The fracture of bulk metallic glasses

The fracture of metallic glasses has received relatively little attention until recently. The development of bulk metallic glasses (BMGs) with more compositions, large sample sizes and diverse fracture behaviors provides a series of ideal model systems for the study of fracture in glassy materials. The fracture toughness of different BMGs varies significantly from approaching ideally brittle to the highest known damage tolerance. Diverse fracture patterns on the fracture surface, fracture modes and dynamic propagation of cracks have been observed in different BMGs. In this review paper, we present a comprehensive view of the state-of-the-art research on various aspects of the fracture of BMGs, including fracture behavior and characteristics, fracture mode, fracture criterion, fracture toughness, and fracture morphology. Accumulated experimental data on BMG fracture are presented and their possible theoretical connections with continuum fracture mechanics and the atomic-scale process are introduced and discussed. Modeling studies of the fracture of BMGs by various computational methods are also reviewed. The review also presents a number of perspectives, including the relation of BMG fracture study to other topics, and unsolved issues for future investigation.     

A microplasticity analysis of shear band cracks in rolled 2024 aluminium alloy

Flow localization in the form of shear bands is an important part of the ductile fracture process. The propagation of shear-band cracks has been followed in the rolling of an annealed and an aged-hardened 2024 aluminium alloy respectively. Shear bands occurred in the alloy that had been aged-hardened prior to rolling but not in the one that had been annealed. There are two types of observed shear bands: grain-scale shear bands and sample-scale shear bands. The shear-band cracks were found to propagate along the directions of the sample-scale shear bands which often deviate from the maximum shear stress plane of 45°. While some grains allowed the cracks to pass through, some grains would stop the cracks from growing. A material based constitutive equation was used to predict the angle of the shear-band cracks and the prediction agreed well with the observed ones. The propagation of the shear-band cracks were found to be sensitive to the local variation of the crystallographic texture proportion. Both the microplasticity and continuum theories point out the importance of strain hardening rate as an important controlling factor in the occurrence of shear band cracks in textured polycrystals.     

Medium-range order dictates local hardness in bulk metallic glasses

Bulk metallic glasses (BMGs) are materials with outstanding strength and elastic properties that make them tantalizing for engineering applications, yet our poor understanding of how their amorphous atomic arrangements control their broader mechanical properties (hardness, wear, fracture, etc.) impedes our ability to apply materials science principles in their design. In this work, we uncover the hierarchical structure that exists in BMGs across the nano- to microscale by using nanobeam electron diffraction experiments. Our findings reveal that local hardness of microscale domains decreases with increasing size and volume fraction of atomic clusters with higher local medium range order (MRO). Furthermore, we propose a model of ductile phase softening that will enable the future design of BMGs by tuning the MRO size and distribution in the nanostructure.     

Continuously distributed dislocation model for shear-bands in softening materials

The classical concept of slip-surfaces in soil mechanics is assigned a physical significance based on frequent observations of rupture in overconsolidated soils and rock masses; propagation of a narrow shear-band (i.e. slip-surface) is commonly the mechanism of failure. Effectively, relative sliding occurs between adjacent material points on the surface, a concept familiar in rack mechanics; just as failure by crack propagation can occur in metals well below net section yielding, so can progressive failure by band propagation cause slope failure at nominal stresses well below the peak strength of the material. The relative sliding is simulated here, for a material response which is linear outside the band, by superposing continuously distributed dislocations; use is made of influence functions, for dislocation and point-force singularities in a variety of materials and geometries. The method has further application in rock mechanics and metallurgy. A stress-displacement constitutive relation is imposed on the line of the band and the resulting non-linear singular integral equations necessitate numerical solutions; the routines are prefected on two problems for which answers are otherwise available. Extension to previously intractable plane-strain problems is then described and results are given for a surface-parallel shear-band corresponding to a flake slide on a long slope.     

