A unified approach to grain boundary sliding in creep and superplasticity

Wafer-Scale Fabrication of 2D Nanostructures via Thermomechanical Nanomolding

With shrinking dimensions in integrated circuits, sensors, and functional devices, there is a pressing need to develop nanofabrication techniques with simultaneous control of morphology, microstructure, and material composition over wafer length scales. Current techniques are largely unable to meet all these conditions, suffering from poor control of morphology and defect structure or requiring extensive optimization or post-processing to achieve desired nanostructures. Recently, thermomechanical nanomolding (TMNM) has been shown to yield single-crystalline, high aspect ratio nanowires of metals, alloys, and intermetallics over wafer-scale distances. Here, TMNM is extended for wafer-scale fabrication of 2D nanostructures. Using In, Al, and Cu, nanomold nanoribbons with widths < 50 nm, depths ≈0.5-1 µm and lengths ≈7 mm into Si trenches at conditions compatible is successfully with back end of line processing . Through SEM cross-section imaging and 4D-STEM grain orientation maps, it is shown that the grain size of the bulk feedstock is transferred to the nanomolded structures up to and including single crystal Cu. Based on the retained microstructures of molded 2D Cu, the deformation mechanism during molding for 2D TMNM is discussed.

Room Temperature Strengthening and High-Temperature Superplasticity of Mg-Li-Al-Sr-Y Alloy Fabricated by Asymmetric Rolling and Friction Stir Processing

Magnesium-lithium alloy is the lightest alloy to date. To explore its room temperature strength and high-temperature ductility, a plate of a new fine-grained Mg-9.13Li-3.74Al-0.31Sr-0.11Y alloy was fabricated by asymmetric rolling, and the rolled plate was subjected to friction stir processing (FSP). The microstructure and mechanical properties at room and elevated temperatures were investigated by optical microscopy, X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and tensile tester. Grain refinement with an average grain size in the α-Mg phase of 1.65 μm and an average grain size in the β-Li phase of 4.24 μm was achieved in the water-cooled FSP alloy. For room temperature behavior, the ultimate tensile strength of 208 ± 4 MPa, yield strength of 193 ± 2 MPa, and elongation of 48.2% were obtained in the water-cooled FSP alloy. XRD and EDS analyses revealed that the present alloy consists of α-Mg and β-Li phases, Al₂Y, Al₄Sr, MgLi₂Al, and AlLi intermetallic compounds. For high-temperature behavior, the maximum superplasticity or ductility of 416% was demonstrated in this fine-grained alloy with an average grain size of 10 μm at 573 K and 1.67 × 10^-3 s^-1. A power-law constitutive equation was established. The stress exponent was 2.29 (≈2) (strain rate sensitivity 0.44), and the deformation activation energy was 162.02 kJ/mol. This evidence confirmed that the dominant deformation mechanism at elevated temperatures is grain boundary and interphase boundary sliding controlled by lattice diffusion.

Room-Temperature Strengthening, Portevin-Le Chatelier Effect, High-Temperature Tensile Deformation Behavior, and Constitutive Modeling in a Lightweight Mg-Gd-Al-Zn Alloy

To explore room-temperature strengthening and high-temperature ductility, a lightweight novel Mg-1.85Gd-0.64Al-0.62Zn alloy was fabricated by innovative multidirectional forging and a hot-rolling technique. Microstructures and mechanical properties were studied at room and elevated temperatures with an optical microscope, an X-ray diffractometer, and a tensile tester. An ultimate tensile strength of 260 MPa, yield strength of 171 MPa, and elongation of 20.4% were demonstrated at room temperature. The room-temperature strengthening mechanisms were identified by strengthening the model estimation. A type C Portevin-Le Chatelier effect was discovered and elucidated in this alloy. X-ray diffraction analysis revealed that the phase composition is α-Mg solid solution and (Mg, Al)₃Gd, Al₇Zn₃, and Al₂Gd intermetallic compounds. Examination of the microstructure at elevated temperatures revealed that dynamic recrystallization and dynamic grain growth occur. In particular, it was discovered that bimodal microstructures or incomplete dynamic recrystallization microstructures exist in high-temperature deformation. A maximum quasi-superplasticity of 228.4% was demonstrated in this alloy at 673 K and 5.0 × 10^-4 s^-1. Flow stress curves showed that the present alloy exhibits Sotoudeh-Bate curves or a long intermediate strain-hardening stage followed by a strain-softening stage. A modified Zerilli-Armstrong constitutive equation incorporating the number of dislocations was established. The power-law constitutive equation was established to identify the deformation mechanism. Both constitutive models had good predictability. At 673 K and 5.0 × 10^-4 s^-1, the stress exponent was 4, and the average deformation activation energy was 104.42 kJ/mol. The number of dislocations inside a grain was 146. This characteristic evidence confirmed that dislocation motion controlled by pipe diffusion dominates the rate-controlling process under this condition.

