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The amorphous/amorphous nanolaminates (A/ANLs) have aroused great attentions owing to their tunable structure and enhanced mechanical properties. However, the plastic deformation mechanism of A/ANLs have yet been clarified. Here, we systematically examined the mechanical properties and deformation behavior of series of NiNb/ZrCuNiAl A/ANLs via nanoindentaion test. It was found that both the amount and morphology of amorphous/amorphous interface (A/AIs) played crucial roles in the plastic deformation of A/ANLs. Less and straighter A/AIs facilitated multiple shear banding deformation, of which the hardness increased with decreasing layer thickness, as the A/AIs hindered the propagation of shear bands (SBs). Whilst, more and wavier A/AIs promoted homogeneous deformation, of which the hardness stayed at a much lower value and was relatively irrelevant with the layer thickness, for the promoted activation of shear transformation zones by A/AIs. Our results provide guidance for modifying the mechanical properties of amorphous alloys with interface engineering design.

Pulse electrodeposition is utilized to prepare Co–Fe–P metallic nano-glasses (NGs) containing glassy grains with an average size D

In this work, the effects of cryogenic cycling rejuvenation on the flow behavior of ZrCuAlNiAg metallic glass (MG) in the range of relaxation region from 0.7 up to 0.9 glass transition temperature (Tg) were investigated. For this purpose, nanoindentation and strain-rate jump tests were carried out to show the strain response of glassy alloy. According to the jump test, the increase in applied temperature leads to the rise in induced strain in both of as-cast and rejuvenated samples. It was also uncovered that the rejuvenated sample is sensitive to the higher strain rates leading to the significant overshoot in the flow stresses. In addition, the nanoindentation results indicated that the loading process induced a significant anelastic strain in the rejuvenated sample at 0.9Tg, demonstrating the possible formation and percolation of shear transformation zones (STZs) in the rejuvenated amorphous structure.

Abstract Strain localization during plastic deformation drastically reduces the shear band stability in metallic glasses, ultimately leading to catastrophic failure. Therefore, improving the plasticity of metallic glasses has been a long-standing goal for several decades. In this regard, nanoglass, a novel type of metallic glass, has been proposed to exhibit differences in short and medium range order at the interfacial regions, which could promote the formation of shear transformation zones. In the present work, by introducing heterogeneities at the nanoscale, both crystalline and amorphous, significant improvements in plasticity are realized in micro-compression tests. Both amorphous and crystalline dispersions resulted in smaller strain bursts during plastic deformation. The yield strength is found to increase significantly in Cu–Zr nanoglasses compared to the corresponding conventional metallic glasses. The reasons for the mechanical behavior and the importance of nanoscale dispersions to tailor the properties is discussed in detail. Graphic Abstract

In this work, we study the deformation behavior in amorphous/crystalline metallic composites (A/C-MCs) through nanoindentation experiments and molecular dynamic (MD) simulations. The atomic deformation processes in both crystalline (C-) and amorphous (A-) phases near the amorphous-crystalline interface (ACI) are investigated and correlated with the material’s overall constitutive behavior at the microscale. Our major findings are (i) the ACIs enable a co-deformation of the A- and C-phases through “stiffening” the soft phases but “softening” the stiff phases in A/C-MCs through different micro-mechanisms; (ii) there exists an ACI-induced transition zone with a thickness of ~ 10 nm; (iii) the strong coupling between shear transformation zones (STZs) and dislocations can be quantified through carefully designed indentation experiments and simulations; and (iv) the nanoscale MD-simulation-predicted mechanisms can be mapped to the “pop-in” or “excursion” events on the force–indentation depth curves extracted from microscale experiments, although there is a length-scale gap in between.

The study on the structure and mechanical properties of Zr55Al10Ni5Cu29Y1 bulk metallic glasses (BMGs) indicates that deep cryogenic cycling treatment (DCT) is an effective method to improve room-temperature plasticity of BMGs containing trace rare earth elements. DCT not only increases the volume and size of STZs in BMGs, promotes the generation of multiple shear bands, but also reduces the maximum shear stress that occurs in pop-in events. Due to the internal stress generated during DCT, rejuvenation causes the larger volume and size of the shear deformation zone, which can promote the shear bands formation and generate multiple shear bands to accommodate plastic deformation. In different BMGs systems, the volume of STZs tends to decrease as v increases. STZ is more sensitive to the composition, and changes in trace elements can cause changes of the volume of STZs.

Orientation microscopy (EBSD) has been used to investigate the structure of austenitic stainless steel containing 18% Cr and 9 wt % Ni after its long-term operation at high temperatures and neutron irradiation. The γ → α shear transformation has been implemented partially in the samples during deformation-free cutting. Austenite has decomposed due to isotropic stresses oriented normally to the surface under study. The precision of orientation relationships (ORs), such as Kurdjumov–Sachs, Nishiyama–Wassermann, Greninger–Troiano, and others during γ → α transformation has been comparatively analyzed. The Greninger–Troiano OR has been found to be the best possible OR describing the transformation in this case. The α phase has been shown to nucleate at coherent twin boundaries (Σ3 in the coincidence site lattice model) between austenite grains. This has determined the variants of the current ORs and the unambiguous crystallographic orientation of α-phase grains.

