Formation, Structure and Properties of Bulk Metallic Glasses

Bulk metallic glasses with up to 72 mm critical section thickness have been obtained by conventional casting techniques and the properties of these materials, particularly the mechanical and magnetic properties have been studied. These materials have been demonstrated to have novel properties which are fundamentally different from their crystalline counterparts. The recent status of research and development in formation, structure and properties of bulk metallic glasses is reviewed. The techniques to produce such bulk glasses are summarized and the glass forming ability and the critical cooling rate of these materials are discussed. Further consideration of the development and application of this new class of materiaIs will be proposed.

Generalized model for laser-induced surface structure in metallic glass

The details of the special three-dimensional micro-nano scale ripples with a period of hundreds of microns on the surfaces of a Zr-based and a La-based metallic glass irradiated separately by single laser pulse are investigated.We use the small-amplitude capillary wave theory to unveil the ripple formation mechanism through considering each of the molten metallic glasses as an incompressible viscous fluid.A generalized model is presented to describe the special morphology,which fits the experimental result well.It is also revealed that the viscosity brings about the biggest effect on the monotone decreasing nature of the amplitude and the wavelength of the surface ripples.The greater the viscosity is,the shorter the amplitude and the wavelength are.

Structure Evolution of Bulk Metallic Glass Zr_(52.5)Ni_(14.6)Al_10Cu_(17.9)Ti_5 during Annealing

Bulk metallic glass Zr_52.5Ni_14.6Al_10Cu_17.9Ti_5 was prepared by melt injection casting method. Its glass transition and crystallization temperatures were determined by differential scanning calorimetry (DSC) to be 631 K and 710 K respectively. By analysis of X-ray diffractometry (XRD) and transmission electron microscopy (TEM ), the predominant crystallized phase of Zr_2Ni0.67O0.33 distributed on glass state matrix was detected after annealing at 673 K for 600 s. The transformation to Zr_2Ni_0.67O_0.33 and a small amount of ZrAl and Zr_2Cu took place after annealing for 600 s at temperature from 703 K to 723 K. With increasing annealing temperature from 753 K to 823 K, the amounts of ZrAl and Zr_2Cu increased, but the size of the crystals did not significantly change. The transformation to Zr_2Ni_0.67O_0.33 is interface-controlled, but is diffusion-controlled to Zr_2Cu and ZrAl. With increasing annealing temperature up to 200 K above T_x, the nanometer grains became very fine because of the increase of nucleation rate for Zr_2Cu and ZrAl.

Structural origin for composition-dependent nearest atomic distance in Cu–Zr metallic glass

We systematically investigate the structures of Cu–Zr metallic glass(MG) by varying the Cu concentration in classic molecular-dynamics simulation. From the pair distribution functions(PDFs), it is found that the nearest atomic distance between Zr atom and Zr atom increases significantly after adding Cu, which is related to the composition-dependent coordination behavior between Cu atom and Zr atom in the nearest neighbors. The portion of PDF related to the nearest connection is decomposed into the contributions from quadrilateral structure, pentagonal structure, hexagonal structure,and heptagonal bipyramid structure. Although the population of denser structures, i.e. 5-, 6-, and 7-number sharing ones,increases with Cu addition increasing, the connection distances between the central atoms in all these bipyramids increase for Zr–Zr pairs, leading to the expansion of Zr–Zr nearest atomic distance. These results unveil the effect of the interplay between chemical interaction and geometric packing on the atomic-level structure in Cu–Zr metallic glasses.

Structure Relaxation of Mg_(65)Cu_(25)Gd_(10) Metallic Glass and Its Effect on Strength

To identify the re-arrangement of constituent atoms of an amorphous Mg65Cu25Gd10 alloy happened with annealing, structure relaxation of the alloy was investigated as a function of an- nealing time at 373 K through extended X-ray absorption fine structure （EXAFS） analysis procedures. To understand the effect of structure relaxation on strength, compression tests were conducted for both the as-cast and the annealed Mg65Cu25Gd10 samples. It is found that short range order around Cu and Gd atoms exhibits different variation trends with increasing annealing time at 373 K, though the structure of the alloy still remains to be amorphous. Based on the fact that the strength of the alloy first exhibits a reduction and then a recovery with annealing time, it is suggested that the enhancement of short range order around Cu should be responsible for the strength reduction, while the enhancement of short range order around Gd should be responsible for the strength recovery.

