Cooperative Shear Research Articles

Amplifying Colossal Thermal Expansion of a Martensitic Molecular Crystal through Interlayer Shear‐Induced Side‐Chain Liberation

AbstractMolecular crystals capable of colossal thermal expansion (TE) are fascinating owing to their substantial and continuous volume changes and reasonably linear responses to temperature. This makes them promising candidates for micromachine applications. Macroscopic motion is driven by subtle yet cooperative movements of molecules that respond to the thermal motions of dynamic functional units. The study of p‐TIPS‐DSB presented here offers a compelling case highlighting the relationship between the degree of dynamicity of functional units and TE behavior. In its α‐phase, the p‐TIPS‐DSB crystal undergoes an irreversible martensitic transition to the β‐phase, accompanied by significant cooperative interlayer shear. This process substantially enhances the mobility of the side‐chains driven by the increased free volume surrounding them. This nearly doubles the volumetric TE coefficient from 255.3 (10) to 444.9 (32) MK−1, particularly in the actuation direction from 175.0 (7) to 291.7 (20) MK−1, enabling about 4.5 % elongation/contraction. As demonstrated here, p‐TIPS‐DSB exhibits a decent force density (>1.4×107 N m−3) and precise motion control capabilities due to its hysteresis‐free and non‐abrupt TE nature. Furthermore, we demonstrated the limited operating distance of colossal TE materials can be amplified by utilizing levers, highlighting the high potential of these materials for use in micromachines.

An Efficient CRISPR/Cas Cooperative Shearing Platform for Clinical Diagnostics Applications.

The CRISPR/Cas system is a powerful genome editing tool and possesses widespread applications in molecular diagnostics, therapeutics and genetic engineering. But easy folding of the target sequences causes remarkable deterioration of the recognition and shear efficiency in the case of single Cas-CRISPR RNA (crRNA) duplex. Here, we develop a CRISPR/Cas cooperative shearing (CRISPR-CS) system. Compared with traditional CRISPR/Cas system, two CRISPR/Cas-crRNA duplexes simultaneously recognize different sites in the target sequence, increasing recognition possibility and shearing efficiency. Cooperative shearing cuts more methylene blue-ssDNA reporters on the electrode, enabling unamplified nucleic acid electrochemical assay in less than 5 minutes with a detection limit of 9.5×10-20 M, 2 to 9 orders of magnitude lower than those of other electrochemical assays. The CRISPR-CS platform detects monkeypox, human papilloma virus and amyotrophic lateral sclerosis with an accuracy up to 98.1%, demonstrating the potential application of the efficient cooperative shearing.

An Efficient CRISPR/Cas Cooperative Shearing Platform for Clinical Diagnostics Applications

The CRISPR/Cas system is a powerful genome editing tool and possesses widespread applications in molecular diagnostics, therapeutics and genetic engineering. But easy folding of the target sequences causes remarkable deterioration of the recognition and shear efficiency in the case of single Cas‐CRISPR RNA (crRNA) duplex. Here, we develop a CRISPR/Cas cooperative shearing (CRISPR‐CS) system. Compared with traditional CRISPR/Cas system, two CRISPR/Cas‐crRNA duplexes simultaneously recognize different sites in the target sequence, increasing recognition possibility and shearing efficiency. Cooperative shearing cuts more methylene blue‐ssDNA reporters on the electrode, enabling unamplified nucleic acid electrochemical assay in less than 5 minutes with a detection limit of 9.5×10−20 M, 2 to 9 orders of magnitude lower than those of other electrochemical assays. The CRISPR‐CS platform detects monkeypox, human papilloma virus and amyotrophic lateral sclerosis with an accuracy up to 98.1%, demonstrating the potential application of the efficient cooperative shearing.

Nanoindentation-induced serrated flow behavior and deformation mechanism in metallic glass: A combined experiments and molecular dynamics simulation

