau strain distribution at the out-of-neck zone throughout necking deformation toward tensile failure for both annealed and UFG aluminum. The DIC method offers an alternative means to demonstrate the transition in necking behaviors of materials by estimating the plastic lateral contraction exponent. This report demonstrates miniature tensile testing coupled with DIC analysis of solid-state welded bulk nanostructured Al alloy through HPT. This study fully utilizes DIC-generated strain data to quantitatively determine the flow and necking behaviors of nanostructured materials, aiming to enhance comprehensive understanding and explore further opportunities to determine the superior ductility of UFG metals.image (c) 2024 WILEY-VCH GmbH

Al-Mg alloy disks were produced from Mg sandwiched between Al through 100 turns of high-pressure torsion (HPT) at 6.0 GPa at room temperature, resulting in high microhardness of Hv 300–350 in regions experiencing a nominal shear strain > 390. While compositional mapping using scanning electron microscopy energy-dispersive spectroscopy (EDS) showed a uniform distribution of Mg through the disk thickness at 1.5 mm and 3.0 mm from the disk center, transmission electron microscopy EDS showed a heterogeneous distribution of Mg remained on the nanoscale. Although HPT induces enough mixing to result in face-center-cubic Al with supersaturations of Mg of up to 20 at.

It is known that the grain size plays a major role in the mechanical properties of magnesium. The aim of the present study is to evaluate its role in long-term corrosion rate. Samples of pure magnesium with grain sizes in the range of 0.9-82 mu m are produced through severe plastic deformation and annealing treatments. The mechanical properties are evaluated using tensile tests and the corrosion behavior is evaluated using immersion tests in Hank's solution. A maximum yield stress of approximate to 150 MPa is observed in the sample with 1.8 mu m of grain size and an elongation larger than 25% is observed in the ultrafine-grained sample. Ultrafine- and fine-grained magnesium display uniform corrosion with a decreasing corrosion rate while coarse-grained magnesium displays localized corrosion with an accelerated corrosion rate. A corrosion rate of approximate to 0.2 mm year-1 is observed in the ultrafine- and fine-grained magnesium. The corrosion product layer of the fine-grained magnesium contains elements absorbed from the media. An analysis of the data in the literature suggests that grain refinement changes the corrosion type from localized to uniform corrosion. The exact relationship between grain size and the corrosion rate remains elusive. Long-term immersion tests are carried out in samples of pure magnesium with grain sizes in the range of 0.9-82 mu m. Results show no clear trend between grain size and corrosion rate. Fine-grained magnesium displays general corrosion while coarse-grained displays localized. image (c) 2024 WILEY-VCH GmbH

Increasing sample dimensions in high‐pressure torsion (HPT) processing affects load and torque requirements, deformation distribution, and heating. Finite‐element modeling (FEM) and experiments are used to investigate the effect of technical parameters on the scaling up of HPT. Simulations confirm that axial load and torque requirements are proportional to the square and the cube of the sample radius, respectively. The temperature rise also displays a pronounced dependency on the radius. Decreasing the diameter‐to‐thickness ratio can cause heterogeneity in strain distribution along the thickness direction at the edges of the sample. Such heterogeneity is governed by friction conditions between the material and the lateral wall of the anvil depression. Simulation of HPT processing of ring‐shaped samples shows that it is possible to reach more homogeneous distribution of strain and flow stress in the processed material. Experiments using magnesium confirm a tendency for strain localization in the early stage of HPT processing but increasing the number of turns increases the homogeneity of the material. The embodied energy in HPT processing is discussed.

The grain size, and therefore the grain boundary density, is known to play a major role in the flow stress of metallic materials. A linear relationship to the inverse of the square root of the grain size was identified about 70 years ago giving rise to the well-established Hall-Petch grain refinement strengthening effect. Nevertheless, grain refinement softening is known to take place at high homologous temperatures and both effects have been given separate treatments. A recent model showed that a general relationship can explain both the Hall-Petch strengthening effect at low temperatures and superplasticity at high temperatures. The present review discusses recent ad-vances in structural and mechanical characterization to provide an updated analysis of trends observed in the relationship between the grain size and the flow stress. The model of grain boundary sliding is evaluated using multiple sets of data in the literature and a general description is provided for the transition between grain refinement hardening and grain refine-ment softening. The analysis incorporate data from over 30 different metals and alloys with different grain sizes and after testing at different strain rates and temperatures. Data from mo-lecular dynamic simulations are also included and show supporting evidence to the model of grain boundary sliding. The thermal contribution of the grain size strengthening and threshold stress is discussed including the trends observed in the strain rate sensitivity of fine-grained materials.

