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The deformation behavior of a ferrite steel with ultrafine‐grained (UFG) lamellar microstructure generated by linear flow splitting is investigated and compared to a coarser cold‐worked reference state, using a set of complementary local deformation and microstructural characterizations methods. The pile‐up around indentations shows a pronounced anisotropy for the UFG lamellar microstructure indicating the relative motion of grains along their elongated boundaries. This observation is confirmed by stepwise compression testing of micropillars along the normal direction of lamellar‐shaped grains using a new faceted pillar geometry to image the initial microstructure and its evolution throughout the test. The surface roughening in pillar compression testing can be categorized into the formation of discrete steps at the surface along particular grain boundaries and a more gradual roughening that is attributed to intragranular dislocation slip. Potential mechanisms for the observed grain boundary sliding are discussed taking several factors such as the strain rate sensitivity and potential Coble creep rates into account. In conclusion, a grain boundary sliding process carried by grain boundary dislocations appears to be the most likely explanation for the observed behavior. [ABSTRACT FROM AUTHOR]

The microstructure and texture are investigated for nanocrystalline Au‐13at%Pd deformed by high‐pressure torsion. The grain size of this alloy is observed to remain below about 20 nm when subjected to severe plastic deformation. Surprisingly, the initial <110> powder compaction texture does not change significantly during shearing. The results are explained in terms of a grain boundary sliding mechanism involving planar interfaces formed by grain boundary migration. [ABSTRACT FROM AUTHOR]

In polycrystalline magnesium (Mg) and Mg alloys, as the grain size decreases, the grain boundary (GB) mediated plasticity including GB sliding and GB migration becomes the dominant deformation mechanism. In this study, the motion of [ 1 1 ¯ 00 ] symmetric tilt GBs in Mg bicrystals is investigated using molecular dynamics (MD) simulations. The effects of GB misorientation angle and temperature are considered. At low/room temperatures and varied GB misorientation angles in the range of θ ≥ 58.36°, the GB migration occurs via the shear coupling with the invariant plane of {0001}; At 35.80° < θ < 58.36°, both the GB migration and GB sliding happen and the invariant plane changes from {0001} plane to [ 11 2 ¯ 2 ] plane; At 26.54° ≤ θ ≤ 35.80°, the GB migrates with the invariant plane of [ 11 2 ¯ 2 ] ; Finally, at θ < 26.54°, the GB sliding becomes the main deformation mechanism. At 700 K, the GB sliding occurs at the misorientation angles in the range of θ < 58.36θ; while the GB migration occurs at the misorientation angles of θ ≥ 58.36°. By comparing the energy barriers of GB migration and GB sliding, it yields that the deformation mode with a low energy barrier always happens, which leads to the transition of deformation modes and agrees well with the MD simulation results. [ABSTRACT FROM AUTHOR]

In this study, the molecular dynamics (MD) method was used to study the tensile deformation of polycrystalline γ-TiAl with complex and random grain orientations. Firstly, the tensile deformation was simulated with different average grain sizes (8.60 nm, 6.18 nm, and 4.50 nm) and strain rates (1 × 108 s−1, 5 × 108 s−1, and 1 × 109 s−1). The results show that the peak stress increases with an increase in tensile strain rate, and the peak stress decreases as the grain size decreases, showing an inverse Hall–Petch effect. Upon observing atomic configuration evolution during tensile deformation, it is found that the grain boundary is seriously distorted, which indicates obvious grain boundary sliding occurring. With a further increase in the loading, some dislocations nucleate at the grain boundaries and propagate towards the interior of the grains along the grain boundaries, which demonstrates that dislocation motion is the primary coordination of the mechanical process of the grain boundaries. The dislocation density near the grain boundaries continues to increase, leading to the generation of micro-cracks and eventually causing material failure. Another interesting phenomenon is that the grains rotate, and the specific rotation angle values of each grain are quantitatively calculated. Grain rotation relaxes the stress concentration near the grain boundaries and plays a toughening role. Consequently, the plastic deformation behaviors of polycrystalline γ-TiAl are achieved through the grain boundary mechanical process, that is, grain boundary sliding and grain rotation. [ABSTRACT FROM AUTHOR]

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.

This study provides the quantitative assessment of the effect of melt on the polycrystal anelasticity by grain boundary sliding, which is necessary to evaluate the effect of melt on seismic wave velocity and attenuation. We measured elasticity, anelasticity, and viscosity of rock analog samples by changing temperature continuously from subsolidus to supersolidus. Our previous studies have shown that when homologous temperature exceeds 0.9, anelastic relaxation by grain boundary sliding is significantly enhanced, probably due to the increase in grain boundary disorder called pre‐melting. In this study, we found that when homologous temperature exceeds 1, grain boundary sliding is further enhanced by melt and that the viscoelastic property at supersolidus temperatures shows the combined total effect of disordered grain boundaries and melt. The relaxation strength due to melt depends on melt fraction, while that due to disordered grain boundaries does not. When melt fraction is very small (≪1%), the melt effect is negligibly small and the effect of disordered grain boundaries is dominant. Based on these experimental results, we present a new viscoelasticity model seamlessly applicable for a broad temperature range from subsolidus to supersolidus temperatures and for a broad frequency range from seismic waves to geodetic deformation. The new model covers an upper mantle region where a moderate amount of melt exists (e.g., near subduction zones). We further compare our data with those of rock samples and infer that a common physical mechanism, subsolidus and supersolidus enhancements on grain boundary sliding, may be dominant in these two data sets. Plain Language Summary: Low seismic velocity and high attenuation are observed beneath volcanoes where a certain amount of melt may be present. However, quantitative analyses of such seismic anomalies are still difficult. This is because the effect of melt on rock anelasticity, which causes velocity dispersion and attenuation of seismic waves, is not well understood due to insufficient experimental data. This study investigated the effect of melt on polycrystal anelasticity by using binary organic aggregates as rock analog in laboratory experiments. Our previous studies have shown that anelastic relaxation by the grain boundary sliding mechanism is significantly enhanced at near solidus temperatures probably due to the increase in grain boundary disorder called premelting. This study shows that grain boundary sliding is further enhanced by melt and that the effect of melt systematically increases with increasing melt fraction. We combine the present result with our previous data and present a new model of viscoelasticity which is seamlessly applicable from subsolidus to supersolidus temperatures. We further compare the anelasticity data from the present analog samples with those of rock samples. The two data sets are similar, suggesting a common anelastic mechanism between these polycrystalline materials. Key Points: We assess the effect of melt on grain boundary sliding and develop a model of viscoelasticity seamlessly applicable across the solidusA very small amount of melt has a negligibly small effect on polycrystal viscoelasticityAnelasticity of partially molten rock and rock analog commonly shows the combined total effect of disordered grain boundaries and melt [ABSTRACT FROM AUTHOR]