块体金属玻璃制备技术的研究进展

主要介绍了块体金属玻璃的连接技术、放电等离子烧结法和电磁振动法等制备块体金属玻璃的新技术手段。块体金属玻璃的连接技术包括激光焊、爆炸焊、电子束焊、熔融液相连接法和摩擦焊等。采用焊接的方法可将块体金属玻璃连接在一起,以形成大尺寸甚至超大尺寸的块体金属玻璃;放电等离子烧结可在很短时间内制备多孔、大尺寸和具有一定塑性的块体金属玻璃,在制备具有优异软磁性能的块体金属玻璃上也具有显著优势;电磁振动法可以有效抑制晶体形核,显著提高块体金属玻璃体系的玻璃形成能力,从而制备更大尺寸的块体金属玻璃。     

A Finite-Deformation Constitutive Model of Bulk Metallic Glass Plasticity

A constitutive model of bulk metallic glass (BMG) plasticity is developed which accounts for finitedeformation kinematics, the kinetics of free volume, strain hardening, thermal softening, rate-dependency and non-Newtonian viscosity. The model has been validated against uniaxial compression test data; and against plate bending experiments. The model captures accurately salient aspects of the material behavior including: the viscosity of Vitreloy 1 as a function of temperature and strain rate; the temperature and strain-rate dependence of the equilibrium free-volume concentration; the uniaxial compression stress-strain curves as a function of strain rate and temperature; and the dependence of shear-band spacing on plate thickness. Calculations suggest that, under adiabatic conditions, strain softening and localization in BMGs is due both to an increase in free volume and to the rise in temperature within the band. The calculations also suggest that the shear band spacing in plate-bending specimens is controlled by the stress relaxation in the vicinity of the shear bands.     

Design of Zr–Al–Ni–Cu bulk metallic glasses with network structures

A series of Zr–Al–Ni–Cu bulk metallic glasses (BMGs) with network structures were carefully designed and their heterogeneous microstructures were investigated by carefully etching the cross sections of these BMGs. It is found that the heterogeneous microstructures of these BMGs can be uncovered by etching with mixed solution of HF and HNO₃. Some of the studied Zr-based BMGs characterize in micrometer network structure. The cell size of the network structure depends on the composition of the BMG and is related with the fractural mode and mechanical property. The found network structure of the Zr-based BMGs benefits for understanding the unique mechanical properties of the Zr-based BMGs.     

Fatigue behavior of Zr-based bulk-metallic glasses

The objective of this paper was to study the fatigue characteristics of zirconium (Zr)-based bulk-metallic glasses (BMGs) and to investigate the mechanisms of fatigue-crack initiation, crack propagation, and fracture in BMGs. The fatigue ratios (fatigue limit/tensile strength) (0.30–0.55) of Zr-based BMGs were found to be generally comparable with those of crystalline alloys, such as steel and titanium alloys. Fatigue cracks typically initiate from shear bands, inclusions, and/or porosities. The striations resulting from the blunting and resharpening of the fatigue-crack tip formed in the fatigue-crack-growth region. The fine striation spacing seems to be comparable to that of the crystalline alloys.     

A plastic FeNi-based bulk metallic glass and its deformation behavior

The strength and plasticity of Fe39Ni39B12.82Si2.75Nb2.3P4.13 bulk metallic glass(BMG)are improved simul-taneously by modulating atomic-scale structure through fluxing treatment.The compression strength increases from 3074 to 4220 MPa,and the plastic strain is enlarged from 10.7％ to more than 50％.The increased mechanical properties of the fluxed FeNiBSiNbP BMG originate from the optimization of atomic-scale structure.More icosahedral-like clusters(ILCs)and crystal-like clusters(CLCs)are found in this FeNi-based BMG with fluxing treatment,and the ILCs are usually surrounded by CLCs.Furthermore,phase separation and a sandwich-like heterogeneous structure of SB are also observed during deforma-tion,indicating the multiscale deformation mechanism and a stable shear-band evolution.The unique"ILC surrounded by CLCs"structure and phase separation lead to a stable plastic deformation process with strong interactions of multiple shear bands,thereby the improved plasticity and strength.This work provides useful guidelines to develop strong and plastic Fe-based BMGs from a structural aspect.     

