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Superplastic forming is a process that enables the production of complex-shaped parts using metallic alloys. To design the optimal forming regimes and ensure the success of forming operations, it is essential to use mathematical models that accurately represent the superplastic deformation behavior. This paper is concerned with the study of the microstructure and superplastic deformation behavior, with the construction of a constitutive model, of Al-Mg-Zn-Cu-Zr aluminum alloys with varying Ni contents. The aluminum solid solution and coarse precipitates of the T(Mg32(Al,Zn)49 and Al3Ni second phases were formed in the studied alloy and Cu dissolved in both second phases. The deformation behavior was investigated in the temperature range of 400–480 °C and the strain rate range of 10−3–10−1 s−1. Due to the fine Al3Zr precipitates, the alloys exhibit a partially recrystallized grain structure before the onset of superplastic deformation. Coarse precipitates of the second phases facilitate dynamic recrystallization and enhance superplasticity at the strain rates and temperatures studied. The alloys with ~6–9% particles exhibit high-strain-rate superplasticity at temperatures of 440–480 °C and strain rates of 10−2–10−1 s−1. The presence of high fractions of ~9% Al3(Ni,Cu) and ~3% T-phase precipitates provided high-strain-rate superplasticity with elongations of 700–800% at a low temperature of 400 °C. An Arrhenius-type constitutive model with good agreement between the predicted and experimental flow stresses was developed for the alloys with different Ni contents. [ABSTRACT FROM AUTHOR]

The paper examines the microstructural evolution of alloy 1565ch of the Al-Mg-Mn-Zn-Zr system during thermomechanical treatment, including severe plastic deformation by high-pressure torsion or equal channel angular pressing according to the Conform scheme and subsequent isothermal rolling at 200°C. Formation of the nanostructured and ultrafine-grained states in alloy 1565ch with the controlled distribution of the Al3Mg2, Al6Mn and Al3Zr phases both inside grains and at their boundaries allows for the effect of superplasticity at the temperatures 250 and 300°C and strain rates 5 × 10–2, 10–2, and 5 × 10–3 s–1. Microstructural analysis by transmission electron microscopy shows that superplastic deformation at the temperatures 250 and 300°C allows a homogeneous ultrafine-grained state to be preserved. The studied ultrafine-grained aluminum alloy 1565ch has a high strength and the ability to relieve stresses, and therefore it can be favorably used as the matrix material in composites reinforced with continuous boron fibers. In the paper, we use this alloy to study special features of production of a multilayer (foil–fiber–foil) metal matrix composite by isothermal pressing under low-temperature superplastic conditions. This method has a positive effect on the mechanical properties of the composite, such as ultimate strength at 200°C, impact strength at room temperature, and fracture toughness at room temperature. [ABSTRACT FROM AUTHOR]

In the present article, insights into the high‐temperature deformation behavior of medium‐Mn steel (MMS), which is prepared via high‐ratio differential speed rolling (HRDSR), are provided. Moreover, through innovative bidirectional jump experiments, variations in the strain rate sensitivity index m under various conditions are obtained. In the research findings, it is indicated that an increase in strain rate (SR) leads to a hardening of the alloy. During high‐temperature deformation, the value of m decreases with the increase in SR, but the rate of decrease gradually slows down. Furthermore, the higher the temperature (T), the greater the impact of changes in SR on m. In addition, the change in deformation mechanism during deformation leads to microstructural changes, and under the main deformation mechanism of grain‐boundary sliding, m generally increases with strain. Interestingly, at a T of 760 °C, the material exhibits a strong texture with high orientation, resulting in larger m values and superior superplasticity (≈691%). This study not only enriches the research content of HRDSR but also has significant implications for the superplasticity research of MMS. Moreover, a reference is provided for the research of other superplastic materials. [ABSTRACT FROM AUTHOR]

This paper proposes an innovative multi-scale method for determining gas pressure parameters of superplastic forming, which is based on the quantitative relationship between the grain growth mechanism and fracture mechanism of Tie6Ale4V alloy. The high-temperature tensile tests were conducted on the material at temperatures ranging from 700, 800, 840, 890, 920, and 950 °C, strain rates were selected as 10-2~10-4/s. The grain size measurements were observed using electron back-scatter diffraction (EBSD). Particularly, the relation between grain size changes and fracture behaviour is specifically discovered using a physically-based dynamic material model (DMM), and the grain size thresholds for each forming limit are proposed. The physical fracture mechanism is named the "Grain growth based fracture (GGBF)" mechanism. Furthermore, an innovative method based on the GGBF mechanism is proposed to design the superplastic forming loading, and practical four-layer hollow structures experiments are applied to validate the fracture mechanism in superplastic forming. In total, A superplastic forming GGBF mechanism has been verified, and it is expected to be helpful for shape and property control in the forming process of complex structures. [ABSTRACT FROM AUTHOR]

In this work, superplastic behavior in tension for the Zn-21Al-2Cu alloy was reviewed as a function of: grain size, temperature and strain rate. The deformation mechanism was studied under conditions where the greatest elongation was reached, characterizing microstructural changes and analyzing the associated mechanical response such as the study of plastic stability. This analysis allowed us to propose a phenomenological model consisting of five steps for the mechanism of superplastic deformation under which dynamic conditions are involved for this alloy. In the first stage, an accommodation of the microstructure was observed, in the second stage sliding by individual grain boundaries (GBS) was activated, which provided the conditions for stationary plastic flow. In the third stage, GBS was hampered by the tendency of grain boundaries remaining from high temperature phase (FβBs) to align at 45°. This fact caused the onset of plastic instability. The fourth stage consisted of a transition in which there was competition between individual and cooperative GBS mechanisms, which increased plastic instability. In the last stage, the FβBs were aligned parallel to tensile direction, which favored the GBS, and an additional diffusion flow mechanism allowed partial recovery of stable plastic flow. [ABSTRACT FROM AUTHOR]

