Mechanical Properties Of Metallic Glasses Research Articles

Modulating the rejuvenation in a Al75Mg25 metallic glass by multiple recovery annealing treatment

Rejuvenation presents a promising way to improve the plasticity of metallic glasses. How modulating the degree of rejuvenation of metallic glasses remains a challenging issue. Here, based on the molecular dynamics simulation, we investigate the evolution of the structural and mechanical properties of a binary Al75Mg25 metallic glass and successfully realized the rejuvenation modulation through the multiple recovery annealing treatment. A range of samples with different degrees of rejuvenation were obtained by successive multiple recovery annealing process. According to our calculations, we found that the degree of rejuvenation is highly correlated with the Al-centered clusters. Microstructural analyses show that a higher degree of rejuvenation is concomitant with less 1551 index of Honeycutt-Andersen indices and a lower content of <0,0,12,0 > in terms of Voronoi Tessellation analysis. Moreover, our calculated results demonstrate that the samples with the aging initial state show a higher degree of rejuvenation as compared to the samples with the rejuvenated initial state. This can be attributed to that there are more icosahedral clusters, which are more likely to change to other types of clusters with low local fivefold symmetry in the aged samples during the subsequent high-temperature annealing process. Our work confirms that proper multiple (no more than three times) recovery annealing treatment can increase the degree of rejuvenation of metallic glasses and can offer a better understanding of the effect of rejuvenation on the microstructures and mechanical properties of metallic glasses.

Structure effect of ENPs on mechanical properties of amorphous CuCo alloys

Nanoparticles play an important role in the properties of metallic glasses (MGs) due to their diversified structures; however, their structure–property relationship is unclear. In this paper, three ex situ metallic glass matrix composites were assembled by three kinds of nanoparticles and Cu50Co50 MG obtained by rapid cooling, and their structural evolution under uniaxial compression is investigated by molecular dynamic simulation. It is found that the activated atoms always preferentially accumulate in the amorphous region near the embedded nanoparticles (ENPs). ENPs hinder the propagation of shear bands and lead to strain-hardening behavior. The fractal structures convert the HCP and tDh atoms into atoms of other structures to improve the anti-deformation ability, and the parallel-twin structure improves the anti-deformation ability through the mutual conversion of the FCC and HCP atoms. These findings provide a new idea for improving the mechanical properties of MGs. The change in the ENP structure provides theoretical support for the design of composite materials with specific requirements for structural evolution.

Unravelling the relation between free volume gradient and shear band deflection induced extra plasticity in metallic glasses

Previous experiments have revealed that the controllable introduction of structural gradients in metallic glasses (MGs) can endow the materials with extra plasticity due to the gradient-induced deflection of shear bands. However, the relation between the spatial structural gradient and the initiation of shear band deflection remains unclear. The current study has been focused on investigating the relationship between the improved mechanical properties of MGs and structural gradients specified by the distribution of the intrinsic free volume. Molecular dynamics (MD) simulations are firstly performed on homogeneous MG models containing various initial free volume values, showing that the shear band angle increases with decreasing free volume under uniaxial compression, whereas higher shear band angle is observed under uniaxial tension with increasing free volume. Based on the asymmetric behaviors of MGs under compression and tension, a theoretical model is developed to quantitatively characterize the influence of free volume on the mechanical response of MGs, which incorporates a failure criterion based on free volume generation during external loadings. The model can be further utilized to interpret and predict the fracture strain, shear band angle, maximum stress, and fracture surface morphology of gradient structured MGs in both simulations and experiments. The relationship between free volume gradient and shear band deflection induced extra plasticity established in this study provides valuable guidance for the structural design of MGs with enhanced mechanical properties.

Molecular dynamics simulation of spall behavior of amorphous/crystalline composites under shock loading