Bulk metallic glass: the smaller the better

Bulk metallic glasses (BMGs) are strong, highly elastic, and resistant to wear but still find limited utility due to their macroscopic brittle nature, high costs, and difficulty of processing, particularly when complex shapes are desired. These drawbacks can be mitigated when BMGs are used in miniature parts (< 1 cm), an application which takes advantage of BMGs' enhanced plasticity at small length scales as well the insignificant material cost associated with such parts. As an alternative to traditional metal processing techniques, thermoplastic forming (TPF)-based microfabrication methods have been developed which can process some BMGs like plastics. In this article, we discuss the properties and fabrication of BMGs on minuscule length scales to explore their prospective application in small-scale devices.     

Metallic glasses from “alchemy” to pure science: Present and future of design,processing and applications of glassy metals

Metallic glasses, first discovered a half century ago, are currently among the most studied metallic materials. Available in sizes up to several centimeters, with many novel, applicable properties, metallic glasses have also been the focus of research advancing the understanding of liquids and of glasses in general.Metallic glasses (MGs), called also bulk metallic glasses (BMGs) (or glassy metals, amorphous metals, liquid metals) are considered to be the materials of the future. Due to their high strength, metallic glasses have a number of interesting applications, for example as coatings. Metallic glasses can also be corrosion resistant. Metallic glasses, and the crystalline materials derived from them, can have very good resistance to sliding and abrasive wear. Combined with their strength – and now, toughness – this makes them ideal candidates for bio-implants or military applications. Prestigious Journals such as “Nature Materials”, “Nature” frequently publish new findings on these unusual glass materials. Moreover Chinese and Asian scientists have also been showing an interest in the study of metallic glasses.This review paper is far from exhaustive, but tries to cover the areas of interest as it follows: a short history, the local structure of BMGs and the glass forming ability (GFA), BMGs’ properties, the manufacturing and some applications of BMGs and finally, about the future of BMGs as valuable materials.     

Roles of minor additions in formation and properties of bulk metallic glasses

Bulk metallic glasses (BMGs) are of current interest worldwide in materials science and engineering because of their unique properties. Exploring BMGs materials becomes one of the hottest topics in the materials science field. To date, there is very active worldwide development of new BMGs, and extensive efforts have been carried out to understand and improve the glass-forming ability of metallic materials supported by large government and industry programs in North America, Asia, and Europe. Minor addition or microalloying technique, which has been widely used in other metallurgical fields, plays effective and important roles in formation, crystallization, thermal stability and property improvement of BMGs. This simple approach provides a powerful tool for the BMG-forming alloys development and design. In this paper, we present a comprehensive review of the history and the recent developments of this technique in the field of BMGs. The roles of the minor addition in the formation and the properties of the BMGs and the BMG-based composites will be discussed and summarized within the framework of thermodynamics, kinetics and microstructure. The empirical criteria, or the principles and guidelines for the applications of the technique in BMG field are outlined.     

Microstructure and compressive/tensile characteristic of large size Zr-based bulk metallic glass prepared by laser solid forming

The large size, crack-free Zr_(55)Cu_(30)Al_(10)Ni_(5) bulk metallic glass(BMGs) with the diameter of 54 mm and the height of 15 mm was built by laser solid forming additive manufacturing technology, whose size is larger than the critical diameter by casting. The microstructure, tensile and compressive deformation behaviors and fracture morphology of laser solid formed Zr_(55)Cu_(30)Al_(10)Ni_5 BMGs were investigated. It is found that the crystallization mainly occurs in the heat-affected zones of deposition layers, which consist of Al_5Ni_3Zr_2, NiZr_2, ZrCu, CuZr_2 phases. The content of amorphous phase in the deposit is about 63%.Under the compressive loading, the deposit presents no plasticity before fracture occurs. The fracture process is mainly controlled by the shear stress and the compressive shear fracture angles of about39?. The compressive strength reaches 1452 MPa, which is equivalent to that of as-Cast Zr_(55)Cu_(30)Al_(10)Ni_5 BMGs, and there exist vein-like patterns, river-like patterns and smooth regions at the compressive fractography. Under the tensile loading, the deposit presents the brittle fracture pattern without plastic deformation. The fracture process exhibits normal fracture model, and the tensile shear fracture angle of about 90?. The tensile strength is only about 609 MPa, and the tensile fractography mainly consists of micro-scaled cores and vein-like patterns, dimple-like patterns, chocolate-like patterns and smooth regions. The results further verified the feasibility and large potential of laser additive manufacturing on fabrication and industrial application of large-scale BMGs parts.     

