Deformation Mechanism Map Research Articles

A Large-Deformation Phase-Field Model for Characterizing Electro-Chemo-Mechanical Failure Mechanisms at Li/Solid Electrolyte Interface

All-solid-state-batteries (ASSBs) are considered the future replacements for traditional lithium-ion batteries, thanks to their superior energy density and enhanced safety. The ionic conductivity of solid electrolytes (SEs) could be improved to match that of liquid electrolytes through careful material design and optimization. However, the relatively sluggish kinetics at the interface between at the active material (lithium metal or cathode materials) and the SE interface remain a hurdle for the wider adoption of ASSBs. The SE’s contact with the anode and cathode materials is inferior compared to that of its liquid counterpart, and improper working conditions can easily lead to the formation of many voids and pores. This results in an increased interface resistance and a deteriorated power density of the ASSB cell. The mechanical contact between the anode material and the SE is more significant due to the larger volumetric expansion and contraction during cycling compared to the cathode side. In addition, high stack pressure and plating current density can also induce the penetration of lithium dendrites. Therefore, it is vital to explore the stability envelope for the anode-SE interface, namely determining the operation condition under which void formation and dendrite growth could be suppressed for the effective design and utilization of ASSBs.Pressure and current density are two controllable condition parameters in the operation of ASSBs1, 2. High current density can lead to the formation of numerous voids at the Li/SE interface during stripping, as well as the initiation and growth of lithium metal dendrite within the SE. This results in increased polarization and potential battery failure. Applying stack pressure to the ASSB can stabilize the Li/SE interface during manufacturing and operation, as the creep of Li metal is believed to occur once its stress surpasses a certain threshold. The plastic ‘flow’ of Li metal could help to hinder the growth of voids2 and keep good contact condition. However, at an excessively large pressure3, 4, dendrites can penetrate the SE, which cause a short-circuit in the ASSB and lead to failure5. Therefore, an electro-chemo-mechanical model that can describe the large-deformation mechanical properties of Li metal and its coupled mechanisms with the reactions at the Li/SE interface is essential for determining the stability envelope for stable stripping and plating of lithium in the ASSBs.In this work, a phase-field electro-chemo-mechanical model is proposed, in which the coupling of void diffusion, lattice annihilation, stripping and plating reactions, and mechanical properties of lithium metal are comprehensively described. The first contribution of this work is a comprehensive summary of the mechanical properties of the lithium metal under different temperatures and deformation rates, generating a unified deformation-mechanism map for the general battery manufacturing and characterizing community. Based on this map, our phase-field electro-chemo-mechanical model is developed to include important features below. The diffusion of vacancies and Li sites in the lithium metal is considered to simulate the lattice annihilation and the void formation during stripping.The flow of lithium metal caused by creeping or plasticity is incorporated in the kinetical equation of the order parameters.The plating and stripping kinetics are described by the modified Butler-Volmer equation in which the effect of vacancies is considered The general theory proposed in this work can simulate the electro-chemo-mechanical effects at different operation conditions for Li metal or other Li alloy anode materials, which is believed to be a powerful tool for the effective design, manufacturing and management of next-generation batteries.

Modeling the electro-chemo-mechanical failure at the lithium-solid electrolyte interface: Void evolution and lithium penetration

The solid-solid contact interface is crucial for the reliability of solid-state energy storage systems. The contact condition becomes more complicated when lithium (Li) metal is used as the anode. The contact between solid electrolyte (SE) and Li metal is inferior compared to the liquid/solid interface in conventional Li-ion batteries. Experimental evidence has shown that improper operating conditions of solid-state batteries can lead to electro-chemo-mechanical failures at the Li/SE interface, including the formation of voids and the penetration of Li. In this study, a unified phase-field model is developed to investigate these two mechanisms. The model considers the coupled electro-chemo-mechanical processes including void diffusion, lattice annihilation, stripping and plating reactions, and plastic deformation of Li metal. The study begins with a revisit of the deformation-mechanism map for Li metal under a wide range of temperatures, stress, and deformation rates. This map serves as the basis for the mechanical characterization in the phase-field model. The large inelastic deformation of Li is considered by introducing an advection term into the Allen-Cahn equation, which is used to describe the dynamic evolution of the Li and void phases. The effects of current density and stack pressure on void evolution and Li penetration are studied based on the model predictions. By combining the simulation results with the experimental data from publications, we obtain the stable operation zone of stack pressure and applied current density. In this zone, the Li/SE interface can enable stable stripping and plating of Li metal. The same phase-field modeling framework is transferred to investigate the Li-Mg alloy/SE interface considering Li-Mg alloy is also used as the anode. The fundamental difference between Li/SE and Li-Mg/SE is analyzed accordingly. This study provides a useful tool for the design, manufacturing, and management of next-generation batteries by providing important scientific insights into the electro-chemo-mechanical processes of different anode materials under various operational conditions.