The industrial application of metallic glass (MG) is greatly limited due to its poor ductility and susceptibility to catastrophic fracture. The mechanical properties of MG can be effectively improved by adding crystal phases to make amorphous/crystalline composites. Meanwhile, spallation is a typical form of dynamic failure of materials under shock loading. However, the spall behavior of amorphous /crystalline composites has been rarely studied. In this work, the shock induced spallation process of Cu50Zr50/copper (Cu) composite materials was investigated through molecular dynamics (MD) simulations. It is found that stress waves transmit and reflect at interface due to the mismatch of acoustic impedance. Besides, MG is “softer” than the single crystal Cu from the perspective of stress wave propagation. By changing the loading direction, it was found that classical spall is more likely to occur in the Cu phase, while micro-spall is more likely to occur in the MG phase. However, as the loading velocity increases, the spall form in the Cu phase will also transform into micro-spall. The spall strength and strain rate through acoustic approximation and MD simulations were also obtained. Finally, it was observed that the tensile stress superposition region caused by interface reflection waves may form new danger points where spallation may initiate, thereby resulting in complex spall phenomena in composite materials. The conclusions obtained reveal the characteristics of the spall phenomenon in Cu50Zr50/Cu composite materials, which is beneficial to further understanding the spall mechanism and providing guidance for designing amorphous/crystalline composite materials under shock loading.

Entropy of metallic glasses and the size effect on glass transition

A statistical description of metallic glasses is proposed based on evaluation of an entropy and using the concept of potential energy landscape (PEL). The PEL is probed using nano-indentation, assuming that serrations observed are related to local energy levels. Serrations sizes follow a Poisson distribution consistent with, that their formation is a rare events. An entropy is derived revealing parameters characterizing the glass structure. The relevance of the approach is tested on the size effect on mechanical properties of metallic glass. It is noticed that the relations explain the effect of film thickness on the glass transition temperature observed for glass polymers.

Surface quality and mechanical property evolution of laser nitrided Zr-based metallic glass induced by laser shock flattening

Laser nitriding is an effective modification technique to improve the comprehensive mechanical properties of metallic glasses (MGs). However, numerous micro-defects, such as pores and humps, usually appear on the laser-nitriding-treated (LNed) MG surface, and therefore additional post-treatment is required. Herein, laser shock processing was implemented to flatten the LNed MG and as well modify its surface mechanical property. The dependence of the laser shock flattening process on the laser energy density was investigated. The results revealed that the surface micro-defects on the LNed MG were significantly reduced after laser shock flattening, and accordingly, the surface roughness (Sa) was decreased, with a maximum reduction of 61.87% (from 0.48 to 0.183 μm) achieved at a relatively high laser energy density of 178.3 J/cm2. Furthermore, nanoindentation measurements indicated that although the surface hardness of the LNed MG was decreased after laser shock flattening, it was still higher than that of the as-cast MG.

Cryogenic thermal cycling effects on serration behavior in wide glass transition temperature Zr-based bulk metallic glasses

Serration behavior serves as a pivotal link connecting micro-deformation mechanism to the mechanical properties of metallic glasses. To elucidate the impact of cryogenic thermal cycling (CTC) and compositional modifications on metallic glasses’ serration behavior, a series of Zr-based metallic glasses, exhibiting a broad range of glass transition temperatures (Tg), were meticulously manufactured. The stress drop magnitude initially decreases before the peak-rejuvenation induced by CTC, then gradually increases, while the number of stress drops exhibits an initial increase and then a decrease. These trends correlate with optimized plasticity of the metallic glasses. Remarkably, metallic glasses with lower Tg predominantly undergo strain hardening during plastic deformation. They exhibit lower viscosity in shear band zones and stress drop compared to others, attributed to the temperature increase around shear band zones approaching Tg. This phenomenon promotes shear band activation in smaller flow units, leading to finer serrations compared to glasses with higher Tg.

The effect of crystalline structure on the mechanical behavior in Zr-based amorphous materials: A molecular dynamics simulation

The structure relaxation in the Zr-based amorphous phase affects the microstructure and mechanical properties of metallic glasses. To clarify the mechanical properties and deformation mechanism of amorphous crystallization, the compression and tensile deformation of the Zr90Cu10 amorphous model with different levels of ordered lattice structure were studied by molecular dynamics simulation. The influence mechanism of crystals on the deformation and mechanical properties of amorphous alloys was investigated using atomic-scale stress‒strain curves, atomic structures, and dislocation line distributions. The results show that the in situ crystallization of amorphous alloys has a positive effect on enhancing the elastic modulus and yield strength of amorphous metals. The high-stress regions in the model are predominantly concentrated within the amorphous phase. The phase dislocation entanglement under stress will improve the plasticity of the amorphous state. This work also discusses the deformation mechanism of nanocrystalline amorphous alloys at the atomic level.