Role of late transition metals on pitting resistance of Zr-Ti-(Cu,Ni,Co)-Al bulk metallic glasses in 0.6 M NaCl aqueous solution

Four quanternary Zr-based bulk metallic glasses(BMGs)were selected,including the Zr_(46)Ti_2Cu_(45)Al_7,Zr_(61)Ti_2Cu_(25)Al_(12),Zr_(55)Ti_4Ni_(22)Al_(19)and Zr_(55)Ti_2Co_(28)Al_(15),due to their robust glass-forming ability and containing a single species of late transition metal(LTM)in compositions.Their pitting resistances in 0.6 M Na Cl aqueous solution were investigated to examine the role of LTM elements in the alloys,with electrochemical measurements,surface morphology observation and x-ray photoelectron spectrometry analysis.It is shown that in comparision with two Cu-bearing BMGs,Zr_(55)Ti_4Ni_(22)Al_(19)and Zr_(55)Ti_2Co_(28)Al_(15)BMGs exhibited significantly superior resistance to pitting.Zr_(61)Ti_2Cu_(25)Al_(12),Zr_(55)Ti_4Ni_(22)Al_(19)and Zr_(55)Ti_2Co_(28)Al_(15)BMGs manifested distinct passivation behaviour,because of the formation of surface passive film mainly comprising of Zr O_2,Ti O_2and Al_2O_3.However,no significant differences in the electrochemical resistive properties and thicknesses of passive films were found between Zr_(61)Ti_2Cu_(25)Al_(12)and Zr_(55)Ti_4Ni_(22)Al_(19)BMGs.Nevertheless,at the passive film/metal interface,copper enrichment took place in Zr_(61)Ti_2Cu_(25)Al_(12),whereas the nickel was slightly deficient at the interface in Zr_(55)Ti_4Ni_(22)Al_(19).During pitting propagation,selective dissolution of the zirconium,titanium and aluminum over the copper took place in Zr_(61)Ti_2Cu_(25)Al_(12),but it was not the case in Zr_(55)Ti_4Ni_(22)Al_(19).For the two Cu-bearing BMGs,reduction of passive base metal elements in composition resulted in local selective dissolution,even absence of the passivation.     

Additive Manufacturing of Advanced Multi‐Component Alloys: Bulk Metallic Glasses and High Entropy Alloys

Bulk metallic glasses (BMGs) and high entropy alloys (HEAs) are both important multi‐component alloys with novel microstructures and unique properties, which make them promising for applications in many industries. However, certain hindrances have been identified in the fabrication of BMGs and HEAs by conventional techniques due to the intrinsic requirements of BMGs and HEAs. With the advent of metal additive manufacturing, new opportunities have been perceived to fabricate geometrically complex BMGs and HEAs with tailorable microstructure theoretically at any site within the specimen, which are not achievable using conventional fabrication techniques. After providing some background and introducing the conventional fabrication techniques for BMGs and HEAs, this review will focus on the current status, development, and challenges in metal additive manufacturing of BMGs and HEAs including different additive manufacturing techniques being used, microstructure design and evolution, as well as properties of the fabricated BMGs and HEAs. A future outlook of metal additive manufacturing of BMGs and HEAs will also be provided at the end.