The elementary excitations in metallic glasses (MGs), i.e., β processes that involve hopping between nearby sub-basins, underlie many unusual properties of the amorphous alloys. A high-efficacy prediction of the propensity for those activated processes from solely the atomic positions, however, has remained a daunting challenge. Recently, employing well-designed site environment descriptors and machine learning (ML), notable progress has been made in predicting the propensity for stress-activated β processes (i.e., shear transformations) from the static structure. However, the complex tensorial stress field and direction-dependent activation could induce non-trivial noises in the data, limiting the accuracy of the structure-property mapping learned. Here, we focus on the thermally activated elementary excitations and generate high-quality data in several Cu-Zr MGs, allowing quantitative mapping of the potential energy landscape. After fingerprinting the atomic environment with short- and medium-range interstice distribution, ML can identify the atoms with strong resistance or high compliance to thermal activation, at a high accuracy over ML models for stress-driven activation events. Interestingly, a quantitative “between-task” transferring test reveals that our learnt model can also generalize to predict the propensity of shear transformation. Our dataset is potentially useful for benchmarking future ML models on structure-property relationships in MGs.

Shear transformation zones (STZs) are widely believed to be the fundamental flow defects that dictate the plastic deformation of amorphous alloys. However, it has been a long-term challenge to characterize STZs and their evolutions by experimental methods due to transient nature. Here we first introduced a consistent, automated, robust method to identify STZs by linear based machine learning outlier detection algorithms. We exemplify these algorithms to identify the atoms of STZs in Cu64Zr36 metallic glass system, and verify this data-driven model with a physical based model. It is revealed that the average STZ size slightly increases with decreasing cooling rate.

The mechanical relaxation behavior of metallic glasses (MGs) is composition sensitive. For example, in dynamic mechanical analysis, La70Ni15Al15 shows a pronounced secondary peak, termed β relaxation, of the normalized loss modulus at high frequency and/or low temperature, while La70Cu15Al15 only exhibits a shoulder. We have determined relaxation-time spectra at room temperature for these alloys over eleven orders of magnitude of time from quasi-static anelastic relaxation measurements. These are employed to characterize shear transformation zone (STZ) properties in both the α and β regimes. The pronounced β relaxation peak, observed for La70Ni15Al15, is a result of both larger volume fraction of fast and small potential STZs and smaller volume fraction of slow and large potential STZs as compared to La70Cu15Al15. Room-temperature tensile tests show increasing plasticity with decreasing strain rates, characteristic of thermally activated processes. La70Cu15Al15 exhibits greater plasticity than La70Ni15Al15, a negative correlation with the intensity of the β relaxation that is opposite to that previously suggested. The STZ spectra are used to explain the plasticity difference between the two alloys. Plasticity predictions from loss modulus measurements need to be revised, and require absolute measurements.

Lattice inhomogeneities, i.e. nitrogen interstitial-induced hexagonal-close-packed martensite (HCP-eN) and shear bands, can form in face-centred cubic nitrogen-expanded austenite (FCC-γN) synthesised on Fe-Cr-Mn and Fe-Cr-Ni austenitic stainless steel (ASS) using triode-plasma nitriding (TPN). Homogenous elemental distribution between HCP-eN and FCC-γN supports the displacive shear transformation mechanism proposed for a high-Mn, low stacking fault energy ASS under nitrogen interstitial-induced deformation. While being a product of transformation-induced plasticity effect, HCP-eN exhibits similar lattice expansion behaviour to the parent FCC-γN. However, inhomogeneous elemental distributions in the shear bands formed in γN layers on a 400°C TPN-treated high-Ni ASS indicate local migration of substitutional elements.

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.

Metallic nanoglasses are non-crystalline solids with interfacial regions, typically characterized by a modified short-range order and compositional gradients. These interfaces can act as nucleation sites for the formation of shear transformation zones during mechanical deformation, which gives rise to a deformation behavior distinct from the bulk glass counterpart. While various studies have investigated nanoglasses experimentally (mostly Fe–Sc) and in computer simulations (typically Cu–Zr), there is hitherto no study comparing compositionally identical nanoglasses and conventional metallic glasses by experiments and simulations. In this contribution, we investigate Pd–Si as a model system and compare nanoglasses produced by inert gas condensation with melt-spun ribbons. Molecular dynamics simulations and atom probe tomography provide evidence that glass–glass interfaces are primarily topological and chemical defects in this particular system. Differential scanning calorimetry shows a decrease in the glass transition and crystallization temperature of the nanoglasses compared to melt-spun ribbons. Nanoindentation and micropillar tests on Pd–Si metallic nanoglasses, however, provide evidence for shear band formation in both sample types, the melt-spun ribbons and nanoglass. Shear bands in the nanoglass samples appear more diffuse as compared to melt-spun ribbons. This is also evident from the reduced strain localization in the nanoglass. It is concluded that the topological inhomogenieties induced by forming glass–glass interfaces significantly affect the mechanical properties of nanoglasses.

This study investigates the effect of C on the deformation mechanisms in Fe–C alloys by molecular dynamics simulations. In uniaxial tensile simulations, the face-centered-cubic (fcc) structures of Fe–C alloys undergo the following deformation processes: (i) fcc→body-centered-cubic (bcc) martensitic transformation, (ii) deformation of bcc phase, and (iii) bcc→hcp martensitic transformation, which are significantly influenced by the C concentration. For the low C concentrations (0–0.8 wt%) fcc phase, the fcc→bcc phase transformation accords a two-stage shear transformation mechanism based on the Bain model, the deformation mechanism of the bcc phase is the first migration of twinning structures and then elastic deformation, and the bcc→hcp phase transformation follows Burgers relations resulting from the shear of the bcc close-packed layers. However, for the fcc phase with high C concentrations (1.0–2.0 wt%), the fcc→bcc phase transformation follows a localized Bain transformation mechanism impeded by the C atoms, the bcc phase only experiences elastic deformation, and the bcc→hcp phase transformation also conforms to Burgers relations but become localized due to the addition of more C atoms. Because of the different phase transformation mechanisms between the high C and low C supercells, the dislocation generation mechanism is also different.