The intermediate-temperature embrittlement range was examined for Fe, Al, Cu, and Ni alloys. It was found that this embrittlement occurs in many alloys, although the causes are very diverse. Embrittlement can be due to fine matrix precipitation, precipitate free zones, melting of compounds at the grain boundaries, segregation of elements to the boundaries, and, additionally for steel, the presence of the soft ferrite film surrounding the harder austenite matrix. Grain boundary sliding and segregation to the boundaries seem to dominate the failure mode at the base of the trough when intergranular failure takes place. When cracking is due to the presence of hydrogen or liquid films at the boundary, then the dissociation along the boundaries is so easy, it is often independent of the strain rate and is always intergranular. In the other cases when failure occurs, if the deformation is carried out at a high strain rate, it is normally transgranular (e.g., hot rolling giving rise to edge cracking). However, when the strain rate is reduced to that of creep (e.g., bending during continuous casting of steel), failure can also take place by grain boundary sliding, and intergranular failure then becomes the favoured mode. [ABSTRACT FROM AUTHOR]

The current work explores the meso-scale deformation behaviour of an additively manufactured CM247 LC at room and high temperatures. In particular, the study focuses on assessing grain boundary sliding (GBS), which can play a crucial role in the high-temperature deformation of superalloys. Specific samples were produced using the Laser Based Powder Bed Fusion technique (PBF-LB), heat treated and tested under monotonic compression in a Gleeble® system. Compression tests were carried out in a wide temperature range at two strain rates and the effect of testing parameters on GBS activity was studied. A thorough microstructural characterization of the PBF-LB material using EBSD and TEM revealed a γ/γ’ microstructure consisting of columnar grains decorated with Hf-rich MC carbides without any segregations of alloying elements. Qualitative and quantitative analysis of GBS was performed using FEG-SEM and AFM, and contribution of GBS into plastic deformation was estimated. It was demonstrated that GBS is activated at 760 °C. A direct correlation between the contribution of GBS into plastic deformation and testing temperature was found, while strain rate has the opposite effect. The highest GBS contribution (∼32%) was recorded at 1093 °C/10−3 s−1. Finally, intergranular microcracking at triple junctions and along grain boundaries was observed when the material was tested at the highest temperatures (871 °C and 1093 °C). The effect of the temperature and the strain rate on the GBS activity in the PBF-LB material is discussed.

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.

High-temperature resistant material is becoming increasingly significant in numerous industrial applications, including aerospace and automotive use, to increase fuel efficiency and decrease CO2 emissions. Metals with nanoparticles to form metal matrix nanocomposite (MMNCs) are promising to enhance metals for structural/industrial applications at high temperatures. This review provides a comprehensive insight into the nanoparticle effect on the creep behavior of metals and alloys. Firstly, it discusses the different creep mechanisms that are activated in response to the creep strain according to the deformation parameters (temperature, load, and microstructure). The most significant creep mechanisms vary according to the grain shape accommodation mechanism required to facilitate the grain boundary sliding into accommodation by diffusion and accommodation by intragranular dislocation activity. Secondly, it reviews the threshold stress contribution in metal matrix composites as well as the load transfer phenomenon, which enhances creep resistance of metals with nanoparticles. Thirdly, the nanoparticles' influences on the creep behavior of metals, especially Al, Mg, Ti, Zn, and solder alloys, are reviewed in detail. Finally, a summary of this review and the future aspects associated with the behavior of metals with nanoparticles at high temperatures are provided.

In this study, the molecular dynamics (MD) method was used to study the tensile deformation of polycrystalline γ-TiAl with complex and random grain orientations. Firstly, the tensile deformation was simulated with different average grain sizes (8.60 nm, 6.18 nm, and 4.50 nm) and strain rates (1 × 108 s−1, 5 × 108 s−1, and 1 × 109 s−1). The results show that the peak stress increases with an increase in tensile strain rate, and the peak stress decreases as the grain size decreases, showing an inverse Hall–Petch effect. Upon observing atomic configuration evolution during tensile deformation, it is found that the grain boundary is seriously distorted, which indicates obvious grain boundary sliding occurring. With a further increase in the loading, some dislocations nucleate at the grain boundaries and propagate towards the interior of the grains along the grain boundaries, which demonstrates that dislocation motion is the primary coordination of the mechanical process of the grain boundaries. The dislocation density near the grain boundaries continues to increase, leading to the generation of micro-cracks and eventually causing material failure. Another interesting phenomenon is that the grains rotate, and the specific rotation angle values of each grain are quantitatively calculated. Grain rotation relaxes the stress concentration near the grain boundaries and plays a toughening role. Consequently, the plastic deformation behaviors of polycrystalline γ-TiAl are achieved through the grain boundary mechanical process, that is, grain boundary sliding and grain rotation.