The study of changes in the dynamic Young's modulus of a typical model superplastic Bi-43 wt.% Sn alloy under the conditions of plastic deformation, under which polycrystalline materials are subjected in order to create a structural-phase state capable of manifesting the effect of superplasticity, is performed. Changes in the Young's modulus are also studied during longterm exposure at room temperature and normal atmospheric pressure, as a result of which the phenomenological indicators of the superplastic flow of the studied alloy are noticeably reduced, but the manifestation of the effect of superplasticity is observed. Acoustic measurements are carried out using the method of a two-component piezoelectric vibrator. An increase in the dynamic Young's modulus as a result of compression by ≅ 70% on a hydraulic press and in the ageing process is found in both cast and compressed samples. The obtained experimental data are analysed taking into account previously obtained data on changes in the phase composition of the alloy under experimental conditions. The results of the analysis show that the increase in the Young's modulus as a result of compression is caused by the appearance of internal stresses in the material. The increase in the Young's modulus during ageing is primarily related to the transition of the alloy from the initial metastable state to the phase state, which is in equilibrium at room temperature. On the kinetic dependences of the modulus of elasticity in both cast and compressed samples, there is an inhibition of its changes at the ageing stag, when the phase equilibrium in the alloy has not yet been established. This is explained by the change in the kinetics of the decomposition of the α(Sn)-phase (a supersaturated solid solution of bismuth in tin) caused by the appearance of phase stresses associated with the volume effect of phase transformation. As shown, such stresses have an inhibitory effect on the progress of decomposition. [ABSTRACT FROM AUTHOR]

The microstructure evolution in VT6 alloy during multiple forging and subsequent isothermal rolling at a temperature of 560 °C is studied. The combined treatment resulted in the formation of an ultrafine-grained (UFG) structure in alloy which made it possible to implement the effect of low-temperature superplasticity (LTSP). The deformation relief formed on the gauge part of UFG samples at the stage of a steady-state LTSP flow is analyzed. It is established that LTSP at 650 °C is carried out by cooperative grain-boundary sliding. UFG alloy was used to study the peculiarities of metal-matrix composite manufacturing according to the scheme "foil–fiber–foil" by pressing in the LTSP mode. Findings testify that selected and justified parameters of isostatic pressing of Ti-B composite under LTSP conditions provide excellent bonding of the matrix material layers smoothly filling the gaps between the fibers and forming reliable connection of the matrix and fibers. [ABSTRACT FROM AUTHOR]

Achieving impressive superplasticity is an important strategy to manufacture Mg alloy products with complex shapes. In the present study, we report that an excellent superplastic deformation with elongation larger than 500% can be achieved at 623 K and 1.0 × 10−3 s−1 in a Mg-1.51Zn-0.59Ca-0.59Al-0.70Mn (wt.%, ZXAM2111) alloy fabricated by equal-channel angular pressing. The superplastic deformation is mainly carried by grain boundary sliding (GBS), accompanied by a grain size growth from ∼3.0 µm to ∼6.0 µm after deformation. Before deformation, the ZXAM2111 alloy is mainly characterized by a strong co-segregation of Zn and Ca atoms at grain boundaries and uniformly distributed β-Mn particles. With deformation proceeding, the β-Mn particles further dynamically precipitate along grain boundaries that parallel the tensile axis, leading to improved resistance to grain coarsening. Although the enhanced stabilizing effects decrease the strain rate sensitivity value, the resulting impressive microstructure stability provides a cornerstone of the active operation of GBS, facilitating the achievement of superplastic deformation. The present work could provide insight into developing low-alloyed Mg alloys with high microstructure thermal stability to achieve superplasticity.

Currently, studies of structural superplasticity (SP) are of great interest, since the use of this mode in metalworking technologies allows for the production of parts of various shapes in one operation (with high repetition accuracy of even very complex shapes), while requiring less energy and material resources (relatively low pressures and tool wear) compared to deformation in the ‘normal plasticity’ mode. Other advantages of using a structural SP are improved physical and mechanical characteristics of the finished product: better surface quality after deformation, high ductility at elevated temperatures, increased strength at temperatures close to room one without reducing ductility (most often there is an increase in ductility), increased cyclic strength, hardness, impact resistance, elevated corrosion-resistance durability, and absence of anisotropy of properties after superplastic deformation.

Achieving impressive superplasticity is an important strategy to manufacture Mg alloy products with complex shapes. In the present study, we report that an excellent superplastic deformation with elongation larger than 500% can be achieved at 623 K and 1.0 × 10−3 s −1 in a Mg-1.51Zn-0.59Ca-0.59Al-0.70Mn (wt.%, ZXAM2111) alloy fabricated by equal-channel angular pressing. The superplastic deformation is mainly carried by grain boundary sliding (GBS), accompanied by a grain size growth from ∼3.0 µm to ∼6.0 µm after deformation. Before deformation, the ZXAM2111 alloy is mainly characterized by a strong co-segregation of Zn and Ca atoms at grain boundaries and uniformly distributed β-Mn particles. With deformation proceeding, the β-Mn particles further dynamically precipitate along grain boundaries that parallel the tensile axis, leading to improved resistance to grain coarsening. Although the enhanced stabilizing effects decrease the strain rate sensitivity value, the resulting impressive microstructure stability provides a cornerstone of the active operation of GBS, facilitating the achievement of superplastic deformation. The present work could provide insight into developing low-alloyed Mg alloys with high microstructure thermal stability to achieve superplasticity. [ABSTRACT FROM AUTHOR]

