Dominant Deformation Mechanism Research Articles

Effects of Temperature and Bimodal Microstructure on the Mechanical Properties and Fracture Mechanisms of High-Strength Al-Ni-Cu-Fe Alloy Fabricated through Laser Powder Bed Fusion

This study investigates the mechanical properties of an Al-Ni-Cu-Fe alloy fabricated using laser powder bed fusion (LPBF) technology, evaluated at temperatures ranging from room temperature to 300°C under various strain rates for potential aerospace and automotive applications. Aluminum alloys often face the challenges of maintaining high-temperature strength and stability, limiting their use in demanding environments. The results demonstrate that the LPBF Al-Ni-Cu-Fe alloy maintains strength above 300 MPa at standard strain rates and reaches up to 400 MPa at high strain rates at 300°C, indicating its suitability for lightweight, high-strength vehicles and thermal engineering systems. The high-temperature strengthening mechanisms are attributed to Al9FeNi submicron cellular eutectic walls that impede dislocation motion and precipitation strengthening from Al2Cu. Fracture analysis reveals that localized brittleness limits elongation to 3% at room temperature, with deformation distributed between coarse-grained (CG) and fine-grained (FG) regions. At elevated temperatures (300°C), partial homogenization of FG regions reduces melt pool constraints, promoting local deformation. Therefore, ductility increases significantly under high strain rates as the dispersed Al9FeNi phases become less effective in resisting instantaneous stresses, allowing deformation to overcome melt pool boundaries. At extreme strain rates during impact testing, twinning becomes the dominant deformation mechanism, contrasting with tensile testing. These findings confirm the potential of the LPBF Al-Ni-Cu-Fe alloy as a heat-resistant printed aluminum alloy for high-temperature applications and provide valuable insights into fracture mechanisms within the bimodal microstructure, guiding future alloy optimization.

Distinct deformation mechanisms of silicate glasses under nanoindentation: The critical role of structure

We use large-scale molecular dynamics simulations to investigate the indentation response of three silica-based glasses with varying compositional complexities. Our primary goal is to clarify the roles of the typical network-modifying species, namely, sodium, and the secondary network-forming species, namely, boron, in influencing the mechanical behavior of the glasses under localized stress. The distinct mechanical responses of the glasses are linked to structural features such as bond strength, network connectivity, and atomic packing density. The enhanced nanoscale ductility of sodium silicate and sodium borosilicate glasses, compared to silica, is attributed to the structural flexibility induced by Na atoms, which depolymerize the network, and by B species in mixed coordination. We also find that shear flow, driven by network flexibility, is the dominant deformation mechanism in the sodium silicate and sodium borosilicate glasses, while densification dominates in silica due to its low packing density. The evolution of short-to-intermediate-range structures is responsible for the distinct deformation behaviors of the glasses. These results highlight the critical role of structure in determining the deformation mechanisms of silicate glasses under sharp contact loads, providing insights for improving the mechanical performance of these materials.

Effect of Vanadium-Alloying on Microstructural Evolution and Strengthening Mechanisms of High-Nitrogen Steel Processed by High-Pressure Torsion

We study the effect of high-pressure torsion on the microstructure, phase composition, microhardness, and strengthening mechanisms of high-nitrogen austenitic steels with different vanadium content: Fe-23Cr-19Mn-0.2C-0.5N, Fe-19Cr-21Mn-1.3V-0.3C-0.8N, and Fe-18Cr-23Mn-2.6V-0.3C-0.8N, wt %. Regardless of the chemical composition of the steels, high-pressure torsion (HPT) causes the refinement of their microstructure due to a high density of dislocations, twin boundaries, and shear bands. Vanadium alloying decreases the stacking fault probability in the structure of the steels and changes their dominating deformation mechanism under high-pressure torsion: from planar dislocation slip and twinning in the vanadium-free steel to dislocation slip with a tendency to shear band formation in the vanadium-alloyed steels. An increase in the vanadium content forces precipitation hardening. Thus, after HPT, the V-alloyed steels have a higher microhardness as compared to the vanadium-free one. Different strengthening factors (strain hardening, solid solution hardening, and precipitation strengthening) govern the value and kinetics of growth of microhardness of the steels processed by high-pressure torsion. Vanadium alloying and increasing its content result in the growth of the contribution of precipitation hardening and decreases strain hardening of high-nitrogen steels.