An amorphous/crystalline metallic composite (A/C-MC) integrates metallic glass with crystalline metals in one system. The amorphous-crystalline interface (ACI) in A/C-MCs under deformation absorbs dislocations and may fundamentally change the dilemma that the strength comes at the expense of the ductility of a material. However, the development of such materials is still at a trial and error stage due to the lack of a clear-cut understanding on how the amorphous component become instable when a dislocation-mediated plasticity flows into the glassy phases. To meet this need, here we focus on gaining the physical insights into the dislocation-ACI reaction in A/C-MCs through atomistic simulations. We have (i) digitally resembled an interface structure close to that in experiments by annealing melted metallic glasses at cooling rates as low as ∼ 104 K/s; (ii) correlated the dislocation absorption events with the activation of shear transformation zones (STZs) in A/C-MCs under a plastic shear; (iii) identified the mechanisms responsible for a continuous dislocation absorption-induced instability in glassy phases; (iv) calibrated a set of constitutive relations, kinetic rules, and model parameters that can be used in an effective temperature concept-based STZ theories at the continuum level; and (v) characterized the local stress states ahead of the instability band and lay the macroscopic-level glass instability criterion on a firm atomistic basis. Our major findings are: (a) there exists a nanoscale structure transition at the ACI when the cooling rate in the atomistic simulations is reduced to an experimentally-comparable level; (b) the number of atoms participating in the STZs exponentially increases with the number of dislocations arriving at the ACI at an early stage of the dislocation-ACI reaction, but is linearly proportional to the number of absorbed dislocations at a later stage; (c) the dislocation absorption-induced instability in metallic glasses occurs through a three-stage process, i.e., the activation of STZs in the region between icosahedral (ICO) clusters, the coalescence of newly formed STZs, and then the breakdown of ICOs; (d) the model parameters in the continuum-level constitutive relations and kinetic rules are found to be sensitive to cooling rates; and (e) the local stress states ahead of the instability band in glassy phases map surprisingly well with the Mohr-Coulomb criterion regardless of the applied stress at the macroscopic level. The gained knowledge may provide a pathway of connecting the atomistic deformation physics of an A/C-MC with its overall mechanical performance, which is currently difficult to achieve in laboratory experiments.

Unlike obviously topological defects in crystals, “defect” identification in metallic glasses (MGs) is still controversial and under investigation. Based on molecular simulations and analysis of MG samples, a structural parameter related to the Laplacian of local equivalent stiffness is proposed for prediction of localized shear deformation in MGs. It is found that local regions with the parameter of positive and relatively large value represent several particular modes of “stiffness valleys” in initial configuration which will strengthen local shear deformation and lead to the formation of shear transformation zones (STZs). More than 80% of the locations where STZs are formed in six types of MG samples under athermal quasi-static shear deformation match well with the regions predicted by the parameter calculated from initial configuration of the samples. The parameter not only reveals the relationship among local heterogeneity, nonaffine displacement, and shear localization, but also provides an efficient way for predicting the activation of STZs in MGs.

Interference between the grids of the camera sensor and the screen cause moire patterns to always appear on photographs captured from a screen, significantly affecting people’s ability to review images. We propose a novel method to remove such a screen moire pattern from a single image. We characterize the degraded image as a composition of two layers: the latent layer and the moire pattern layer. Because the screen moire pattern is global and content-independent, we regard it as a group of sublayers , and we find that each sublayer after the shear transformation has a low-rank property . Combined with the piecewise constant feature of the latent layer, a convex model is proposed to solve the demoireing problem. Experiments on synthetic and real data demonstrate its feasibility and efficiency.

Using molecular dynamics simulations, we provide an atomistic description of the shear band multiplication mechanisms in a heterogeneous metallic glass consisting of two distinct amorphous regions with different amounts of free volume and degrees of short-range order. The structural differences and elastic fluctuations encountered by the developing shear band while crossing the interface between the two regions alter the autocatalytic activation of the shear transformation zones (STZs) and, subsequently, lead to shear band branching. The two-unit STZ-vortex mechanism sheds light on the correlation between the strain distribution during tensile deformation and the directional characteristics of the STZ activation process.

The incipient plasticity of an amorphous solid represents the onset of shear deformation, which is a stochastic progress closely related to microstructure evolution. It is well known that the cooling rate plays a crucial role in the microstructure evolution of a glass. Nevertheless, how the cooling rate affects the incipient plasticity is still unclear. In this work, the incipient plastic deformation of the amorphous phase in Cu47.5Zr48Al4Nb0.5 bulk metallic glass composites (BMGCs) cast with different cooling rates is systematically studied. The incipient plasticity of the glassy matrix of the BMGCs is found to occur via a first displacement burst, the occurrence of which is more stochastic as the cooling rate decreases. According to the auto-correlation functions and cooperative shear model, the stochastic behavior is attributed to the effects of the cooling rate on the initial free volume and subsequent activated shear transformation zones (STZs). A higher content of free volume is present in the glassy matrix of the studied samples cooled at a faster rate, which facilitates the operation of STZs. Moreover, the larger incipient burst size is correlated with the higher content of free volume and larger STZs in the glassy matrix. The larger STZs result in more preferable propagation of multiple shear bands, leading to less stochastic deformation and more obvious plastic deformation. This study provides further understanding on the incipient plasticity of glassy matrix in BMGCs.