To achieve higher strength and better durability, ultra-high performance concrete (UHPC) typically employs a relatively small water-binder ratio. However, this generally leads to an undesired increase in the paste viscosity. In this study, the effects of liquid and powder polycarboxylate superplasticizers (PCE) on UHPC are compared and critically discussed. Moreover, the following influential factors are considered: air-entraining agents (AE), slump retaining agents (SA), and defoaming agents (DF) and the resulting flow characteristics, mechanical properties, and hydration properties are evaluated assuming UHPC containing 8‰ powder PCE (PCE-based UHPC). It is found that the spread diameter of powder PCE is 5% higher than that of liquid PCE. Among the chemical admixtures studied, AEs have the best effect on improving UHPC workability, while DFs have the worst effect. When the addition of AE and SA is 1.25‰ and 14.7% of PCE, paste viscosity reduces by 35% and 19%, respectively compared to the paste with only 8‰ PCE. A low AE dosage (1.25‰) decreases compressive strength by 4.1%, while SA (8.1%) increases UHPC compressive strength by 9.1%. Both AE and SA significantly delay the UHPC hydration process, reducing the hydration heat release peaks by 76% and 27%, respectively. [ABSTRACT FROM AUTHOR]

We continue the systematic analytical study of the nonlinear Maxwell-type constitutive equation for shear flow of tixotropic viscoelastoplastic media formulated in the previous article. It accounts for interaction of deformation process and structure evolution, namely, the influence of the kinetics of formation and breakage of chain cross-links, agglomerations of molecules, and crystallites on viscosity and shear modulus and deformation influence on the kinetics. The constitutive equation is governed by an increasing material function and six positive parameters. Assuming that the stress is constant (in order to simulate creep conditions), we formulate the set of two nonlinear differential equations for two unknown functions (namely, strain and cross-links density) and obtain its exact general solution in explicit form. We examine the properties of creep curves generated by the model for arbitrary material function and material parameters and analyze dependence of creep curves and cross-links density on time, stress level, initial cross-links density, and material parameters governing the model. Thus, we prove that the model not only describes basic phenomena observed for simple shear flow of shear thinning fluids, but can simulate creep, relaxation, and other phenomena observed for solid bodies. [ABSTRACT FROM AUTHOR]

The aim of this work is to predict, for the first time, the high temperature flow stress dependency with the grain size and the underlaid deformation mechanism using two machine learning models, random forest (RF) and artificial neural network (ANN). With that purpose, a ZK30 magnesium alloy was friction stir processed (FSP) using three different severe conditions to obtain fine grain microstructures (with average grain sizes between 2 and 3 µm) prone to extensive superplastic response. The three friction stir processed samples clearly deformed by grain boundary sliding (GBS) deformation mechanism at high temperatures. The maximum elongations to failure, well over 400% at high strain rate of 10−2 s−1, were reached at 400 °C in the material with coarsest grain size of 2.8 µm, and at 300 °C for the finest grain size of 2 µm. Nevertheless, the superplastic response decreased at 350 °C and 400 °C due to thermal instabilities and grain coarsening, which makes it difficult to assess the operative deformation mechanism at such temperatures. This work highlights that the machine learning models considered, especially the ANN model with higher accuracy in predicting flow stress values, allow determining adequately the superplastic creep behavior including other possible grain size scenarios.

Superplastic forming is considered a highly efficient technique for shaping intricate components from titanium matrix composites (TMCs). In this work, high-temperature TMCs with 5 vol% (TiBw + TiCp) reinforcements underwent superplastic tensile tests at various temperatures and strain rates. The results demonstrated that at all deformation temperatures (900 °C–1050 °C) and strain rates (5 × 10−3 s−1 to 10−4 s−1), the elongation of composite sheets surpassed 100%. The strain rate of 10−3 s−1 and a temperature of 1000 °C were found to yield the maximum elongation of 328.1%. At 900 °C, the matrix grains maintain a lamellar morphology during the main stage of deformation, and dynamic recovery (DRV) is the primary mechanism of matrix softening. At 1000 °C, wide-range dynamic recrystallization (DRX) takes place, and grain boundary slip coordinated by grain rotation is the main mechanism of deformation. At 1050 °C, the matrix grains undergo DRX and grow rapidly by migration, and the number of grain boundaries decreases dramatically, resulting in poor superplastic qualities. In the early stages of superplastic deformation, micropores sprout at the triple grain boundaries and at the interfaces between the reinforcements and the matrix. As the deformation proceeds, the micropores extend along the tensile direction and connect with each other to form cavity stringers (CS). The distance between the CS rapidly narrows as the stretching process continues, and eventually the CS links horizontally to form cavity coalescence (CC). Micropore dilatation around the reinforcements causes debonding and cavitation, leading to material failure.

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 last two decades have witnessed significant progress in the development of severe plastic deformation techniques to produce ultrafine-grained materials with new and superior properties. This review examines works and achievements related to the low-temperature superplasticity of ultrafine-grained aluminum alloys. The examples are provided of the possibility to observe low-temperature superplasticity in aluminum alloys at temperatures less than 0.5 Tmelt and even at room temperature, and herein, we demonstrate the cases of achieving high ductility and high strength in aluminum alloys from processing utilizing severe plastic deformation. Special emphasis is placed on recent studies of the formation of segregations of alloying elements at grain boundaries in UFG Al alloys and their influence on the development of grain boundary sliding and manifestation of low-temperature superplasticity. In addition, the current status and innovative potential of low-temperature superplasticity in aluminum alloys are observed. [ABSTRACT FROM AUTHOR]

Currently, studies of structural superplasticity (SP) are of great interest, since the use of this mode in metalworking technologies allows for the production of parts of various shapes in one operation (with high repetition accuracy of even very complex shapes), while requiring less energy and material resources (relatively low pressures and tool wear) compared to deformation in the 'normal plasticity' mode. Other advantages of using a structural SP are improved physical and mechanical characteristics of the finished product: better surface quality after deformation, high ductility at elevated temperatures, increased strength at temperatures close to room one without reducing ductility (most often there is an increase in ductility), increased cyclic strength, hardness, impact resistance, elevated corrosion-resistance durability, and absence of anisotropy of properties after superplastic deformation. [ABSTRACT FROM AUTHOR]

