Shear Transformation Zones Research Articles

Local structural ordering determines the mechanical damage tolerance of amorphous grain boundary complexions

Amorphous grain boundary complexions act as toughening features within a microstructure because they can absorb dislocations more efficiently than traditional grain boundaries. This toughening effect should be a strong function of the local internal structure of the complexion, which has recently been shown to be determined by grain boundary crystallography. To test this hypothesis, molecular dynamics are used here to simulate dislocation absorption and damage nucleation for complexions with different distributions of structural short-range order. The complexion with a more disordered structure away from the dislocation absorption site is actually found to better resist crack nucleation, as damage tolerance requires delocalized deformation and the operation of shear-transformation zones through the complexion thickness. The more damage tolerant complexion accommodates plastic strain efficiently within the entire complexion, providing the key mechanistic insight that local patterning and asymmetry of structural short-range order controls the toughening effect of amorphous complexions.

In situ investigation of the evolution of shear transformation zones with different elastic tensile strains in metallic glass

Shear transformation zones are usually thought as the fundamental unit characterizing microscopic atomic rearrangement motion during deformation in amorphous materials and the corresponding evolution process determines the macroscopic mechanical behaviors. However, the evolution process of shear transformation zones under tensile loading are difficult to be experimentally detected. Here, one in-situ tensile platform coupled with nanoindenter was introduced to conduct a series of nanoscale continuous loading and creep tests on Cu50Zr50 metallic glass film under different pre-tensile strains within the elastic deformation region. The effects of loading rate and pre-tensile strain on the continuous loading and creep behaviors were systematically investigated. Moreover, the corresponding shear transformation zone volume, the activation volume and the relaxation time spectrum during tensile deformation were determined based on the Cooperative Shearing model and Maxwell-Voigt model. It was found that both of the shear transformation zone volume, activation volume and the peak intensity for the relaxation time spectrum increase with pre-tensile strain. Finally, one scheme is proposed to illustrate the microscopic mechanism for the evolution of shear transformation zones during tensile deformation. The current study offers one effective experimental strategy to in-situ investigate the mechanical behaviors and to reveal microscopic deformation mechanism in amorphous materials.

A scheme for achieving strength-ductility trade-off in metallic glasses

The strength-ductility paradox in structural materials remains a long-standing dilemma in advanced materials synthesis. Metallic glasses (MGs) that are structural materials of remarkably high-strengths are not exclusive in structural applications due to their inherently poor plasticity. This study addresses this conundrum by presenting a scheme for achieving excellent strength-ductility synergy in MGs from structural and energetic standpoints. By applying a two-step pathway of mechanical rejuvenation and cyclic stress loadings upon a Zr2Cu as-constructed MG model, deformation behavior was analyzed by molecular dynamics simulations. As higher energy states of the MG characterized by enhanced compressive plasticity was found by mechanical rejuvenation due to internal stress accumulation accompanied with an unusual increase in free volume and atomic energy, cyclic stress loading upon rejuvenation annihilated loosely-packed regions to advance a more stable state with less energy to enhance structural stability with an improvement in elastic strain limit and strength. By theoretical modelling, strength-ductility trade-off was demonstrated by the delay in shear transformation zone activities that orchestrated the generation and intersection of multiple shear bands. As a result, a two-step scheme in which soft regions are aggregated to a concentered mass intensified to higher potential energy followed by dissipating the consolidated mass into randomly dispersed soft regions is the key to achieve the strength-ductility trade-off in MG materials. The outcome of this study will further advance structure-property characterization in amorphous solids.

