Evolution Of Metallic Glasses Research Articles

Physics-motivated fractional viscoelasticity model for dynamic relaxation in amorphous solids

Dynamic mechanical relaxation is an important metric to understand the mechanical/physical properties of amorphous solids which are of viscoelastic nature. Due to the heterogenous microstructure, the relaxation behavior of amorphous solids usually shows strong deviation from the Debye relaxation. The distribution of relaxation time derived from either the stretched exponential function (KWW function) or the power law form is probably the most adopted paradigm to describe the non-Debye relaxation. They are essentially the continuous spectrums given in analytical forms. However, whether a real amorphous material conforms to such distribution law remains to be discussed. Here we test the assumption in typical metallic glasses (MGs) as representatives of the general amorphous solids. The mechanical spectrum of a Cu46Zr47Al7 MG in wide frequency domain is probed by the dynamic mechanical analysis technique. It is found that both the KWW function and the modified fractional (MF) model based on power law can well describe the experimental data. As a step forward, we combine the quasi-point defect theory with the MF model to theoretically reveal the feature of temperature-dependent structural evolution in MG. Finally, the distribution of relaxation time corresponding to the experimental data is discretized to argue the theoretically predicted microstructural heterogeneity in the MGs.

Achieving structural rejuvenation in metallic glass by modulating β relaxation intensity via easy-to-operate mechanical cycling

Structural rejuvenation is an effective measure to optimize the mechanical properties of metallic glasses (MGs). Sophisticated solutions to rejuvenation include thermal cycling, laser shocking, and multiaxial stress loading. Here, we propose an easy-to-operate mechanical cycling as an alternative strategy to tailor the mechanical relaxation, deformation, and structural heterogeneity of MGs. Structural rejuvenation in a La-based MG is achieved via mechanical cycling even at very few cycles (102 tension load cycles) and low frequencies (10−3 Hz). The results manifest intuitively the competition between structural relaxation and rejuvenation, which constitutes the structural evolution in MGs. A theoretical model is constructed which reveals a scenario that mechanical cycling wakes up frozen flow defect, accelerating creep and, thus, enhancing the β relaxation in MGs. Therefore, this handy anti-ageing methodology provides an alternative pathway to optimize the mechanical properties of MGs. It also contributes to a more comprehensive understanding of the structure-property relationship in amorphous materials, especially with regard to the correlation between structural rejuvenation and relaxation behavior in such topologically disordered materials.

Temperature-dependent plasticity and fracture mechanism under shear loading in metallic glass

The fracture surface morphology under shear loading is important for understanding the plasticity and the deformation mechanism of metallic glasses (MGs). However, it is difficult to carry out the shear fracture test for most MGs by conventional test methods due to their limited glass forming ability. In this work, through a unique anti-symmetrical four-point bend method, the shear fracture test is carried out under a quasi-static loading at a wide temperature range from 220 K (0.31Tg, Tg: glass transition temperature) to 620 K (0.88Tg) for a Zr-based MG. Basing on the examination on the fracture surface morphologies, we found three different fracture mechanisms with temperature: at lower temperatures (220 ~ 260 K, 0.31 ~ 0.37 Tg), both flower-like and shear-driven vein patterns appear on the fracture surface, which displays a mix of shear- and dilatation- dominated fracture; at intermediate temperatures (300 ~ 570 K, 0.42 ~ 0.80 Tg), the shear-driven vein patterns dominate on the fracture surface, indicating the shear-dominated fracture; at higher temperatures (600 ~ 620 K, >0.80 Tg), a mass of melt droplets indicate the transition from inhomogeneous to homogeneous deformation. From the Weibull statistical analysis on the main fracture features at intermediate temperatures, we found that the size of shear-driven vein patterns increases and the data distribution is sparser with the temperature increasing. The temperature dependence of fracture surface morphology as well as the relationship between the fracture surface feature and the microstructural evolution in MG were also interpreted from the shear transformation zone theory.

Atomic structure evolution in metallic glasses under cyclic deformation

Because of the limited experimental capability as well as computing power, the understanding of atomic structure evolution in metallic glasses (MGs) under cyclic loading is still lacking, which in turn makes the fatigue mechanism of MGs remaining largely elusive. Here we perform molecular dynamics simulations of tension-compression fatigue tests in Cu50Zr50 MGs under strain-controlled cyclic loading. When the applied strain lies in the elastic regime, no noticeable changes in the atomic structure of MGs in terms of pair distribution function and the fraction of various Voronoi polyhedra have been found during fatigue tests. When the applied strain lies in the plastic regime, however, shear band (SB) thickening is the intrinsic fatigue mechanism for MGs. It is shown that after its formation, the SB does not propagate along the shearing plane or develop into fatigue cracks with further cyclic loading. Instead the SB expands spatially and eventually evolves into the soft second phase with increased free volume within. The present study provides an in-depth atomic understanding of the structural evolution in MGs under low-cycle fatigue deformation.