Amorphous alloys possess increased mechanical strength. Currently, the processing and shaping of amorphous alloys employ their superplasticity in the supercooled liquid phase, which has pushed research into the evolution behavior of the supercooled liquid area during the processing of amorphous alloys. In this study, a molecular dynamics technique was applied to analyze the development rules of the supercooled liquid area and the "shear transformation zone" during laser-assisted scratching of Cu50Zr50 amorphous alloy. It was discovered through research on temperature fluctuations that laser irradiation raises the temperature of the processing region, and the formation of the supercooled liquid region moves the characteristic point of material removal to an earlier stage. As the local temperature rises, the distribution of the supercooled liquid region changes from dispersed to an angle of about 30° with regard to the scratching direction, and the angle subsequently decreases. The variable laws of the "shear transformation zone" during the scratching process were explored based on the von Mises strain theory. The findings demonstrate that when laser energy rises, the Newtonian layer warms up and the "shear transformation zone" distribution angle gradually decreases. At 20 eV/ps, the angle between the "shear transformation zone" and the scratching direction achieves its smallest value of 33°, resulting in the largest overlap with the supercooled liquid region and the lowest content of the "shear transformation zone". By constructing a mathematical model for material removal efficiency, it was established that energy in the range of 20 eV/ps to 30 eV/ps demonstrates higher removal efficiency and a steady processing process while reducing surface roughness by 6-7%. [ABSTRACT FROM AUTHOR]

A systematic analytical study of the mathematical properties of the previously constructed nonlinear model for shear flow of thixotropic viscoelastic-plastic media is continued. For arbitrary six material parameters and an (increasing) material function that control the model, the basic properties of the families of stress-strain curves at constant strain rates and relaxation curves generated by the model, and the features of the evolution of the structuredness under these types of loading are analytically studied. The dependences of these curves on time, shear rate, initial strain and initial structuredness of the material, as well as on the material parameters and function of the model, are studied. Several indicators of the applicability of the model are found which are convenient to check with experimental data. It was examined what effects typical for viscoelastic-plastic media can be described by the model and what unusual effects (unusual properties) are generated by a change in structuredness in comparison with typical stress-strain curves and relaxation curves of structurally stable materials. In particular, it has been proved that stress-strain curves can be both increasing functions and can have decreasing sections resembling a "yield tooth" or damped oscillations, that all stress-strain curves (SSCs) possess horizontal asymptotes (steady flow stress), monotonically dependent on shear rate, and flow stress increases with shear rate growth, that the instantaneous shear modulus, on the contrary, depends on the initial structuredness, but does not depend on shear rate. Under certain restrictions on the material parameters, the model is also capable of providing a bilinear form of stress-strain curves, which is typical for an ideal elastoplastic model, but with strain rate sensitivity. It has been established that the family of stress-strain curves does not have to be increasing either in initial structuredness or in shear rate: in a certain range of shear rates, in which the equilibrium position is a "mature" focus and pronounced oscillations of stress-strain curves are observed, it is possible to intertwine stress-strain curves with different shear rates. It is proved that for any material parameters and functions, all stress relaxation curves decrease and have a common zero asymptote as time tends to infinity. The analysis proved the ability of the model to describe behavior of not only liquid-like viscoelastoplastic media, but also solid-like (thickening, hardening, hardened) media: creep, relaxation, recovery, a number of typical properties of experimental relaxation curves, creep and stress-strain curves, strain rate and strain hardening, flow under constant stress and so on. [ABSTRACT FROM AUTHOR]

• ZK30 Mg alloy after severe friction stir processing, FSP, becomes superplastic. • Grain size distributions are fine, but wide, and prone to instability at high temperature. • Extensive grain coarsening modified conventional strain rate-stress behaviour. • ML ANN model was able to predict superplasticity even with grain coarsening. • ANN model predicts the absence of change in deformation mechanism at high temperature. The aim of this work is to predict, for the first time, the high temperature flow stress dependency with the grain size and the underlaid deformation mechanism using two machine learning models, random forest (RF) and artificial neural network (ANN). With that purpose, a ZK30 magnesium alloy was friction stir processed (FSP) using three different severe conditions to obtain fine grain microstructures (with average grain sizes between 2 and 3 µm) prone to extensive superplastic response. The three friction stir processed samples clearly deformed by grain boundary sliding (GBS) deformation mechanism at high temperatures. The maximum elongations to failure, well over 400% at high strain rate of 10−2 s−1, were reached at 400 °C in the material with coarsest grain size of 2.8 µm, and at 300 °C for the finest grain size of 2 µm. Nevertheless, the superplastic response decreased at 350 °C and 400 °C due to thermal instabilities and grain coarsening, which makes it difficult to assess the operative deformation mechanism at such temperatures. This work highlights that the machine learning models considered, especially the ANN model with higher accuracy in predicting flow stress values, allow determining adequately the superplastic creep behavior including other possible grain size scenarios. [Display omitted] [ABSTRACT FROM AUTHOR]

Introduction. The combined effect on the phase transformation process, involving a combination of heat treatment and external action, is a major technology solution for obtaining the required properties of steel products. When hardening steel in a constant magnetic field with a strength of 1–2 MA/m, martensite formation is observed at higher temperatures. In addition, when compared to conventional hardening, there are changes in structure and properties. Such effects cannot be explained only in terms of thermodynamics, since the expected shift in the equilibrium temperature between austenite and martensite in a magnetic field of such strength does not exceed 4–8°C. To explain the effects that occur during hardening in a magnetic field, it is proposed to consider the features of martensitic transformation in highspeed steel when exposed to an external magnetic field in the temperature range of austenite superplasticity. This research was aimed at identifying the features of martensitic transformation in the presence of a constant magnetic field in steel with account for the phenomena occurring in the premartensitic state.Materials and Methods. Samples made of steel R6M5 were used. Characteristics of the martensitic transformation were studied using the potentiometric method of electrical resistance. The data were recorded using an L-CARD E14-440 analog-to-digital converter with the LGraph2 software package. The sample was heated by passing current. The sample was placed in the interpolar space of an open-type laboratory electromagnet FL-1, which provided the creation of a magnetic field with a strength of 1.2 MA/m.Results. The obtained differentiated dependences were characterized by electrical resistance anomalies (low-temperature peaks) at a temperature corresponding to the appearance of a ferromagnetic phase as a result of martensitic transformation. In a magnetic field, the development of martensitic transformation began at a higher temperature, which could not be explained in terms of thermodynamics. Thus, the formation of stress martensite was observed in microvolumes of austenite with ferromagnetic ordering, which perceived the energy of the external field through magnetostrictive stresses. Under conditions of superplastic austenite, such stresses were sufficient to initiate shear transformation. The minimum possible size of lattice instability fluctuations (1.372 nm) was determined.Discussion and Conclusion. Exposure to a magnetic field under hardening intensified the processes of some magnetic decomposition of austenite. At temperatures close to the beginning of the martensitic transformation, the existing areas of magnetic inhomogeneity were superimposed on the effects of the phenomenon of instability of the crystal lattice. In the temperature range Md-Mn, when austenite exhibited superplasticity, the formation of stress martensite in microvolumes of austenite with ferromagnetic ordering was significantly facilitated