Structural mechanisms of enhanced mechanical property in ZrCu metallic glass at low temperatures

The application of traditional crystalline materials as structural materials cannot meet critical design challenges due to low-temperature brittleness. To address this issue, this study investigates the deformation behavior of Zr2Cu metallic glass (MG) model at ambient and low temperature states by molecular dynamics (MD) simulations to unravel the governing mechanisms associated with mechanical performance. Structurally representative amorphous models of the MG were developed and their evolutions under tensile loadings at ambient (300 K) and low temperatures (250 K, 200 K, 150 K, 100 K, 50 K) were captured. Notably, significant variations in maximum stress and corresponding strains characterized the stress-strain behavior as elastic-plastic properties increased with decreasing temperatures. Stress activations of the atomic mass were rapidly incipient at the ambient condition, but largely constrained at low temperatures by which the density variations of activated zones and the emerging strain of first shear transformation zones (STZ's) established a negative correlation with temperature as inferred by the wider strain range of deformation. Disparities in strain thresholds accounted for the higher free volume content at the ambient state, but as temperatures declined, fine structural differences stimulated the formation of densely-packed structures in the amorphous model at low temperatures by the spontaneous annihilation of excessive free volumes. Consequently, the higher growth rates of the average atomic potential energy and the retarded emergence of high-potential-energy atomic mass suggest that at low temperatures, there is a higher resistance to shear band nucleation and higher barrier for atomic migration. As a result, the generation of free volumes, formation of STZs and shear bands were retarded, leading to the enhanced ductility. Thus, at low temperatures, unique tuning mechanisms of structural performance were evolved in the amorphous model that suitably enhanced the constitutive properties of Zr2Cu MG.

Unveiling the effect of interface on torsional behavior of crystalline Al-Al90Sm10 metallic glass nanolaminates

ABSTRACT Influence of configurational design of single crystal Al-Al90Sm10 metallic glass nanolaminates on torsion deformation behaviour of Al/Al90Sm10 nanolaminate (Configuration 1) and Al90Sm10/Al nanolaminate (Configuration 2) from a structural evolution aspect have been analysed by employing Molecular Dynamics for a torsion speed of 1/600 revolution/ps. Adaptive common neighbour (a-CNA) analysis, Dislocation extraction algorithm (DXA), atomic shear strain analysis, and Voronoi Polyhedral (VP) analysis have been carried out to reveal the structural evolution in the nanolaminates specimen subjected to torque. As a consequence of dislocation density localisation under torsional loading in Al/Al90Sm10 nanolaminate high atomic strain gradient is developed in the nanolaminate specimen causing torsional buckling of the Al/Al90Sm10 nanolaminate. The localisation of dislocation density rings induces the formation of dislocation substructure in Al/Al90Sm10 nanolaminate. The crystalline/amorphous interface serves as a free surface and encourages the formation of such dislocation substructure. The collective nucleation, coalescence, and growth of shear transformation zones (STZs) leading to the formation of thick shear bands on either end of Al90Sm10/Al nanolaminate inducing an almost homogenous atomic strain gradient across the surface of the nanolaminate specimen thereby averting torsional buckling. The C/A interface serves as a nucleation site for the generation STZs in Al90Sm10/Al nanolaminate. VPs such as <0, 0, 4, 6>, <0, 3, 6, 4>, <0, 3, 6, 5> <0, 2, 8, 2> have the load bearing capacity and are resistant to fragmentation under the subjugation of torsion loading.

Structural properties and tensile deformation mechanism of amorphous Si3B3N7: Insights from molecular dynamics simulations

We conducted molecular dynamics simulations to study an amorphous model of Si3B3N7. The model consists of SiNx (x = 3, 4, and 5) and BNy (y = 2, 3, and 4) units, with 93.50% SiN4 and 99.13% BN3, forming a network structure. The splitting of the first peak of the gSi−Si(r) is caused by corner- and edge-sharing bonds, while N atoms belonging to NSim linkages or NBn and NSipBq linkages cause the first peak splitting of the gN−N(r). The model includes Si-rich and B-rich regions, and three SiN, BN, and SiBN regions that can be identified through simplex analysis. Overlapping simplexes with RS≥2.0 Å form a chain that spreads throughout the model. B atoms cause more large simplexes with RS≥2.6 Å than Si atoms. We subjected the model to a uniaxial tensile test, which resulted in elastic and plastic deformations. Under tensile loading, the bond lengths of atoms belonging to large simplexes are stretched more than the rest. Si-N bonds are more easily broken than B-N bonds, indicating heterogeneous deformation. Large simplexes gather to form shear transformation zones, which evolve into shear band at strains above 0.143. The cluster of large simplexes grows in the shear band, causing crack propagation at high strains.