Most images have lots of linear information, which plays an important role in the tasks of image processing and pattern recognition. However, due to the interference of complex background in the images, the multiple directional characteristics of the linear information, and the problems of directional limitations in traditional methods, all these lead to the incomplete and discontinuities existed in the extracted linear information. To solve these problems, this paper puts forward a general model for improving the performance of the linear information extraction methods (LIEM) by utilizing the shear transformation. In this model, the shear transformation can transform an object (a filter or an image) in multiple directions, which directly or indirectly increases the directional characteristics of the traditional LIEM, and improves their performances to extract directional linear information and weak linear information. This paper elaborates on the basic principles of the model and its two implementations for the specific tasks. Furthermore, a variety of experiments are made to verify the versatility and effectiveness of the proposed model.

Aiming at the problem of irregular shape and low contrast of the scratches in the image of mobile phone screen, a method based on machine vision was proposed to detect the surface scratches of mobile phone screen. Firstly, the PatMax algorithm and affine transformation were adopted to preprocess the screen images of mobile phone. Then the shear transformation was used to decompose the image into two parts: low frequency and high frequency, the element shapes in four directions of 0°, 45°, 90° and 135° were constructed to perform gray-scale closing operation on the low frequency part, and the N × M median filter denoising operation was performed on the high frequency part, the enhanced image was generated by the inverse shear transformation. Finally, the improved Otsu double threshold method was used to extract the target. 450 pieces of mobile phone screen images were randomly selected for experiments, and the highest detection rate is 98.7%. The results show that this method can effectively enhance the detail information of the images, which greatly guarantees the integrity of the scratch defects.

In this work, we have synthesized dual-phase high entropy alloys (DP-HEAs) multilayers with a tunable layer thickness through a bottom-up approach. The DP-HEAs' multilayers reach the maximum hardness of 13.3 GPa at a critical thickness of 10 nm. The main strengthening mechanism is that dislocation movement is locked by both interior columnar grain boundaries and heterogeneous interfaces. A crystalline-to-amorphous transition occurs when layer thickness is below the critical thickness. The formation of amorphous DP-HEAs deformed by shear transformation zones with collective atomic rearrangement is responsible for the softening behavior.

In this work, the structural mechanisms of the cooling rate effect on the deformation behaviors in metallic glasses (MGs) is studied, by performing the synchrotron radiation-based experiments coupled with a series of simulations. It is found that a MG prepared at lower cooling rate has the higher yield strength and is more likely to soften itself, resulting in lower plasticity. This is because some atomic-to-cluster level structural factors, such as coordination numbers, atomic packing efficiencies, cluster concentrations and regularities. In addition, a quantitative analysis reveals that higher cooling rate leads to more free volumes, and significantly affect the evolution of free volumes during the compressive deformation, tuning the formation and the evolution of shear transformation zones, as well as the yield strength and the plasticity.

Metallic glasses (MGs) have many attractive advantages such as high yield strength, high hardness, high wear resistance, high fracture toughness, and low friction coefficient. However, the localized deformation and poor ductility due to shear bands prohibit the further applications of MGs. The localized deformation could be controlled by the residual stresses in MGs. In this study, a mesoscale model which combines the kinetic Monte Carlo algorithm (kMC) and the finite element method (FEM) is constructed to investigate the effects of residual stresses on deformation behaviors of MGs. The developed computational framework is implemented by integrating in-house Matlab program for kMC and commercial finite element software, Abaqus. In the mesoscale model, a shear transformation zone (STZ) with nanoscale volume is the basic deformation unit and is represented by an element in the FEM model. Each element whether is transformed into an STZ is determined by the kMC algorithm according to its energy state. The developed mesoscale model can simulate the deformation process of MGs with the time and length scales which are greater than those by atomistic modeling. Finally, a sanity check was performed to verify the accuracy of FEM in solving Eshelby’s inclusion problem. Then the developed computational framework is applied to simulate the uniaxial tension test of the metallic glass specimen with different residual stresses. Simulation results demonstrate that the residual stresses could dominate the occurrences of STZs and change the stress–strain curve of MGs.

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An anomaly in martensitic transformation (the effect of martensitic two-peak splitting in the temperature-dependent thermal expansion coefficient) in complex alloyed 12% chromium steels Fe-12%Cr-Ni-Mo-W-Nb-V-B (ChS-139), Fe-12%Cr-Mo-W-Si-Nb-V (EP-823) and Fe-12%Cr-2%W-V-Ta-B (EK-181) was investigated in this study. This effect is manifested in steels with a higher degree of alloying (ChS-139). During varying temperature regimes in dilatometric analysis, it was found that the splitting of the martensitic peak was associated with the superposition of two martensitic transformations of austenite depleted and enriched with alloying elements. The anomaly was subsequently eliminated by homogenization of the steel composition due to high-temperature aging in the γ-region. It was shown that if steel is heated to 900 °C, which lies in the (α + γ) phase region or slightly higher during cooling, then the decomposition of austenite proceeds in two stages: during the first stage, austenite is diffused into ferrite with carbides, during the second stage, shear transformation of austenite to martensite occurs.

Construction machines are in a state of vibration during operation, which causes the misaligned horizontally of image pairs taken by the binocular camera installed on machines. Therefore, epipolar rectification must be carried out before stereo-matching. We propose a novel approach for epipolar rectification, which can be divided into two steps: the rectification of the left image and the corresponding rectification of the right image. The rectification of the left image consists of the transformation of mapping the epipole to infinity and the shear transformation. The rectification of the right image can be achieved on the constraint of zero vertical disparity. The images can be resampled by the transformation matrix, which realizes epipolar rectification of the image pairs. The experimental results show that the proposed algorithm has good accuracy under the premise of minimal geometric distortion, and the linear least square method is used only once in the algorithm, which shows the fewer computing cost and better stability.