It is mathematically shown that ductile fracture after finite plastic strain is a necessary consequence of the polycrystalline nature of the materials. A closed–form equation for the plastic strain to fracture of a fine–grained polycrystal with no voids is derived. The mathematical model for the plastic deformation is grounded on the physical hypothesis that adjacent grains slide with a relative velocity proportional to the local shear stress resolved in the plane of the shared grain boundary, when exceeds a finite threshold. Hence plastic flow is governed predominantly by the in–plane shear forces making grain boundaries to slide, and the induced local forces responsible for the continuous grain reshaping are much weaker. The process is shown to produce a monotonic hydrostatic pressure variation with strain that precludes a stationary flow. The hydrostatic pressure dependence on strain has two solutions. One of them leads to superplasticity, the other one is shown to diverge logarithmically at a finite fracture strain and then represents ductile behaviour. Emphasis is done in the mathematical aspects of the deformation of the polycrystal up to the initiation of fracture. Although theoretical predictions agree well with mechanical tests of commercial alloys, technical issues like the effects of the presence and evolution of porosity and other imperfections, or how fracture evolves after initiation are left for a more specific communication.

Zn alloys are desirable candidates for biodegradable materials due to their great biocompatibility and sufficient mechanical properties. Nevertheless, the most popular strengthening method by grain refinement after cold processing is usually ineffective in Zn alloys. Besides highly anisotropic deformation through a dislocation slip, grain boundary sliding (GBS) plays an important role in total deformation in fine-grained Zn alloys at room temperature (RT). Herein, Zn–0.5Cu (wt. %) alloy is fabricated by RT equal channel angular pressing, and its deformation mechanisms in tension were systematically analyzed at strain rates from 10–4 s−1 to 100 s−1. GBS contribution in total deformation was measured using surface markers and atomic force microscopy. In addition, dislocation slip activity was evaluated via electron-backscattered diffraction-based slip trace analysis. As a result, investigated alloy presents the GBS contribution in a total deformation at RT from 35% at the strain rate 100 s−1 to 70% at 10–4 s−1. Simultaneously, the number of slip-deformed grains decreased from 97.5% to 8%. Moreover, the basal slip system was dominant at all strain rates, while the prismatic and the pyramidal < c + a > slip systems were activated at the higher strain rates. The results presented here for the first time clearly show the complexity of deformation mechanisms in fine-grained Zn–0.5Cu, at significantly different strain rate conditions. [ABSTRACT FROM AUTHOR]

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 Mg 5 Gd-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. [ABSTRACT FROM AUTHOR]

High residual porosity in superplastically deformed brass carries the risk of reducing the mechanical properties. Multicomponent brasses demonstrate lower residual porosity, associated with a lower grain size and more effective accommodation of grain boundary sliding. In this paper, the microstructural evolution of the surface and bulk structure of the binary brass and aluminum-bearing brass during steady-state superplastic deformation is compared. After superplastic deformation, dislocation pile-ups and dislocation walls are revealed in the α grains of both alloys, indicating the activation of the dislocation slip/creep mechanism. It is shown that aluminum reduces the contribution of grain boundary sliding along the phase boundaries from ~75 to ~30% and causes strain localization in the β-phase region with the formation of ultrafine grains with the size below ~300 nm as a result of dynamic recrystallization. Alloying with 0.4% Al reduces the flow stress by 20%, increases the relative elongation by a factor of 1.5, and decreases the fraction of residual porosity by a factor of 3. This leads to a much lower loss of room temperature strength in superplastically deformed alloys. [ABSTRACT FROM AUTHOR]

Superplastic aluminum (Al) alloys can be used in the forming processes to fabricate complex geometry components for a wide range of applications in the automobile industry, where light weight and high stiffness are needed. Those alloys exhibit extreme tensile elongation of more than 300% at a high homologous temperature and appropriate low strain rate. Superplasticity occurs in Al alloys via the mechanisms of grain boundary sliding, solute drag creep and diffusion creep. Grain boundary sliding usually leads to extensive superplasticity. The activation of grain boundary sliding depends on grain size, strain rate sensitivity, deformation temperature and alloy chemical composition. A complete understanding of influencing factors on Al alloy superplasticity is the key to developing novel superplastic Al alloys. This review discusses the superplastic behavior of several Al alloys, especially focusing on Al-Mg 5xxx alloys. It highlights the mechanisms that govern superplasticity of Al alloys at a low and high strain rate. The factors which influence superplasticity are analyzed. As practice industrial applications, high-cycle-time superplastic forming operations such as quick plastic forming and high-speed blow forming are briefly discussed. [ABSTRACT FROM AUTHOR]