Quantitative structure-plasticity relationship in metallic glass:A machine learning study

The lack of the long-range order in the atomic structure challenges the identification of the structural defects,akin to dislocations in crystals,which are responsible for predicting plastic events and mechanical failure in metallic glasses(MGs).Although vast structural indicators have been proposed to identify the structural defects,quantitatively gauging the correlations between these proposed indicators based on the undeformed configuration and the plasticity of MGs upon external loads is still lacking.Here,we systematically analyze the ability of these indicators to predict plastic events in a representative MG model using machine learning method.Moreover,we evaluate the influences of coarse graining method and medium-range order on the predictive power.We demonstrate that indicators relevant to the low-frequency vibrational modes reveal the intrinsic structural characteristics of plastic rearrangements.Our work makes an important step towards quantitative assessments of given indicators,and thereby an effective identification of the structural defects in MGs.

STRUCTURE AND GLASS FORMING ABILITY(GFA)OF AMORPHOUS ALLOYS

A new microstructure model is developed for amorphous alloys,so called Cluster medel, in which the amorphous phase is thought of composing of randomly distributed ordered clusters of different sizes.Thermodynamic calculation on this model deduces a parameter describing the glass forming ability of metallic alloys:α_c=(1-2.08/Φ_m)T_g/T_m,where T_g is gass transition temperature,T_m is the melting temperature,and Φ_m is entralpy change of melting.It is believed that easy glass forming alloy systems have larger values of a_c.This new criterion of GFA not only provides the theoretical background for several GFA criteria in the literature cited,but also can predict the GFA of many alloy systems more reasonably and accurately.

Effect of melt cooling rate on glass transition kinetics and structural relaxation of Vit1 metallic glass

The thin ribbons and the bulk cylindrical rods with diameters of 2 mm and 10 mm of the Vit1 metallic glass(MG)were prepared by the single roller melt spinning method and the copper mold injection casting method,respectively.The cooling rates of the samples during melt solidification were evaluated.The glass transition behaviors of three groups of MG samples with different solidification cooling rates were studied by differential scanning calorimetry(DSC)at different heating rates.The effects of melt cooling rate on the glass transition kinetic parameters such as apparent activation energy(E)and fragility parameter(m)of the Vit1 MG were studied using the Kissinger and the Vogel-Fulcher-Tammann(VFT)equations.Additionally,the structural relaxation enthalpy(ΔHrel)of three groups of MG samples was quantitatively analyzed by DSC through multi-step temperature rise and fall measurements.Results show that the melt cooling rate(R)has a significant effect on the glass transition kinetics and the structural relaxation of the Vit1 MG.As R decreases in the order of magnitude,the glass transition temperature(Tg),E,m,andΔHrel of the Vit1 MG gradually decreases.Furthermore,in the range of the experimental cooling rates,E,m,andΔHrel all have an approximately linear relationship with lgR.

Thickness Effect of Nanocrystalline Layer on the Deformation Mechanism of Amorphous/Crystalline Multilayered Structure

Different thickness of amorphous/nanocrystalline multi-layered structure can be used to modulate the strength and ductility of the composite materials.In this work,molecular dynamics simulations were conducted to study the thickness effect of nanocrystalline layer on mechanical properties and deformation behavior of the Cu64Zr36/Cu multi-layer structure.The stress-strain relationship,local stress,local strain,and deformation mechanism are investigated.The results reveal that the change of thickness of the crystalline layer significantly affects the mechanical properties and deformation behavior.As the strain at the elastic region,the amorphous Cu64Zr36 layer dominates the mechanical behavior,leading the fact that Young’s modulus,first yielding stress,and first yielding strain are close to that of Cu64Zr36 BMG.As the strain at the plastic region,the contribution of the crystalline layer on the mechanical behavior becomes more and more significant with increasing the thickness of the crystalline layer.For the thickness ratio(amorphous/crystalline)of 4,the shear band deformation of amorphous layer dominates the mechanical properties.For the thickness ratio is 1,the glide dislocation of the crystalline layer dominates the stress-strain behavior.