Yielding under compression and the polyamorphic transition in silicon

We investigate the behavior of amorphous silicon under hydrostatic compression using molecular simulations. During compression, amorphous silicon undergoes a discontinuous nonequilibrium transition from a low-density to a high-density structure at a pressure of around $13$-$16$~GPa. Ensemble-averaged density and elastic constants change discontinuously across the transition. Densification of individual glassy samples occurs through a series of discrete plastic events, each of which is accompanied by a vanishing shear modulus. This is the signature of a series of elastic instabilities, similar to shear transformation zones observed during shear yielding of glasses. We compare the structure obtained during compression with a near-equilibrium form of amorphous silicon obtained by quenching a melt at constant pressure. This gives structures identical to nonequilibrium compression at low and high pressure, but the transition between them occurs gradually rather than discontinuously. Our observations indicate that the polyamorphic transition is of a nonequilibrium nature, and it has the characteristics of a yield transition that occurs under compression instead of shear.

Unexpected creep behavior in a rejuvenated metallic glass

Rejuvenation, bringing metallic glasses (MGs) to the younger and higher energy states, provides an alternative avenue to explore the interplay between the property and microstructures of MGs. In this study, the creep behavior of the Zr69.5Cu12Ni11Al7.5 MGs was experimentally examined by controlling the energy state in terms of structural rejuvenation and thermal annealing. It is found that compared to the as-cast counterpart, the annealed MG at a lower energy state exhibits a higher hardness, a smaller displacement, and a lower creep rate due to the decreased free volume and the inhibited activation of the shear transformation zone. Conversely, the rejuvenated MG at a high energy state displays lower hardness and increased free volume content, yet it demonstrates superior creep resistance compared to its as-cast counterpart, which deviates from conventional understanding. This unexpected phenomenon occurs as the initial high-content free volume annihilates during creep, and strain hardening takes precedence over strain softening as the prevailing process during creep deformation, leading to a superior creep performance in extremely rejuvenated MGs.

Study on mechanical properties of high-entropy crystal/amorphous composites with pre-existing hole

In recent years, experiments have shown that it is possible to develop crystal/ amorphous composites (CAC) with high strength and ductility by combining high-entropy alloys and metallic glasses (HE-MGs). In this paper, the rapid solidification of CoNiCrFeMn is simulated by molecular dynamics simulations to obtain HE-MGs, and subsequently holes of different sizes were added inside it for uniaxial tension. The result indicates that the larger hole has better plasticity, but the plasticity increases at the cost of the decrease of strength and hardness. Based on the consideration of strength and plasticity, CoNiCrFeMn HE-MGs with a hole radius of 7.06 Å are selected to examine the effect of the crystal layer thickness on the mechanical properties of CAC. The different distance of the crystal/amorphous interface from the hole is due to the crystal layer thickness. And the interface disperses the localized strain to a larger area by hindering the extension of shear transformation zone, resulting in a more uniform distribution of strain and improving the strength of CAC. In addition, the interface is composed of FCC and other amorphous clusters, and there are three connection modes among them. The growth of interfacial clusters will enhance the strength of CAC. The current work helps to understand the deformation mechanisms of high-entropy CAC at the nanoscale, and may advance the development of metallic materials with a synergy of high strength and ductility.