Superplastic forming is a process that enables the production of complex-shaped parts using metallic alloys. To design the optimal forming regimes and ensure the success of forming operations, it is essential to use mathematical models that accurately represent the superplastic deformation behavior. This paper is concerned with the study of the microstructure and superplastic deformation behavior, with the construction of a constitutive model, of Al-Mg-Zn-Cu-Zr aluminum alloys with varying Ni contents. The aluminum solid solution and coarse precipitates of the T(Mg32(Al,Zn)49 and Al3Ni second phases were formed in the studied alloy and Cu dissolved in both second phases. The deformation behavior was investigated in the temperature range of 400–480 °C and the strain rate range of 10−3–10−1 s−1. Due to the fine Al3Zr precipitates, the alloys exhibit a partially recrystallized grain structure before the onset of superplastic deformation. Coarse precipitates of the second phases facilitate dynamic recrystallization and enhance superplasticity at the strain rates and temperatures studied. The alloys with ~6–9% particles exhibit high-strain-rate superplasticity at temperatures of 440–480 °C and strain rates of 10−2–10−1 s−1. The presence of high fractions of ~9% Al3(Ni,Cu) and ~3% T-phase precipitates provided high-strain-rate superplasticity with elongations of 700–800% at a low temperature of 400 °C. An Arrhenius-type constitutive model with good agreement between the predicted and experimental flow stresses was developed for the alloys with different Ni contents.

The superplasticity and microstructure evolution during superplastic deformation for two Al-Cu-Mg-Zr-Mn-Y and Al-Cu-Mg-Zr-Mn-Er alloys were compared. The heterogeneous microstructure was formed in both alloys. Coarse particles of the Cu and Y/Er-bearing and Mn, Fe, Si-bearing phases of solidification origin with a mean size of 1.1/1.4 µm and volume fraction of ~ 9% and fine precipitates of the Mn- and Zr-bearing phases were observed. Precipitates with Zr demonstrated L12 structure and contained Cu, Mg, and Y or Er. The residual elements Si and Fe were found in these precipitates for the alloy with Y. Due to PSN effect of coarse particles and Zener pinning effect of fine precipitates, a fine-grained structure with a mean size of ~ 6.5 µm was formed. Distribution of coarse particles in the aluminum solid solution was more homogeneous for Y-bearing alloy, which exhibited more uniform grain structure and a higher grain size stability with much better superplastic properties. The alloy with Y demonstrated strain rate sensitivity of 0.45–0.55 and elongation to failure of 400–550% at 5 × 10−4–1 × 10−2 s−1 and 575°C. Grain elongation to the tensile direction, dislocation activity in the grain interior and formation of low-angle grain boundaries were observed during superplastic deformation. [ABSTRACT FROM AUTHOR]

The phasing of the friction process provides for the formation of the intersurface phase of the rubbing surfaces of the friction pair, the "third body". In essence, the third body is a hinge of internal friction of a developed stage of joint plastic deformation of surfaces. Being a consequence of the self-organization of surface adaptation processes in the presence of environmental chemistry, the third body can have a wide range of tribological properties. These properties, in turn, are a consequence of the formation of a wide range of optimal (compatible) structures of the third body, which determines the controllability of friction. The relationship of structure and properties as the basic principle of tribomaterial science provokes the question of what are the limits of this relationship? For example, what are the minimum properties of friction and how are they reflected in the essence of the structural structure and behavior of the friction contact? The equation of a quasi-ideal solid third body under friction is obtained, which proves the existence of an elementary structural element of a solid body under friction (deformation), a mechanical (nano) quantum. A mechanical quantum, as an ideal (theoretical) crystal of atomically rough and spherical shape, is an oscillator of dynamic dissipative friction structures. The potential energy accumulated initially during the evolution of contact is further dissipated in the area of compatibility into these formed mechanical quanta of the third body (elementary tribosystem) in the form of surface energy, creating prerequisites for their elastic reversals with an abnormally low coefficient of friction between them. The efficiency of the third body is proportional to the spectrum of elastic reversals of structural shapes. [ABSTRACT FROM AUTHOR]