Coupling effects of temperature and strain rate on the mechanical behavior and microstructure evolution of a powder-plasma-arc additive manufactured high-entropy alloy with multi-heterogeneous microstructures

A single-phase or simple-structured alloy does not always possess outstanding combinations of strength and ductility over a wide range of temperatures and strain rates for engineering applications. In the present work, a high-entropy alloy with multi-heterogeneous microstructures was in-situ fabricated via powder-plasma-arc additive manufacturing. The compressive behavior of the additive manufactured high-entropy alloy over a wide range of temperatures and strain rates was studied, using an improved split Hopkinson bar system and electronic universal testing machine. It shows exceptional combination of strength and ductility within the selected temperature and strain rate ranges. Microstructural evolution was characterized at various temperatures and strain rates, providing insight into the intricate relationship between microstructure and property. The multicomponent Laves phase is hard yet deformable, while the multicomponent FCC phase is soft and ductile. The deformation twins observed all over the selected temperature and strain rate ranges and dynamic recrystallization appearing at high temperatures in the FCC phase enhance the ductility of the FCC phase and rise the crack-arresting capability. The third-type strain aging occurs at different strain rates, which shifts to a higher temperature range as strain rate increases. Ta and impurity atom, Si, acting as “solute atoms” form atom atmosphere and silicide, pinning the moving dislocations in the FCC phase. Finally, a deformation mechanism map was proposed over a wide temperature and strain rate range. The study explored a potentially new avenue to design alloys with exceptional combinations of strength and ductility over a wide range of temperatures and strain rates.

Dynamic globularization behavior of the dual-phase TiZrV medium entropy alloy during hot working

Globularization is an advantageous way to improve the formability of dual-phase materials. Dynamic globularization behavior of the TiZrV medium entropy alloy (MEA) with dual-phase was investigated at the temperature range of 500 ℃ ∼ 700 ℃ and strain rate of 0.1 s-1 ∼ 10 s-1 by the double-cone compression test. The finite element software was used to simulate the hot compression process of the double-cone specimen. Transmission electron microscope (TEM) was used to study the deformation microstructure features. The double-cone test can produce a wide strain range and corresponding deformation microstructure. The highest globularization ratio of 72.4% was obtained under the strain of about 0.88 at 500 ℃ and 10 s-1. The hot deformation mechanism map in the temperature range of 500 ℃ ∼ 700 ℃ and the strain range of 0.2 ∼ 1.2 at 10 s-1 was constructed by combining the double-cone test with finite element simulation. The hot deformation mechanism map is divided primarily into two parts. At 500 ℃, in the strain region of 0.2 ∼ 0.88, the deformation mechanism is dynamic recovery (DRV), and in the strain region of 0.88 ∼ 1.2, the deformation mechanism is dynamic recrystallization (DRX). It is considered that DRX plays a key role in dynamic globularization of the TiV phase during hot working and its production is summarized.