Inhibiting creep in nanograined alloys with stable grain boundary networks

Creep, the time-dependent deformation of materials stressed below the yield strength, is responsible for a great number of component failures at high temperatures. Because grain boundaries (GBs) in materials usually facilitate diffusional processes in creep, eliminating GBs is a primary approach to resisting high-temperature creep in metals, such as in single-crystal superalloy turbo blades. We report a different strategy to inhibiting creep by use of stable GB networks. Plastic deformation triggered structural relaxation of high-density GBs in nanograined single-phased nickel-cobalt-chromium alloys, forming networks of stable GBs interlocked with abundant twin boundaries. The stable GB networks effectively inhibit diffusional creep processes at high temperatures. We obtained an unprecedented creep resistance, with creep rates of ~10 –7 per second under gigapascal stress at 700°C (~61% melting point), outperforming that of conventional superalloys.

High Strain Rate Quasi-Superplasticity Behavior in an Ultralight Mg-9.55Li-2.92Al-0.027Y-0.026Mn Alloy Fabricated by Multidirectional Forging and Asymmetrical Rolling

To explore new approaches to severe plastic deformation and the ductility of a multicomponent magnesium-lithium alloy, an ultralight microduplex Mg-9.55Li-2.92Al-0.027Y-0.026Mn alloy was made by novel multidirectional forging and asymmetrical rolling, and the superplasticity behavior was investigated by optical microscope, hot tensile test, and modeling. The average grain size is 1.9 μm in this alloy after multidirectional forging and asymmetrical rolling. Remarkable grain refinement caused by such a forming, which turns the as-cast grain size of 144.68 μm into the as-rolled grain size of 1.9 μm, is achieved. The elongation to failure of 228.05% is obtained at 523 K and 1 × 10^-2 s^-1, which demonstrates the high strain rate quasi-superplasticity. The maximum elongation to failure of 287.12% was achieved in this alloy at 573 K and 5 × 10^-4 s^-1. It was found that strain-induced grain coarsening at 523 K is much weaker than the strain-induced grain coarsening at 573 K. Thus, the ductility of 228.05% is suitable for application in high strain rate superplastic forming. The stress exponent of 3 and the average activation energy for deformation of 50.06 kJ/mol indicate that the rate-controlling deformation mechanism is dislocation-glide controlled by pipe diffusion.

Different Types of Particle Effects in Creep Tests of CoCrFeNiMn High-Entropy Alloy

Compressive creep tests were performed on a CoCrFeNiMn equiatomic alloy with the dispersion of (i) aluminum nitride or (ii) boron nitride at temperatures of 973 K and 1073 K. The results are compared with previously published creep rates of the unreinforced matrix alloy and the alloy when strengthened by yttrium + titanium oxides. The comparison reveals that the creep rate is essentially unchanged by the presence of aluminum nitride particles, whereas it is reduced by the presence of oxide particles. Boron nitride particles do not influence the creep rate at low stresses but reduce it substantially at high stresses.

Reveal the Viscoplastic Behaviour and Microstructure Evolution of Stainless Steel 316L

Stainless steel 316L is a widely used structural material in the nuclear industry because of its excellent corrosion resistance and mechanical properties. However, very little research can be found on its viscoplastic behaviour and microstructure evolution at warm and hot deformation conditions, which hinder the possible application of advanced manufacturing technologies for producing complex parts, such as superplastic forming or hydroforming. The aims of this study are to explore stainless steel 316L's viscoplastic behaviour, to determine its strain rate sensitivities, and to reveal its underlying microstructure evolution; this will allow appropriate manufacturing (forming) technologies and the optimal forming condition to be determined. Hence, isothermal tensile tests at 700 °C, 800 °C, 900 °C, and 1000 °C at strain rates of 0.01 s^-1 and 0.001 s^-1 have been conducted. Moreover, the corresponding microstructure evolution, including the grain orientation and geometrically necessary dislocation density, has been revealed by the electron backscatter diffraction method. The data show the viscoplastic behaviour of stainless steel 316L under various thermomechanical deformation conditions and how microstructure evolution influences the viscoplastic flow stress.