Amorphous/crystalline nanolaminate composites have aroused extensive research interest because of their high strength and good plasticity. In this paper, the nanoindentation behavior of Cu64Zr36/Cu amorphous/crystalline nanolaminates (ACNLs) is investigated by molecular dynamics (MD) simulation while giving special attention to the plastic processes occurring at the interface. The load–displacement curves of ACNLs reveal small fluctuations associated with shear transformation zone (STZ) activation in the amorphous layer, whereas larger fluctuations associated with dislocations emission occur in the crystalline layer. During loading, local STZ activation occurs and the number of STZs increases as the indentation depth in the amorphous layer increases. These STZs are mostly located around the indenter, which correlates to the high stresses concentrated around the indenter. When the indenter penetrates the crystalline layer, dislocations emit from the interface of amorphous/crystalline, and their number increases with increasing indentation depth. During unloading, the overall number of STZs and dislocations decreases, while other new STZs and dislocations become activated. These results are discussed in terms of stress distribution, residual stresses, indentation rate and indenter radius.

Johari–Goldstein (JG) relaxation closely relates to the shear transformation zones (STZs) in the flow of metallic glasses (MGs). However, the actual role played by JG relaxation in the STZ-evoked plastic deformation of MGs remains unknown. In this work, by substituting the transition metal (TM) element in La(TM)Al MGs (TM = Ni, Co, Cu, CuNi) to tune the feature of JG relaxation, how JG relaxation alters the deformation behaviors of MGs is tentatively examined with nanoindentation. The results demonstrate that JG relaxation modulates the anelastic to plastic transition rate in the plastic deformation accommodation of MGs via a characteristic relaxation time.

A novel Cu–Zr nanoglass consisting of glassy nano-cells connected by oxide interfaces is proposed. Compared to conventional nanoglasses, the novel oxide-connected nanoglass presents ultrahigh tensile strength and superior tensile plasticity at ambient temperature. Subjected to tensile loading, the oxide interfaces are found to promote the nucleation of shear transformation zones (STZs) due to the existence of excess free volume. Meanwhile, the strong bonding between metallic and oxygen atoms in the oxide interface makes it difficult for STZs to propagate through. Thus, the STZs are effectively proliferated and confined inside the cell interior without any mature shear band (SB) formed. The results provide new ideas for toughening metallic glasses with a decent combination of plasticity and strength, thus making it possible to overcome the longstanding strength-ductility trade-off dilemma.

We report martensitic shear transformation of strongly segregated block copolymer micelles on face-centered cubic (FCC) lattices to hexagonally close-packed (HCP) structures elucidated by X-ray sca...

The plastic deformation of metallic glasses (MG) is well-known to occur via shear transformation zones (STZs) on the scale of atomic clusters, yet fracture of MG takes place via shear bands of the micron scale. So far, understanding on how the operation of STZs leads to shear localization and fracture remains limited. In this work, tensile tests on Cu/Zr-based MG micro-wires show that both the first-yield and fracture stress exhibit the Weibull distribution, and fractography reveals that shear localization in the form of intense shear bands leads to shear fracture. Molecular dynamics (MD) simulations show that shear bands form via the correlated emergence and operation of discrete STZs close to one another. To describe how the stochastic yet correlated occurrence of the discrete STZs leads to shear localization, a model is constructed to relate the probability of the successive operation of discrete STZs, to their nucleation density. The model predicts that, if nucleation density of the STZs grows along the strain path, as prior shear events triggers the emergence of new STZs, then successive occurrence of discrete shear events speeds up rapidly to an asymptotic state which is exactly the condition of shear localization and shear banding. Furthermore, the MD results suggest an exponential growth law for the occurrence of the STZs along the strain path, which also gives predictions in good agreement with the experimental Weibull distributions of the first-yield stress of Cu/Zr-based MGs.

The effects of loading rate on creep behavior and shear transformation zone (STZ) in magnetron sputtered La-Co-Al and Zr-Cu-Ni-Al amorphous alloy thin films were characterized through instrumented nanoindentation with a spherical indenter. It was revealed that with an increase in loading rate, both thin films became harder with a narrower distribution of mechanical response, and exhibited more pronounced creep displacement with higher creep strain rate. Based on cooperative shear model, both STZ volume and activation energy were calculated by measuring the strain rate sensitivity during creep, showing an increasing tendency with loading rate. However, the strain rate sensitivity was found to decrease with loading rate. Furthermore, structural heterogeneity density, estimated by analyzing the stressed volume at yielding, decreased with loading rate. Therefore, with increasing loading rate, the STZ event can only be activated at detectable structural heterogeneities with relatively larger volume, and the number of available fertile sites was decreased, resulting in postponed shear band formation and suppressed deformation mode transition.

Dual-phase crystalline/amorphous nanostructure model is an effective method to improve the mechanical properties of Mg alloys. The effects of amorphous thickness and crystal orientation on the plastic deformation mechanism of dual-phase crystalline/amorphous Mg/Mg-Al nanolaminates under tensile loading are investigated by using molecular dynamics simulation method. The results indicate that for the model in which the crystal phase orientation is [0 0 0 1] along tensile direction, with the increase of amorphous thickness, the deformation mechanism of samples changes from localized deformation dominated by generalized shear band (GSB) to the homogeneous plastic deformation. Here, the uniform plastic deformation is achieved by combining complete basal-prismatic (BP) transformation in crystal phase and uniformly distributed shear transformation zones in amorphous phase. The results also show that for the large amorphous thickness model, there is an obvious secondary hardening stage in the plastic deformation process, and the large amorphous phase is conducive to the formation of BP interface. However, when the crystal orientation is [ 1 - 0 1 0] along tensile direction, all samples undergo local plastic deformation dominated by GSB, which indicates that the plastic deformation mechanisms of dual-phase nanolaminates depend not only on the thickness of amorphous phase, but also on the orientation of crystalline phase.