Recent progress in the study of lattice-preferred orientation of olivine

Recent progress on the study of olivine LPO after (Karato et al., 2008) is reviewed with the emphasis on three issues: (i) LPO formed by the rotation of olivine crystals with anisotropic shape (euhedral crystals) in diffusion creep (Miyazaki et al., 2013), (ii) B-type LPO in the olivine + basaltic melt (Holtzman et al., 2003), and (iii) pressure change in the influence of LPO (Ohuchi and Irifune, 2013). Regarding the role of euhedral crystals, we show that euhedral olivine crystals occur in a mixture of forsterite and diopside (used by (Miyazaki et al., 2013)) but not in a mixture of olivine and enstatite. Consequently, the results by reported by (Miyazaki et al., 2013) are not applicable to the Earth’s upper mantle where olivine co-exists mostly with enstatite. Also we show that the LPO reported by (Miyazaki et al., 2013) is not consistent with the shape of olivine, and the observed LPO is likely due to dislocation glide (A-type fabric) under the conditions near the diffusion-dislocation creep regime boundary.Regarding the LPO of olivine with the presence of melt, (Qi et al., 2018)’s experimental study with the torsion geometry did not reproduce the B-type fabric reported by (Holtzman et al., 2003) indicating that the B-type fabric reported by (Holtzman et al., 2003) was indeed an artifact of the direct shear experiments. The weak LPO found by (Qi et al., 2018) (compared to that by (Zimmerman et al., 1999)) can be explained by the smaller grain size in their experiments. I conclude that a majority of the experimental results on olivine LPO at relatively low pressures (<2 GPa) can be understood based on the basics of deformation mechanism map and LPO caused by various slip systems in olivine. Regarding a claim by (Ohuchi and Irifune, 2013) that the A-type LPO (a-slip) dominates at high water content and c-slip dominates at low water content at pressures higher than ∼7 GPa, a compilation of experimental studies by (Masuti et al., 2019) and the observed LPO of the ultra-deep xenolith do not support their claim. However, experimental studies under these high-pressure conditions are limited and there remain large uncertainties regarding the LPO at high pressures (P>3 GPa).

Quantifying dislocation drag at high strain rates with laser-induced Microprojectile impact

As deformation rate increases, the thermally activated dislocation glide gives way to a continuous glide of dislocations governed by their interactions with phonons. Understanding the dislocation-phonon drag regime is critical for designing metallic materials for extreme deformations rates. However, it has proven challenging to study empirically, partly due to the resource intensive nature of the experimental approaches targeting this regime. Here, we develop an impression-based experimental approach combining laser-induced microprojectile impact (Hassani et al., 2020a) and spherical nanoindentation to characterize the dislocation-phonon drag regime. We also develop a physically based constitutive framework that, when integrated with our experimental measurements, can quantify the dislocation-phonon drag regime. We isolate the effect of dislocation-phonon drag by leveraging the similar deformation geometries and length scales but different operative mechanisms during spherical nanoindentation and microprojectile impact. We discuss mechanistic contributions to the plastic work for microprojectile impacts in a range of impact velocities producing strain rates up to 109 s−1. We also develop a deformation mechanism map focused on the transition from thermal activation to dislocation drag for a model FCC metal, copper.

Lath decomposition and dispersion induced by heterostructure deformation and associated interactions in a lamellae metastable-β Ti alloy during subtransus processing

The mechanisms of lath decomposition and dispersion were hitherto rarely explained due to limited understanding into the heterostructure deformation between two phases and associated interactions. In this work, the lath decomposition and dispersion were systematically investigated in a wide subtransus processing range of 0.001 s−1–1 s−1 and 745 °C–820 °C with the consideration into the heterostructure deformation and associated interactions. The relationship between flowing behavior and microstructure evolution were discussed. Results indicated that the initial buckling of α laths caused high peak strength. The rotation and twist of α laths are helpful to decrease the flow stress, but it caused serious extra deformation and dislocation storage in the β matrix. The lath decomposition dominated by the ablation was greatly contributed by the adiabatic temperature rise (ATR) near boundaries with high energies. The α laths could rotate under low strain rates, and numerous residual laths (RLs) could be obtained. The increment in strain rates to 0.1 s−1 resulted in earlier lath decomposition and softening, as well as to disperse αp particles and homogenize the β matrix. But an overhigh strain rate of ∼1 s−1 caused adiabatic shear bands. The low temperatures are unfavorable to the lath decomposition, while over-high temperatures diminished the extra deformation and ATR. On this basis, a deformation mechanism map for the subtransus processing of lamellae Ti-55531 alloy was established. This investigation sheds deeper insight into the tailoring of microstructure and prediction of flowing behavior for the subtransus processing of lamellae metastable-β Ti alloys.

