Dominant Deformation Mechanism Research Articles

Analysis of the Rotation Bending Test Method and Characterization of Unidirectional Carbon Fiber-Reinforced Polycarbonate Tapes at Processing Temperatures.

Bending is one of the dominant material deformation mechanisms that occurs during the forming process of unidirectional (UD) thermoplastic tapes. Experimental characterization of the bending behavior at processing temperatures is crucial to obtaining close-to-reality data sets for process analysis or material modeling for process simulation. The main purpose of this study is to characterize to a high degree of accuracy the temperature-dependent bending behavior of single and multi-ply specimens of carbon fiber-reinforced polycarbonate (PC/CF) UD tapes at processing temperatures, which implies a molten state of the thermoplastic matrix. The application of the rotation bending test using a customized fixture may come with systematic deviations in the measured moment that result from a pivot offset or an effective clearance that is unknown under realistic test conditions. The present research analyzes these effects with analytical methods, experimental investigations, and simulations using a finite element model. In this context, a compensation method for the toe-in effect is evaluated. With this approach, we were able to obtain reliable data and characterize the bending resistance within the desired processing window. The data reveal a major drop in bending resistance between 200 °C and 250 °C and a less significant decrease between 250 °C and 300 °C. Analysis of the thickness-normalized bending resistances indicates a non-linear relationship between specimen thickness and measured moment but an increasing shear-dominated characteristic at higher temperatures.

Effect of strain rate on compressive behavior and deformation mechanism of CoCrNi medium entropy alloy fabricated via cold spray additive manufacturing

The equiatomic CoCrNi medium entropy alloy (MEA) has shown better mechanical properties than many MEAs and high entropy alloys (HEAs) at room temperatures. However, the mechanical behaviors and deformation mechanisms of CoCrNi MEAs have yet to be studied, especially at different strain rates. In this study, cold spray additive manufacturing (CSAM) was applied to fabricate freestanding CoCrNi deposits. The investigation of compressive behaviors and deformation mechanisms were studied at a low-strain rate of 0.001 s−1 and at a high strain rate ranging from 2300 s−1to 4200 s−1. The as-fabricated sample exhibited refined grains, unfused powders, and large porosity, resulting in limited metallurgical bonding within the bulk CoCrNi deposit and deformed particle interface. Therefore, post-heat treatment at 1350 °C was applied to improve the interparticle bonding and strengthen the as-fabricated CoCrNi deposits. As expected, the annealed deposit significantly improved the ultimate compressive strength (UCS) and strain elongation of the as-fabricated deposits from ∼90 MPa and ∼5% to ∼1400 MPa, and ∼70% at a strain rate of 0.001 s−1. Besides, the annealed samples showed improvement twofold in UCS and ductility of the as-fabricated deposits from ∼67 MPa and ∼8.6% to ∼1266 MPa and ∼34% at a strain rate of 4200 s−1 and 3800 s−1 respectively. The TEM image of the annealed CoCrNi sample showed deformation twining, nano twins, planar slips, and microbands, which are the dominant deformation mechanisms that help to promote the plasticity and the work hardening behavior of this alloy. This work validates the compressive behavior and deformation mechanism of cold-sprayed CoCrNi MEA associated with post-heat treatment at different strain rates.

Molecular dynamics simulation of deformation behavior of nanocrystal CuNiAlCo medium-entropy alloys

Applying the molecular simulation method, the melting, elastic, and plastic properties of nanocrystal CuNiAlCo medium entropy alloy are investigated in this work. The results show that the melting point in nanocrystal CuNiAlCo is decreased with decreasing crystallite size from 5.0 to 3.1 nm. Compared to the bulk modulus, the crystallite size has a discernible influence on both the shear and Young's modulus. The uniaxial tension calculations suggest that the tensile strength decreases with decreasing crystallite size, and the inverse Hall–Petch relationship is found in nanocrystal CuNiAlCo. The polyhedral template matching analysis illustrates that the FCC constitution of CuNiAlCo gradually decreases with the decrease in crystallite size. When the strain is 30%, the amorphization would occur in uniaxial tension. The atomic structural evolution of CuNiAlCo with the crystallite size of 5.0 nm under the uniaxial strain verifies that the stacking fault reconstruction and amorphization are the dominant plastic deformation mechanism for nanocrystal CuNiAlCo medium entropy alloy. The shockley dislocation plays an essential role in uniaxial tension.