First-principles atomic level stresses: application to a metallic glass under shear

Unlike crystalline alloys, metallic glasses (MGs) do not possess distinctive defects but exhibit a highly heterogeneous response to shear deformation. The difficulties in describing such non-uniform behaviour hamper the prediction of the mechanical properties of MGs. Using the first-principles athermal quasi-static shear simulation on a CuZr glass, we investigate the mechanical responses of various atomic-level parameters, such as the first-principles atomic stresses and electronic properties (an atomic charge, chemical bonds, etc), and their correlations. We find that the atomic von Mises stress is correlated with a parameter, which is commonly employed and also serves as a unique measure of the degree of non-uniform responses. We also show little correlation between the mechanical and electronic properties during the relaxation process, while we perceive a high correlation between the change in chemical and topological bonds. We discuss the physical insights behind these correlations.

High-tenacity in-situ Ti/Zr-based bulk metallic glasses composites fabricated by industrial high-pressure die casting

The glass-forming ability and mechanical properties of metallic glasses and their composites are well known to be sensitive to the preparation conditions and are highly deteriorated by industrial preparing conditions such as low-purity raw materials and low vacuum. Here, we showed that a series of in-situ bulk metallic glass composites (BMGCs) which exhibit excellent ductility and segmental work hardening were successfully developed utilizing a high vacuum high-pressure die casting (HV-HPDC) technology along with industrial-grade raw materials. The tensile properties of these BMGCs are systematically investigated and correlated with the alloy microstructure. As compared with the copper mold suction casting method, the volume fraction difference of the dendrite phase for the BMGCs with the same composition is not significant when fabricated by the HV-HPDC, whereas the size of the β-phase is generally larger. In-situ BMGCs with the composition of Ti48Zr20(V12/17Cu5/17)19Be13 obtained by the HV-HPDC process show ductility up to 11.3% under tension at room temperature and exhibit a certain amount of work hardening. Two conditions need to be met to enable the BMGCs, which are prepared by vacuum die-casting to retain favorable ductility: (1) The volume fraction of β phase stays below 62% ± 2%; (2) The equiaxed crystals with a more uniform size in the range of 5–10 μm. Meanwhile, the results of the present study provided guidance for developing BMGCs with good ductile properties under industrial conditions.

Effects of rejuvenation modes on the microstructures and mechanical properties of metallic glasses

This study systematically investigates the effects of three rejuvenation modes, namely biaxial loading, thermal pressure loading, and cryogenic thermal cycling, on the microstructures and mechanical properties of Cu64Zr36 metallic glasses (MGs) using molecular dynamics (MD) simulations. The simulation results demonstrate that biaxial tension application during loading results in a rejuvenated glassy state with high strength. Specifically, we find that the yield stress, Young's modulus, and shear modulus of the rejuvenated glass obtained by biaxial tensile loading decreased by 2.8%, 2.1%, and 2.7%, respectively, compared to an as-cast MG reference. We also find that MGs achieve the greatest degree of rejuvenation, at the expense of strength, with the use of thermal pressure loading. Interestingly, the application of cryogenic thermal cycling results in a rejuvenated state with a lower strength drop compared to thermal pressure loading. Furthermore, analysis of the potential energy and medium-range-order (MRO) size indicates that the microstructures of MGs achieves a high-energy ordered glass state after thermal pressure loading, but a high-energy disordered glass state after biaxial tensile loading and cryogenic thermal cycling. These findings provide meaningful insights into the effect of rejuvenation modes on the microstructures and mechanical properties of MGs, as well as useful guidelines for designing MGs with high strength and superior plasticity for prospective applications in industry.

Direct observation of quadrupolar strain fields surrounding Eshelby inclusions in metallic glasses.

For decades, scanning/transmission electron microscopy (S/TEM) techniques have been employed to analyze shear bands in metallic glasses and understand their formation in order to improve the mechanical properties of metallic glasses. However, due to a lack of direct information in reciprocal space, conventional S/TEM cannot characterize the local strain and atomic structure of amorphous materials, which are key to describe the deformation of glasses. For this work, we applied 4-dimensional STEM to map and directly correlate the local strain and the atomic structure at the nanometer scale in deformed metallic glasses. We observe residual strain fields with quadrupolar symmetry concentrated at dilated Eshelby inclusions. The strain fields percolate in a vortex-like manner building up the shear band. This provides a new understanding of the formation of shear bands in metallic glass. This article is protected by copyright. All rights reserved.