Discrete defect plasticity and implications for dissipation

Inelastic deformation of solids is almost always, if not always, associated with the evolution of discrete defects such as dislocations, vacancies, twins and shear transformation zones. The focus here is on discrete defects that can be modeled appropriately by continuum mechanics, but where the discreteness of the carriers of plastic deformation plays a significant role. The formulations are restricted to small deformation kinematics and the defects considered, dislocations and discrete shear transformation zones (STZs), are described by their linear elastic fields. In discrete defect plasticity both the stress–strain response and the partitioning between defect energy storage and defect dissipation are outcomes of an initial/boundary value problem solution. For such defects an explicit expression for the dissipation rate is presented and, because there is a length scale associated with the discrete defects, the stress–strain response and the evolution of the dissipation rate can be size dependent. Discrete dislocation plasticity modeling results are reviewed that illustrate the implications of defect dissipation evolution for friction, fracture, fatigue crack growth and thermal softening. Examples are also given of the consequences in constrained shear of three modes of the evolution of discrete defects for the size dependence of the stress–strain response and the associated dissipation. For a purely mechanical formulation the Clausius–Duhem inequality specializes to the requirement that the dissipation rate is non-negative. Requiring a non-negative dissipation rate for all points of a body and for all time imposes restrictions on kinetic relations for the evolution of discrete defects. In statistical mechanics, the Clausius–Duhem inequality can be violated for sufficiently small regions for a sufficiently short time. Explicit kinetic relations for discrete dislocation plasticity dissipation and for discrete STZ plasticity dissipation identify conditions that can lead to a negative dissipation rate. In continuum mechanics, at least in some circumstances, satisfaction of the Clausius–Duhem inequality can be regarded as a stability requirement. Nevertheless, simple one-dimensional continuum calculations illustrate that there can be a negative dissipation rate over a small region and for a short time period with overall stability maintained. Implications for discrete defect plasticity modeling are briefly discussed.

Shear Transformation Zone and Its Correlation with Fracture Characteristics for Fe-Based Amorphous Ribbons in Different Structural States

Fe-based amorphous alloys often exhibit severe brittleness induced by annealing treatment, which increases the difficulties in handling and application in the industry. In this work, the shear transformation zone and its correlation with fracture characteristics for FeSiB amorphous alloy ribbons in different structural states were investigated. The results show that the bending strain decreases sharply with the annealing temperature increase, accompanied by decreased shear band density and the induced plastic deformation zone. Furthermore, the microscopic fracture surface features transform from a micron-scale dimple pattern to nano-scale dimples and periodic corrugations. According to nano-indentation results, the strain rate sensitivity and shear transformation zone volume change significantly upon annealing treatment, which is responsible for the deterioration of bending ductility and the transition of microscopic fracture surface features.

Effects of void shape and location on the fracture and plastic deformation of Cu (crystalline) /Cu64Zr36 (amorphous) composites

Voids are probably the most widely studied type of manufacturing defects since they are often formed during material processing. In this paper, the fracture and plastic deformation of Cu/Cu64Zr36 crystalline/amorphous composites with different void locations and shapes are investigated by molecular dynamics simulations. The results show that changes of the void location and shape in the composites significantly change the dislocation motion, shear transformation zones (STZs) activation, shear band propagation, and fracture strength of the composites. The largest shear strain is always found around the void, where it is most likely to activate dislocation motion, STZs and shear band formation. A stepwise increase of the shear localization is mainly related to the nucleation and emission of dislocations, while the very rapid increase is caused by shear band formation. Moreover, the variation of dislocation density and activity of shear bands are mainly responsible for the stress fluctuations in the composites. This paper provides important information for understanding and improving the plastic and fracture behaviors of composites by changing the location and shape of void from an atomistic perspective.

A criterion of the critical threshold of the maximum shear stress in bulk metallic glasses with cryogenic thermal cycling by statistics in nanoindentation

The shear transformation zones of metallic glasses have been widely studied by nanoindentation. In this work, the mathematic relationship between the relevant parameters is analyzed emphatically. Bulk metallic glasses were given different activation volumes by the cryogenic thermal cycling treatment. Nanoindentation experiments were carried out in a load-controlled loading mode. The critical threshold of the maximum shear stress, (τmax)C, was obtained by the noise-reduction procedures and the statistical analysis. And the activation volume, V*, was obtained from the nonlinear slope by introducing a first-order polynomial. Based on the mean-filed theory, a criterion is successfully established, (τmax)C=V*−λ, where λ=0.30±0.10. According to lots of previous studies on (τmax)C and V* of metallic glasses, three applicable conditions of the criterion are extracted.

Effect of crystalline phase on deformation behaviors of amorphous matrix in a metallic glass composite

In metallic glass composites (MGCs), the existence of crystalline phases causes the structures of amorphous matrix to become relatively heterogeneous near the crystalline phase, resulting in the difference of mechanical properties in micro- or nano-scale in amorphous matrix near crystalline/matrix interface. In this study, this difference was characterized by nanoindentation technology, and the pile-up height was quantitatively related to the activation volume of shear transformation zones (STZs). Molecular dynamics simulations were conducted to investigate the activation behaviors of STZs during nanoindentation process of amorphous matrix. The influences of crystalline phase on deformation behaviors of amorphous matrix near the interface in MGCs were analyzed in details.