Strain rate-dependent tensile response and deformation mechanism of laser powder bed fusion 316L stainless steel

The laser powder bed fusion (LPBF) process is renowned for producing metal parts with exceptional mechanical properties, attributed to their unique microstructures. However, there is a lack of discussion in the literature regarding the plastic deformation mechanism at wide strain rates. To address this research gap, this study employs both quasi-static tensile (QST) tests and dynamic Split-Hopkinson tensile bar (SHTB) experiments to investigate the tensile properties and deformation mechanisms of a well-characterized material, 316L stainless steel (SS), across an extensive range of strain rates spanning from 10−3 s−1 to 103 s−1. Our results reveal the superior quasi-static and dynamic stretching behavior exhibited by LPBF specimen in comparison to the wrought 316L counterpart. This enhanced behavior can be primarily attributed to the cellular and hierarchical structures inherent to LPBF specimen. Notably, the cellular structure emerges as the primary controlling factor in mechanical deformation due to its capacity for increasing the critical strain required for deformed twinning and martensite transition. Consequently, under quasi-static loading, twinning and martensite transition act as the dominant deformation mechanism, while dynamic loading conditions are characterized by a prevalence of dislocation interactions. This comprehensive understanding of the deformation mechanism and strain rate-dependent behavior of LPBF 316L SS provides valuable insight for optimizing its mechanical properties under a wide range of loading conditions.

Effect of Ce on microstructure and dynamic compression properties of ZK60 alloy

The Mg–6Zn– xCe–0.6Zr ( x = 1, 2) alloys were subjected to a dynamic compression experiment, and the change in microstructure was noticed and studied. The primary components of two alloys are α-Mg matrix, Mg7Zn3 phase, and (Mg1− xZn x)11Ce phase. The two alloys have grain diameters of 2.8 and 3.2 μm, respectively. The dynamic compressive mechanical characteristics of the two alloys show a positive strain-strengthening effect, with Mg–6Zn–1Ce–0.6Zr alloy having somewhat higher values than Mg–6Zn–2Ce–0.6Zr alloy. At a strain rate of 500 s−1, {10[Formula: see text]2} tensile twin is the dominant deformation mechanism for two alloys, but the Mg–6Zn–2Ce–0.6Zr alloy is more prone to produce {10[Formula: see text]2} tensile twin. With the increase of the strain rate, the dominant deformation mechanism changes from {10[Formula: see text]2} tensile twin to pyramidal slip.

Hot isostatic pressing induced precipitation strengthening at room and high temperature of Ni-Fe-Cr-Al-V high-entropy alloy manufactured by laser powder bed fusion

The highly adjustable properties of high-entropy alloys (HEAs) offer great potential for developing superior materials for critical structural applications in high-temperature conditions. In this study, Ni-Fe-Cr-Al-V HEA was manufactured by laser powder bed fusion, which has a unique microstructure composed of dislocation substructure and Face-centered cubic + L12 coherent structure and achieves the desired strength-ductility combination. Hot isostatic pressing post-treatment was applied on the laser powder bed fusion-processed HEA to further improve the relative density and optimize the mechanical properties. During the hot isostatic pressing process, the precipitation of L12 and B2 phases can be ascribed to the precipitation modes dominated by continuous precipitation and discontinuous precipitation, respectively. With the tensile deformation temperature increasing from 773-1,173 K, the softening degree of HEA increases continuously, and the dominant deformation mechanism evolves from intragranular dislocation slip to grain boundary sliding. At temperatures below 773 K, precipitation strengthening significantly improves tensile strength and ductility. At 1,173 K, the grain boundary strength decreases and grain boundary area increases, which promotes grain boundary sliding and contributes to plastic deformation, resulting in significant softening.