Relationship of glass forming ability and local structural properties of liquid Cu-Zr alloys

Molecular dynamics(MD) simulations were performed to investigate the glass forming ability(GFA) and microscopic structural properties of liquid Cu-Zr alloys.Based on the analysis of composition dependences of the reduced glass transition temperatures and the excess volume,we found that the Cu-Zr glasses have the largest GFA at Cu65Zr35 composition.To get more detailed information of local structure,we calculated the pair correlation functions,partial pair correlation functions,the excess entropy,chemical order parameter,coordination number,and Voronoi index of Cu-Zr liquids.We found that there exists an obvious and close relationship among the GFA,the excess entropy calculated using the total pair correlation functions,chemical order parameters,and some Cu centered cluster with Voronoi index <0,2,8,1> and Zr centered cluster with Voronoi index <0,3,6,4>,which all have nonlinear dependences on Cu/Zr concentration and have extreme values at liquid Cu65Zr35 composition.

Tailoring mechanical heterogeneity,nanoscale creep deformation and optical properties of nanostructured Zr-based metallic glass

Metallic glasses are spatially heterogeneous at the nanometer scale.However,the effects of external excitation on their structural and mechanical heterogeneity and the correlation to their properties are still unresolved.Nanoindentation,atomic force microscopy(AFM) and high-resolution transmis sion elec tron micro scopy(HRTEM) were carried out to explore the effects of cryogenic thermal cycling(CTC) on mechanical/structural heterogeneity,nano sc ale creep deformation and optical properties of nano structured metallic glass thin films(MGTFs).The results indicate that CTC treatment alters the distribution fluctuations of hardness/modulus and energy dissipation and results in an increase-then-decrease variation in mechanical heterogeneity.By applying Maxwell-Voigt model,it can be shown that CTC treatment results in a remarkable activation of more defects with longer relaxation time in soft regions but has only a slight effect on defects in hard regions.In addition,CTC treatment increases the transition time from primary-state stage to steady-state stage during creep deformation.The enhanced optical reflectivity of the MGTFs after 15 thermal cycles can be attributed to increased aggregation of Cu and Ni elements.The results of this study shed new light on understanding mechanical/structural heterogeneity and its influence on nanoscale creep deformation and optical characteristics of nanostructured MGTFs,and facilitate the design of high-performance nanostructured MGTFs.

Secondary relaxation and dynamic heterogeneity in metallic glasses：A brief review

Understanding mechanical relaxation, such as primary（α） and secondary（β） relaxation, is key to unravel the intertwined relation between the atomic dynamics and non-equilibrium thermodynamics in metallic glasses. At a fundamental level, relaxation, plastic deformation, glass transition, and crystallization of metallic glasses are intimately linked to each other, which can be related to atomic packing, inter-atomic diffusion, and cooperative atom movement. Conceptually, βrelaxation is usually associated with structural heterogeneities intrinsic to metallic glasses. However, the details of such structural heterogeneities, being masked by the meta-stable disordered long-range structure, are yet to be understood. In this paper, we briefly review the recent experimental and simulation results that were attempted to elucidate structural heterogeneities in metallic glasses within the framework of β relaxation. In particular, we will discuss the correlation amongβ relaxation, structural heterogeneity, and mechanical properties of metallic glasses.

Thermal effects and evolution of the defect concentration based on shear modulus relaxation data in a Zr-based metallic glass

A relationship between thermal effects and relaxation of the high-frequency shear modulus upon heat treatment of bulk Zr48(Cu5/6Ag1/6)44Al8 metallic glass is found.This relationship is attributed to the relaxation of a interstitial-type defect system frozen-in from the melt upon glass production.Calorimetric data show that thermal effects occurring on heating include heat release below the glass transition temperature,heat absorption above it and heat release caused by crystallization.The equation derived within the Interstitialcy theory can be used to calculate the shear modulus relaxation using the calorimetric data.The obtained results are used to trace the defect concentration as functions of temperature and thermal prehistory.

Five-fold local symmetry in metallic liquids and glasses

The structure of metallic glasses has been a long-standing mystery. Owing to the disordered nature of atomic structures in metallic glasses, it is a great challenge to find a simple structural description, such as periodicity for crystals, for establishing the structure–property relationship in amorphous materials. In this paper, we briefly review the recent developments of the five-fold local symmetry in metallic liquids and glasses and the understanding of the structure–property relationship based on this parameter. Experimental evidence demonstrates that five-fold local symmetry is found to be general in metallic liquids and glasses. Comprehensive molecular dynamics simulations show that the temperature evolution of five-fold local symmetry reflects the structural evolution in glass transition in cooling process, and the structure–property relationship such as relaxation dynamics, dynamic crossover phenomena, glass transition, and mechanical deformation in metallic liquids and glasses can be well understood base on the simple and general structure parameter of five-fold local symmetry.