Molecular dynamics (MD) simulations are conducted to investigate the influence of various critical structural aspects such as crystallographic direction, spatial distribution and size/volume fraction of nanoscale crystalline secondary phase on the mechanical behavior of dual-phase nanocrystal-metallic glass composites (NCMGCs). The results of MD simulations show that the ‘softer’ amorphous matrix and amorphous-crystal interfaces serve as the nucleation sites of the shear transformation zones, and eventually developing into mature shear bands (SBs). We find that the different crystallographic directions of the nanocrystals have great influence on elastic modulus of composites. Moreover, the spatial distribution of nanocrystals plays an important role in SBs propagation direction. In particular, by increasing the volume fraction of the crystalline phase, plastic co-deformation of the SBs and stacking faults is achieved for the NCMGCs with nanocrystals of [1 1 0] and [1 1 1] directions. Moreover, the presence of stacking faults releases most of the stress of nanocrystals and retards the formation of a mature SB. By systematically varying the characteristics of the B2 CuZr nanoscale crystalline phase one can design composites with improved mechanical properties.

The shear transformation zone (STZ) and ductility of Zr-based bulk metallic glass with varying hydrogen content were analyzed using serrated flow in nanoindentation, with each serration representing the formation and expansion of one or more shear bands. An exponential model was employed to extract the serrations and obtain a certain range of shear stress, and STZ size was estimated via thorough statistical data analysis. The experimental results demonstrated a striking decrease in the serration number and width of the load-displacement curve after hydrogenation, which is similar to the effects of the nanoindentation loading rate. The calculated STZ volume and atomic number were in good agreement with those reported in the literature. The STZ volume, size, activation energy, and activation volume also increased with the increased hydrogen content, leading to shear band formation and proliferation as well as increased shear band density, plastic zone, and plasticity. At the microscopic level, the hydrogen addition accelerated free volume formation and enhanced flow ability between the atoms, as characterized by the improvement of plasticity. At the macroscopic level, hydrogen addition promoted shear band nucleation and multiplication and increased shear band density, plastic zones, and plasticity. The metallic glass plasticity was greatly improved after hydrogenation as assessed via compression tests at room temperature, confirming the rationale of the experimental results.

Fe-based amorphous alloys with excellent soft magnetic properties always suffer catastrophic annealing embrittlement. The soft magnetic and mechanical properties of a series of Fe84.3-xCoxSi2B13Cu0.7 (x = 0, 2, 4, 6, 8) amorphous alloy ribbons were investigated. With appropriate substitution of Co for Fe, the BS of the alloys shows a slight increase and the HC remains at a low level. From two-point bending tests, a sharp transition from ductility to brittleness can be observed upon annealing. The embrittlement can be attributed to localized shear bands due to shear bands branching during deformation. It is found that the ductile-brittle transition temperature increases slightly by applying cryogenic thermal cycling treatment to annealed ribbons, indicating that the annealing embrittlement can be improved to some extent by cryogenic thermal cycling treatment. According to nanoindentation results, the increased shear transformation zone volume is responsible for the bending ductility increment. Ultimately, a ductile Fe80.3Co4Si2B13Cu0.7 amorphous alloy ribbons with a BS of 1.68 T and a HC of 4.8 A/m were obtained by cryogenic thermal cycling treatment after annealing.

The observation of negative strain rate sensitivity during nanoindentation is a common, yet poorly understood phenomenon in metallic glasses. In this study, a combination of experimental investigation and atomistic simulation has been employed to understand the mechanism behind this perplexing effect. The results suggest that the negative strain rate sensitivity arises out of the influence of a slow time-dependent relaxation process during the driven deformation at a controlled rate. By complementing the experimental outcome by simulation results, it is possible to show that this unification of the rate-driven and relaxation mechanisms is the consequence of their overlapping timescales. In addition, we demonstrate that by using the numerical results of simulated nanoindentation, it is possible to calculate the size and activation volume of the shear transformation zone, which is otherwise difficult using only experimental data in the regime of negative strain rate sensitivity.

cited By 1; Rate-dependent plastic deformation behavior of TiZrHfCuNiBe high entropy bulk metallic glass (HE-BMG) is investigated by the nanoindentation and quasi-static compression tests. The experimental results reveal that the pop-in sizes during nanoindentation and the serrations sizes under compression will reduce when the loading/strain increases. The strain rate sensitivity during nanoindentation creep is enhanced as loading rate increases, which represents more homogeneously deformation of TiZrHfCuNiBe HE-BMG at higher loading/strain rate. The average serrations sizes of TiZrHfCuNiBe HE-BMG are significantly smaller than that of traditional bulk metallic glasses. The plastic deformation behavior of the TiZrHfCuNiBe HE-BMG is discussed with respect to the self-organized critical (SOC) state behavior. The results demonstrated that the shear-band interactions of multiple shear bands could affect the SOC behavior and inhomogeneous deformation. Research suggests the shear transformation zone (STZ) sizes become smaller as the loading rate increases based on the statistical analysis. It is ascribed to have a larger shear band density at lower loading/strain rate. The current research is in coincidence with the shear-band patterns shown in microscopic micrographs of specimen surfaces. More shear-banding distributions at the lower loading/strain rate will result in more inhomogeneous deformation. © 2019 Elsevier B.V.