Microstructural evolution during superplastic deformation of commercial Al5083 alloy

The aluminum alloy SPF process has found extensive application in the aviation and rail transportation industries. The commercial Al5083 alloy presents the advantages of low density, high specific strength, a simplified manufacturing technique, and low cost, thus making it an appealing option for producing components with minor strain and complex configurations. This study focused on examining the mechanical properties of the commercially available Al5083 alloy. Different temperatures (450 °C, 480 °C, 510 °C) and strain rates (0.01 s−1, 0.005 s−1, 0.001 s−1, 0.0005 s−1) were employed to investigate the behavior of the material. Furthermore, a constitutive equation was developed to describe the alloy's response under superplastic conditions. The utilization of EBSD allowed for the investigation of the evolution of grain and dislocation density, while XRD was employed to determine the development of macroscopic texture strength and texture type. The dominant mechanism of superplastic deformation in the commercial Al5083 alloy was identified as dynamic recrystallization and intra-crystalline slip, which distinguishes it from conventional fine-grained aluminum alloys. SEM was used to establish the exponential relationship between void volume fraction and true strain. The results provide valuable insights into the evolution of the macro-mechanical properties and microstructure of commercial Al5083 alloy during superplastic forming.

Enhanced mechanical properties and microstructural stability of ultrafine-grained biodegradable Zn–Li–Mn–Mg–Cu alloys produced by rapid solidification and high-pressure torsion

Zinc alloys have emerged as promising candidates for biodegradable materials due to their remarkable biocompatibility and favorable mechanical characteristics. The incorporation of alloying elements plays an essential role in advancing the tensile strength of Zn alloys. Nevertheless, achieving uniform dispersion of these elements poses challenges due to chemical segregation during solidification. In this study, rapid solidification followed by high-pressure torsion was successfully employed to fabricate Zn–Li–Mn–Mg–Cu alloys characterized by ultrafine-grained microstructures with evenly distributed nanometric intermetallic phases. A comprehensive examination, including phase composition, microstructural evolution, tensile properties and deformation mechanisms, was conducted. The impact of varying annealing temperatures on microstructural stability was systematically examined. The combined implementation of rapid solidification and high-pressure torsion yielded alloys with an average grain size below 360 nm, thereby demonstrating exceptional mechanical properties including yield stress (YS), ultimate tensile strength (UTS), and elongation to failure (Ef) equal to at least 325 ± 6 MPa, 350 ± 8 MPa and 40 ± 11 %, respectively. Heat treatment notably augmented the mechanical properties, resulting in a YS = 440 ± 11 MPa and UTS = 491 ± 6 MPa, while preserving plasticity (Ef = 23 ± 4 %) in the Zn-0.33Li-0.27Mn-0.14Mg-0.1Cu alloy. Nanoindentation strain rate jump tests identified thermally activated mechanisms and grain boundary sliding as dominant deformation mechanisms.

Deformation microstructures of low- and high-strain epidote-blueschist (Ryukyu arc, Japan): Implications for subduction interface rheology

We present field and microstructural data from an exhumed subduction complex in the Ryukyu arc, Japan, where epidote-blueschist, Triassic, Tomuru metamorphic rocks with block-in-matrix structure crop out. With the aim to constrain epidote-blueschist rheology, we investigate fabric development and infer deformation mechanisms of blocks and matrix through microstructural analyses on the main fabric-forming minerals: glaucophane, epidote and albite. The blocks have a poorly developed, discontinuous foliation. In contrast, the matrix has a well-developed, continuous foliation. Glaucophane is the principal foliation-forming phase, arranged in interconnected layers that surround epidote and albite, which suggests that glaucophane controls epidote-blueschist rheology. Deformation mechanisms inferred from petrographic and electron backscatter diffraction (EBSD) analysis are similar in blocks and matrix, with glaucophane deformed by diffusion creep accompanied by reaction-driven creep, albite by diffusion creep, and epidote by brittle fracture and incipient dislocation creep, based on degrees of internal misorientations, lattice preferred orientation (LPO) development and grain size. During deformation, shear stress must have been relatively low, which is implied by the predominance of diffusion and reaction creep as the dominant deformation mechanisms, and by the small viscosity contrast between blocks and matrix, reflected by their similarity in dominant deformation mechanism.

