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.     

Effect of prior compression treatment on the deformation behavior of Zr based bulk metallic glass

In the present work, the plasticity of Zr_64.2Cu_11.2 Ni_14.6Al₁₀ bulk metallic glass was enhanced through prior compression treatment. A considerably large compressive plastic deformation (over 6.5%) was achieved by pressing Zr_64.2Cu_11.2 Ni_14.6Al₁₀ bulk metallic glass laterally in specially designed tool steel die before compression test. Numerical analysis was also carried out to investigate the stress distribution under same mechanical conditions. It was revealed that the lateral pressing induced structural heterogeneity and high stress gradients facilitate large plastic strains through the generation of dense multiple shear band network.     

Plasticity improvement of an Fe-based bulk metallic glass by geometric confinement

In this paper, the effect of nickel coating on the plasticity of a brittle Fe-based bulk metallic glass (BMG) was investigated. By electrodepositing a nickel coating onto an Fe₇₅Mo₅P₁₀C_8.3B_1.7 BMG, the plasticity of the BMG is shown to increase dramatically from ~ 0.5% to ~ 5.0%. Without reducing the strength of the BMG, the findings demonstrate that the electrodeposited nickel is an effective way to improve prominently brittle BMGs. The increased compressive plastic strain is believed to be attributed to the nucleation, intersection and bifurcation of multiple shear bands due to the exterior impedance of the surrounding coating.     

Development of quaternary Fe-based bulk metallic glasses

Fe-based bulk metallic glasses have been designed using several modeling criteria. Quaternary alloys were successfully developed in the Fe-rich compositional region where the only solutes used were Cr, Nb, and B. Furthermore, the exceedingly high level of Fe, exceeding 75 wt.% in some alloys, makes this series of alloys, compositionally very close to conventional steel alloys. Therefore, their production using conventional facilities and materials is more easily attainable than previously developed bulk metallic glasses. The modeling criteria used, simultaneously analyzes the thermodynamics and kinetics of the vitrification behavior in a potential glass-forming alloy. A liquidus model, which determines and ranks the presence of deep eutectics, is used to determine the optimal compositional region. This criterion is cross-checked with an elastic strain model. Alloys compositionally located near a deep eutectic, while simultaneously containing a topology that induces significant elastic strain in a developing crystalline lattice exhibits experimentally good glass forming ability.     

Melt-spinning technique for preparation of metallic glasses

Rapid solidification of metallic melts at a rate ≳10⁵ °K/sec has gained considerable interest for casting non-equlibrium crystalline structures in general and metallic glasses in particular. Of the variety of techniques used for rapid solidification, melt-spinning and melt-extraction are widely used. This paper describes the design and development of a melt-spinning technique in our laboratory. Optimization of spinning parameters for smooth and continuous ribbons is discussed. Glassy ribhons of Cu-Zr and Pd-Ge alloys have been made under optimized conditions. A brief account of some of the structural and electronic properties of these glasses is illustrated.     

Study on continuous casting of bulk metallic glass

Continuous casting method for massive production of steel, aluminum, copper or other crystalline alloy ingot is a very important industrial technology for its low energy consumption and high productivity. In this study, a new continuous casting method was developed for the massive production of bulk metallic glass ingot with centimeter-scale diameter without length limitation. An intermittent withdrawal procedure was practiced for continuous casting of bulk glassy alloy. The new developed continuous casting method can provide a cooling speed as high as that provided by the stationary mold casting method. A Zr-based glassy alloy rod with a diameter of about 10 mm and length of tens of centimeters was prepared by the continuous casting method for the first time.     

Interlaminar shear properties of composite insulation systems for fusion magnets at cryogenic temperatures

This paper reports the cryogenic interlaminar shear properties of composite insulation systems for the superconducting magnet coils in the International Thermonuclear Experimental Reactor (ITER). Short beam shear tests were performed at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) on three insulation systems consisting of woven glass fiber reinforced plastic (GFRP) composites with different polymer resins and polyimide films, and the dependence of their apparent interlaminar shear strength on the temperature and the polymer resins was discussed. A detailed observation of failed specimens was made to verify the failure mechanisms. Insulation systems subjected to gamma irradiation at room temperature were also considered, and the effect of irradiation on the cryogenic interlaminar shear properties was examined.     