Deformation mechanism diagram and deformation instability of a Ti-5Al-5Mo-5V-1Cr-1Fe titanium alloy during the hot compression

Controlling the hot deformation mechanism of the titanium alloy and avoiding deformation instability are the key points in the hot forging of the titanium alloy. A high throughput experimental method was used to establish a deformation mechanism map efficiently. A continuously varying strain distribution from 0.3 to 1.4 was obtained by the deformation of the double-cone specimens. At the temperature of 910 °C and the strain rate of 1.37 s−1, Combining the analysis of metallography and strain-stress curves, the deformation mechanisms in different regions of the double-cone specimens were divided. Dynamic recovery (DRV) and dynamic recrystallization (DRX) work together in the low strain region (＜0.62), and DRX gradually becomes the dominant deformation mechanism in the medium strain region (0.62–1.02). In the large strain region (1.02–1.4), amounts of deformation bands (DBs) formed. Electron back scatter diffraction (EBSD) technique was used for the microstructure observation and the analysis of deformation mechanism. The results demonstrate that with an increase in strain, the recrystallization fraction increased from 83.8% to 96.3%. Combining the several results of the double-cone specimens, the hot deformation mechanism diagram with a temperature range of 850–930 °C, and a strain range of 0.3–1.4 was established. When the strain reaches a certain degree, it is difficult to further form new recrystallized grains at the grain boundaries. The formation of recrystallized grains within the deformation band has been characterized using transmission electron microscopy (TEM). Under the action of shear force, fine recrystallized grains form within the elongated grains. The concentrated fine recrystallized grains aggravate the microstructural nonuniformity of the Ti-55511 titanium alloy in this area, which finally leads to the deformation instability.

Effects of Ag on High-Temperature Creep Behaviors of Peak-Aged Al-5Cu-0.8Mg-0.15Zr-0.2Sc(-0.5Ag)

The tensile creep of Al-5Cu-0.8Mg-0.15Zr-0.2Sc(-0.5Ag) was tested at 150–250 °C and 125–350 MPa, and the effect of Ag on the high-temperature creep of Al-Cu-Mg alloys was discussed. After the addition of Ag, the high-temperature creep performances of the alloy were significantly improved at 150 °C/300 MPa and 200 °C/(150 MPa, 175 MPa). Then, constitutive relational models of the alloy during high-temperature creep were built, and the activation energy was calculated to be 136.65 and 104.06 KJ/mol. Based on the thermal deformation mechanism maps, the high-temperature creep mechanism of the alloy was predicted. After the addition of Ag, the creep mechanism of the alloy at 150 °C transitioned from lattice diffusion control to grain boundary diffusion control. At 250 °C, the mechanism was still controlled by grain boundary slip, but as the stress index increased and after Ag was added, the alloy fractures lead to the formation of dimples, thus improving the high-temperature creep performance.

Special hot working plastic deformation behavior and microstructure evolution mechanism of single-phase BCC structure AlFeCoNiMo0.2 high-entropy alloy

The single-phase body-centered cubic (BCC) structured high-entropy alloys are considered to be typically difficult-to-deform processing alloy because they can still crack during hot deformation at high temperatures and low strain rates. However, we found that the hot working formability of this type of alloy is very sensitive to temperature, and there is a unique microscopic transformation. In this paper, the as-cast single-phase BCC-structured AlFeCoNiMo0.2 high-entropy alloy was investigated with the single-pass hot-compression simulation experiment (deformation of 0.6), at temperatures and strain-rate ranges of 900 – 1150 ℃ and 0.001 – 0.1 s−1, respectively. The hot-deformation behavior and microstructure-evolution mechanisms were studied. The Arrhenius constitutive relation model was revised and established. The processing maps of Prasad, Gegel, Malas, and Murty with different instability criteria were constructed. The optimal thermal-processing parameters (temperatures of 1070 – 1150 ℃ and strain rates of 0.001 – 0.1 s−1) were provided. It was great to find that the alloy has a narrow temperature window effect of hot working with the Ruano-Wadsworth-Sherby (R-W-S) deformation-mechanism map established by incorporating the dislocation quantity. The deformation mechanisms of the alloy at 900 °C, 1000 °C, 1050 °C, and 1100 °C were predicted. The single-phase (BCC) structure of the alloy has strong stability during hot deformation. In a narrow range of hot-working temperatures, the microstructure has a necklace-like structure, and its substructure has a strong textured effect, impeding grain-boundary slip. In the optimized processing interval, discontinuous dynamic recrystallization (DDRX) occurs, and the necklace-like structure disappears. The recrystallization mechanism is related to grain-boundary sliding, caused by dislocation sliding.