Effect of microstructure and temperature on the bulk and small-scale deformation behavior of 2nd and 3rd generation advanced intermetallic TiAl alloys

Advanced intermetallic TiAl alloys have attracted considerable attention as a high-temperature structural material in aerospace and automobile sectors owing to their low density, elevated temperature strength, superior creep, and oxidation resistance. The present study is focused on assessing the effect of alloy composition, heat treatment and microstructural variation on the bulk and small-scale deformation behavior of commercial grade 2nd generation and newly developed 3rd generation TiAl alloys at 25 °C (298 K), 300 °C (573 K), and 500 °C (773 K). Twinning is noted to be the dominant deformation mechanism irrespective of the temperature. The highest yield strength and hardness are noted for the as-cast 3rd generation TiAl alloy owing to the finer microstructure and compositional variation. Heat treatment however modifies the TiAl microstructure significantly resulting to unique bilamellar-biglobular (BLBG) microstructure, devoid of the brittle β0 phase. Consequently, nano twins- and micro twins-induced enhanced deformability is noted for such microstructure, as compared to the as-cast counterparts. Interestingly, an anomalous increase in strength with an increase in temperature is observed for the studied alloys owing to the presence of high-density twins and Kear-Wilsdorf locks. Most importantly, a temperature dependant hardness anomaly is noted in the TiAl alloys that opens new avenues for further research in nanoscale deformation. This piece of research potentially strengthens the understanding of the role of constituent phases and temperature in the deformation behavior of complex TiAl alloys. Twin-induced deformation mechanism noted for the novel BLBG microstructure, can also be valid for different generations of TiAl alloys to achieve enhanced deformability that paves the way for the development of next-generation material.

Research Progress on Ultra-Low Temperature Steels: A Review on Their Composition, Microstructure, and Mechanical Properties

To address global environmental concerns and reduce carbon dioxide emissions, countries worldwide are prioritizing the development of green, eco-friendly, and low-carbon energy sources. This emphasis has led to the growing importance of promoting clean energy industries like hydrogen energy and natural gas. These gases are typically stored and transported at cryogenic temperatures, making ultra-low temperature alloys indispensable as essential materials for the storage and transportation of liquid gas energy. With the temperature decreasing from room temperature (RT) to liquid nitrogen temperature (LNT), the dominant deformation mechanism in high-manganese steels undergoes a transformation from dislocation slip to deformation twinning, resulting in exceptional cryogenic mechanical properties. Consequently, high-manganese steel has emerged as an excellent material candidate for cryogenic applications. This report focuses on establishing the composition of high-manganese steel suitable for cryogenic applications and provides a comprehensive review of its microstructure and mechanical properties at both RT and LNT. Furthermore, it offers a prospective outlook on the future development of cryogenic high-manganese steels.