Mechanical behaviours of workhardening and worksoftening bulk metallic glasses

Abstract

The workhardening Cu_47·5Zr_47·5Al₅ and the worksoftening Zr_52·5Cu_17·9Ni_14·6Al_10·0Ti_5·0 bulk metallic glasses before and after precompression deformation were characterised for thermal and mechanical behaviours. The predeformation introduces excessive free volume in both glasses. Cu_47·5Zr_47·5Al₅ and Zr_52·5Cu_17·9Ni_14·6Al_10·0Ti_5·0 exhibit substantial workhardening and worksoftening behaviours respectively. For Cu_47·5Zr_47·5Al₅, the precompression has a negligible effect on serrations in the plastic flow during nanoindentation, which is related to the hardening of a shear band, while for Zr_52·5Cu_17·9Ni_14·6Al_10·0Ti_5·0, the precompression moderates serrations in the plastic flow during nanoindentation, which is associated with the softening of a shear band. Strengthening from mechanically induced nanocrystallites at shear bands is responsible for the workhardening of Cu_47·5Zr_47·5Al₅, which overwhelms softening due to the introduction of excessive free volume.     

Synthesis and characterization of boron-oxygen-hydrogen thin films at low temperatures

We have studied the influence of synthesis temperature on chemical composition and mechanical properties of X-ray amorphous boron-oxygen-hydrogen (B-O-H) films. These B-O-H films have been synthesized by RF sputtering of a B-target in an Ar atmosphere. Upon increasing the synthesis temperature from room temperature to 550 °C, the O/B and H/B ratios decrease from 0.73 to 0.15 and 0.28 to 0.07, respectively, as determined by elastic recoil detection analysis. It is reasonable to assume that potential sources of O and H are residual gas and laboratory atmosphere. The elastic modulus, as measured by nanoindentation, increases from 93 to 214 GPa, as the O/B and H/B ratios decreases within the range probed. Hence, we have shown that the effect of impurity incorporation on the elastic properties is extensive and that the magnitude of the incorporation is a strong function of the substrate temperature.     

Numerical study of shear banding evolution in bulk metallic glass composites

In this article, a systematic simulation was performed to demonstrate the detailed shear banding evolution in bulk metallic glass (BMG) composites subjected to the tension, and the relation between microstructures and tensile ductility was therefore elucidated. Free volume was adopted as an internal state variable to characterize the shear banding nucleation, growth and coalescence in BMG matrix with the help of free volume theory, which was incorporated into ABAQUS finite element method (FEM) code as a user material subroutine. The present numerical method was firstly verified by comparing with the corresponding experimental results, and then parameter analyses were conducted to discuss the impacts of particle volume fraction, particle shape, particle orientation and particle yielding strength on the enhancement efficiency of the tensile ductility of BMG composites.     

Mechanical behavior of Zr-based bulk metallic glasses

Bulk metallic glasses have a very high corrosion resistance and mechanical strength. Bulk metallic glasses show elastic-perfectly plastic behavior with an extended region of elastic strain (≈ 2%). But at room temperature their macroscopic plasticity is weak even though a local plastic strain is observed in shear bands. A relaxation analysis allowed studying micro-mechanisms of plastic deformation and estimating the apparent activation volume (≈ 2000 ų). __________ Translated from Problemy Prochnosti, No. 1, pp. 167–170, January–February, 2008.     