In this study, the deformation behaviors of bulk metallic glass (BMG) during the nanoindentation are presented via the experiments and molecular dynamics (MD). The relationship between shear transformation zone (STZ) formation and serrated flow dynamics are developed to explain the deformation characterization evolution of BMG. It is found that as the peak load increases, the hardness and elastic modulus significantly decrease. Creep behavior at room temperature was also revealed by analyzing the creep displacement curve and the stress index under different peak loads.The accumulation of free volume during the deformation process promotes more uniform creep deformation. Furthermore, the serrated flow behaviors were analyzed innovatively using the shear stress drops further statistically. At lower loads, the occurrence of serrated retention is due to energy exceeding the potential barrier required for flow unit activation, ultimately released in the form of kinetic energy. At higher loads, the transition from intermittent curve to smooth curve exhibits a typical self-organized critical (SOC) state dynamic characteristics, and ISE phenomenon is also observed. The results also suggested that the shear deformation of the spherical indenter is more pronounced, with the shear band forming at a 45° angle to the direction of the downward pressure. And based on the Cooperative shear model (CSM) theory, the STZ size was estimated and verify that the increment of STZ makes it easier to generate stronger plastic strain to dissipate STZ, ultimately leading to complete plastic deformation. Combining five fold symmetry and gradient atoms, the majority of atoms with five fold symmetry (LFFS>0.5) have stronger resistance to plastic deformation when the load rate is low.

Amplifying Colossal Thermal Expansion of a Martensitic Molecular Crystal through Interlayer Shear-Induced Side-Chain Liberation.

Molecular crystals capable of colossal thermal expansion (TE) are fascinating owing to their substantial and continuous volume changes and reasonably linear responses to temperature. This makes them promising candidates for micromachine applications. Macroscopic motion is driven by subtle yet cooperative movements of molecules that respond to the thermal motions of dynamic functional units. The study of p-TIPS-DSB presented here offers a compelling case highlighting the relationship between the degree of dynamicity of functional units and TE behavior. In its α-phase, the p-TIPS-DSB crystal undergoes an irreversible martensitic transition to the β-phase, accompanied by significant cooperative interlayer shear. This process substantially enhances the mobility of the side-chains driven by the increased free volume surrounding them. This nearly doubles the volumetric TE coefficient from 255.3 (10) to 444.9 (32) MK-1, particularly in the actuation direction from 175.0 (7) to 291.7 (20) MK-1, enabling about 4.5 % elongation/contraction. As demonstrated here, p-TIPS-DSB exhibits a decent force density (>1.4×107 N m-3) and precise motion control capabilities due to its hysteresis-free and non-abrupt TE nature. Furthermore, we demonstrated the limited operating distance of colossal TE materials can be amplified by utilizing levers, highlighting the high potential of these materials for use in micromachines.

Deformation behavior of thermally rejuvenated Zr-Cu-Al-(Ti) bulk metallic glass

The deformation behavior of metallic glasses has been shown in prior studies to be often dependent on its structural state, namely higher energy “rejuvenated” state versus lower energy “relaxed” state. Here, the deformation behavior of thermally rejuvenated Zr-Cu-Al-(Ti) bulk metallic glasses (BMGs) was evaluated. Rejuvenation was achieved by cryogenic thermal cycling with increase of free volume measured in terms of enthalpy of relaxation. Hardness, stiffness, and yield strength of the BMGs were all found to decrease while plasticity increased after rejuvenation. More free volume in the rejuvenated BMG resulted in homogeneous plastic deformation as was evident from the high strain rate sensitivity and more pronounced shear band multiplication during uniaxial compression. Shear transformation zone (STZ) volume was calculated by cooperative shear model and correlated well with the change in structural state after rejuvenation. The enhanced plasticity with the addition of 1 at. % Ti as well as after cryogenic thermal cycling was explained by lower activation energy for shear flow initiation due to increased heterogeneity induced in the system. Molecular dynamics simulation demonstrated that the variation in plastic deformation behavior is correlated with local atomic structure changes.

Micromechanical response of an electrodeposited NiP metallic glass by pillar compression under extreme conditions

In this work, the mechanical properties of pulse electrodeposited amorphous nickel phosphorous (NiP), with ∼16 wt% P, was investigated for an improved understanding of the deformation behavior of metallic glasses at extreme conditions, particularly at the microscale. Upon decreasing the temperature to sub-ambient, both the yield strength and the shear band density increase. While the change of yield strength with temperature can be well explained by a cooperative shear model, the increase in the number of shear bands can be related to an increase in potential sites for activation of shear transformation zones due to the enhanced structural heterogeneity at lower temperatures. This, however, hinders the percolation of deformation units and leads to a decrease in the shear band propagation speed, from ∼50 nm/s at 295 K to ∼20 nm/s at 195 K. By testing micropillars across seven orders of magnitude in strain rate, a two-stage strain rate sensitivity is observed, from nearly negligible at the quasi-static range (m1 = 0.001 ± 0.002 for 10−3 s−1 to 10−1 s−1) to highly positive at dynamic high strain rate range (m2 = 0.02 ± 0.004 for 1 s−1 to 1000 s−1). This transition implies a change in the yield-controlling process, from the diffusion-assisted relaxation at quasi-static conditions to the rate of energy barrier crossing of shear transformation zones at high strain rates. These findings highlight several distinctive features of microscale metallic glasses, including the reduced shear band velocity at sub-ambient temperature and the two-stage strain rate sensitivity.