Research progress on the shear band of metallic glasses

Metallic glasses have received considerable attention in the fields of materials science and condensed matter physics. The disordered structure of metallic glasses gives rise to unique mechanical properties, such as high strength, large elasticity and a distinct deformation mechanism in contrast to conventional crystalline alloys. The plastic deformation of metallic glasses at temperatures well below the glass transition is generally carried out through the formation of highly localized shear bands. Therefore, a thorough understanding of shear banding behavior is vital for improving the mechanical properties of metallic glasses and tailoring the properties of post-deformed metallic glasses. This review article aims to provide an up-to-date overview of the atomic mechanisms underlying shear band nucleation, propagation and interaction. Additionally, the dynamics of shear bands and their correlation with plasticity are also presented. This article also focuses on the progress in the fundamental structure and properties of mature shear bands, such as density fluctuation, medium-range order, shear-band affected zone, and diffusivity. Furthermore, the fracture mechanism based on the development of cavities is elaborated. Finally, we identify a number of key unsolved issues on this subject.

Atomistic Study on Microstructure Control and Deformation Behaviors of Metallic Glasses

Mechanical properties of metallic glasses depend on their microstructure. Rejuvenation recently has attracted much attention because it is a potential tool for controlling the microstructure and properties of metallic glasses. Here I explain molecular dynamics studies on microstructure control and deformation behaviors of metallic glasses. From atomistic simulation approaches, I revealed the thermal rejuvenation process in the heterogeneous microstructure of metallic glasses, the formation of nanocrystals in thermal processing for rejuvenation, and the effects of rejuvenation and nanocrystallization on deformation behaviors.

Effect of Fe addition on the glass-forming ability, stability, and mechanical properties of Zr50Cu34-xFexAl8Ag8 metallic glasses

Zr50Cu34-xFexAl8Ag8 (x = 0, 2, 4, 6, and 8) alloy bars of different diameters were prepared by copper mold suction casting and copper mold casting. The compressive mechanical properties of metallic glass samples with different Fe contents were determined by a universal testing machine. The microstructure and thermodynamics of the alloy samples were analyzed by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and differential scanning calorimetry. According to the experimental results, Fe doping had a significant effect on the glass-forming ability, stability, and mechanical properties of these metallic glasses. The maximum diameter of the Zr50Cu32Fe2Al8Ag8 metallic glass bar was 15 mm, and the cooling rate of the alloy sample was 7.7 K/s. The addition of Fe inhibited the formation of stable crystalline phases inside the alloy sample and improved the glass-forming ability of the metallic glass. With the addition of Fe, the DSC curve of the alloy sample moved to higher temperatures, and the characteristic temperatures and the stability of the metallic glass increased. The fracture strength (σmax) and ultimate strain (εf) of the metallic glass samples reached their maximum values of 1917.6 MPa and 3.97 %, respectively, for Zr50Cu30Fe4Al8Ag8. Compared with the Fe-free Zr50Cu34Al8Ag8 metallic glass, σmax and εf increased by 51.4 MPa and 1.19 %, respectively. With the addition of Fe, the change in the distribution and content of free volume inside the metallic glass sample is the main reason for the improvement of its mechanical properties. This study shows that trace element doping has an important effect on the properties of metallic glasses.

Rejuvenation by enthalpy relaxation in metallic glasses

Structural relaxation can be induced by annealing and usually leads to embrittlement in metallic glasses (MGs). Here, we show that a relaxed MG obtained by annealing an as-cast MG at a temperature (annealing temperature, Ta) below its fictive temperature can be suitably rejuvenated by a thermal treatment of the relaxed MG at a temperature higher than Ta. The effect is driven by enthalpy relaxation, an endothermic reaction upon heating relaxed MG. This rejuvenation lifts the energy state of the relaxed MG towards that of the as-cast MG. Importantly, we found that the plasticity and fracture toughness of the optimally rejuvenated samples exceed those in the as-cast sample, reversing the relaxation-induced embrittlement in the relaxed sample. An analysis of the structural changes shows that the packing density, rather than the ordering of the atomic structure, is responsible for the effects of rejuvenation. The observed rejuvenation may originate from the recovery of loosely packed regions in the MG atomic structure that were densified during annealing. Our findings are significant for tailoring the mechanical properties of MGs.