In the plastic deformation of amorphous materials, dilatation occurs universally with softening. However, the critical question that whether softening is caused by dilatation or not remains an ansatz and requires experimental examination. In this work, by examining the deformation behaviors of 3 metallic glasses (MGs) with distinct Johari-Goldstein (JG) relaxation characteristics before and after crystallization with nanoindentation, it is found that the origin of softening is probably not dilatation but the collective mode in the activation dynamics of shear transformation zones (STZs). Since the deformation in the creep stage of nanoindentation of the MGs originates from the “delayed plasticity” from the loading stage and depends on the softened deforming state of the MGs, the creep depth vs. time curves in the creep stage of nanoindentation of the MGs are examined and display similar loading rate dependent softening phenomena for both the glassy and crystallized MGs. The stretched exponent and relaxation time characterizing the deformation dynamics in the creep stage of nanoindentation are derived by fitting the creep depth vs. time curves with the Kohlrausch-Williams-Watts (KWW) equation. The stretched exponents indicate similar collective modes in the activation dynamics of the elementary deformation units (STZs in glassy MGs and dislocations in crystallized MGs). The relaxation times manifest similar softening effects in the deformation accommodation processes of both the glassy and crystallized MGs, except for a glassy La MG of which the relaxation time is extraordinary for its pronounced JG relaxation. The extraordinary relaxation time of the glassy La MG precludes the possibility of shear dilatation being the main cause for the softening of the glassy La MGs indicated by the loading rate dependent creep depth vs. time curves. The stretched exponent and the relaxation time indicate the homological softening dynamics in the plastic deformation of the glassy and crystallized MGs. For the distinct elementary deformation units, i.e., STZs in glassy MGs and dislocations in crystallized MGs, noting the absence of dilatation in dislocation movements, the softening phenomena in the plastic deformation of both the glassy and crystallized MGs is suggested to originate from the collective mode in the activation dynamics of the elementary deformation units, rather than from local properties of elementary deformation units, such as dilatation in STZs.

In the work reported in this paper, high-pressure torsion (HPT) was conducted on Ti-22Al-25Nb alloy under the applied pressure of 6 GPa at ambient temperature in 1, 5, and 10 turns. Investigation of the microstructural characteristics and microhardness evolution of the HPT-processed Ti-22Al-25Nb alloy showed that the lamella B2-phase could be transformed to O-phase at ambient temperature. This was due to severe shear transformation of the B2 lattice caused by the high density of dislocations generated during HPT. Significant grain refinement was achieved, from grain size of ~67 μm in the initial annealed specimen to ~53 nm after 10 turns in the HPT process. In addition to dislocations, nanotwins played an important role in the grain refining process. With increasing accumulation of equivalent strain, the microhardness value of this alloy increased and reached a saturated value of about ~600 HV when the equivalent strain reached ~20. This is higher than that obtained by conventional processes. The hardening mechanisms were primarily attributed to pronounced grain refinement and the high density of dislocations induced by the HPT process.

Nanolaminate composites consisting of alternate layers of Nanoglasses (NGs) and metallic glasses (MGs) have shown enhanced tensile ductility without great penalty on strength. Recent atomistic simulations on such NG-MG nanolaminate composites have revealed that peak stress attained in these materials do not follow rule of mixture. Also, a transition in deformation behaviour takes place when MG layer thickness is reduced below a threshold level which is correlated with average size of glassy grain in NG layer. However, the mechanistic reasons for this correlation is not well understood. Therefore, continuum simulations of tensile loading on NG-MG laminate composites are performed using thermodynamic consistent non-local plasticity model. Results show that interaction stress associated with flow defects such shear transformation zones (STZs) plays a pivotal role in the deformation response of laminate composites. Also, shear band width in these materials, scales with intrinsic material length associated with the interaction stress. Further, the material length with respect to MG layer thickness governs the transition in deformation behaviour. The present work may provide guidelines in developing highly ductile NG-MG laminate composites for practical engineering applications.

To uncover the physical correlation between intrinsic structural heterogeneity and mechanical properties of metallic glasses, we performed high-pressure torsion (HPT) to tailor their microstructure with a rejuvenated state. Nanoindentation results show that the change of mechanical properties results from the convolution of the intrinsic thermal activation process and the structural inhomogeneity in the glassy phase. The spatial heterogeneity induced by severe plastic deformation can markedly operate to reduce the energy barrier for shear transformation zone (STZ) activation and increase the number of STZ events during tensile testing to accommodate the plastic strain, leading to a brittle-to-ductile transition. We highlight that by properly controlling the distribution of structural heterogeneities, one can purposely tune the mechanical performance of metallic glasses.

Metallic glasses with embedded crystalline phases have been experimentally reported to exhibit enhanced mechanical properties. To further explore this observation, we employed molecular dynamics simulations to study Cu50Zr50/B2 CuZr nanolaminates subjected to tensile tests under iso–stress and iso–strain conditions. The onset of plasticity, martensitic transformation, and failure mechanisms were inspected at atomic level. It was found that most of the B2 CuZr phase undergoes martensitic transformation, enhancing the strength of the nanolaminate thanks to the second elastic regime developed in the crystalline layer. Interestingly, this transformation is promoted due to the rearrangement of Cu atoms at the amorphous/crystalline interface, without the direct influence of shear transformation zones. Regarding the failure mechanism, it was observed that it depends on the deformation condition: the iso–stress condition leads to void formation at the interface, whereas the iso–strain condition triggers B2 phase bands amorphization. Finally, tension–compression tests on Cu50Zr50/B2 CuZr nanolaminates under iso–strain revealed that the crystalline layer undergoes reversible B2–monoclinic transformation, decreasing the dissipated energy during mechanical loading when compared to the pure metallic glass sample.