A novel Hf 6 Ta 2 O 17 ceramics is prepared by a solid-state reaction method. High-temperature creep behavior of Hf 6 Ta 2 O 17 and 8YSZ ceramics are investigated by compressive creep test combined with a digital image correlation (DIC) method. It is found that the creep mechanism of Hf 6 Ta 2 O 17 ceramics is controlled by grain boundary sliding associated with dislocation movement (stress exponent ∼2-3, and activation energy of 600–620 kJ/mol). Grain boundary sliding accommodated to the interface reaction is the main creep mechanism of 8YSZ ceramics (stress exponent ∼2, and activation energy of 425∼465 kJ/mol). Hf 6 Ta 2 O 17 ceramics have higher creep resistance than 8YSZ ceramics under the same conditions. [ABSTRACT FROM AUTHOR]

Shaping of dense ceramics is difficult due to their inherent brittleness. Nanograined ceramics like tetragonal zirconia (TZP) can be superplastically deformed and shaped at high temperatures owing to grain boundary sliding (GBS). Herein, the enhanced plasticity of gadolinium‐doped ceria (GDC) ceramics under mild and strong AC electric current in terms of steady state creep rate under both compressive and tensile loading is demonstrated. A current density of 25 and 200 mA mm−2 is used for the creep deformation. The creep rate increases by up to two orders of magnitude under electric current. The stress exponent remains unchanged for creep experiments at 1200 °C with and without electric current, suggesting a GBS mechanism of plastic deformation in both cases. The field‐enhanced creep rate is attributed to the interaction of space–charge layer and the electric field resulting in enhanced GBS. A higher current density results in enhanced ductility of GDC even when the Joule heating effect is compensated by reducing the furnace temperature. [ABSTRACT FROM AUTHOR]

The deformation behavior of ZrO 2 ceramics were systematically investigated at different strain rates and current densities under a high electric field. We showed that the ceramic exhibited plastic deformation when the strain rate was lower than a certain value. Microstructure and hardness measurements revealed that rapid and stable deformation can be obtained at high current density and moderate strain rate. The strain rate of 7.2 × 10−3 s−1 can be achieved at the furnace temperature of 600 °C without decrease of mechanical properties. The deformation mechanisms were dislocation accommodated grain boundary sliding (GBS) with different control processes. The results showing here are very important to solve the critical problems of difficult to form ceramic materials. [ABSTRACT FROM AUTHOR]

The influence of nickel on superplasticity characteristics, microstructure evolution and the contribution of acting superplastic deformation mechanisms in the Al–Zn–Mg–Cr based alloys has been studied. In the Al–Zn–Mg–Cr alloy, dispersoids with an average size of 140 nm containing aluminum, chromium, magnesium, and a small amount of zinc are precipitated. In the Al–Zn–Mg–Cr–Ni alloy, additionally an Al3Ni phase has formed. Nickel aluminide provides a more homogeneous and stable grain structure at elevated annealing temperatures and during superplastic deformation at 440°C. An alloying with Ni reduced the average grain size from 7.7 to 7.3 μm before deformation and from 10 to 8.6 μm after straining to 0.69. An increased dislocation density has been found near the Al3Ni particles after deformation. At comparable values of the strain rate sensitivity coefficient (m ≈ 0.6), the presence of Al3Ni particles results in a higher contribution of GBS, and a lower contribution of intragranular dislocation slip, as compared to the alloy without these particles. The alloying with Ni provided a more equiaxed fine-grained structure and an increase in elongations-to-failure. [ABSTRACT FROM AUTHOR]

A warm equal-channel angular pressing (ECAP) procedure was applied to a new Mg–6.5Y–1.2Er–1.6Zn (WEZ612)–0.5Ag alloy with a long-period stacking ordered (LPSO) phase. The microstructure, superplasticity, and deformation mechanism of the WEZ612–0.5Ag alloy were then systematically investigated. Three different precipitation phases formed in the alloy with trace Ag addition, namely LPSO, γ, and nanoprecipitate phases. A remarkable elongation of 726% was attained at 623 K and a strain rate of 0.01 s−1. The strain rate sensitivity index (m) and activation energy (Q) under these deformation conditions were 0.461 and 133.08 kJ mol−1, respectively. These m and Q values confirm that the primary deformation mechanism of the WEZ612–0.5Ag alloy at 623 K was grain boundary sliding assisted by lattice diffusion. The highly stable LPSO phases, γ precipitates, and nanoprecipitates synergistically improved the superplasticity of the alloy by retarding crack propagation and grain growth and by promoting the basal plane slip.

In the last three decades, several severe plastic deformation (SPD) procedures have been developed and applied in materials science to create bulk, ultrafine-grained (UFG) structures. As a consequence of the SPD, not only submicron and even nanometer grain sizes can be obtained, but also new grain boundaries with non-equilibrium structures in a material may be formed. Therefore, both the strength and ductility of the UFG materials can change significantly compared to the coarse-grained counterparts. This review is focused on unusual mechanical behaviors of UFG Al and Al alloys, associated with grain boundary structure and segregations which are responsible for the modification of the Hall-Petch relationship and the so-called size-effect for UFG structure. Unusually high strain rate sensitivity, intensive grain boundary sliding and superplasticity at low temperature are described and surveyed. In addition, innovation potential of these phenomena is also briefly considered. [ABSTRACT FROM AUTHOR]

In tensile testing, polycrystalline materials generally fail at relatively low total elongations but under some limited conditions it is possible to achieve exceptionally high elongations of up to and exceeding 400%. This superplastic condition is important scientifically but also it has important uses through the industrial development of superplastic forming operations. This overview traces the historical development of this superplastic effect and it provides a summary of the main characteristics of the superplastic flow process. Conventional thermomechanical processing is not able to produce exceptionally small grain sizes within the submicrometer or nanometer range but this limitation was effectively overcome through processing using severe plastic deformation (SPD). The advantages of SPD processing are discussed and examples are presented. Finally it is demonstrated that the experimental data may be easily and effectively displayed through the construction of deformation mechanism maps based on combinations of stress, grain size and temperature. [ABSTRACT FROM AUTHOR]