A maximum temperature rise model of the shear band in bulk metallic glasses

Based on mechanisms applicable to bulk metallic glasses (BMGs), such as a superposition principle of an instantaneous finite surface heat source temperature field and a flow model about the shear-band velocity, a maximum temperature rise model of shear bands in BMGs was deduced, ΔTmax=σycpΔupl·vSB2πα. The model describes the dependence of the maximum temperature rise on the yield strength and the shear-band velocity in BMGs. It can be calculated that the maximum temperature rise of BMGs is between 1 K ∼ 30 K at room temperature through these two parameters. A critical temperature, TC, of almost 0.8∼0.9Tg is introduced, at T<TC, the change of the yield strength with temperature in BMGs basically conforms a cooperative shear model (CSM), and at T>TC, a significant reduction of the yield strength in BMGs can be attributed to avoiding failure due to localized material softening.

Shear transformation zones and serrated flow dynamics of metallic glasses revealed by nanoindentation

The relationship between shear transformation zones (STZs) and serrated flow dynamics of Co-, Fe-, Zr-, La- and Al-based metallic glasses (MGs) with different plasticity were analyzed by nanoindentation. The serrated flow dynamic behaviors were analyzed using statistical analysis of shear stress drops, Δτ. The STZ size was estimated via the nanoindentation creep method based on the cooperative shear model. Examination of two distinct types of serrated flow dynamics were performed. For MGs (e.g., Al- or Zr-based) where STZs could be easily activated or at low loading rates, shear stress exhibited a decreasing trend and followed a power-law distribution, which was an indicator of a self-organized critical (SOC) state in the dynamics. In contrast, a Gaussian law distribution was found for STZs in hard MGs (e.g., La-, Co-, and Fe-based) or at high loading rate. In the case of Gaussian distribution case, serrated flow dynamics were in a chaotic state, which indicated that STZ aggregation reduced the stresses required for plastic deformation.

Indentation creep dynamics in metallic glasses under different structural states

In this study, the effects of the structural energy states (as-cast and relaxed) of Zr50Cu40Al10 metallic glass were investigated using depth-sensing nanoindentation. In terms of cooperative shear model, we performed experimental characterization of shear transformation zone for plastic flow of metallic glass. The shear transformation zone volume of the relaxed metallic glass is larger than that of as-cast one, while the strain rates sensitivity displays the opposite tendency. The plastic deformation in the creep stage of nanoindentation originates from the “delayed plasticity” inherited from the loading process. The creep depth shows strain rates and microstructural states dependence. The characteristic relaxation time and creep depth limit were determined by fitting the displacement versus time curves with Kelvin model are employed to characterize the creep dynamics of metallic glasses. It was found that the softening effects in the plastic deformation originates from the collective movements of the deformation carriers (shear transformation zones). The different softening behaviors were discussed from the perspective of soft zones in the framework of the free volume model. Our experimental as well as the theoretical analysis deepens the fundamental understanding of plasticity deformation mechanism in metallic glasses.

Effect of nanoindentation experimental parameters on the estimation of the plastic events in metallic glasses employing various analysis methods

The term Shear Transformation Zone (STZ) refers to the atomic cluster that dissipates external stress through cooperative shear. The STZ is the process that describes the local atomic rearrangement mechanism of metallic glasses (MGs). Based on the cooperative shear model (CSM), the size of an STZ has been estimated using the rate jump method (RJM), nano-scale creep method (NCM), first pop-in statistical method (FSM) and serration-flow statistic method (SSM), respectively. In the nanoindentation process, different sizes of STZs can be elicited by employing different experimental conditions (indentation depth and loading rate). In addition, different characterization methods exhibit various degrees of response to changes in the experimental conditions. In order to study the heterogeneity of the STZ of MGs using different nanoindentation methods, in this study, three types of Zr-based MGs with different Poisson's ratios were used as the research objects, and the sizes of the STZs of these amorphous alloys were calculated using the previously mentioned methods. By applying different maximum loads and loading rates, the effect of the nanoindentation test parameters on the size of the metallic glasses' STZs were characterized using different nanoindentation methods. Under the same testing conditions, the volume of the STZs was calculated using each method that exhibited improvement with the increase of the Poisson's ratio. The calculated results obtained using the RJM, NCM and SSM were found to be much greater than those obtained using the FSM. With the increase in the loading rate, the results obtained using NCM decreased, whereas the statistic method results slightly increased. At the maximum load, the STZ volume calculated using the JRM, NCM and SSM increased significantly. The FSM was not affected by the maximum load. Finally, these differences are discussed from the aspects of the calculation principles and the activation and evolution of the STZ.