Stress-induced elastic modulus evolution in metallic glasses

In this study, flexible Cu50Zr50 metallic glass fibers (MGFs) with a diameter of 50 μm were prepared by a melt extraction method. The microfibers were axially and radially uniform with a smooth surface. It was found that the elastic modulus of the micron-sized Cu50Zr50 MGFs was low and could increase towards the value of the bulk counterparts with multiple-loading tension. High resolution transmission electron microscopy (HRTEM) experimental observation verified that tensile stress could induce the nanostructural rearrangements in metallic glasses from disorder to short- and medium-range order. Meanwhile, this unique fact was explained by the potential energy landscape with multiple metastable states.

Radiation-induced structural evolution in Zr2Cu metallic glass

Seeking nuclear materials that possess a high resistance against particle radiation damage is a long-standing issue. In this work, to reveal the radiation effect on the metallic glasses (MGs), the atomistic structural evolution of MGs induced by particle radiation is investigated, by performing a molecular dynamics simulation. It is found that radiation-induced vacancy-like defects appear in the MG structural model during the collision cascades. However, these defects are transient and unstable that they are fully annihilated. In particular, free volumes which are specific structural characteristics in MGs can annihilate these transient defects. In addition, there is a rearrangement of free volumes that large free volumes change into small ones and are apt to distribute homogeneously in the amorphous model after structural relaxation, so that the problems of radiation-induced structural instability and energy imbalance are solved. This work will shed light on evaluating the structural stability of MGs under particle radiation.

Probabilistic Modelling of Microstructural Evolution in Zr Based Bulk Metallic Glass Matrix Composites during Solidification in Additive Manufacturing

Bulk metallic glass and their composites (BMGMCs) are a new class of materials which possess superior mechanical properties as compared to existing conventional materials. Owing to this, they are potential candidates for tomorrow’s structural applications. However, they suffer from poor ductility and little or no toughness which render them brittle and they manifest catastrophic failure under applied force. Their behavior is dubious, unpredictable and requires extensive experimentation to arrive at conclusive results. In present study, an effort has been made to design bulk metallic glass matrix composites by the use of modeling and simulation. A probabilistic cellular automaton (CA) model is developed and described in present study by author which is used in conjunction with earlier developed deterministic model to predict microstructural evolution in Zr based BMGMCs in additive manufacturing liquid melt pool. It is elaborately described with an aim to arrive at quantitative relations which describe process and steps of operations. Results indicate that effect of incorporating all mass transfer and diffusion coefficients under transient conditions and precise determination of probability number play a vital role in refining the model and bringing it closer to a level that it could be compared to actual values. It is shown that proposed tailoring can account for microstructural evolution in metallic glasses.

Electrical Resistance Relaxation in La<sub>55</sub>Al<sub>25</sub>Ni<sub>10</sub>Cu<sub>10</sub> Bulk Metallic Glass

Electrical resistance is always related to the electronic structure of metallic glass and sensitive to structural changes, which provides a more intuitive approach to investigate structure evolution of metallic glasses upon structural relaxation. Electrical resistance relaxation of the La55Al25Ni10Cu10 bulk metallic glass was studied using the standard four-probe method. The electrical resistance of La55Al25Ni10Cu10 bulk metallic glass decreases significantly with the structural relaxation below the glass transition temperature at 445 K. During the subsequent continuous heating, the relaxed specimen shows a reduction in the resistivity decrease at the glass transition. The relaxed electrical resistance caused by the structural relaxation during the isothermal measurement equals the changes of the electrical resistance reduction in the glass transition region in the subsequent isochronal measurements. The calculated relaxed electrical resistance as a function of the annealing time can be fitted by Kohlrausche-Williams-Watts (KWW) equation. The equilibrium value, time constant and the stretched exponent for the isochronal electrical resistance measurements at 445 K are 0.0384, 2390s and 0.66, respectively. The in-situ electrical resistance data recorded in the isothermal annealing process show the same relaxation behavior with fitting parameters 0.0362, 2033 s, and 0.74, respectively.

Elastic Anomaly and Polyamorphic Transition in (La, Ce)-based Bulk Metallic Glass under Pressure

Pressure-induced polyamorphism in Ce-based metallic glass has attracted significant interest in condensed matter physics. In this paper, we discover that in association with the polyamorphism of La32Ce32Al16Ni5Cu15 bulk metallic glass, the acoustic velocities, measured up to 12.3 GPa using ultrasonic interferometry, exhibit velocity minima at 1.8 GPa for P wave and 3.2 GPa for S wave. The low and high density amorphous states are distinguished by their distinct pressure derivatives of the bulk and shear moduli. The elasticity, permanent densification, and polyamorphic transition are interpreted by the topological rearrangement of solute-centered clusters in medium-range order (MRO) mediated by the 4f electron delocalization of Ce under pressure. The precisely measured acoustic wave travel times which were used to derive the velocities and densities provided unprecedented data to document the evolution of the bulk and shear elastic moduli associated with a polyamorphic transition in La32Ce32Al16Ni5Cu15 bulk metallic glass and can shed new light on the mechanisms of polyamorphism and structural evolution in metallic glasses under pressure.