Molecular dynamics study of nano-indentation deformation behavior of Al/Al90Sm10 nanolaminate.

Molecular dynamics-based investigation has been carried out to simulate the nano-indentation loading in crystalline Al-amorphous Al90Sm10 metallic glass (MG) with an aim to investigate the effect orientation of crystalline-amorphous (C/A) interface orientation on the nano-indentation behavior of the C/A Al-Al90Sm10 nanolaminate for varying indenter speeds. Post-analysis techniques like adaptive-common neighbor analysis (a-CNA), atomic strain, dislocation extraction algorithm (DXA), and Voronoi polyhedral analysis (VP) have been employed to capture the structural evolution during simulated nano-indentation loading. C/A Al-Al90Sm10 nanolaminate with C/A interface orientated perpendicular to the indenter exhibits the presence of elastic regime followed by plastic curve, whereas load versus depth curve behaves plastically since the beginning in case of C/A Al-Al90Sm10 nanolaminate with C/A interface orientated parallel to the indenter. The dislocation density growth is slower in case of C/A Al-Al90Sm10 nanolaminate with C/A interface orientated perpendicular to the indenter attributed to the sinking of dislocations into MG counterpart of the nanolaminate, thereby triggering shear transformation zone activation. Whereas, the dislocation generation is delayed in case of C/A Al-Al90Sm10 nanolaminate with C/A interface orientated parallel to the indenter by virtue of amorphous Al90Sm10 MG coating on crystalline Al but is extensive and rapid. The disintegration of ICO-like structures and mixed clusters and growth of crystal-like clusters is discernible in C/A Al-Al90Sm10 nanolaminate with C/A interface orientated perpendicular to the indenter. On the other hand, the VP population exhibits cyclic variation in C/A Al-Al90Sm10 nanolaminate with C/A interface orientated parallel to the indenter. A transformation pathway of VPs has been mapped out for C/A Al-Al90Sm10 nanolaminate under nano-indentation loading. The simulations have been carried out by employing Molecular Dynamics using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) platform. Post-analysis techniques like adaptive-common neighbor analysis (a-CNA), atomic strain, dislocation extraction algorithm (DXA), and Voronoi polyhedral analysis (VP) have been employed to capture the structural evolution during simulated nano-indentation loading.

Experimental and modelling investigation of vibration‐induced fluidization in sheared granular soils

AbstractVibration‐induced fluidization (ViF) is a phenomenon where a granular medium completely loses shear resistance or flows continuously under vibration and thus behaves like a fluid without invoking remarkable excess pore pressure. This paper attempts to investigate ViF through a series of modified triaxial tests and using an extended shear‐transformation‐zone (STZ) model that correlates macroscopic plastic deformation to the motion of internal mesoscopic weak spots (i.e., STZs) within granular materials. The test results revealed that the ViF may take place when the vibration is applied at either critical or non‐critical states. Theoretical analyses using the extended STZ model show that the vibration intensity required to cause fluidization increases linearly with the initial quasi‐static shear stress level at which vibration is imposed. The model results are generally consistent with experimental data, indicating that the extended STZ model has a desirable performance in simulating the fluidization of granular soil subjected to quasi‐static shearing and vibration simultaneously.