The article describes and analyses the changes in the shape of the cantilever made of α-PdHn alloy at a temperature of 240°C with the use of a video recording of the experiment. The experiment is performed for a cantilever made of following alloys: α-PdH0.0105, α-PdH0.0198, α-PdH0.026, α-PdH0.0326, α-PdH0.0464, α-PdH0.0644. The cantilever is covered on one side with a copper film with a thickness of 0.75 μm, which does not allow hydrogen to pass through and does not affect the magnitude of the shape change. Saturation of the palladium cantilever to the α-PdHn alloys is carried out in a chamber at a constant temperature of 240°C. Next, the resulting alloys are kept in isothermal conditions, after which the cantilever is degassed on one side. It is experimentally recorded that, when the pressure changes by ΔРH2 = 0.03, 0.1, 0.15, 0.2, 0.3, 0.43 MPa, the value of the maximum bends for the cantilever increases from -1.6 to -7.05 mm. As established, during the degassing of hydrogen for the cantilever made of α-PdHn alloys, the bend develops in two different time stages. The first stage lasts a very short time (9-15 seconds) and is characterized by the rapid achievement of the maximum bending of the cantilever. The second stage lasts much longer (more than 100 seconds) and is marked by the formation of a plateau and reverse straightening of the cantilever. During the second stage, the cantilever returns to its initial state or reaches a stationary state with minimal deviation from the initial position. As also recorded, the degree of reversibility of cantilever bending during degassing increases with increasing hydrogen pressure. As determined, the internal hydrogen-concentration stresses that arise during degassing of hydrogen from alloys α-PdH0.0105, α-PdH0.0198, α-PdH0.026, α-PdH0.0326, α-PdH0.0464, α-PdH0.0644 exceed the elastic characteristics of gradient alloy α-PdHn (200 MPa) and are in the range from 88 to 539 MPa. The process of formation of the maximum bending of the cantilever in the degassing process for α-PdHn alloys is due to the diffusion transfer of hydrogen, the redistribution of internal stresses in the plate during its bending (straightening) and the corresponding restructuring of the hydrogen-concentration field, which changes the internal conditions of the diffusion transfer of hydrogen into the layers of the α-PdHn alloy. [ABSTRACT FROM AUTHOR]

A homogenized, supersaturated AlZnMgZr alloy was processed via severe plastic deformation (SPD) using a high-pressure torsion (HPT) technique for different revolutions at room temperature to obtain an ultrafine-grained (UFG) microstructure. The microstructure and mechanical properties of the UFG samples were then studied using transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and tensile and hardness measurements. The main purpose was to study the effect of shear strain on the evolution of the microstructure of the investigated alloy. We found a very interesting evolution of the decomposed microstructure in a wide range of shear strains imposed by HPT. While the global properties, such as the average grain size (~200 nm) and hardness (~2200 MPa) appeared unchanged, the local microstructure was continuously transformed. After 1 turn of HPT, the decomposed UFG structure contained relatively large precipitates inside grains. In the sample processed by five turns in HPT, the segregation of Zn atoms into grain boundaries (GBs) was also observed. After 10 turns, more Zn atoms were segregated into GBs and only smaller-sized precipitates were observed inside grains. The intensive solute segregations into GBs may significantly affect the ductility of the material, leading to its ultralow-temperature superplasticity. Our findings pave the way for achieving advanced microstructural and mechanical properties in nanostructured metals and alloys by engineering their precipitation and segregation by means of applying different HPT regimes. [ABSTRACT FROM AUTHOR]

The superplastic behavior and associated deformation mechanisms of a fine-grained Mg-10.1 Li-0.8Al-0.6Zn alloy (LAZ1011) with a grain size of 3.2 µm, primarily composed of the BCC β phase and a small amount of the HCP α phase, were examined in a temperature range of 473 K to 623 K. The microstructural refinement of this alloy was achieved by employing high-ratio differential speed rolling. The best superplasticity was achieved at 523 K and at strain rates of 10−4 -5 × 10−4 s−1, where tensile elongations of 550–600% were obtained. During the heating and holding stage of the tensile samples prior to tensile loading, a significant increase in grain size was observed at temperatures above 573 K. Therefore, it was important to consider this effect when analyzing and understanding the superplastic deformation behavior and mechanisms. In the investigated strain rate range, the superplastic flow at low strain rates was governed by lattice diffusion-controlled grain boundary sliding, while at high strain rates, lattice diffusion-controlled dislocation climb creep was the rate-controlling deformation mechanism. It was concluded that solute drag creep is unlikely to occur. During the late stages of deformation at 523 K, it was observed that grain boundary sliding led to the agglomeration of the α phase, resulting in significant strain hardening. Deformation mechanism maps were constructed for β-Mg-Li alloys in the form of 2D and 3D formats as a function of strain rate, stress, temperature, and grain size, using the constitutive equations for various deformation mechanisms derived based on the data of the current tests.

Microstructure evolution and mechanical properties of Zn-22Al alloy after post-ECAP natural/artificial aging were investigated. A homogenization treatment was applied to the casting samples. In addition, after preparing the samples for the ECAP, secondary homogenization treatment was done and then the samples quenched in the water to form a fine grain structure. After 8 passes of ECAP, some ECAPed samples were naturally aged and some ECAPed samples were artificially aged. Natural aging after 8 passes of ECAP showed that Zn-22Al alloy has a quasi-stable microstructure because limited grain growth occurred. Two-phase structure of Zn-22Al alloy prevented excessive grain growth after natural aging. On the other hand, artificial aging after 8 passes of ECAP caused a relatively much grain growth took place. In shorter times of artificial aging, the grain growth rate is faster due to the high surface energy of grain boundaries. On the contrary, as the time of artificial aging increased, the surface energy of grain boundaries decreased, which leads to a decrease in the grain growth rate. In addition, texture evolution was studied after aging artificial. Therefore, the main texture of α and η phases was determined.

To explore high temperature ductility, a new Mg-2.70Zn-1.34Y-0.37Ca-0.02Zr (wt.%) alloy has been fabricated by novel multidirectional forging (MDF) and hot rolling. The microstructure and mechanical properties were investigated. The average grain size of MDF + hot rolled alloy is 10.89 ± 0.90 μm refined from the as-cast average grain size of 60.03 ± 0.72 μm. The ultimate tensile strength of 260.51 ± 1.03 MPa, yield strength of 192.29 ± 1.21 MPa, and elongation of 17.83 ± 0.72 % were obtained at room temperature. For high temperature tensile behavior, microstructural examination revealed that continuous dynamic recrystallization and discontinuous dynamic recrystallization are the main softening mechanism in the temperature range of 573–673 K, while dynamic grain growth with bimodal grains and twins is discovered at 723 K in this alloy. X-ray diffraction and scanning electron microscopy –energy dispersive spectroscopy examinations revealed that the constituent phases are composed of α-Mg solid solution and intermetallic compounds of Ca2Mg6Zn3, Mg3YZn6 (I-phase), and Mg3Zn3Y2 (W-phase). The microstructural evolution, such as dynamic recrystallization and dynamic grain growth at desired tensile temperatures, is related to the thermal stability of constituent phases. The elongation to failure of 215.4 % was demonstrated at 673 K and 1.67 × 10−2 s−1, exhibiting high strain rate quasi-superplasticity. A power-law constitutive equation was established. The deformation activation energy of 177.948 kJ/mol and stress exponent of 4.494 revealed that the dominant deformation mechanism of this alloy at elevated temperatures of 573–723 K is dislocation climb controlled by lattice diffusion.