High-Temperature Deformation Behavior of the AZ31 Alloy Processed by Double-Sided FSW Technology

Generally, AZ31 magnesium alloys have poor formation ability near room temperature. This material, with ultrafine grains, usually exhibits excellent superplasticity at high temperature. Therefore, the preparation of materials with suitable microstructures to obtain the superplasticity is an important goal. In this study, the double-side FSW (Friction Stir Processing) process was applied on the AZ31Mg alloy to obtain the microstructure with ultra-fine grains. The effect of the FSW on the microstructure and the mechanism of microstructure evolution was elaborated. Meanwhile, the effects of deformation parameters, temperature, and strain rate on flow behavior and superplasticity of the joint were systematically and comparatively studied. It was found that the microstructure at the joint center with double-side FSW could obtain much finer grains with an average grain size of 9.6 μm compared with the rolled materials (25.9 μm). The high temperature deformation results showed that the optimum elongation (446%) was achieved with the deformation temperature of 450 °C and strain rate of 0.0003 s−1, which was far greater than the elongation of the room temperature (20.8%). The mechanism of parameters on deformation behavior of the joint samples was elaborated.

Microstructural Evolution and Tensile Testing of a Bi–Sn (57/43) Alloy Processed by Tube High-Pressure Shearing

Tube high-pressure shearing (t-HPS) processing was performed on a eutectic Bi–Sn (57/43) alloy for 0.25, 1, 5 and 20 turns. The selected samples were stored at room temperature for up to 56 days to examine the strain weakening and self-annealing behavior of the alloy. The results showed that t-HPS processing gradually refined the microstructure and led to decreasing of microhardness, but microhardness increased slowly during the subsequent storage at room temperature. Shear localization of the eutectic structure during t-HPS processing was observed as large amounts of narrow dense lamellar zones were visible in the deformed microstructures. The Bi–Sn (57/43) alloy processed by t-HPS exhibited significantly enhanced superplastic properties with elongations up to >1800% in a sample after t-HPS processing for 20 turns. This high elongation is attributed to the breaking of the lamellar structure and the very small grain size.

Grain boundary-induced premelting and solid ↔ melt phase transformations: effect of interfacial widths and energies and triple junctions at the nanoscale

The strong effect of dry grain boundary (GB) and isolated solid-melt interface widths and energies and the triple junction energy on GB-induced premelting and the transformations between solid, premelt, and melt at the nanoscale are studied using a thermodynamically consistent original phase field approach of Ginzburg-Landau type. One order parameter is considered for describing solid ↔ melt transformation and another N- 1 independent order parameters are used to describe N grains in the samples. A variable energy of the GB regions is assumed which depends on the order parameter related to solid ↔ melt transformation, and the energy of the GBs reduces from their dry state energy γ_gb in solid to zero at melt. The complex 'phase diagrams', showing the variation of the transformation temperatures with the non-dimensional width ratio (ratio of dry GB width and isolated solid-melt interface width), are plotted for 2D bicrystals with a planar GB and a circular GB for the cases when (i) solid-melt interface width is fixed and GB width varies, and (ii) GB width is fixed and solid-melt interface width varies. A bistable region, where the premelt spontaneously appears or transforms back to the solid, is shown indicating that these transformations accompany hysteresis. The effect of the width ratio on the effective melt layer thickness and the disjoining potential between the solid-melt interfaces on premelting/melting is also studied. The effect of triple junctions on the transformations is studied in tricrystals having three planar GBs meeting at the interior of the sample with equal dihedral angles. A premelt/melt pocket appears at the junction in a jump-like manner at temperatures significantly lower than the temperatures at which the GBs premelt. The premelt/melt that appeared at the junction eventually wets the adjacent GBs, and finally, the entire sample melts at the melting point. The results show a plausible reason for differences in the premelting and melting temperature ranges of the GBs observed in various experimental and atomistic studies.

Microphysical Modeling of Carbonate Fault Friction at Slip Rates Spanning the Full Seismic Cycle