Due to the characteristics of high strength and high toughness, fine‐grained metals have a high application value in aerospace and defense industries. There are numerous studies to characterize the effects of temperature or grain size on yield strength separately. However, the model that can consider the combined effects of temperature and grain size is rarely reported. Herein, based on the temperature‐dependent yield strength model and elastic modulus model established by the force–heat equivalence energy density principle, and considering the effect of grain boundary sliding, a temperature and grain size‐dependent yield strength model of fine‐grained metals which can take grain boundary sliding into account is developed. The model can predict the yield strength of fine‐grained metals using an easily obtained yield strength at the coarse‐grain size and an arbitrary reference temperature (usually room temperature). The predictions are in good agreement with all the available experimental data, which verifies the rationality of the proposed model. The quantitative influences of Young's modulus and fracture toughness on the yield strength at different temperatures and grain sizes are discussed, and useful suggestions for the preparation of fine‐grained metals with higher yield strength from 77 K to 0.4 Tm are put forward. [ABSTRACT FROM AUTHOR]

Rare earth (RE) can produce excellent precipitation hardening in Mg alloys. However, when forming a solid solution, it also deteriorates formability, a problem that can usually be overcome by raising deformation temperature. Here we report an unexpected observation of high temperature brittleness in a Mg-Gd-Y-Ag alloy. As the temperature reached 500 °C, the formability decreased drastically, leading to severe intergranular fracture under only 0.5% strain. This was caused by failure of grain boundaries, which are weakened by segregated interfacial compounds.

The stress relaxation (SR) test during plastic deformation is a useful technique for studying deformation processes. SR tests were performed during plastic deformation of pure magnesium, magnesium alloys and magnesium alloys-based composites over a wide temperature interval from room temperature up to 300 °C. Various theoretical approaches were applied for the estimation of characteristic parameters of thermally activated processes and finding of stress components. The aim of this study was to reveal the main features of deformation processes in hexagonal close-packed magnesium alloys and composites. The role of solute atoms in dynamic strain aging phenomena is discussed. In ultrafine-grained magnesium, grain boundary sliding during the stress relaxation tests was observed. The paper analyzes extensive set of results obtained by authors on cast and processed magnesium materials. [ABSTRACT FROM AUTHOR]

ZK60 magnesium alloy is produced via submerged friction stir processing (SFSP), and its microstructural characteristics and deformation behaviors at elevated temperature are investigated. Due to the severe plastic deformation with enhanced cooling rate, dynamic recrystallization occurs but grain growth is significantly retarded. Therefore, the as‐cast microstructure is greatly refined after SFSP, and an average grain size of 1.84 μm is attained. High‐strain‐rate superplasticity (HSRS) is observed at temperature of 573–723 K. A maximum elongation of 1205% is obtained at 1 × 10−2 s−1 and 673 K. The excellent HSRS is attributed to the uniform fine‐grained structure and the relatively high fraction of high‐angle grain boundaries (71.5%), which facilitate grain boundary sliding. The comparatively high portion of dispersed fine particles/precipitates also enhances the microstructural stability of the SFSP specimens. Grain boundary sliding is the dominated deformation mechanism for superplasticity and the failure of the deformed samples is due to the coalescence of cavities. [ABSTRACT FROM AUTHOR]

The intermediate-temperature embrittlement range was examined for Fe, Al, Cu, and Ni alloys. It was found that this embrittlement occurs in many alloys, although the causes are very diverse. Embrittlement can be due to fine matrix precipitation, precipitate free zones, melting of compounds at the grain boundaries, segregation of elements to the boundaries, and, additionally for steel, the presence of the soft ferrite film surrounding the harder austenite matrix. Grain boundary sliding and segregation to the boundaries seem to dominate the failure mode at the base of the trough when intergranular failure takes place. When cracking is due to the presence of hydrogen or liquid films at the boundary, then the dissociation along the boundaries is so easy, it is often independent of the strain rate and is always intergranular. In the other cases when failure occurs, if the deformation is carried out at a high strain rate, it is normally transgranular (e.g., hot rolling giving rise to edge cracking). However, when the strain rate is reduced to that of creep (e.g., bending during continuous casting of steel), failure can also take place by grain boundary sliding, and intergranular failure then becomes the favoured mode.

This work explores the possibility of manufacturing dense and nanocrystalline hydroxyapatite (HA) large monoliths by spark plasma extrusion (SPE). This method combines uniaxial mechanical compression, high temperature, and electromagnetic field to promote the extrusion and sintering of HA powder in one single step. The results show that the binder-free extrusion of pre-compacted HA powder is feasible at a temperature similar to the temperature at which nanocrystalline HA shows superplastic behavior. The extrusion continues throughout the sliding and rotation of the particles, and also due to the grain boundary sliding, up to the point where no more material is available, thus producing monoliths of nearly 30 mm in length and 10 mm in diameter. The dehydration and smooth surface of the powder appear as paramount factors to facilitate the HA extrusion without additives. The extruded HA preserved the stoichiometry and nanometric grain size and exhibited preferential microstructural alignment in the direction of extrusion. The material experiences local thermal and pressure gradients during extrusion, producing different densification and hardness along its length. The SPE of HA will benefit the healthcare field by offering new processing approaches of bone substitutes and osteosynthesis devices.