Active, long-lived upper-plate splay faulting revealed by thermochronology in the Alaska subduction zone

The lack of subaerial forearc geological records in active subduction zones has hindered our understanding of the roles of upper-plate structures and their interactions with plate interface processes in accommodating forearc deformation. Forearc splay faults, a type of upper-plate structure, are of particular interest due to their high efficiency in triggering tsunamis during great earthquakes. The coastal area of the Kodiak Islands, Alaska, USA exhibits stratigraphic and geomorphologic records of Miocene to Recent vertical tectonism and Quaternary thrust faults, suggesting potential splay-fault-involved deformation over geological timescales. To better understand the mechanisms of forearc long-term strain accumulation and the roles of splay faults, we investigate the spatial and temporal pattern of recent forearc exhumation in the Kodiak accretionary prism by conducting zircon and apatite (U-Th)/He (ZHe and AHe) thermochronologic analyses and thermal history modeling. These results are supplemented by field investigations, detrital zircon geochronology analyses and offshore active fault mapping. Most of the ZHe ages record cooling through the ZHe closure temperature in the late Eocene-early Oligocene, temporally and spatially consistent with the Eocene-early Oligocene broad antiformal exhumation previously documented by zircon and apatite fission track thermochronological ages. However, the AHe ages record cooling through the AHe closure temperature from early Miocene to Pliocene and exhibit an overall trenchward younging trend, with all the Pliocene ages (3-5 Ma) in the regions closest to the trench. Our thermal history modeling and field survey suggest that the trenchward coastal area of the Kodiak Islands experienced a change from early-middle Miocene basin subsidence to recent deformation and rapid uplift from 6-7 Ma to recent, while the rest of the island experienced an early-middle Miocene decrease in the prolonged exhumation from the Eocene-Oligocene. The newly revealed long-term exhumation pattern resembles the estimated uplift patterns based on elevated marine terraces and geodetic data. The early-middle Miocene change in exhumation pattern might be caused by a change in the dominant deformation mechanism affecting the Kodiak Islands, from broad underplating along the subduction interface mainly during the Eocene-Oligocene to hanging-wall uplift due to an active crustal splay thrust fault system since the late Miocene (the Kodiak Shelf Fault). We further discuss the dip-slip rate and geometry of the Kodiak Shelf Fault system and how inherited forearc upper-plate structures and lithology may affect forearc fluid distribution and facilitate the development and persistent deformation of the Kodiak Shelf Fault system.

Kinematic GNSS inversion of the large afterslip (Mw 6.4) following the 2019 Mw 6.2 Hualien earthquake (Taiwan)

The postseismic deformation following the April 2019 Mw 6.2 Hualien earthquake presents an unique opportunity to investigate the mechanisms by which the northern section of the Longitudinal Valley accommodates lithospheric deformation. We apply a variational Bayesian independent component analysis approach to displacement time-series to infer a 6-month long afterslip. Kinematic inversion shows that displacements are well explained by widespread afterslip (∼60 km in the along-strike direction) with limited slip (≤0.1 m) surrounding the coseismic slip area. The total geodetic moment relieved by afterslip (M0 ∼ 4.6 × 1018 Nm, i. e., Mw ∼ 6.4) is twice as large as the mainshock seismic moment, which represents a rare exception of a moderate magnitude event for which the afterslip moment exceeds that of the seismic moment. Then, combining geodetic and seismological analysis, we infer that afterslip is the dominant mechanism of near-to intermediate-field postseismic deformation and also likely represents the driving force that controls aftershock productivity and the spatiotemporal migration of seismicity. Besides, the fault zone frictional stability parameter a-b of rate-and-state dependent friction (a-b ∼ 0.0067–0.02) is comparable with previous estimates in the Longitudinal Valley. Finally, the study demonstrates that the northern Longitudinal Valley region hosts complex seismogenic structures that display a variety of slip behaviors.