A steady-state irradiation creep and thermal creep model for zirconium alloys

In this work, a mechanistic steady-state creep model is developed to characterize the macroscopic strain rate of metallic materials affected by irradiation flux, testing temperature and applied stress. Thereinto, the steady-state strain rate is obtained by considering the density evolution of mobile dislocations, which involve the mechanisms of dislocation multiplication, dynamic annihilation and time-dependent dislocation annihilation. In order to effectively analyze the irradiation creep behavior in the low and high stress region, main attentions are focused on the influence of dislocation mobility on the process of time-dependent dislocation annihilation. At low stress, dislocation mobility is affected by dislocation climb and thermally activated glide. For the former, the absorption of irradiation-induced point defects can promote dislocation climb; whereas, thermally activated dislocation glide might be inhibited by the existing defect clusters. At high stress, the dominant deformation mechanism changes from dislocation climb to displacement cascade unpinning. For the latter, an explicit formula for dislocation mobility is deduced to address the influence of dislocation unpinning from the barriers on irradiation creep. To further verify the established model, thermal creep and irradiation creep data of zirconium alloys are considered to compare with the theoretical results. A good agreement is achieved over a wide range of temperature and stress indicating that the model can well describe the deformation behavior of steady-state creep. In addition, contribution of the dominant dislocation mobility components and dislocation density evolution components is compared under both thermal and irradiation creep, which can facilitate the comprehension of fundamental creep mechanisms of metallic materials.

Hydration State and Rheologic Stratification of the Lithospheric Mantle Beneath the North Anatolian Fault, Turkey

AbstractWe present constraints on the hydration state and rheology of the lithospheric mantle beneath the North Anatolian fault zone (NAFZ). Peridotite xenoliths from the Biyikali and Çorlu volcanic centers record deformational microstructures consistent with shearing in a lithosphere‐scale transcurrent fault system. Analysis by Fourier transform infrared spectroscopy indicates that nominally anhydrous phases retain some OH−, but bulk rock concentrations are generally restricted to <50 ppm H2O by weight. From the rock microstructure, we determined differential stress magnitude and active deformation mechanism(s); combined with estimates of hydration state, we constrained the rheology. Recrystallized grain size piezometry shows that the mantle beneath the NAFZ sustained differential stresses of 10–20 MPa, largely independent of depth. The dominant deformation mechanism(s) change with depth; xenoliths extracted from shallower depths record evidence for grain size‐sensitive creep possibly in the presence of melt. At intermediate depths, both dislocation creep and grain size‐sensitive mechanisms were active, and we did not observe evidence for deformation in the presence of melt. The deepest samples were dominated by dislocation creep. The strong temperature sensitivity of creep mechanisms, combined with the low variability in differential stress, contributes to a stratified viscosity profile ranging from 1018 Pa s for the deepest samples to >1022 Pa s at shallower depths (assuming a melt‐free rheology). Although difficult to quantify from the rock record, melt likely reduced the viscosity of the shallow lithospheric mantle. Vertical stratification in viscosity beneath the NAFZ, the result of melt‐present deformation and/or transitions in the dominant deformation mechanism, has important consequences for the seismic cycle of strike‐slip fault systems.

Hot Deformation Behavior and Mechanisms of SiC Particle Reinforced Al-Zn-Mg-Cu Alloy Matrix Composites

A systematic and comprehensive analysis of the hot deformation and mechanisms of SiC particle-reinforced aluminum matrix composites is significant for optimizing the processing of the composites and obtaining the desired components. Based on this, related research on 11 vol% SiCp particle-reinforced 7050Al matrix composites was carried out. Hot compression experiments were carried out on the Gleeble-3500 thermal simulator to study the hot deformation behavior of composites at the temperature of 370–520 °C and strain rate of 0.001–10 s−1. The hyperbolic sine constitutive equation of the material was established, and the processing map was calculated. Combining the typical metallograph and misorientation angle distribution, the microstructure evolution mechanism of composites was analyzed, and the effect of particles on recrystallization behavior was investigated. Under certain process conditions, the dominant deformation mechanism of composites changed from dynamic recovery (DRV) to dynamic recrystallization (DRX), and the grain boundary sliding mechanism began to play a role. In addition, high temperature tensile and elongation at break were tested, and it was found that the dominant form of fracture failure changed from brittle fracture of the particles to ductile fracture of the matrix as the temperature increased.