Fabrication of Li anode metal via bulk mechanical property analysis

Two types of lithium (Li) metals with a different average grain size, i.e., ∼10 μm in orders and above 100 μm, are successfully produced by extrusion process and rapid molding process. The results obtained from nano-indentation tests show that hardness is influenced by grain size, and it decreases with a finer grained structure, which is not the general trend as those of other light-weight pure metals. It is remarkable to observe that these Li metals have a large strain rate dependence (a large m-value) even at room-temperature, irrespective of the grain size. Compared with the Frost-Ashby deformation mechanism map, the dominant deformation mechanism is the dislocation climb. On the contrary, this unusual behavior in the fine-grained Li metal is due to the partial contribution of grain boundary sliding to deformation. This behavior is associated with the enhancement of grain boundary diffusivity and high density of grain boundaries. Concerning the effect of grain size on electrochemical characteristics, similar to the mechanical property, the results obtained from electrodeposition and dissolution tests display that the fine-grained Li metal has much better over-potential of charging and discharging operation as well as voltage response.

Temperature dependence of tensile deformation mechanisms in a powder metallurgy Ni–Co–Cr based superalloy with Ta addition

To further develop Ni-based powder metallurgy (PM) superalloys for aero-engines, it is critical to understand their deformation mechanisms at various service temperatures. For turbine disks, the tensile strength and absorbed impact energy are critical parameters. The varying stress during tensile deformation makes it essentially different from creep. In this study, a high-performance PM superalloy with various Ta contents was evaluated by tensile tests at different temperatures from room temperature to 815 °C. The relationship between the temperature and plastic deformation mechanisms was explored at multiple scales using scanning electron microscopy and transmission electron microscopy, and the temperature dependence of the deformation mechanism map was established. The results demonstrated that the main deformation mechanism gradually changed from antiphase boundary shearing to superlattice stacking fault shearing as the temperature increases. The extended stacking faults (ESFs) were the source of the microtwins, which extended within the individual grains and thickened with increasing temperature. In addition, a novel mechanism of ESF formation was observed, i.e., Shockley shearing separated by one layer of atoms. Furthermore, the effects of Ta on the tensile strength and plasticity in PM superalloys were discussed.

Creep deformation behavior of the Ni–Fe-based GH984G alloy

The creep deformation behavior and microstructure evolution of the Ni–Fe-based GH984G alloy were investigated at temperatures of 700 °C–775 °C, with the stress ranging from 150 MPa to 400 MPa. The results demonstrate that the apparent stress exponent and apparent creep activation energy are 6.53 ± 0.74 and 546.7 ± 6.8 kJ/mol, respectively. The creep activation energy is higher than that of lattice self-diffusion, which is related to the interaction between dislocation and γ′ precipitates and can be corrected by threshold stress. The threshold stresses are 109.1 MPa, 51.5 MPa and 24.8 MPa at 700 °C, 750 °C and 775 °C, respectively. The true stress exponent is 4.95 ± 0.14 and the true creep activation energy is determined to be 288.3 ± 4.3 kJ/mol. The microstructure analysis demonstrates that the climbing of a/2 <110> perfect dislocation is the primary mechanism of creep at 700 °C. The dislocation climbing and a/6 <112> partial dislocation shearing into γ′ are both significant factors during creep deformation at 750 °C. The creep damage tolerance of 6.4 ± 1.1 indicates that microstructural degradation, including γ′ and M23C6 coarsening, is the primary creep damage mechanism for the GH984G alloy. The deformation mechanism map for GH984G alloy is established and the creep mechanism within the investigated conditions locates in the power-law creep domain controlled by dislocation climbing.