Plasticity improvement in metallic glasses resulting from cycling treatment between room and liquid nitrogen temperature has been reported and attributed to rejuvenation due to a non-uniform thermal expansion coefficient. However, the detailed microscopic effect is still unclear. The present study focuses on the microscopic effect of room-temperature ageing and cryogenic cycling. La70Cu15Al15 and La70Ni15Al15 metallic glasses were subjected to varying ageing times, after which some were also cryogenically cycled. Quasi-static anelastic relaxation measurements were then employed to characterize the time-constant spectra over seven orders of magnitude. The overall anelastic strain decreases with increasing ageing time, but is not noticeably affected by cryogenic cycling. On the other hand, while ageing also causes an increase in characteristic time constants, cycling reverses this effect. The new details shed light on the effect of structural relaxation and rejuvenation on the properties of shear transformation zones.

Shock loading causes complicated twinning processes in Ti and Ti alloys. In this work, two types of {10 1 ¯ 2} sequential twinning processes are for the first time reported according to electron backscatter diffraction (EBSD) characterization of titanium sheets that are subjected to shock loading. One is described as {11 2 ¯ 1} ⇒ P → {10 1 ¯ 2} ( T i I I ⇒ P → T j I ), i.e., {10 1 ¯ 2} sequential twinning is activated in the parent grain (P) along with {11 2 ¯ 1} twins. The other is described as {11 2 ¯ 2} ⇒ {11 2 ¯ 4} → {10 1 ¯ 2} ( C i I ⇒ C j I I → T k I ), i.e., {10 1 ¯ 2} secondary twinning is stimulated inside {11 2 ¯ 4} twin by co-zone {11 2 ¯ 2} twin. A statistical analysis according to EBSD characterization reveals the well-defined crystallographic relations between sequential {10 1 ¯ 2} twin variants and the primary/incoming twins, i.e., T i I I ⇒ P → T i I o r T i + 1 I and C i I ⇒ C i + 3 I I → T i I or T i + 1 I . We proposed and examined two sequential twinning mechanisms, (i) emissary twinning disconnections at steps along twin boundary for the first case and (ii) shear transformation into the primary twin for the second case, based on dislocation theory and the deformation accommodation ability of the sequential {10 1 ¯ 2} twinning to the shear deformation of the primary/incoming twin. Crystal plasticity modelling is performed to calculate the local stress field associated with the proposed twinning mechanisms. The results demonstrate that the preferred twin variant observed in experiments has the maximum resolved shear stress among the six twin variants. Our work suggests that complicated twinning processes under shock loading obey crystallographic relations according to deformation accommodation ability. The findings from this study can be implemented into meso-/macro-scale crystal plasticity models for predicting mechanical behaviors and texture evolution of polycrystalline aggregates of hexagonal materials.

To investigate the intrinsic effect of the pre-stress (σpre) on the onset of yielding, nanoindentation experiments are performed on two metallic glasses (MGs) which are elastically-bent and constrained within a home-made apparatus. Statistical calculations for the first pop-ins are made and noticeable decrease in maximum shear stress (τmax) is observed under tensile pre-stress, but only slight increase under compressive one, without considering the pre-stress effect on the ratio (0.29) of τmax to the maximum contact pressure (p0). This asymmetry phenomenon is related to the anharmonic and asymmetric nature of the interatomic interaction potential caused by the applied pre-stress. Moreover, finite-element analysis is conducted to obtain the elastic stress field of the pre-stressed region upon indentation, which becomes anisotropy and quarter-symmetric. τmax for the specimen is defined as 0.29p0 + 0.50σpre for the tensile-stressed region and 0.29p0 for the compressive-stressed region within elastic strain limit of MGs, which requires the correction for the intrinsic τmax with tensile σpre, but not for compressive σpre. The corrected τmax and activation energy barrier of shear transformation zone are found to be independent on σpre, suggesting that the intrinsic shear stress is a constant and the yielding of MGs obeys the maximum shear stress criterion.

Aluminum nitrides (AlNs) have many unique properties for broad applications. However, their mechanical behaviour has not been well understood. In this work, the notch effects on the deformation of wurtzite AlN (w-AlN) and amorphous AlN (a-AlN) under tension were investigated using molecular dynamics simulations. As compared to w-AlN, lower strength sensitivity to the notch and the improved fracture toughness are observed in a-AlN, which are attributed to the short-range ordered atomic arrangement, rupture, reconstruction and rotation of bonds during deformation. The introduction of notch leads to obviously reduced strength and ductility in the w-AlN and a-AlN, respectively. The deformation in intact a-AlN is dominated by the formation of shear bands, followed by void coalescence. On the other hand, the deformation in notched a-AlN starts from the development of shear transformation zone near to the notches, followed by void coalescence. These results are useful in design of AlN based systems and devices.

The effects of interfaces in crystalline Cu/amorphous Cu50Zr50 on shear banding dynamics are investigated using molecular dynamics simulations. The results show that the introduction of amorphous-crystalline interfaces can effectively promote the homogeneous distribution of shear transformation zones (STZs). We found that the high potential energy combined with the defects of the interfaces promote the nucleation of STZs. Further, the amorphous-crystalline interfaces are transition regions that mediate the transmission of dislocations in the crystallites into STZs in the metallic glasses. The accumulation of STZs and dislocations at the interfaces provide the sources for the enhanced plastic deformation in nanocomposites. These results allow us to determine the fundamental atomic structure of amorphous-crystalline interfaces in nanocomposites.