An image processing technique is proposed to measure the deformation of polycrystalline materials based on correlating the grains in reference and deformed SEM images. The advantage of this technique compared to the conventional subset‐based Digital Image Correlation (DIC) is that it can be applied when speckle patterning is not efficient or when studying boundary‐related mechanics is the objective. The technique is based on correlating grains by defining their boundaries rather than just subsets of image pixels. It reveals the anisotropy inherent in the polycrystals since it allows the analysis to specify each grain separately without averaging the results. The technique is applied by detecting the approximate grain boundaries edges and then refining their location with high accuracy. The correlation is performed between points calculated from each grain in the reference and deformed images as a Point Set Registration (PSR) problem. Finally, the displacements and strains are calculated from the resulting transformation matrix. A benchmark problem was developed to discuss the error over a strain range of 0.02 to 0.2 and showed that the resulting strains are reasonably accurate. Also, an in situ experiment was conducted to demonstrate the implementation of the technique using a specimen with fine‐grained Zirconia polycrystals. The technique successfully revealed the crack tip plastic zone, and strain mismatch between grains. [ABSTRACT FROM AUTHOR]

In this study, dynamic hot forging (DHF) was proposed and applied to make super-strong zirconia (ZrO 2) ceramics. The flexural strength of the sample forged at 1400 ℃ reached 1860 MPa, which was more than 3 times higher than that of the unforged commercial zirconia ceramics and 120 MPa higher than the conventional hot forged zirconia ceramics. The microstructural analysis suggested that the anomalously high strength is due to grain boundary strengthening caused by grain boundary sliding and grain strengthening caused by work hardening. It is anticipated that DHF could be applicable to other material systems to make super-strong materials. [ABSTRACT FROM AUTHOR]

The tensile elongation behavior and deformation mechanisms of superplastic Mg alloys and Mg composites were examined by extensively reviewing the literature published from the time of the first report on the superplasticity of Mg alloys to the present day. Studies on the superplasticity of Mg alloys have been conducted mainly on Mg-Al-Zn (AZ) series alloys, Mg-Zn-Zr (ZK), Mg-Li and Mg-RE (rare earth) alloys, and in recent years, Mg-RE alloys have attracted the greatest attention. The effect of grain size and the type and amount of secondary phase particles on the superplasticity of Mg alloys was systematically examined and reviewed. The alloys processed by severe plastic deformation (SPD) and powder-metallurgy methods have smaller grain sizes and exhibit superior superplasticity compared to conventionally processed (by extrusion and rolling) Mg alloys. For the AZ alloys, as the volume fraction of the Mg17Al12 phase increases, smaller grains are obtained, and the low-temperature superplasticity (LTS) and high-strain-rate superplasticity (HSRS) characteristics become enhanced. The ZK60 alloy with finely dispersed MgZn2 particles exhibits excellent LTS, while the Mg-RE alloys with a high fraction of thermally stable particles exhibit excellent HSRS. Mg-Li alloys can exhibit LTS even at room temperature due to the presence of a high-volume fraction of the body centered cubic (BCC) phase where atomic diffusivity is high. Grain boundary diffusion- and lattice diffusion-controlled grain boundary sliding are found to operate as the dominant deformation mechanisms below ~ 473 K and above ~ 673 K, respectively, at small grain sizes. Deformation mechanism maps were constructed based on the analysis of the deformation behavior of superplastic Mg alloys, and from the maps, the critical conditions for achieving LTS, HSRS and simultaneous achievement of LTS and HRSR were calculated and proposed, and their importance was discussed.

Experiments were conducted to systematically assess the superplastic properties and the microstructural changes of an Al-3Mg-0.2Sc alloy processed by 10 HPT revolutions at room temperature (RT ≈ 300 K) or at 450 K after subsequent tensile testing at temperatures from 473 to 723 K over a wide range of strain rates. The HPT processing at RT led to the development of elongated grains with an average size of ∼140 nm whereas the grain structures were equiaxed and slightly larger (∼150 nm) after HPT at 450 K. After HPT processing at RT, the Al-Mg-Sc alloy exhibited true superplastic flow at low homologous temperatures and attained a maximum elongation of ∼850 % at 523 K. Nevertheless, the elongations decreased at temperatures T ≥ 623 K and an elongation higher than 400 % was only achieved at 673 K for a strain rate of ε ˙ = 4.5 × 10−3 s−1. The material processed by HPT at 450 K displayed superior microstructural stability and substantially higher superplastic ductilities. Elongations of >1100 % were attained at 673 K for strain rates from 3.3 × 10−4 to 1.0 × 10−1 s−1 and a record elongation of ∼1880 % for an HPT-processed metal was achieved at 1.5 × 10−2 s−1 at 673 K. High strain rate superplasticity was also reached for an extended range of temperatures and strain rates. Analysis of the data confirms a stress exponent of ∼2 which is consistent with superplastic flow by grain boundary sliding accommodated by dislocation glide and climb. [ABSTRACT FROM AUTHOR]

An as-extruded Mg-5Bi-3Al-1Zn (wt%) alloy containing a coarse-grained structure and a large amount of nano-scale Mg 3 Bi 2 precipitates showed 330 % tensile elongation-to-failure at 200 °C and 10-4 s-1. Superplasticity was mainly due to grain boundary sliding (GBS) and grain matrix slip deformation (GMD) at low and moderate strains, along with grain refinement at high strain making more GBS be activated. • An as-extruded BAZ531 alloy had a coarse-grained structure and massive nano-precipitates. • At 200 °C and 10-4 s-1, the BAZ531 alloy exhibited a superplasticity of 330%. • At low/moderate strains, both the GBS and GMD were activated. • At high strain, more LAGBs formed and gradually evolved into HAGBs, causing grain refinement. • Grain refinement further induced more GBS. [ABSTRACT FROM AUTHOR]