The Strain Rate Sensitivity of Heterogeneous Thin Film Metallic Glasses: Interplay Between Nanoscale Heterogeneity and Dynamic Plasticity

The dynamic mechanical properties of metallic glasses (MGs) are crucial to capturing the deformation signatures as well as for structural and functional applications. In this work, we investigate the influence of nanoscale structural heterogeneity of MGs on dynamic plasticity, focusing on the variation in strain rate sensitivity and the transition of deformation mechanisms, using a combined experimental and simulation approach. The Cu50Zr50 thin-film MGs with different nanoscale heterogeneities are synthesized using magnetron sputtering and further characterized using dynamic force microscopy and nanoindentation. All the films exhibit a strain rate hardening effect, but a transition in strain rate sensitivity as the indentation rate increases has been found in the MG with a higher heterogeneity. To understand the underlying mechanisms, mesoscale shear transformation zone dynamics simulations are performed on model Cu50Zr50 MGs. The simulation results are able to capture the experimental trend. Notably, the transition in strain rate sensitivity for a heterogenous MG stems from a change in deformation mechanisms: from structure-dictated strain localization at a lower strain rate to stress-dictated strain percolation into a shear band at a higher strain rate. The observed strain rate sensitivity and the corresponding mechanisms are summarized in a deformation mechanism map where nanoscale structural heterogeneity and strain rate are varied. We envision our study not only providing insights into the structure and property relationship of MGs on the nanoscale but also will facilitate the design of heterogeneous MGs for dynamic applications.

Nanoscale insights into the damage tolerance of Cantor alloys at cryogenic temperatures

High-entropy alloys deform plastically and may also be at risk to fracture in extreme environments. In this paper, the damage tolerance of nanocrystalline Cantor alloys under mode I loading at cryogenic temperatures is investigated via molecular dynamics. We find that the damage tolerance is improved significantly with the decrease of temperature, in contrast to conventional metals. Deformation mechanism maps are constructed to assess the plasticity based on the grain size and temperature. Results show that the plastic deformation is governed by a synergy of face-centered-cubic to hexagonal closed-packed martensite transformation, twinning, stacking fault formation, grain boundary (GB) plasticity, and especially dynamic recovery. Specifically, a crossover from shear localization to solid-state amorphization is identified with the decrease of temperature and grain size. The amorphization at GBs dissipates larger strain energy, leading to precursor retardation and hence a high damage tolerance. A final deformation mechanism map is constructed to combine the deformation mechanisms and the crossover pathway from localization to amorphization. This map has great implications on improving the damage tolerance of Cantor alloy at cryogenic temperatures.

Response Surface of Speed-Loading Path to Grain Refinement during Current-Heating Compression at SAE 5137H Steel.

In thermal deformation of materials, grain refinement induced by dynamic recrystallization (DRX) is often pursued to obtain excellent mechanical properties. Here, the thermal deformation behaviors of SAE 5137H steel were investigated and characterized at temperature and strain rate range of 1123–1483 K and 0.01–10 s−1. Meanwhile, a design approach in speed-loading paths for grain refinement during current-heating compression was proposed, and these paths are linked to a typical three-dimensional (3D) response surface. Depending on the acquired stress–strain curves, the flow behaviors of this steel were analyzed and the typical 3D processing map was constructed to clarify the stable processing parameter domains during the continuous deformation process. Then, by the typical 3D processing map and microstructure observation, the 3D deformation mechanism map was constructed to connect the processing parameters and microstructural mechanisms. Subsequently, the 3D activation energy map was constructed to evaluate these deformation mechanisms, and the enhanced deformation mechanism map was constructed. Eventually, based on the enhanced deformation mechanism map, the speed-loading paths for SAE 5137H steel during current-heating compression were designed and they are mapped in a 3D response surface.

Viscosity of bridgmanite determined by in situ stress and strain measurements in uniaxial deformation experiments.