     

Correlation between deformation behavior and atomic-scale heterogeneity in Fe-based bulk metallic glasses

The correlation betweent deformation behavior and atomic-scale heterogeneity of bulk metallic glasses(BMGs)is critical to understand the BMGs'deformation mechanism.In this work,three typical[(Fe_0.5Co_0.5)_0.75B_0.2Si_0.05]₉₆Nb_4,Fe₃₉Ni₃₉B_14.2Si_2.75P_2.75Nb_2.3,and Fe₅₀Ni₃₀P₁₃C₇BMGs exhibiting different plasticity were selected,and the correlation between deformation behavior and atomic-scale heterogeneity of Fe-based BMGs was studied.It is found that the serrated flow dynamics of Fe-based BMGs transform from chaotic state to self-organized critical state with increasing plasticity.This transformation is attributed to the increasing atomic-scale heterogeneity caused by the increasing free volume and short-to-medium range order,which facilitates a higher frequency of interaction and multiplication of shear bands,thereby results in a brittle to ductile transition in Fe-based BMGs.This work provides new evidence on heterogeneity in plastic Fe-based BMGs from the aspects of atomic-scale structure,and provides new insight into the plastic deformation of Fe-based BMGs.     

Mechanical properties for bulk metallic glass with crystallites precipitation and second ductile phase addition

Monolithic phase bulk metallic glasses (BMGs) produced by a copper mold casting method and BMG composites containing in-situ brittle crystallites and out-situ tungsten fiber produced by a water quenching method were obtained. Mechanical properties including cyclic deformation and fracture toughness were investigated. Under symmetrically cyclic stress control, the life of tungsten fiber reinforced amorphous alloy is much longer than that of the monolithic amorphous alloy. The composite containing tungsten fibers that retard the crack propagation exhibits cyclic softening while the partially crystallized amorphous alloy exhibits stable cycling. The regions of crack initiation, stable propagation and final fracture were observed on the fracture surface. Crystalline brittle phases do not retard the crack propagation but become sites of crack initiation. Tungsten fiber reinforced BMG has the largest fracture toughness while BMG with quenched-in crystallites the smallest. Tungsten fibers stabilize crack growth in the matrix and extend the strain to failure of the composite, while brittle crystallites speed up the crack propagation even though they act as obstacles when shear bands reach them in some cases.     

3D printing metals like thermoplastics: Fused filament fabrication of metallic glasses

Whereas 3D printing of thermoplastics is highly advanced and can readily create complex geometries, 3D printing of metals is still challenging and limited. The origin of this asymmetry in technological maturity is the continuous softening of thermoplastics with temperature into a readily formable state, which is absent in conventional metals. Unlike conventional metals, bulk metallic glasses (BMGs) demonstrate a supercooled liquid region and continuous softening upon heating, analogous to thermoplastics. Here we demonstrate that, in extension of this analogy, BMGs are also amenable to extrusion-based 3D printing through fused filament fabrication (FFF). When utilizing the BMGs’ supercooled liquid behavior, 3D printing can be realized under similar conditions to those in thermoplastics. Fully dense and amorphous BMG parts are 3D printed in ambient environmental conditions resulting in high-strength metal parts. Due to the similarity between FFF of thermoplastics and BMGs, this method may leverage the technology infrastructure built by the thermoplastic FFF community to rapidly realize and proliferate accessible and practical printing of metals.     

Role of minor alloying additions in formation of bulk metallic glasses: A Review

Minor alloying addition or microalloying technology has already shown dramatic effects on glass formation and thermal stability of bulk metallic glasses (BMGs). This paper intends to provide a comprehensive review of recent developments of this technology in the field of BMGs. The beneficial effects of minor alloying additions on the glass formation and the thermal stability of BMGs will be summarized and analyzed. In addition, principles and guidelines for future application of this technology will also be proposed.     

A gradient flow theory of plasticity for granular materials

Summary A flow theory of plasticity for pressure-sensitive, dilatant materials incorporating second order gradients into the flow-rule and yield condition is suggested. The appropriate extra boundary conditions are obtained with the aid of the principle of virtual work. The implications of the theory into shear-band analysis are examined. The determination of the shear-band thickness and the persistence of ellipticity in the governing equations are discussed.