Laboratory studies suggest that seismogenic rupture on faults in carbonate terrains can be explained by a transition from high friction, at low sliding velocities (V), to low friction due to rapid dynamic weakening as seismic slip velocities are approached. However, consensus on the controlling physical processes is lacking. We previously proposed a microphysically based model (the "Chen-Niemeijer-Spiers" [CNS] model) that accounts for the (rate-and-state) frictional behavior of carbonate fault gouges seen at low velocities characteristic of rupture nucleation. In the present study, we extend the CNS model to high velocities (1 mm/s ≤ V ≤ 10 m/s) by introducing multiple grain-scale deformation mechanisms activated by frictional heating. As velocity and hence temperature increase, the model predicts a continuous transition in dominant deformation mechanisms, from frictional granular flow with partial accommodation by plasticity at low velocities and temperatures, to grain boundary sliding with increasing accommodation by solid-state diffusion at high velocities and temperatures. Assuming that slip occurs in a localized shear band, within which grain size decreases with increasing velocity, the model results capture the main mechanical trends seen in high-velocity friction experiments on room-dry calcite-rich rocks, including steady-state and transient aspects, with reasonable quantitative agreement and without the need to invoke thermal decomposition or fluid pressurization effects. The extended CNS model covers the full spectrum of slip velocities from earthquake nucleation to seismic slip rates. Since it is based on realistic fault structure, measurable microstructural state variables, and established deformation mechanisms, it may offer an improved basis for extrapolating lab-derived friction data to natural fault conditions.

Plastic Forming of Metals at the Nanoscale: Interdiffusion-Induced Bending of Bimetallic Nanowhiskers

Controlled plastic forming of nanoscale metallic objects by applying mechanical load is a challenge, since defect-free nanocrystals usually yield at near theoretical shear strength, followed by stochastic dislocation avalanches that lead to catastrophic failure or irregular, uncontrolled shapes. Herein, instead of mechanical load, we utilize chemical stress from imbalanced interdiffusion to manipulate the shape of nanowhiskers. Bimetallic Au-Fe nanowhiskers with an ultrahigh bending strength were synthesized employing the molecular beam epitaxy technique. The one-sided Fe coating on the defect-free, single-crystalline Au nanowhisker exhibited both single- and polycrystalline regions. Annealing the bimetallic nanowhiskers at elevated temperatures led to gradual change of curvature and irreversible bending. At low homological temperatures at which grain boundary diffusion is a dominant mode of mass transport this irreversible bending was attributed to the grain boundary Kirkendall effect during the diffusion of Au along the grain boundaries in the Fe layer. At higher temperatures and longer annealing times, the bending was dominated by intensive bulk diffusion of Fe into the Au nanowhisker, accompanied by a significant migration of the Au-Fe interphase boundary toward the Fe layers. The irreversible bending was caused by the concentration dependence of the lattice parameter of the Au(Fe) alloy and by the volume effect associated with the interphase boundary migration. The results of this study demonstrate a high potential of chemical interdiffusion in the controlled plastic forming of ultrastrong metal nanostructures. By design of the thickness, microstructure, and composition of the coating as well as the parameters of heat treatment, bimetallic nanowhiskers can be bent in a controlled manner.

Using High-Pressure Torsion to Achieve Superplasticity in an AZ91 Magnesium Alloy

An AZ91 magnesium alloy (Mg-9%, Al-1% Zn) was processed by high-pressure torsion (HPT) after solution-heat treatment. Tensile tests were carried out at 423, 523, and 623 K in the strain rate range of 10−5−10−1 s−1 to evaluate the occurrence of superplasticity. Results showed that HPT processing refined the grain structure in the alloy, and grain sizes smaller than 10 µm were retained up to 623 K. Superplastic elongations were observed at low strain rates at 423 K and at all strain rates at 523 K. An examination of the experiment data showed good agreement with the theoretical prediction for grain-boundary sliding, the rate-controlling mechanism for superplasticity. Elongations in the range of 300–400% were observed at 623 K, attributed to a combination of grain-boundary-sliding and dislocation-climb mechanisms.

Recent Development of Superplasticity in Aluminum Alloys: A Review

Aluminum alloys can be used in the fabrication of intricate geometry and curved parts for a wide range of uses in aerospace and automotive sectors, where high stiffness and low weight are necessitated. This paper outlines a review of various research investigations on the superplastic behavior of aluminum alloys that have taken place mainly over the past two decades. The influencing factors on aluminum alloys superplasticity, such as initial grain size, deformation temperature, strain rate, microstructure refinement techniques, and addition of trace elements in aluminum alloys, are analyzed here. Since grain boundary sliding is one of the dominant features of aluminum alloys superplasticity, its deformation mechanism and the corresponding value of activation energy are included as a part of discussion. Dislocation motion, diffusion in grains, and near-grain boundary regions being major features of superplasticity, are discussed as important issues. Moreover, the paper also discusses the corresponding values of grain size exponent, stress exponent, solute drag creep and power law creep. Constitutive equations, which are essential for commercial applications and play a vital role in predicting and analyzing the superplastic behavior, are also reviewed here.