Influence of icosahedron and icosahedron-like on plastic deformation of Co50Mn50 alloy under compressive conditions

The establishment of the cluster-based structure-property relationship is of great significance to enhance the mechanical properties of metallic glasses (MGs). In this paper, molecular dynamics simulation has been used to obtain the uniaxial compression deformation process at four cooling rates. The results show that the molar volume and atomic average packing density increase with the increase of the cooling rate. The yield strength decreases, and the deformation mode gradually changes from localized deformation to delocalized deformation. The increase of icosahedron and icosahedron-like within the system impedes the plastic flow of MGs. The large-sized nanoclusters composed of icosahedron and icosahedron-like are destroyed during compression, resulting in a concentration of stress localization. The system has high initial resistance to plastic flow. The shear transition zone is concentrated to form shear bands, which reduces the plasticity of MGs. These results have marvelous theoretical guidance for improving the plasticity of MGs.

Gamma relaxation in Dy-based metallic glasses and its correlation with plasticity

The discovery of secondary relaxation in metallic glass (MG) breaks the longstanding cognition that MG, as a simple glassy system, exhibits only α relaxation and contributes significantly to the understanding of the structure and mechanical properties of MG. In this work, two independent fast secondary relaxation peaks corresponding to γ and β′ relaxation were detected by dynamic mechanical measurement in several Dy-based MGs at temperatures far below the glass transition temperature. Compared with β′ relaxation, the novel γ relaxation is activated at lower temperatures and possesses smaller activation energy approximately 2/3 of that for β′ relaxation. By exciting γ relaxation, loosely packed regions evolve into vein-like patterns in the fracture surface, leading to unusual plasticity at cryogenic temperatures. Furthermore, a holistic relaxation spectrum including boson peak, fast γ and β′ relaxation, slow β relaxation and primary α relaxation in MG was depicted.

Modulating mechanical performances of metallic amorphous materials through phase gradient

Increasing strength is usually at the cost of sacrificing ductility in structural materials. The tradeoff becomes even conspicuous in a category of metallic amorphous materials, the so-called metallic glasses (MGs) featured without any atomic-scale translational symmetry. Therefore, there remains little room for simultaneous optimization of strength and ductility in MGs through tailoring the morphology and kinetics of structural imperfections. Here we propose an alternative strategy for modulating the mechanical properties of MGs through introducing proper content of compositional or phase gradient inspired by the mechanistic strain gradient theory. We design two types of CuZr-based phase gradient metallic glasses (PGMGs) with different compositional concentration gradient directions in either continuous or stepped gradient form. Extensive molecular dynamics simulations demonstrate that phase gradient raises the concentration of mechanically stable icosahedral and icosahedron-like Voronoi polyhedra and, thus, increases the strength of MGs. In terms of plastic deformation, free volume mismatch between phases invalidates the autocatalytic activation mechanism of shear transformation zones, resulting in greater resistance to shear band propagation. The phase gradient also encourages branches of shear band and nucleation of multiple shear bands, which mechanism delocalizes deformation and postpones failure. These mechanisms lead to an improvement in the overall ductility of the PGMGs. The present strategy sheds light on evading the long-standing strength-ductility tradeoff in amorphous metals through extrinsic chemical and geometrical modulation that can be handled by appropriate thermal processing and fabrication technique.

Large tensile plasticity induced by pronounced β-relaxation in Fe-based metallic glass via cryogenic thermal cycling

The effects of cryogenic thermal cycling (CTC) treatment on tensile plasticity and structure change of [(Fe0.5Co0.5)0.75B0.2Si0.05]96Nb4 metallic glass (MG) ribbons were systematically investigated. Pronounced β-relaxation peaks locating at 643 and 693 K were observed after CTC treatment with diverse thermal temperatures. Specifically, the CTC treated samples exhibiting most remarkable β-relaxation showed tensile plastic strain of 5.2% at 643 K and 4% at 693 K, while the as-quenched sample only showed tensile plastic strain of 3% at 643 K and 2.1% at 693 K. The pronounced β-relaxation and enhanced tensile plasticity are considered to be originated from the increased structural heterogeneity. The separate β-relaxation peaks result from the different sizes/types of shear transformation zones induced by oscillatory (rejuvenation-relaxation) behavior during thermal cycling. Our results indicate that the CTC treatment has a significant impact on the energy states and structural heterogeneity, which makes it a powerful means to modify the mechanical properties of MGs.