Rheology of hydrated plagioclase at lower crustal conditions: Cataclasis, creep and transformational plasticity

Plagioclase feldspar is one of the main rock-forming minerals of the lower continental crust. Assessing its rheology is therefore of particular importance for understanding the mechanical processes acting in convergence zones, where pressure and strain rate increases are often associated with metamorphic reactions and fluid fluxes. In this prospect, we performed axial shortening experiments on “as-is” (∼0.05 wt% H2O) and water-added (∼0.29 wt% H2O) natural plagioclase samples in a Griggs-type apparatus equipped with an acoustic emission monitoring system. Samples were deformed at ∼ 10−6 and ∼ 10−5 s−1, 700–800 °C, and 1.0 GPa in the plagioclase stability field. Experiments have also been performed at 1.5 GPa in order to study the rheological impact of the breakdown reaction Ca-rich plagioclase + water → zoisite + kyanite + quartz + Na-rich plagioclase. Mechanical data indicate that at the conditions studied, the rheology of plagioclase is strongly affected by water content. Microstructural observations confirm that under dry conditions, plagioclase aggregates deform by cataclastic flow. Under wet conditions, dislocation creep appears to be the dominant deformation mechanism, and plagioclase rheology is best described by the following equation, where strain rate, in s−1, is expressed as:ε·=Aσ3.0exp(−(168kJ/mol)/R/T), where σ is the stress in MPa, R is the gas constant, T is the temperature in K, and the pre-exponential term A=10−3.9MPa−3.0s−1. At 1.5 GPa, the reaction is faster under wet conditions but the rheology observed remains identical to that of wet plagioclase in its stability field. In the low water content experiments, plagioclase breakdown induces transient weakening due to fast zoisite nucleation and significant reaction volume change.

Switching from the top-NW backthrusting to top-SE post-orogenic deformation in the Aegean domain: Insights from the calcite microfabric and siliciclastic rocks of the Amorgos Unit (SE Cyclades, Greece).

Detailed microstructural analysis was conducted in the imbricated Amorgos stratigraphy to decipher the complex deformation history of the calcitic mylonitic zones related to different tectonic events. Four major thrust zones were developed during syn-metamorphic tectonic burial by top-NW general shear deformation. Dynamic recrystallization is the dominant deformation mechanism, evolving from the SGR to the BLG field, characterizing a downward increase in strain from the highest Potamos to the lower Paradesia thrusts under low-T conditions. Mesoscopic kinematics within these thrusts reveal top-NW shearing. However, in microscale, both top-NW and top-SE kinematics are observed. In most cases, this top-SE reversal resulted from strain partitioning due to a strong pure shear component. However, the Mouros and Richti shear zones present dominant top-SE kinematics in all scales. Additionally, overprinting relationships within the metaflysch of the Mouros shear zone suggest that the top-SE kinematics of this zone post-dates the former top-NW ones. Therefore, we suggest that a switch from the top-NW general shear deformation during the syn-orogenic compressional phase to a late top-SE extensional deformation occurred in the early Miocene, resulting in the exhumation of the Amorgos Unit below 200 °C, via the Santorini Detachment System.

Micro-scale mechanism for cyclic deformation behavior in Zirconium under low-cycle-fatigue loading

Cyclic deformation behaviors and damage mechanisms of pure Zr were systemically investigated at fully reversed strain-controlled tension-compression fatigue experiments (R = −1), followed by detailed microstructural characterization using scanning electron microscopy with electron backscatter diffraction and transmission electron microscopy. The results demonstrated that three stages (Ⅰ, Ⅱ, Ⅲ) of cyclic response behavior were observed for all applied strain amplitudes, and the trend of the cyclic stress response mainly originated from the substructure evolutions. The initial cyclic softening was attributed to the formation of alternating high- and low-dislocation-density regions, enhancing the free path of dislocation motion. The fatigue life prediction model was established based on the hysteresis loops strain energy. SEM/EBSD slip traces analysis and TEM double beam diffraction analysis showed that prismatic <a> dislocation slip was the dominating deformation mechanism, and pyramidal <c + a> dislocations and 101¯2 twins played an essential role in accommodating plastic deformations. Further, fatigue damage behavior was not only influenced by initial texture, but also other factors played an essential role.