We investigated the effect of C content on the superplasticity of medium Mn steel through high-temperature tensile testing of Fe–7Mn-(0–0.35)C (wt%) steels at temperatures ranging from 490 °C to 720 °C with an initial strain rate of 1.0 × 10−3 s−1. Superplasticity became evident at 550 °C for C-added steels and at 634 °C for the C-free steel. As all the C-added steels exhibited the highest elongation at 650 °C, we conducted in-depth analyses using the specimens tested at 650 °C. The elongation significantly increased with increasing C content up to 0.20 wt%. The results of strain rate sensitivity, apparent activation energy, and microstructural observations indicated that the low elongation of the 0C steel was attributed to the occurrence of dislocation creep, whereas the high elongations of both 0.10C and 0.20C steels were due to the occurrence of grain boundary sliding. The 0.20C steel exhibited higher elongation than the 0.10C steel due to its smaller average grain size and a larger area of α/γ interphase boundaries compared to the 0.10C steel. Meanwhile, the elongation of the 0.35C steel was not significantly improved compared to the 0.20C steel, despite having a smaller average grain size. This was because the 0.35C steel had a lower area of α/γ interphase boundaries, which resulted from the higher γ fraction. We believe the optimal C content for enhancing the superplasticity of Fe–7Mn steel is approximately 0.20 wt%, considering the 0.35C_T steel requires an additional tempering process, and its elongation was comparable to the 0.20C steel.

An extruded Mg−6Gd−3Y−1.5Ag (wt%) alloy was processed by 6 passes of equal channel angular pressing (ECAP) at 553 K using route Bc to refine the microstructure. Electron back-scattered diffraction (EBSD) analysis showed a fully recrystallized microstructure for the extruded alloy with a mean grain size of 8.6 µm. The microstructure of the ECAP-processed alloy was uniformly refined through dynamic recrystallization (DRX). This microstructure contained fine grains with an average size of 1.3 µm, a high fraction of high angle grain boundaries (HAGBs), and nano-sized Mg5Gd-type particles at the boundaries of the DRXed grains, detected by transmission electron microscopy (TEM). High-temperature shear punch testing (SPT) was used to evaluate the superplastic behavior of both the extruded and ECAP-processed alloys by measuring the strain rate sensitivity (SRS) index (m-value). While the highest m-value for the extruded alloy was measured to be 0.24 at 673 K, the ECAP-processed alloy exhibited much higher m-values of 0.41 and 0.52 at 598 and 623 K, respectively, delineating the occurrence of superplastic flow. Based on the calculated average activation energy of 118 kJ mol−1 and m-values close to 0.5, the deformation mechanism for superplastic flow at the temperatures of 598 and 623 K for the ECAP-processed alloys was recognized to be grain boundary sliding (GBS) assisted by grain boundary diffusion.

Double-pass friction stir processing (FSP) engendered intense plastic deformation in Mg–3Al–1Zn alloy with variation in the most influential process parameters, namely tool rotation speed and tool traverse speed. The effect of change in the FSP processing route on the resulting grain size was also analysed. Uniformly distributed fine equiaxed grains (average grain size ∼ 6.15 μm) were acquired in the stir-zone (SZ) due to extensive dynamic recrystallization (DRX). Attainment of large grain refinement encouraged for further investigation of the alloy's superplastic behaviour at elevated temperatures. Microstructural examination was followed by uniaxial tensile tests that were carried out at three distinct temperatures of 350, 400 and 450 °C at the constant strain rate of 1.3 × 10−3 s−1. With the rising deformation temperature, reduction in the flow stress led to significant increase in the tensile elongations of all the processed specimens. Micro-grain superplasticity (elongation >200% under tension) was observed at the highest deformation temperature for the friction stirred specimen with the finest grains.

• Superplasticity of fine-grained Mg-10Li alloy was studied. • Severe plastic deformation was used to achieve the fine grains. • Its deformation mechanisms for superplastic flow were analyzed. • Deformation mechanism maps were constructed in 2D and 3D. The superplastic behavior and associated deformation mechanisms of a fine-grained Mg-10.1 Li-0.8Al-0.6Zn alloy (LAZ1011) with a grain size of 3.2 µm, primarily composed of the BCC β phase and a small amount of the HCP α phase, were examined in a temperature range of 473 K to 623 K. The microstructural refinement of this alloy was achieved by employing high-ratio differential speed rolling. The best superplasticity was achieved at 523 K and at strain rates of 10−4 -5 × 10−4 s−1, where tensile elongations of 550–600% were obtained. During the heating and holding stage of the tensile samples prior to tensile loading, a significant increase in grain size was observed at temperatures above 573 K. Therefore, it was important to consider this effect when analyzing and understanding the superplastic deformation behavior and mechanisms. In the investigated strain rate range, the superplastic flow at low strain rates was governed by lattice diffusion-controlled grain boundary sliding, while at high strain rates, lattice diffusion-controlled dislocation climb creep was the rate-controlling deformation mechanism. It was concluded that solute drag creep is unlikely to occur. During the late stages of deformation at 523 K, it was observed that grain boundary sliding led to the agglomeration of the α phase, resulting in significant strain hardening. Deformation mechanism maps were constructed for β-Mg-Li alloys in the form of 2D and 3D formats as a function of strain rate, stress, temperature, and grain size, using the constitutive equations for various deformation mechanisms derived based on the data of the current tests. [ABSTRACT FROM AUTHOR]