To understand mantle dynamics, it is important to determine the rheological properties of bridgmanite, the dominant mineral in Earth’s mantle. Nevertheless, experimental data on the viscosity of bridgmanite are quite limited due to experimental difficulties. Here, we report viscosity and deformation mechanism maps of bridgmanite at the uppermost lower mantle conditions obtained through in situ stress-strain measurements of bridgmanite using deformation apparatuses with the Kawai-type cell. Bridgmanite would be the hardest among mantle constituent minerals even under nominally dry conditions in the dislocation creep region, consistent with the observation that the lower mantle is the hardest layer. Deformation mechanism maps of bridgmanite indicate that grain size of bridgmanite and stress conditions at top of the lower mantle would be several millimeters and ~105 Pa to realize viscosity of 1021–22 Pa·s, respectively. This grain size of bridgmanite suggests that the main part of the lower mantle is isolated from the convecting mantle as primordial reservoirs.

Accelerated Creep Testing of Inconel 718 Using the Stepped Isostress Method (SSM)

Abstract This paper demonstrates the stepped isostress method (SSM), an accelerated creep test (ACT) for the rapid assessment of metallic materials. The SSM test is based on the time-temperature-stress superposition principle in which temperature, stress, or both, are step increased to accelerate the time to rupture. The SSM test has proven successful for the ACT of polymers and polymeric composites but has yet to be proven for metallic materials. In this study, new test matrix design rules for the SSM of metals are established based on deformation mechanism, time-temperature transformation, and time-temperature precipitation maps. A test matrix of SSM and conventional creep tests (CCTs) are executed for alloy Inconel 718 at 750°C (1,382°F) with stress levels ranging from 100 to 350 MPa. Validation CCT data are gathered from the Japan National Institute of Material Science. Material constants for the Sine-hyperbolic (Sinh) constitutive model are calibrated using the SSM data and employed to predict the conventional creep response. When blindly compared with the CCT data, the SSM-calibrated Sinh model can accurately predict the conventional creep response across logarithmic decades and, thus, accelerate the capture of conventional creep data. Fractography indicates ductile fracture by transgranular microvoid coalescence with the same fracture mode observed in both SSM and CCT specimens. Creep cavitation is indicated by the population, smoothness, and size of microvoids.

Experimental decoding of grain boundary-based plastic deformation

Here, we apply the electron backscatter diffraction (EBSD) technique to identify grain boundaries (GBs) in a metal sheet surface, and the metal sheet is subsequently deformed via contact with a hard nanomold. Quantified by the length of molded nanorods, combining with molecular dynamics (MD) simulations and transmission electron microscopy (TEM) characterization, the microstructure evolution and the important influence of individual GBs on plastic deformation during nanomolding of crystalline Ag at different temperatures and stresses are revealed. Diffusion-based mechanisms become dominant once the temperature is above a critical value (Ttran∼0.54Tm), and the GB-affected zone in this temperature range is measured as several micrometers, approximately 3-4 orders of magnitude larger than the structural width of GBs. Finally, benefiting from the decoded GB-based deformation mechanism, we demonstrate that the prevalent deformation mechanism map can be experimentally constructed with high efficiency based on the proposed method. Our findings provide new insights into the individual GB-based deformation mechanism at high temperature and show the importance of developing new methods for constructing deformation mechanism maps to experimentally quantify specific deformation mechanisms.

An Innovative Approach of Parameter Loading Path Design for Grain Refinement and Its Application in Ni80A Superalloy.

In order to obtain the desired mechanical properties of products, an innovative method of loading parameter designs for acquiring the desired grain refinement is proposed, and it has been applied in the compression process of Ni80A superalloy. The deformation mechanism maps derived from processing maps based on the Dynamic Materials Model (DMM) theory were constructed, since the critical indicator values corresponding to dynamic recrystallization (DRX) and dynamic recovery (DRV) mechanisms were determined. The processing-parameter domains with DRX mechanisms were separated from the deformation mechanism map, while such domains were chaotic and difficult to apply in innovative parameter loading path design. The speed-loading path derived from strain rate-loading path in a compression process was pursued. The grain refinement domains are discretized into a finite series of sub-domains with clear processing parameters, and the optimal strain rate of each sub-domain is determined by step-by-step finite element simulation. A 3D response surface of the innovative optimal loading path of strain rate was fitted by interpolating methods. Finally, the isothermal compression experiments for Ni80A superalloy were conducted, and the microstructure observations indicated that the desired grain refinement was achieved. This innovative method of parameter loading path design contributes to the microstructure adjustment of the alloys with DRX mechanism.