On Forming Characteristics of Hems by Means of Incremental Sheet Forming

Given the need for versatile joining processes, form-fit joining is gaining increasing importance. Although it has known limitations and complexity, roller hemming remains widely used due to its flexibility. Here, the novel Incremental Sheet Forming (ISF) hemming technique has the potential to expand the range of applications and process limits. It has already proven effective in preliminary works for joining comparatively small radii without wrinkles and cracks. However, a deeper understanding of the dominant material flow and deformation mechanism during forming is required to fully exploit its potential. This study aims to conduct a detailed examination of this technology through experimental and numerical investigations. Strain measurements on convex and concave hems provide insights into the material flow. A comparison of the forming mechanism for both processes is made using straight hems. The results show that ISF hemming has a favorable material flow for compensating cracks and wrinkles in curved hems. Additionally, it induces strains across the entire hem area, reaching higher values than those achieved with roller hemming. One reason for this is the forming mechanism, which combines tension, compression and shear, whereas roller hemming primarily involves bending and compression of the hemming radius.

Understanding and Controlling Stack Pressure Inhomogeneity in Anode-Free Solid-State Batteries

Lithium (Li) metal anodes are widely studied as replacements for current graphite anodes as they have projected higher energy density. However, Li-metal batteries face issues with dendrite propagation and the eventual formation of dead Li. Solid-state batteries (SSBs) are a promising pathway to realize Li-metal batteries, which is attributed to their ability to improve safety and stability through the use of solid-state electrolytes. In particular, “anode-free” SSBs can enable high energy densities by forming a Li metal anode in situ. The initial Li nucleation, plating, and stripping behaviors in anode-free SSBs is influenced not only by the interfacial electrochemistry, but also by the mechanical stresses present along the current collector interface. Mechanical stress also plays a key role in Li metal anode deformation, where creep is the dominant deformation mechanism. However, to date, stack pressure is typically reported as a singular value, rather than considering the temporal and spatial variations in interfacial stress as the battery cycles.This work investigates the role of inhomogeneous stack pressure on Li anode formation and dissolution against a sulfide solid electrolyte. To compensate for this inhomogeneous stack pressure, elastomeric interlayers are used to increase the uniformity of stress along the interface, and thus improve electrochemical cyclability. To analyze the impact of elastomeric interlayers on areal Li plating coverage, post-mortem optical microscopy images of the current collector are captured, showing an improvement in areal coverage from 49% to 70%. The reversible capacity also increases from 89% to 94% with inclusion of an elastomer layer. To confirm the trends in an elastomer layer on increasing stack pressure homogeneity, the mechanical stress at the anode-free interface is modeled using finite-element simulations under different stack pressure geometries. System-level simulations illustrate tradeoffs with respect to the uniformity of mechanical stress and energy density. This work demonstrates the importance of studying external auxiliary components and packaging in SSBs to optimize anode-free SSB performance.

Relative hardening contributions and ductility of as-prepared and annealed nanocrystalline Co-P electrodeposits

The effect of annealing up to 538 °C (1000 °F) on the hardness and ductility of two electrodeposited bulk nanocrystalline cobalt-phosphorus alloys (L-nCoP, 0.14 at%P and H-nCoP, 2.17 at%P) was investigated. Through a combination of electron microscopy and X-ray/synchrotron diffraction characterization, hardness and bend ductility measurements as well as density functional theory calculations, it is shown that hardness is controlled by four additive contributions. At 0.14 at%P, grain size strengthening and likely grain boundary relaxation are the dominant strengthening mechanisms. In the 2.17 at%P alloy, the contributions from solute strengthening and cobalt phosphide precursors and precipitates are much more significant and typical age strengthening behavior is observed with a peak hardness temperature of 371 °C (700 °F). The bend ductility at 0.14 at%P was in the 12–17 % range up to 482 °C (900 °F). In contrast, the ductility at 2.17 at%P decreased rapidly with increasing annealing temperatures approaching values less than 1 % at 427 °C (800 °F) due to excessive cobalt phosphide precursor and precipitate formation. The dominant deformation mechanism in both alloys was found to be basal dislocation slip.