A fine-grained micro-duplex AlCoCrFeNi2.1 eutectic high entropy alloy exhibited high strain rate superplasticity with an elongation to failure of ~ 960 pct at 1173 K and a strain rate of 10-1 s-1. Optimum superplasticity was associated with a strain rate sensitivity of ~ 0.5, and there were transitions to non-superplastic flow with strain rate sensitivities of < 0.5 at both low and high strain rates. Superplasticity is attributed to grain boundary sliding with the observed retention of an equiaxed grain morphology, with some grain growth. Cavities with dimensions in the range of 1 to 5 μm were observed in specimens pulled to failure. Although analysis revealed that cavity nucleation is likely under the experimental conditions, cavity growth was slow because of control by a plasticity growth rate that was proportional to the cavity size. [ABSTRACT FROM AUTHOR]

This study focuses on investigating the effect of Mg and Si content in different ratios on the microstructure evolution and superplasticity after processing by multidirectional isothermal forging of the Al–Mg–Si–Cu based alloys with additions of Fe and Ni forming large particles of phases of crystallization origin and dispersoid-forming elements Sc and Zr. The study examined alloys with the following Mg/Si content: 1.2/0.4 (3), 1.2/0.7 (1.7), and 2.0/0.7 (2.8) (wt %). The alloys underwent six cycles of isothermal multidirectional forging (MDF) at a temperature of 325°C with cumulative deformation of up to ∑ε = 14.4. During the MDF process, fragmentation occurred in particles of Mg2Si and Al9FeNi eutectic phases, resulting in the formation of particles with sizes of 0.6–0.7 and 1.2–1.5 µm, respectively. Due to particle stimulated nucleation mechanism and suppressed grain growth by nanoscale dispersoids the finegrained structure with a mean grain size of 2 μm was formed in alloys. An increase in Si concentration at constant Mg content, as well as an increase in Mg and Si concentration at a close Mg/Si ratio leads to an increase in fraction of recrystallized grains and a decrease in grain size after MDF. The alloy with the lowest Mg and Si content has shown the highest elongation under superplastic deformation, which is explained by the reduced fraction of Mg2Si phase particles of crystallization origin. [ABSTRACT FROM AUTHOR]

Body-centered cubic refractory metallic materials exhibit excellent high-temperature strength, but often suffer from brittle intergranular fracture due to the recrystallization-induced enrichment of trace elements at grain boundaries (GBs). Here, we report a fully-recrystallized pure molybdenum (Mo) material with room temperature (RT) superplasticity, fabricated by a facile method of powder metallurgy, Y-type hot rolling and annealing. By engineering the ultralow concentration of O at GBs, the inherent GB brittleness of Mo can be largely eliminated, which, in conjunction with high fractions of soft texture and low angle GBs, enables a significant development of ordered dislocation networks and the effective dislocation transmission across low angle GBs. Synergy of these factors greatly suppress the brittle intergranular fracture of Mo, contributing to an enhanced deformability of 108.7% at RT. These findings should have general implication for fabricating a broad class of refractory metals and alloys toward harsh applications. Body-centered cubic refractory metals are known to exhibit excellent high-temperature strength, but often suffer from brittle fracture. Here, the authors produce a bulk pure Mo with stable fine-grain structure after powder metallurgy, Y-type hot rolling and ultra-high temperature annealing (1000~1700 °C) that exhibits room temperature superplasticity. [ABSTRACT FROM AUTHOR]

Microstructure evolution and mechanical properties of Zn-22Al alloy after post-ECAP natural/artificial ageing were investigated. A homogenization treatment was applied to the casting samples. In addition, after preparing the samples for the ECAP, secondary homogenization treatment was done and then the samples were quenched in the water to form a fine-grain structure. After 8 passes of ECAP, some ECAPed samples were naturally aged and some ECAPed samples were artificially aged. Natural ageing after 8 passes of ECAP showed that Zn-22Al alloy has a quasistable microstructure because limited grain growth occurred. The two-phase structure of Zn-22Al alloy prevented excessive grain growth after natural ageing. On the other hand, artificial ageing after 8 passes of ECAP caused relatively much grain growth to take place. In shorter times of artificial ageing, the grain growth rate is faster due to the high surface energy of grain boundaries. On the contrary, as the time of artificial ageing increased, the surface energy of grain boundaries decreased, which led to a decrease in the grain growth rate. In addition, texture evolution was studied after ageing artificially. Therefore, the main texture of α and η phases was determined. [ABSTRACT FROM AUTHOR]

The Zn22Al4Ag alloy was studied to analyze the evolution through plastic deformation and static compression test. First, the master alloy comes from pure Zn, Al, and Ag elements; later, it rolled down to twelve passes. The microstructural characteristics were analyzed by scanning electron microscopy (SEM), Scilab algorithm image processing. Finally, static compression is carried out, including the as-cast, homogenized, and tempered conditions at different loads to measure microhardness. The SEM and XRD image analysis output a 97% plasticity increase in the Zn22Al4Ag alloy due to the rearrangement of the Al, ZnAl, and ZnAg3 phases, observing a phases homogenization from the sixth to the twelfth deformation pass. This deformation allowed the softening of the material as indicated by the microhardness results, relating this behavior to the inverse of the H a l l - P e t c h relationship with hardness. [ABSTRACT FROM AUTHOR]

Abstract—The deformation behavior of an ultrafine-grained (UFG) 08Kh18N10T steel at elevated temperatures (450–900°C) has been studied. The maximum elongation to failure (~250%) is detected at a temperature of 750°C. The deformation of the UFG steel at elevated temperatures is controlled by the intensities of simultaneous processes of grain-boundary sliding and power-law creep. The contribution of each mechanism depends on the grain growth rate under superplasticity conditions, which affects the rate of defect accumulation at migrating grain boundaries. The fracture of the UFG steel has a cavitation character: the fracture and specimen surfaces after high-temperature tests contain large elongated pores having formed on nonmetallic inclusions and submicron pores having formed on σ-phase particles. [ABSTRACT FROM AUTHOR]