The superplasticity of Ti-based alloys is improved due to grain refinement, an increase in grain size stability, and an increase in the β -phase fraction up to ∼0.5. Alloying with β -stabilizers reduces the β -transus temperature and increases the β -phase fraction at low forming temperatures, thus improving the superplasticity and lowering its temperature. Both grain growth and superplastic deformation mechanisms are diffusion-controlled phenomena, and, therefore, the diffusivity of the alloying elements should have a significant effect on superplasticity. This work deals with the influence of the high-diffusive elements Fe, Co, and Ni on the grain structure and superplasticity of titanium alloys. For this purpose, the Ti-Al-Mo-V alloy, and alloys with proportional replacement of low-diffusive Mo by Fe, Co, or Ni, exhibiting an interstitial diffusion mechanism, were studied. The alloying elements content was selected to ensure the same β -phase fraction at a deformation temperature of ∼875 °C. The microstructure evolution after thermomechanical processing, annealing, and superplastic deformation, as well as post-deformation room-temperature mechanical properties of the studied alloys, were compared. The actual phase ratio was reconstructed by comparing scanning electron microscopy images in backscattered electrons and electron backscatter diffraction data for the same fragments. It was found that Mo replacement for highly diffusive elements accelerated dynamic grain growth during superplastic deformation at an elevated temperature of 875 °C but improved superplasticity at a lower temperature of 775 °C. The results confirmed that alloying with high-diffusive elements is a promising strategy for the design of low-temperature superplastic forming alloys. [Display omitted] • Ti-Al-V-Mo and alloys with proportional replacement of Mo by Fe/Co/Ni were studied. • High diffusive elements accelerated dynamic grain growth for the Mo-free alloys. • Fe/Co promote excellent superplasticity with a stable flow at low temperatures. • Fe/Co/Ni significantly accelerate recrystallization/globularization and GBS. • Ti 2 Ni precipitates significantly influence the grain structure formation. [ABSTRACT FROM AUTHOR]

Grain size effects on the early plastic strain localization and slip transfer at grain boundaries were investigated for the Alloy 718 Ni-based superalloy at 650 °C. Three microstructures with different grain sizes underwent monotonic tensile tests at 650 °C, both in air and under vacuum, until rupture. All the microstructure variants exhibit fully intragranular fracture under vacuum and partially intergranular fracture in air. In this latter case, predominant intergranular fracture mode was found in the fine-grain microstructures. Interrupted tensile tests were also conducted under vacuum with ex-situ SEM high-resolution digital image correlation (HR-DIC) measurements to assess in-plane kinematics fields at the microstructure scale. Out-of-plane displacement jumps were obtained using laser scanning confocal microscopy. Both crystallographic slip within grains and near Σ 3 twin boundaries (TBs) and morphological sliding happening at grain boundaries (GBs) were documented. Statistical analysis of all plastic events aimed at quantifying strain localization distribution as a function of the microstructure. The fine-grain microstructure was found to have extensive strain localization at grain boundaries, while the coarse-grain microstructure is more prone to intragranular slip development and slip localization near TBs. Different scenarios of slip band/grain boundary interactions were evidenced. [ABSTRACT FROM AUTHOR]

In this work, a fine-grained Ti-15 V-3Cr-3Al-3Sn alloy with 0.93 μm in average grain size and 75.6% of high-angle grain boundaries was achieved by ultra-low heat input friction stir processing (FSP). The superplastic behavior of the friction stir processed (FSPed) sample was investigated at the temperature of 600 °C–700 °C and strain rates range of 1 × 10−4 s−1-3 × 10−3 s−1. The FSPed sample exhibited the largest elongation of 1110% at 675 °C and 3 × 10−4 s−1. This elongation was the largest reported for the alloy to date. The main reasons could be drawn as follows: Firstly, the α phase precipitated during the heating process before superplastic deformation pinned the β grain boundaries in the subsequent superplastic deformation process. Secondly, the dynamic recrystallization occurred during superplastic deformation, which significantly refined the grains and kept the ratio of high-angle grain boundaries >80%. Thirdly, an obvious diffusional creep of the β grains was observed in the late superplastic deformation period, which coordinated the sliding of β grain boundaries. In summary, the superplastic deformation mechanism of the FSPed sample deformed at 675 °C and 3 × 10−4 s−1 was grain boundary sliding accompanied by dynamic recrystallization and diffusional creep. • A fine-grained Ti-15-3 alloy was obtained via ultra-low heat input friction stir processing. • An excellent superplasticity with an elongation of 1110% was obtained at 675 °C and 3 × 10−4 s−1. • The superplastic deformation behaviors and the superplastic deformation mechanism were revealed. [ABSTRACT FROM AUTHOR]