Effect of grain size on mechanical characteristics and work-hardening behavior of fine-grained Mg-0.8Mn alloy via adjusting extrusion temperature

In this study, the Mg-0.8Mn (M08, wt%) alloy is subjected to hot extrusion at various temperatures ranging from 150 ℃ to 300 ℃, and the correlations between the microstructure evolution, tensile properties, and work hardening behavior at ambient temperature are elaborated. The M08 alloy extruded at 150 ℃ exhibits a bimodal microstructure, which included undynamic recrystallized (unDRXed) grains and fine dynamic recrystallized (DRXed) grains with an average size of 1.4 μm. As the extrusion temperature exceeds 210 °C, fully DRXed grains can be observed with grains growth becoming more pronounced with the increase of extrusion temperature. Notably, Mn atomic segregations are evident at the grain boundaries (GBs) in the M08 alloy extruded at 150 °C. It is also essential to highlight that the propensity for GB segregation is diminished with higher extrusion temperature, and the amount of α-Mn precipitates within grain interiors is increased in the M08 sample extruded at 300 ℃. An intriguing observation is the abnormal rise in yield strength (YS) with grain size as the extrusion temperature is increased. This phenomenon can be attributed to the diminishing effect of GB sliding and the concurrent enhancement of GB strengthening effect. However, as the extrusion temperature is raised from 150 °C to 300 °C, the ductility of M08 alloy experiences a substantial decline from 74 % ± 4–16 % ± 3 %. Meanwhile, the grain growth can bring about an increased strain hardening rate, which is primarily dependent on the transition in the dominant deformation mechanism from grain boundary sliding (GBS) to dislocation slip. This transition leads to a more substantial accumulation of dislocations within the coarser grains, thereby affecting the mechanical behavior of M08 alloy. This work provides valuable insights into the influence of extrusion temperature on the microstructure evolution and mechanical properties of Mg-0.8Mn alloy, offering a foundation for optimizing the processing parameters to achieve desired mechanical performance.

Thermal processing behaviour and microstructural evolution of high W and high γ’ content extruded nickel-based powder metallurgy superalloy FGH4109

ABSTRACT Nickel-based powder metallurgy superalloys with high W and high γ’ content exhibit excellent high-temperature properties, but their high deformation resistance poses significant challenges for hot processing. In this study, the hot compression behaviour of an extruded nickel-based powder superalloy FGH4109 with high W (6.1%) and high γ’ phase contents (60%) was systematically investigated at temperatures ranging from 1060 ℃ ~ 1140 ℃, strain rates from 0.001 s−1 ~1 s−1, and maximum true strains of 0.7. Combined with the hot working map and the microstructure evolution in different hot working zones, the optimal hot working window for the alloy was determined to be in the temperature range of 1060 ℃ ~ 1140 ℃ and strain rates of 0.001 s−1 ~0.012 s−1, where the power dissipation efficiency η was greater than 0.5, and the dominant deformation mechanism was discontinuous dynamic recrystallisation.

The microstructure evolution and mechanical properties of Ti–1300 alloy during ECAP deformation and subsequent aging treatment

The effect of Equal channel angular pressing (ECAP) deformation on the microstructure, mechanical properties, and aging behavior of solution treated Ti–1300 alloy was investigated. The results show that the grains are not broken during ECAP deformation due to coordination of grain rotation. The dominant deformation mechanism involved dislocation slip and twinning. The ECAP deformation has a significant effect on the subsequent aging behavior. The microstructure of aged samples comprised αs phase in β matrix. The precipitation of αs phase in the pre-deformed sample is higher than that in the solution treated sample, which is reflected in the improvement of microhardness of the pre-deformed + aging sample. The microhardness of the alloy after ECAP deformation increases with decreasing aging temperature and increasing aging time.