Recent studies show significant advances in improving the mechanical properties of magnesium and its alloys. While many papers deal with different alloy compositions, it is apparent that grain size plays a key role in the mechanical behavior of these materials. The ability to produce samples with very fine grain sizes leads to observations of high strength and/or high elongations. There are recent reports of exceptional elongations of over 100% in pure magnesium and a few alloys. These recent findings are critically reviewed in the present study. The experimental data from over 300 papers are collected, and trends between flow stress, elongation, strain rate sensitivity, and grain size are identified. The role of alloy content is examined. The data clearly shows a transition in the flow stress vs. grain size relationship which is attributed to a change in deformation mechanism from twinning controlled in coarse grained to slip controlled in fine and ultrafine grained samples. The slip controlled deformation agrees with the model of grain boundary sliding, which has shown good agreement with multiple metallic materials. It is shown that the elongations display a maximum in the grain size range in which there is a transition in the deformation mechanism. Three strategies are described for achieving high strength, high ductility, and good strength-ductility combination. Keywords: magnesium; grain refinement; mechanical properties; Hall-Petch

Abstract. Grain boundary sliding plays a key role on the high temperature deformation of fine grained materials. This mechanism is related to a high strain rate sensitivity of approximately 0.5 and usually gives rise to high superplastic elongations. The rate controlling equation for the mechanism of grain boundary sliding has shown good agreement with experimental data for multiple materials, with different grain sizes and tested at different strain rates. However, the predictive ability of the rate controlling equation seems to deteriorate at low temperatures. Although there are experimental evidences of high strain-rate sensitivities in ultrafine grained materials tested at low temperatures, this parameter does not reach values near 0.5 and also there seems to be disagreement in stress level in many conditions. The present overview evaluates the occurrence of grain boundary sliding in ultrafine grained materials at low temperatures considering an adapted rate controlling equation which display good agreement with experimental data. A gradual transition from grain refinement softening at high temperature to grain refinement hardening at low temperatures and a gradual increase in strain rate sensitivity with increasing temperature are observed.

Magnesium and its alloys display a non-usual relationship between flow stress and grain size at room temperature. Breaks in the Hall-Petch relationship have been reported in the literature. Inverse Hall-Petch behavior in which flow stress reduces with grain size decreasing has also been reported in pure magnesium and magnesium alloys with ultrafine and nanocrystalline structures. The present overview discusses these effects in terms of controlling deformation mechanisms. The distinct strength observed in pure magnesium and magnesium alloys with ultrafine grained structure is also discussed. It is shown that experimental data for fine and ultrafine grained magnesium alloys agree with a model suggested recently based on the mechanism of grain boundary sliding. It is also exhibited that the stability of the grain structure might control the strength of ultrafine grained samples.

The effect of high pressure torsion processing on mechanical properties and corrosion behavior of pure magnesium and Mg-Zn, Mg-Zn-Ca, Mg-Li-Y and Mg-Y-RE alloys is investigated. Micro-tomography and SEM characterization are used to estimate corrosion rate and evaluate non-uniform corrosion features. The results show that severe plastic deformation processing improves the strength of all magnesium alloys, but deformation localization can take place in the Mg-Zn-Ca and Mg-Y-RE alloys. The occurrence of deformation localization is associated with low strain rate sensitivity in these alloys and with severe corrosion localization. Pure magnesium and Mg-Zn and Mg-Li-Y alloys display good corrosion resistance with a low corrosion rate and maintain integrity after 28 days of immersion in Hank's solution.

Severe plastic deformation through high-pressure torsion is used to refine the grain structure of the Mg–Al–Zn alloy down to ~ 140 nm and low temperature annealing is used to produce samples with different grain sizes, within the ultrafine range. The mechanical behavior is investigated using different testing techniques including microhardness, indentation creep, plane-strain compression, creep, and miniaturized tensile testing. The results allow a comprehensive analysis of the deformation mechanism. It is shown that the relationship between the flow stress and the inverse of the square root of the grain size is not linear in the ultrafine grained range and depends on temperature and strain rate. Grain refinement hardening and grain refinement softening can be observed at different temperatures and strain rates. There is an increase in strain-rate sensitivity and a decrease in apparent activation volume with decreasing the grain size. These experimental observations agree with the deformation mechanism of grain boundary sliding provided the thermal contribution for the threshold stress is taken into account.