Formation and crystallization of Zr-Ni-Ti metallic glass

The metallic Zr65Ni25Ti10（mole fraction, %） glass has been fabricated by a single roller melt-spinning method. The glass forming ability（GFA） and thermal stability of the Zr65Ni25Ti10 melt-spun ribbons were investigated by using X-ray diffraction（XRD） and differential scanning calorimetry（DSC） in the mode of continuous heating. It is shown that the reduced glass transition temperature (Trg) is 0.506 and the supercooled liquid region （ΔTx） is 30 K. Two exothermic peaks were observed in the DSC curves of the as-quenched ribbon, which indicates that the crystallization process undergoes two different stages. The phase transformation during the isothermal annealing was investigated by X-ray diffraction（XRD） and transmission electronic microscope（TEM）. It is observed that the metastable FCC Zr2Ni（Fd3m, a=12.27 ） precipitated while annealing in the suppercooled region（615 K） and the stable BCT Zr2Ni（I4/mcm, a=6.499 , c=5.270 ） precipitated while annealing at higher temperature（673 K or 723 K）. The crystallines are on nanoscale, with grain size of 1530 nm. The reason for the precipitation of the different structural Zr2Ni from the glassy matrix under different annealing conditions was discussed based on the concept of multi-component chemical short range order（MCSRO）.

Preferred clusters in metallic glasses

In this work, we present a feasible scheme based on framework of the sophisticated Voronoi tessellation method in order to evaluate what clusters should be preferred for building blocks in any given metallic glass, by analysing the fivefold-symmetry axes as well as the degree of structural regularity in various clusters. This scheme is well proved by a group of experiments and calculations, which may have broad implications for exploration of obtaining explicit and proper structural pictures, and understanding the structural origin of the unique properties and glass forming ability in these novel amorphous alloys.

Crystallization evolution and relaxation behavior of high entropy bulk metallic glasses using microalloying process

The role of the microalloying process in relaxation behavior and crystallization evolution of Zr_(20) Cu_(20) Ni_(20) Ti_(20) Hf_(20) high entropy bulk metallic glass(HEBMG) was investigated. We selected Al and Nb elements as minor elements, which led to the negative and positive effects on the heat of mixing in the master HEBMG composition, respectively. According to the results, both elements intensified β relaxation in the structure;however, α relaxation remained stable. By using different frequencies in dynamic mechanical analysis, it was revealed that the activation energy of β relaxation for the Nb-added sample was much higher, which was due to the creation of significant structural heterogeneity under the microalloying process. Moreover, it was found that Nb addition led to a diversity in crystallization stages at the supercooled liquid region.It was suggested that the severe structural heterogeneity in the Nb-added sample provided multiple energy-level sites in the structure for enhancing the crystallization stages.

Atomic structures of Zr-based metallic glasses

The atomic structures of Zr-Ni and Zr-Ti-Al-Cu-Ni metallic glasses were investigated by using classical molecular dynamic (MD),reverse Monte Carlo (RMC),ab initio MD (AIMD) simulations and high resolution transmission electron microscopy (HRTEM) techniques. We focused on the short-range order (SRO) and medium-range order (MRO) in the glassy structure. It is shown that there are icosahedral,FCC-and BCC-type SROs in the Zr-based metallic glasses. A structural model,characterized by imperfect ordered packing (IOP),was proposed based on the MD simulation and confirmed by the HRTEM observation. Furthermore,the evolution from IOP to nanocrystal during the crystallization of metallic glasses was also ex-plored. It is found that the growth from IOP to nanocrystal proceeds through three distinct stages: the formation of quasi-ordered structure with one-dimensional (1D) periodicity,then 2D periodicity,and finally the formation of 3D nanocrystals. It is also noted that these three growth steps are crosslinked.

Structure Heterogeneity in Metallic Glass: Modeling and Experiment

Despite the great efforts dedicated to metallic glasses （MGs）, their structure still remains a mystery to be understood. With comparison to the existing mJciomechanical models, such as the free-volume and shear transformation zone （STZ） models, we first discuss in this article our recently proposed ＇core-shell＇ model, which contains a solid-like matrix and liquid-like inclusions. This serves as the theoretical basis to understand the structural heterogeneity in MGs in our analytical framework. After that, a scanning ultrafast nanoindentation technique is used to map out the structure heterogeneity in a Zr-based bulk metallic glass （BMG）. With these ongoing research efforts, we hope that more research work could be stimulated in the pursuit of the structure-property relation in MGs.