Pursuing ultrahigh strength–ductility CoCrNi-based medium-entropy alloy by low-temperature pre-aging

Developing high-performance alloys with gigapascal strength and excellent ductility is crucial for modern engineering applications. The concept of multi-component high/medium entropy alloys (H/MEAs) provides an innovative approach to designing such alloys. In this work, we developed the Co1.5CrNi1.5Al0.2Ti0.2 MEA, which exhibits outstanding mechanical properties at room temperature through low-temperature pre-aging followed by annealing treatment. Tensile testing reveals that the MEA possesses an ultrahigh yield strength of 20±0785 MPa, an ultimate tensile strength of 2365 ± 70 MPa, and exceptional ductility of 15.8% ±1.7%. The superior tensile properties are attributed to the formation of fully recrystallized heterogeneous structures (HGS) composed of ultrafine grain (UFG) and fine grain (FG) regions, along with discontinuous precipitation of coherent nano-size lamellar L12 precipitates. The mechanical incompatibility between the UFG region and the FG regions during deformation induces the accumulation of a large number of geometrically necessary dislocations at the interface, resulting in strain distribution and hetero-deformation-induced (HDI) stress accumulation, contributing significantly to HDI strengthening. HDI strengthening, precipitation strengthening, and grain boundary strengthening are the primary mechanisms responsible for the ultra-high yield strength of the MEA. During deformation, the dominant deformation mechanisms include dislocation slip, deformation-induced stacking faults, and Lomer–Cottrell locks, with minor deformation twinning. The synergistic interaction of these multiple deformation modes provides the MEA with excellent work hardening capability, delaying plastic instability and achieving an excellent combination of strength and ductility. This study provides an effective strategy for synergistically strengthening MEAs by combining HDI strengthening with traditional strengthening mechanisms. These findings pave the way for the development of advanced structural materials with high performance tailored for demanding applications in engineering.

Exceptional Strength-Ductility Combinations of a CoCrNi-Based Medium-Entropy Alloy via Short/Medium-Time Annealing after Hot-Rolling.

Strong yet ductile alloys have long been desired for industrial applications to enhance structural reliability. This work produced two (CoCrNi)93.5Al3Ti3C0.5 medium-entropy alloys with exceptional strength-ductility combinations, via short/medium (3 min/30 min) annealing times after hot-rolling. Three types of intergranular precipitates including MC, M23C6 carbides, and L12 phase were detected in both samples. Noticeably, the high-density of intragranular L12 precipitates were only found in the medium-time annealed sample. Upon inspection of the deformed substructure, it was revealed that the plane slip is the dominant deformation mechanism of both alloys. This is related to the lower stacking fault energy, higher lattice friction induced by the C solute, and slip-plane softening caused by intragranular dense L12 precipitates. Additionally, we noted that the stacking fault and twinning act as the mediated mechanisms in deformation of the short-time annealed alloy, while only the former mechanism was apparent in the medium-time annealed alloy. The inhibited twinning tendency can be attributed to the higher energy stacking faults and the increased critical twinning stress caused by intragranular dense L12 precipitates. Our present findings provide not only guidance for optimizing the mechanical properties of high/medium-entropy alloys, but also a fundamental understanding of deformation mechanisms.

Atomistic investigation on the anisotropic elastic and plastic responses of nanotwinned metals

Introducing nanotwins into materials is one of the important strengthening methods to achieve the synergistic improvement of strength and ductility. The anisotropic mechanical behaviors of nanotwinned materials have been widely studied by experimental and computational methods. The dominant deformation mechanisms about dislocation slippages can be effectively switched among three modes, and controlled by twin spacing and the angle between twin boundaries (TBs) orientation and loading direction. Particularly, most of previous researches mainly focused on the deformation mechanisms during the plastic flow stage and researchers paid little attention on the anisotropic characteristics of TBs at the elastic stage which are also essential to manufacture the high-performance materials. Therefore, this study is aiming to systematically investigate the anisotropic effect of TBs both at the elastic and plastic stages within the single crystalline or polycrystalline systems by molecular dynamics (MD) simulations. It is revealed that, when the loading direction is parallel to twin planes, the introduction of TBs in single crystalline models will significantly affect the characteristics of atomic bond rotation and elongation dominated elastic deformation, which can alter the Poisson’s ratio of materials, generate elastic-softening behavior and inhomogeneous elastic deformation. At the plastic flow stage, the deformation mechanism transforms from trans-twin dislocation slippage into the coexistence of Hard Mode I and threading dislocation slippage (Hard Mode II) when the loading direction changes from parallel to perpendicular direction with respect to TBs. Moreover, the dislocation segments at the conjunction of trans-twin dislocation play a momentous role in enhancing material strength. The results for polycrystalline models are consistent with that of single crystalline ones. These findings are expected to be beneficial for the development of high-performance nanostructured materials for structural and functional applications by strain engineering and defect regulation.