Nanomechanical on-chip testing is combined with nanoscale in situ digital image correlation and automated crystal orientation mapping in TEM to deliver novel statistically representative quantitative data about the deformation mechanisms in nanocrystalline aluminum films. The films are very ductile, with a rare stable multiple necking process with local strains reaching up to 0.45 and macroscopic elongation up to 0.17. The strain fields with resolution below 100 nm are related to the underlying microstructure and crystallographic orientation maps. This reveals nanoscopic shear bands forming preferentially along GB with high misorientations, tilted at +/− 45° with respect to loading direction. The analysis of these data prove that the strong strain delocalization process is promoted by GB migration and grain rotation, leading to large strain rate sensitivity. The distribution of misorientation angles between grains evolve during deformation. The GBs with misorientation between 20° and 40°, which are the GBs with highest energy, involve the largest strains. [Display omitted] [ABSTRACT FROM AUTHOR]

Superplastic aluminum (Al) alloys can be used in the forming processes to fabricate complex geometry components for a wide range of applications in the automobile industry, where light weight and high stiffness are needed. Those alloys exhibit extreme tensile elongation of more than 300% at a high homologous temperature and appropriate low strain rate. Superplasticity occurs in Al alloys via the mechanisms of grain boundary sliding, solute drag creep and diffusion creep. Grain boundary sliding usually leads to extensive superplasticity. The activation of grain boundary sliding depends on grain size, strain rate sensitivity, deformation temperature and alloy chemical composition. A complete understanding of influencing factors on Al alloy superplasticity is the key to developing novel superplastic Al alloys. This review discusses the superplastic behavior of several Al alloys, especially focusing on Al-Mg 5xxx alloys. It highlights the mechanisms that govern superplasticity of Al alloys at a low and high strain rate. The factors which influence superplasticity are analyzed. As practice industrial applications, high-cycle-time superplastic forming operations such as quick plastic forming and high-speed blow forming are briefly discussed.

Using molecular dynamics simulations, we investigate shock-induced sliding of a set of (0 0 1) twist grain boundaries (GBs) in Cu bicrystal, with shock direction parallel to GB and low shock strength. The GB sliding is produced via transverse particle motion in constituent grains and resisted by GB viscosity. Its utmost magnitude su increases first and then decreases with increasing of the angle between [1 0 0] direction in grain and shock direction. Quantitative acoustic wave equation analysis reveals that transverse particle motion exists because of the anisotropic elastic stiffness and particular grain orientation. For GBs of noteworthy sliding, boundary viscosity η is estimated according to the deceleration of volume element in grain, and decreases piecewisely with increasing misorientation angle. su is jointly determined by η and transverse particle velocity dependent on grain orientation.

• The coarse-grained overaged WE54 alloy was refined to 1 µm by severe FSP. • Thus, deformation mechanism at high temperature changed from solute-drag creep to GBS. • Comparison of Mg alloys showed that heavier solutes give less superplastic response. • The accommodation process for GBS is hindered by low solute diffusion rates. The coarse-grained WE54 magnesium alloy was heat treated in order to have minimum hardness minimizing the effects of precipitates and solid solution. Friction stir processing (FSP) was applied in severe conditions to obtain fine, equiaxed and highly misoriented grains, with grain sizes even less than 1 µm. The high severity of processing demonstrated to have a strong impact in the microstructure. Consequently, the processed materials exhibited excellent superplasticity at the high strain rate 10−2 s−1, and temperatures between 300 and 400 °C. The maximum tensile superplastic elongation of 756% was achieved at 400 °C thanks to the operation of grain boundary sliding mechanism (GBS). Besides the new data obtained through tensile testing, the paper deals with a transcendental question regarding the large differences in strain rate values at a given stress in the superplastic regime at maximum elongation compared to other magnesium-based alloys. With this is mind, 19 magnesium alloys from 22 different investigations were analyzed to give some light to this behavior that never was treated before. It is proposed that this behavior has to be attributed to the accommodation process, necessary for GBS to occur, which is hindered by reinforcing solutes. [ABSTRACT FROM AUTHOR]

• An unexpected high temperature brittleness of Mg-Gd-Y-Ag alloy deformed at 500 °C. • Softening of segregated compounds resulting in severe intergranular fractures during high temperature deformation. • A systemic characterization and in-depth symmetry analysis of the segregated compounds with periodic microstructure. Rare earth (RE) can produce excellent precipitation hardening in Mg alloys. However, when forming a solid solution, it also deteriorates formability, a problem that can usually be overcome by raising deformation temperature. Here we report an unexpected observation of high temperature brittleness in a Mg-Gd-Y-Ag alloy. As the temperature reached 500 °C, the formability decreased drastically, leading to severe intergranular fracture under only 0.5% strain. This was caused by failure of grain boundaries, which are weakened by segregated interfacial compounds. [ABSTRACT FROM AUTHOR]

Ultrafine-grained materials display almost no strain hardening, an enhanced strain rate sensitivity and grain boundary offsets during plastic deformation. It is expected that dislocation climb is active in order to enable prompt recovery. The present analysis proposes a deformation mechanism that includes these effects and follows from the mechanism for high temperature grain boundary sliding. This mechanism predicts the relationship between strain rate, flow stress, grain size, temperature and basic material properties such as the Burgers vector modulus, the shear modulus and the grain boundary diffusion coefficient. The model may be used to estimate the final grain size achieved by severe plastic deformation and the strain rate sensitivity. An analysis shows that the predicted behavior agrees with the data from multiple experimental investigations and provides a good estimate of the Hall–Petch slope for different materials which includes breakdown and inverse Hall–Petch behavior under some conditions. The incorporation of a threshold stress provides an opportunity to predict the relationship between flow stress and grain size for a broad range of grain sizes, strain rates and temperatures. An excellent agreement is observed between the predictions of the model and experimental data for Al, Cu, Fe (α), Fe(γ), Mg, Ni, Ti and Zn.