Strain rate effects on the yielding strength and maximum temperature at shear bands in a Zr-based bulk metallic glass

The effects of strain rate on the yielding strength and maximum temperature at shear bands in a typical Zr41.2Ti13.8Ni10Cu12.5Be22.5 (Vit 1) bulk metallic glass are investigated under tension and compression over a wide range of strain rates at ambient temperature. Using the modified cooperative shear model incorporating the notable internal thermal effect at high strain rates, the transition of the strain rate effect of yielding strength from the sudden decrease to the subsequent slow change with increasing the strain rate is quantitatively characterized. The fracture surface temperature evolution under different shear band evolution times is captured by a hierarchical multi-scale model of heat conduction. Dynamic strain rates shorten the shear band evolution time, leading to an increase in the maximum temperature at shear bands compared to quasi-static loadings.

Manipulating internal flow units toward favorable plasticity in Zr-based bulk-metallic glasses by hydrogenation

• Hydrogen effects on plastic deformation and mechanical properties were studied. • The creep properties were strongly influenced by the hydrogen content. • Serration-flow statistics are used to determine the internal flow unit. This work intends to manipulate the internal flow units in Zr 55 Cu 30 Ni 5 Al 10 bulk-metallic glasses (BMGs) through plasma-assisted hydrogenation to generate a positive microalloying effect on plasticity. Based on the cooperative shear model theory, serration-flow statistics during nanoindentation loading and creep tests during the holding stage were used to analyze the influence of hydrogen on the behavior of flow units in BMGs. Experimental observations showed that the hydrogen in the Zr 55 Cu 30 Ni 5 Al 10 BMGs caused mechanical softening, plasticity improvement, and structural relaxation. Analysis also showed that the average volume, size, and activation energy of internal flow units in the BMGs all increased as a result of the increase in the hydrogen content. The hydrogenation in the BMGs was found to lead to a proliferation of shear bands, which promoted plasticity. The aggregation of these internal flow units reduced the stress required for plastic deformation through shear bands, ultimately causing softening and structural relaxation.

Time-dependent deformation mechanism of metallic glass in different structural states at different temperatures

There is limited understanding of time-dependent plastic deformation behavior of amorphous alloys as a function of their structural state. Here, the creep behavior of Zr52.5Ti5Cu17.9Ni14.6Al10 bulk metallic glass was investigated in its as-cast and relaxed states using nanoindentation technique with applied load in the range of 500–1500 mN and temperature in the range of room temperature to 573 K. The creep displacement increased with increasing load and temperature since the creep process is thermally activated and diffusion rate is enhanced at elevated temperature and higher load. The creep strain rate sensitivity, which is a measure of the creep mechanism, increased with increase in temperature and decrease in applied load. This was attributed to the transition from localized to more homogeneous creep. Reduction in free volume for the relaxed alloy resulted in lower creep displacement and larger strain rate sensitivity compared to its as-cast counterpart. The results suggest that diffusion-based deformation dominate at higher temperature in contrast to shear transformation mediated plasticity at room temperature. The volume of shear transformation zone for the metallic glass was calculated using cooperative shearing model and correlated with the structural state for fundamental insights into the deformation process as a function of temperature and load.

Cavity-mediated cooperative shear transformation in metallic glasses

Molecular dynamic simulation was performed to study the correlation between atomic packing and shear transformation under compressive deformation in apparent elastic regime in CuZr metallic glass. The packing feature of atomic structures was characterized in terms of cavities in metallic glass. It is found that while atoms surrounded by larger cavity volumes, i.e., loosely packed regions, show very small nonaffine displacements, some atoms surrounded by very small cavities, i.e., densely packed regions, undergo very large nonaffine displacement and form cooperative shear transformations with large scale in space. The size of shear transformation zones monotonically increases with decreasing cavity volume. However, shear transformations rarely occur in either densely packed or loosely packed regions with very small probability. In addition, metallic glasses are revealed to possess characteristic cavity volumes around which atoms have more probability to undergo relatively larger nonaffine displacements. It is found that more neighboring atoms together with these central atoms experience cooperative shear transformations. These findings are general in different metallic glasses and provide a general underlying structural basis for different sizes of shear transformation zones observed in previous studies.