Hot deformation behavior and processing map of the nanocrystalline pure Mg

The thermal compression test of the nanocrystalline, as-extrusion and as-cast pure Mg was carried out by the AGX-XD universal testing machine. The results demonstrated that flow stress of pure Mg with different grain sizes decreased with increasing temperature and decreasing strain rate. Accurate prediction of the thermal deformation behavior was achieved through establishing the constitutive equations. In comparison, the nanocrystalline pure Mg represents stronger strain hardening rate and weaker sensitivity to strain rate. Based on processing map, the optimal processing area for the nanocrystalline pure Mg is at 300–400 °C, 0.01– 0.1 s−1. The grain distribution of the nanocrystalline pure Mg remains stable and uniform after hot compression, and the grain boundary migration is the dominant deformation mechanism.

Unveiling mechanical response and deformation mechanism of extruded WE43 magnesium alloy under high-speed impact

To clarify the mechanical behavior and deformation mechanism of rare earth magnesium (Mg) alloy WE43 under extreme service loads, high-speed impact tests under various deformation temperatures and loading paths were conducted using a split Hopkinson pressure bar. The flow stress along extrusion direction (ED) and extrusion radial direction (ERD) decreases apparently with deformation temperature. Compared with conventional Mg alloys, it exhibits a slight anisotropy and an unusual C-shaped characteristic. Cellular dislocation, mechanical twin and fine grain that occur after high-speed impact deformation are insensitive to the loading direction, but strongly dependent on the deformation temperature, especially superimposed with adiabatic temperature rise. As a result, dynamic recrystallization (DRX) occurs even at an ambient temperature of 25 °C. Double twinning and prismatic slip or pyramidal slip are the dominant deformation mechanisms at 25 °C. These twins induce mechanical cutting refinement to form some fine-grained structures, accompanied by a small number of fine grains by twinning induced DRX. In contrast, the deformation at 250 °C is mainly controlled by prismatic slip and pyramidal slip, accompanied by various types of twinning in early deformation stage. Compared with 25 °C, more fine-grained microstructures are formed at 150 and 250 °C through a synergy mechanism of twinning induced mechanical cutting and twinning induced DRX.

TPMS-based strut-shell interpenetrating lattice metamaterial with wide-range customizable mechanical properties and superior energy absorption

Mechanical metamaterials (MMs) are artificially designed structures with superior properties that originate from unit cells. Compared with the widely studied single-morphology MMs, multi-morphology interpenetrating MMs offer, by the virtue of their fused lattice characteristics, the potential for customizable mechanical properties and new application fields. Inspired by the fact that the isosurface of triply periodic minimal surfaces (TPMS) can divide a lattice into two independent regions, this paper proposes a novel TPMS-based strut-shell interpenetrating (TSSI) lattice metamaterial. This study demonstrates that the TSSI lattice has a larger specific surface area than traditional TPMS lattices. Compared with the volume fraction, the fusion proportion parameter is not only more conducive to effectively adjusting the mechanical properties in a wide range and controlling the anisotropy of the lattice metamaterials to achieve elastic isotropy but significantly changes the dominant mechanism of deformation under uniaxial and shear loads. Moreover, the TSSI lattice metamaterial can avoid sharp stress drops due to the mutual support and wrapping of the internal interpenetrating lattices after failure, resulting in a more stable energy absorption efficiency and a maximum increase in energy absorption of 74%. This work provides new opportunities to design lightweight components with a wide range of customizable mechanical properties and superior energy absorption.