The Effect of Ultrafine-Grained Microstructure on Creep Behaviour of 9% Cr Steel

The effect of ultrafine-grained size on creep behaviour was investigated in P92 steel. Ultrafine-grained steel was prepared by one revolution of high-pressure torsion at room temperature. Creep tensile tests were performed at 873 K under the initially-applied stress range between 50 and 160 MPa. The microstructure was investigated using transmission electron microscopy and scanning electron microscopy equipped with an electron-back scatter detector. It was found that ultrafine-grained steel exhibits significantly faster minimum creep rates, and there was a decrease in the value of the stress exponent in comparison with coarse-grained P92 steel. Creep results also showed an abrupt decrease in the creep rate over time during the primary stage. The abrupt deceleration of the creep rate during the primary stage was shifted, with decreasing applied stress with longer creep times. The change in the decline of the creep rate during the primary stage was probably related to the enhanced precipitation of the Laves phase in the ultrafine-grained microstructure.

Room-Temperature Superplasticity in an Ultrafine-Grained Magnesium Alloy

Superplasticity, a phenomenon of high tensile elongation in polycrystalline materials, is highly effective in fabrication of complex parts by metal forming without any machining. Superplasticity typically occurs only at elevated homologous temperatures, where thermally-activated deformation mechanisms dominate. Here, we report the first observation of room-temperature superplasticity in a magnesium alloy, which challenges the commonly-held view of the poor room-temperature plasticity of magnesium alloys. An ultrafine-grained magnesium-lithium (Mg-8 wt.%Li) alloy produced by severe plastic deformation demonstrated 440% elongation at room temperature (0.35 T_m) with a strain-rate sensitivity of 0.37. These unique properties were associated with enhanced grain-boundary sliding, which was approximately 60% of the total elongation. This enhancement originates from fast grain-boundary diffusion caused by the Li segregation along the grain boundaries and the formation of Li-rich interphases. This discovery introduces a new approach for controlling the room-temperature superplasticity by engineering grain-boundary composition and diffusion, which is of importance in metal forming technology without heating.

Dislocation-accommodated grain boundary sliding as the major deformation mechanism of olivine in the Earth's upper mantle

Understanding the deformation mechanisms of olivine is important for addressing the dynamic processes in Earth's upper mantle. It has been thought that dislocation creep is the dominant mechanism because of extrapolated laboratory data on the plasticity of olivine at pressures below 0.5 GPa. However, we found that dislocation-accommodated grain boundary sliding (DisGBS), rather than dislocation creep, dominates the deformation of olivine under middle and deep upper mantle conditions. We used a deformation-DIA apparatus combined with synchrotron in situ x-ray observations to study the plasticity of olivine aggregates at pressures up to 6.7 GPa (that is, ~200-km depth) and at temperatures between 1273 and 1473 K, which is equivalent to the conditions in the middle region of the upper mantle. The creep strength of olivine deforming by DisGBS is apparently less sensitive to pressure because of the competing pressure-hardening effect of the activation volume and pressure-softening effect of water fugacity. The estimated viscosity of olivine controlled by DisGBS is independent of depth and ranges from 10(19.6) to 10(20.7) Pa·s throughout the asthenospheric upper mantle with a representative water content (50 to 1000 parts per million H/Si), which is consistent with geophysical viscosity profiles. Because DisGBS is a grain size-sensitive creep mechanism, the evolution of the grain size of olivine is an important process controlling the dynamics of the upper mantle.

Microstructure Development and Superplasticity in a Zn-22% Al Eutectoid Alloy Processed by Severe Plastic Deformation

Severe plastic deformation (SPD) is an attractive processing method for refining microstructures to have ultrafine grain sizes within the submicrometer or even the nanometer levels. In SPD, the most promising techniques are equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). A conventional superplastic Zn-22% Al eutectoid alloy was processed by ECAP and HPT. Experiments were conducted to demonstrate the evolution of hardness and microstructure and the enhancement of superplastic properties of the Zn-Al alloy after processing by the SPD techniques. In addition, flow mechanisms of the Zn-22% Al alloy are discussed by utilizing a deformation mechanism map.

Characteristics of High Temperature Creep in Pure Aluminum Processed by Equal-Channel Angular Pressing

High purity aluminum was processed by equal-channel angular pressing (ECAP) to reduce the grain size to ~1.3 m. Tensile specimens were cut from the as-pressed billets and these specimens were tested under conditions of high temperature creep. The results show excellent creep properties with a well-defined region of steady-state flow. The flow behavior is analyzed by comparing the creep data with the predicted behavior for different fundamental creep mechanisms and by plotting a deformation mechanism map to provide a visual representation of the creep properties.