Strain rate-dependent tensile response of glassy silicon nanowires studied by accelerated atomistic simulations

Mechanical properties of glassy nanowires have been intensively investigated recently by both nanomechanical experiments and atomic-level simulations. Unfortunately, there exists a huge gap in the strain rate of the nanomechanical tests between experiments and simulations, which makes it difficult to compare results even for the same material system. Using accelerated atomistic simulations based on a self-learning metabasin escape algorithm, here, we report the tensile mechanical properties of amorphous Stillinger–Weber silicon nanowires with different intrinsic ductility under strain rates ranging from 1010 to 10−1 s−1. It is found that both brittle and ductile glassy silicon nanowires display weakened strength with a decreasing strain rate, in agreement with the cooperative shear model. Moreover, as the strain rate decreases, the amount of plasticity remains unchanged for the brittle nanowires, yet it decreases for the ductile ones. Such deteriorated plasticity in ductile glassy nanowires is caused by enhanced strain localization at low strain rates. Lastly, we show that via the distance matrix of nonaffine displacement, a more hierarchical potential energy landscape is responsible for the higher strain localization propensity in ductile silicon glassy nanowires.

Strain-rate effect on yielding behavior of an in-situ Ti-based metallic glass matrix composite upon dynamic compression

An in-situ Ti-based metallic glass matrix composite, Ti43Zr32Ni6Ta5Be14, presents strain hardening and plastic flow upon dynamic compression. The strain hardening is mainly attributed to the competition between the dislocation mechanism within the crystalline dendrites and the thermal and structural softening mechanisms within the glass matrix. The plasticity of the composite is semi-quantitatively analyzed by the difference in shear band toughness between the glass matrix and crystalline dendrites. Furthermore, strain-rate effect on yield strength is characterized by employing the dislocation-based Zerilli and Armstrong (ZA) model, the cooperative shear model (CSM) associated with the adiabatic temperature rise in the shear transformation zone (STZ), together with the modified CSM model related to viscosity, respectively.

Revealing the structural heterogeneity of metallic glass: Mechanical spectroscopy and nanoindentation experiments

Microstructural heterogeneity is an important issue in understanding the mechanical and physical properties of metallic glasses (MGs). Theoretically, physical aging below the glass transition temperature relax the MGs systems to a lower potential energy state on the potential landscape (PEL) while the cold rolling rejuvenates MGs to a higher potential energy state. In the current work, we systemically turned the structural heterogeneity through dynamic mechanical relaxation and rejuvenation. Correlations among the structural heterogeneity, internal friction, and shear transformation zones (STZs) of the model alloy was connected. Based on the cooperative shear model and statistical analysis, the scale variation of the STZs was revealed. This study demonstrated that the spatial structural heterogeneity can be enhanced by severe plastic deformation which can reduce the energy barrier for STZs activation. And we proposed the barriers decrease and more nucleation sites of STZs from physical aging to rejuvenation, which may be potential mechanism for improving the plasticity in MGs.

Cooperative Shear in Bulk Metallic Glass Composites Containing Metastable β-Ti Dendrites.

We report a novel plastic deformation mechanism of bulk metallic glass composites (BMGCs) containing metastable β-Ti dendrites. Plastic deformation of the BMGCs beyond the ultimate tensile strength is mediated by cooperative shear events, which comprise a shear band in the glassy matrix and a continuous ω-Ti band with a thickness of ∼10 nm in the β-Ti dendrite. The cooperative shear leads to serrated shear avalanches. The formation of narrow ω-Ti bands is caused by high local strain rates during the cooperative shear. The cooperative shear mechanism enriches the deformation mechanisms of BMGCs and also deepens the understanding of ω-Ti formation.

Shear transformation zone dependence of creep behaviors of amorphous phase in a CuZr-based bulk metallic glass composite

The creep behaviors of the amorphous phase in a CuZr-based bulk metallic glass composite (BMGC) are studied by nano-indentation. Samples fabricated via higher cooling rates are found to exhibit more prominent creep, but a smaller shear viscosity. The volume of the shear transformation zones (STZs) in the amorphous phase calculated based on a cooperative shear model increases with the cooling rate. The evolution of excess free volume created during creep deformation is clarified. A looser atomic arrangement leads to a larger STZ volume, thus facilitating creep deformation. This study gives a better understanding of the deformation behaviors of the amorphous phase in BMGCs.