Composition-dependent fracture energy in metallic glasses

The interplay between metallic glasses (MGs) mechanical properties, fracture energy $(G)$, and glass-forming ability (GFA) and their dependence on alloy composition remain poorly understood. Here, we perform molecular dynamics simulations to investigate the intrinsic composition dependence of $G$ in ${\mathrm{Cu}}_{x}{\mathrm{Zr}}_{100\ensuremath{-}x}\mathrm{MGs}$ $(x=20,30,40,50,64)$. The results indicate that the value of $G$ increases with Cu content. In addition, it is revealed that MGs with higher $G$ values display higher Poisson's ratio (\ensuremath{\nu}) and GFA, suggesting a close correlation between fracture toughness, mechanical properties, and GFA. This correlation between G, \ensuremath{\nu}, and GFA can be understood based on the fragility ($m$) of supercooled liquids, which is directly related to the structural heterogeneity in MGs. Larger $m$ values are related to dynamic slowdown and supercooled liquid stabilization, which enhance GFA and the formation of pronounced structural heterogeneity, comprised of loosely packed regions that favor \ensuremath{\beta} relaxation and the activation of shear transformation zones. Those concurrently promote the expansion of the plastic zone at the crack tip, enhancing the observed value of $G$. These simulation results shed light on the intrinsic relationship between fracture toughness, mechanical properties, and alloy composition in MGs.

Synthesis and mechanical properties of highly structure-controlled Zr-based metallic glasses by thermal rejuvenation technique

A novel thermal rejuvenation treatment facility for Zr-based bulk metallic glass (BMG) was developed, consisting of a rapid heating and indirect liquid nitrogen quenching process. The re-introduction of free volume into thermally rejuvenated BMG results in more disordered state. The rejuvenation improves ductility, implying that the re-introduced free volume aids in the recovery of the shear transformation zone (STZ) site and volume. Actually, it is confirmed that relaxation significantly reduces STZ volume; however, it is recovered by thermal rejuvenation. Molecular dynamics simulations also indicate that rejuvenation enhances homogeneous deformation. The current findings indicate that the thermal rejuvenation method is extremely effective for recovering or improving the ductility of metallic glass that has been lost due to relaxation.

Influence of the Hydrogen Doping Method on the Atomic Structure, Mechanical Properties and Relaxation Behaviors of Metallic Glasses

The interaction of metallic glasses (MGs) with hydrogen can trigger many interesting physical, chemical and mechanical phenomena. However, atomic-scale understanding is still lacking. In this work, molecular dynamics (MD) simulations are employed to study the atomic structure, mechanical properties and relaxation behaviors of H-doped Ni50Al50 MGs doped by two methods. The properties of H-doped MGs are determined not only by the hydrogen content but also by the doping method. When H atoms are doped into the molten state of samples, H atoms can fully diffuse and interact with metallic atoms, resulting in loose local atomic structures, homogeneous deformation and enhanced β relaxation. In contrast, when H atoms are doped into as-cast MGs, the H content is crucial in affecting the atomic structure and mechanical properties. A small number of H atoms has little influence on the elastic matrix, while the percolation of shear transformation zones (STZs) is hindered by H atoms, resulting in the delay of shear band (SB) formation and an insignificant change in the strength. However, a large number of H atoms can make the elastic matrix loose, leading to the decrease in strength and the transition of the deformation mode from SB to homogeneous deformation. The H effects on the elastic matrix and flow units are also applied to the dynamic relaxation. The deformability of H-doped Ni50Al50 MGs is enhanced by both H-doping methods; however, our results reveal that the mechanisms are different.

Revealing multiple strengthening transitions in crystalline-amorphous nanolaminates through molecular dynamics

Crystalline-amorphous nanolaminates allow for a unique combination of high strength and good ductility. This promising mechanical behavior is attributed to the different deformation mechanisms that occur in the crystalline and amorphous layers, however, a mechanistic understanding of the plastic strain transmission among these layers does not exist. In the present study a new nanoindentation configuration is employed that allows for the continuous emission of a pair of edge dislocations in the same slip planes as plastic sources, revealing the different interaction mechanisms between dislocations and amorphous layers of varying thickness. This allows to study the transmission of plastic strain from the crystalline to amorphous, and amorphous to crystalline layers. It is shown that the thickness of the amorphous layer controls the deformation mechanism, since thin amorphous layers act as obstacles to dislocation motion, while thick ones as dislocation sinks. As a result, more shear transformation zones and mature shear bands form in the amorphous layer, but less dislocations are nucleated in the lower crystalline layer as its thickness increases. These findings can be used to guide the design of crystalline-amorphous nanolaminates with desirable mechanical properties by precisely controlling the thickness of the amorphous layer.