Microstructural evolution, high temperature tensile deformation behavior, and deformation mechanism in an Mg–Zn–Y–Ca–Zr alloy processed by multidirectional forging and hot rolling

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

Effects of hydrothermal alteration on shear localization and weakening in the mantle lithosphere

Hydrothermal alteration occurs in upper-mantle fault zones and significantly modifies the rheological properties of ultramafic rocks. To understand how hydrothermal alteration affects shear localization and the strength of the fault zones, we conducted simple-shear deformation experiments on water-saturated harzburgite (70 wt% olivine +30 wt% orthopyroxene) gouges sandwiched between three types of shear pistons (tungsten, yttria-stabilized zirconia, and corundum) using two deformation-DIA apparatuses at confining pressures of 1–4 GPa, temperatures of 500–580 °C, average shear strain rates of 6.1 × 10−6 s−1 to 1.0 × 10−4 s−1, and water contents of 4–30 wt%. The harzburgitic gouges with water contents of 10 and 30 wt% exhibited steady-state sliding or strain-hardening behavior, and the shear strength of the latter sample was less than one-third of that of the former sample, possibly reflecting the difference in pore fluid pressure. Olivine/orthopyroxene grains exhibit distributed microfracturing and grain rotation towards P foliation, indicating the operation of cataclastic flow. Shear displacement is also accommodated by the development of serpentine- or talc-bearing multiple shear zones including B, R1, and/or Y shears, where intense cataclasis-related grain size reduction and dissolution/precipitation occur. For experimental runs using tungsten and zirconia shear pistons, the serpentine or talc blades that wrap finer-grained and dissolved olivine/orthopyroxene grains form an interconnected network, indicative of frictional sliding or dislocation glide on their basal planes as a dominant deformation mechanism. Raman spectra of the serpentine minerals indicate that they represent antigorite with low crystallinity. These findings suggest that when deep lithospheric fault zones undergo hydrothermal alteration, deformation is controlled by competition between cataclastic flow in olivine/orthopyroxene matrix and slip or glide of antigorite or talc in semi-brittle fault zones, the predominance of which depends on the amount of the hydrous phyllosilicates and the magnitude of pore fluid pressure.

Towards strength-ductility synergy in a weak-textured Mg–3Y extruded sheet via pre-twinning and annealing

Mg-RE (magnesium-rare earth) binary alloys, such as Mg–Y alloys, usually exhibit weakened texture and excellent plasticity compared with traditional RE-free Mg alloys. However, the weakened texture could lead to a significant decrease in the strength of Mg-RE binary alloys. In this work, the weak-textured Mg–3Y (wt. %) extruded sheet displayed good strength-ductility synergy with a 41% increase in the yield strength and a good ductility similar to the initial sheet via simple pre-twinning and annealing treatment (PT). After PT, the average grain size of the sheet decreased from 11.2 μm to 6.0 μm and 34.2% initial soft texture component (<11–21> //normal direction (ND)) was transformed into the hard texture component (<0001> //transverse direction (TD)). The effect of pre-twinning on deformation mechanisms was systematically investigated by slip trace analysis and activation stress (τ) analysis. Basal slip was the dominant deformation mechanism during the tension of the as-extruded (AE) specimen while pre-twinning promoted the activation of non-basal slip, especially prismatic slip in the PT specimen. The texture transformation and reduced critical resolved shear stress (CRSS) ratio between non-basal slip and basal slip induced by pre-twinning were primarily responsible for the high activity of non-basal slip. The underlying strengthening mechanism and the reason for good ductility were also rationally discussed. The strengthening effect of single grain refinement was relatively lower in the weak-textured Mg–3Y alloy. Texture hardening could effectively improve the effect of fine-grain strengthening. The excellent deformation compatibility between basal slip of parent grains and prismatic slip of twins as well as relatively higher activity of the second-order pyramidal slip was the origin of the good ductility of the PT specimen.