Computational modeling sheds light on structural evolution in metallic glasses and supercooled liquids

This article presents an overview of three challenging issues that are currently being debated in the community researching on the evolution of amorphous structures in metallic glasses and their parent supercooled liquids. Our emphasis is on the valuable insights acquired in recent computational analyses that have supplemented experimental investigations. The first idea is to use the local structural order developed, and in particular its evolution during undercooling, as a signature indicator to rationalize the experimentally observed temperature-dependence of viscosity, hence suggesting a possible structural origin of liquid fragility. The second issue concerns with the claim that the average nearest-neighbor distance in metallic melts contracts rather than expands upon heating, concurrent with a reduced coordination number. This postulate is, however, based on the shift of the first peak maximum in the pair distribution function and an average bond length determined from nearest neighbors designated using a distance cutoff. These can instead be a result of increasing skewness of the broad first peak, upon thermally exacerbated asymmetric distribution of neighboring atoms activated to shorter and longer distances under the anharmonic interatomic interaction potential. The third topic deals with crystal-like peak positions in the pair distribution function of metallic glasses. These peak locations can be explained using various connection schemes of coordination polyhedra, and found to be present already in high-temperature liquids without hidden crystal order. We also present an outlook to invite more in-depth computational research to fully settle these issues in future, and to establish more robust structure-property relations in amorphous alloys.

Thermally induced failure mechanism transition and its correlation with short-range order evolution in metallic glasses

The effect of temperature on the short-range order (SRO) structures, deformation mechanisms and failure modes of metallic glasses (MGs) is of fundamental importance for their practical applications. However, due to lack of direct structural information at the atomistic level from experiments and the absence of previous molecular dynamics (MD) simulations to reproduce experimental observations over a wide range of temperature, this issue has not been well understood. Here, by carefully constructing the atomistic models of Cu64Zr36 and Fe80W20 MGs, we are able to reproduce the major deformation modes observed experimentally, i.e. single shear banding (SB) at low temperatures, multiple shear-bandings at intermediate temperatures and homogeneous plastic flow at elevated temperatures. By examining the evolution of SRO, we find that the different failure modes exhibit distinctively different full-icosahedron (FI) evolution pathways at different temperatures. Specifically, at low temperatures, the FI concentration first deceases to a minimum, then recovers slightly, and finally comes to a plateau; at intermediate temperatures, it first decreases and then reaches a plateau; while at elevated temperatures, it decreases simply monotonically. These different pathways arise from the dynamic competition between the destruction and recovering of FI clusters. We further show that the local softening caused by the destructions of FI clusters is crucial for the formation of localized shear planes and further shear bands. Since our simulations exhibit the same trend for both MG systems, it is expected that these findings may be generic for a wide range of MGs.

A molecular dynamics analysis of internal friction effects on the plasticity of Zr65Cu35 metallic glass

Abstract The effects of internal friction (IF) on Zr 65 Cu 35 metallic glass plasticity are investigated through molecular dynamics simulations. Results show that the Voronoi polyhedron 〈0, 3, 6, 3〉 increases as IF increases, thereby effectively inhibiting localized deformation and improving metallic glass plasticity. The simulations allow reproduction of images of IF evolution in metallic glasses subjected to isothermal annealing at 730 K and 850 K respectively, which can help explain the experimental observations. IF could be adjusted by selecting suitable annealing temperatures and cooling rates. The results of this work provide a strong foundation for future metallic glass designs.

Stress-versus temperature-induced structural evolution in metallic glasses

Structure evolution induced by shear deformation was investigated via molecular dynamic simulation on CuZr metallic glass system and compared with that induced by temperature. Voronoi tessellation analysis found that the local structures evolve to a liquid-like state as shear stress increases, similar to the temperature-induced structure evolution. However, shear stress induces little change to the radial distribution functions (RDFs) compared to temperature, indicating that the global glassy state still sustains. This is in contrast to the opinion that shear deformation may be similar to temperature increase and induce a metallic glass system to experience glass-liquid transition. The analysis of anisotropic part of RDFs reveals that shear deformation can induce significant anisotropic structural evolution, while pure increase of temperature cannot. Our findings demonstrate that shear deformation may induce different structural evolution in metallic glasses compared to temperature.

Free Volume Evolution in Metallic Glasses Subjected to Mechanical Deformation

We define the Tumbull-Cohen free volume as the critical excess of the Voronoi volume of an atom less its core volume. Using molecular dynamics simulation we calculated the free volume change in two model binary metallic glasses undergoing tension and shear deformation. We show that the free volume change is an integral part of the deformation process; and the shear localization manifested as a shear band is directly related to the inhomogeneous distribution of the free volumes. Shear band formation may consist of two stages: the initial free volume production in the amorphous solids and the liquefaction of the regions with accumulated deformation strains. We show, for the first time, the formation of voids and the vein patterns on fracture surfaces at atomic scales; they are the combined result of the free volume change and loading and sample conditions.