Tailoring size and fraction of coherent L12 nanoprecipitates to achieve strong hardening in medium entropy alloys with heterogeneous grain structures

Heterogeneous grain structures with coherent L12 nanoprecipitates were constructed in a medium entropy alloy with chemical composition of Co34.5Cr32Ni27.5Al3Ti3 (at. %). The volume fraction and size of L12 nanoprecipitates are observed to become higher and larger, and the interspacing is found to become smaller with increasing aging time, resulting in a severer heterogeneity. The domain of tensile properties was expanded by varying the size and volume fraction of coherent L12 nanoprecipitates after aging treatment. The samples after longer-time aging treatment show stronger strain hardening as compared to those after shorter-time aging treatment. The hetero-deformation-induced (HDI) hardening plays a more vital role in the longer-time aged samples than the shorter-time aged samples, especially at the elasto-plastic transition stage. The dominant precipitation hardening mechanism is observed to transit from dislocation-cutting to Orowan dislocation-looping with increasing precipitation size, as predicted by a theoretical model and validated by experimental results. Multiple deformation defects, such as deformation twins, stacking faults and Lomer-Cottrell locks, are the dominant deformation mechanisms for all samples, while interactions between these defects and L12 nanoprecipitates were observed to be more intensive in the longer-time aged samples than in the shorter-time aged samples, resulting in stronger precipitation and HDI hardening.

Anomalous size dependent softening behaviors and related deformation mechanisms of Zr/Mo multilayers

As a representative of hcp/bcc metallic multilayers, the correlation between microstructures and mechanical properties of nanostructured Zr/Mo multilayers with were studied. Zr/Mo multilayers with different layer thicknesses were prepared by magnetron sputtering method. XRD and TEM analyses revealed that Zr/Mo multilayers have Zr (0002) and Mo (110) out-of-plane crystalline texture. The nanoindentation hardness of Zr/Mo multilayers is in the range of ∼9.9–10.8 GPa at h ≤ 10 nm, while decreases with reducing layer thickness at larger h, which is on the contrary of the common rule - smaller is stronger. The aforementioned strengthening mechanism applicable in multilayers, i.e., dislocation bowing-out in nanolayers is not suitable to explain this size dependent softening behavior, while the IBS model fits well with the measured hardness at h = 10 nm. This anomalous softening behavior at large layer thickness scales may be caused by its special structures and deformation mechanisms. Dislocations interact with the grain boundaries, instead of the interfaces, and grains rotation/grain boundaries sliding are the dominant deformation mechanism at large layer thickness scales. The size-dependent strain rate sensitivity and activation volume were also explored, to revealing the rate dependent deformation mechanisms.

Activation of Dissolution‐Precipitation Creep Causes Weakening and Viscous Behavior in Experimentally Deformed Antigorite

AbstractAntigorite occurs at seismogenic depth along plate boundary shear zones, particularly in subduction and oceanic transform settings, and has been suggested to control a low‐strength bulk rheology. To constrain dominant deformation mechanisms, we perform hydrothermal ring‐shear experiments on antigorite and antigorite‐quartz mixtures at temperatures between 20 and 500°C at 150 MPa effective normal stress. Pure antigorite is strain hardening, with frictional coefficient (μ) > 0.5, and developed cataclastic microstructures. In contrast, antigorite‐quartz mixtures (10% quartz) are strain weakening with μ decreasing with temperature from 0.36 at 200°C to 0.22 at 500°C. Antigorite‐quartz mixtures developed foliation similar to natural serpentinite shear zones. Although antigorite‐quartz reactions may form mechanically weak talc, we only find small, localized amounts of talc in our deformed samples, and room temperature friction is higher than expected for talc. Instead, we propose that the observed weakening at temperatures ≥200°C primarily results from silica dissolution leading to a lowered pore‐fluid pH that increases antigorite solubility and dissolution rate and thus the rate of dissolution‐precipitation creep. We suggest that under our experimental conditions, efficient dissolution‐precipitation creep coupled to grain boundary sliding results in a mechanically weak frictional‐viscous rheology. Antigorite with this rheology is much weaker than antigorite deforming frictionally, and strength is sensitive to effective normal stress and strain rate. The activation of dissolution‐precipitation in antigorite may allow steady or transient creep at low driving stress where antigorite solubility and dissolution rate are high relative to strain rate, for example, in faults juxtaposing serpentinite with quartz‐bearing rocks.