Ductile Fe-based bulk metallic glasses at room temperature

In this work, Fe₇₀Ni₁₀P₁₃C₇ and Fe₆₀Ni₂₀P₁₃C₇ bulk metallic glasses (BMGs) with high ductility are synthesised through the appropriate adjustment of flux conditions. Microstructure analyses indicate that the ductility of these two Fe-based BMGs is primarily determined by Fe–Ni and Ni–Ni metal–metal bonds, which enable the glasses to undergo a strain of more than 50% in the absence of fracture, as well as extensive bending ductility, similar to metals such as Fe and Ni. This study aims to find a solution to the brittleness problem in Fe-based BMGs, and the results have implications for understanding the deformation mechanism in Fe-based BMGs.     

Deformation dynamics at low and ambient temperatures

The variation of tensile yield stress at a constant strain rate as a function of temperature for well-annealed pure metals show, with increasing temperatures, a rather sharp drop in yield stress (low temperature regime), followed by the intermediate temperature regime where yield stress decreases more slowly (and the ratio of yield stress to shear modulus remains more or less constant), which in turn is followed by the high temperature regime where the yield stress drops again rather sharply. The paper discusses the phenomenological framework for studying deformation dynamics in the low and intermediate temperature regimes. The approach adopted is the well-known state variable approach, where the evolutionary nature of deformation structure is described by one or more structure variables such that the current values of mechanical variables and structure variables together completely define the current state of deformation. A critical analysis of experimental results available suggest that at least for deformation at low strain rates, stress-rate is probably not a state variable of deformation. Thus deformation is most conveniently studied in terms of TASRA (thermally activated strain rate analysis) where the stress, plastic strain rate, temperature and structure are interrelated through a Gibb’s free energy for thermal activation by an Arrhenius equation. The stress-dependence of Gibb’s free energy and its maximum value then form the basis of identifying the rate-controlling obstacles. The need for careful experimentation and systematic analysis is illustrated by the example of low temperature deformation of hard hep metals. Modelling for the evolution of deformation structure is also touched upon.     

Fabrication and mechanical characterization of a series of plastic Zr-based bulk metallic glass matrix composites

Coupling the high yielding strength with enhanced plasticity under compression at ambient temperature, a series of Zr-based bulk metallic glass matrix composites are designed based on a pseudo ternary phase diagram. The largest compressive fracture plastic strain of 17.0% with the yielding strength of 1070 MPa is available for Zr_60.0Ti_14.7Nb_5.3Cu_5.6Ni_4.4Be_10.0 bulk metallic glass matrix composite. The relationship between the cooling rate and the microstructure, the microstructure and the mechanical properties, and the fractographs of the composites is carefully identified.     

Ultra-high strength Mg-Li based bulk metallic glasses: Preparation and performance research

In this paper, new Mg-Li based bulk metallic glasses (BMGs) are prepared by conventional copper mold injection casting method. The alloys exhibit excellent mechanical properties, such as ultra-high compressive fracture strength (maximal 729 MPa), high Vickers hardness (>2 GPa) and low elastic modulus (∼35 GPa). Compared with the corresponding crystal alloys, the density of the amorphous alloy samples is reduced by about 1.5% due to their free volume. Thus, it is believed that this new BMGs with these outstanding properties will broaden Mg-Li based alloys’ application fields.     

An investigation on the fracturing behavior of bulk metallic glass with large plasticity

A bulk metallic glass (BMG) with large plasticity was prepared and its fracturing behavior was observed at a strain rate larger than ∼ 10^− 3 s^− 1 under uniaxial compression. Even in this strain rate condition, the BMG still exhibits an excellent plastic deformability. The BMG exhibits a large elastic limitation of about 2.43% for engineering strain and 2.46% for true strain. The engineering and true plastic strains are 3.05% and 3.18%, respectively, and the maximum compressive strength is 1810 MPa. High dense shear bands appear in the outer surfaces of the failed BMG, of which the fracture surface exhibits melting and subsequently tearing-up signs with low vein height and small droplet, orientating significantly. The fracture angle is about 54°, deviating from the theoretical fracture angle by 9°. These may be related to the unique performance characteristics and the micro-structure of the BMG.     

Thermoelastic analysis of cracked woven GFRP laminates at cryogenic temperatures

We deal with the thermal-mechanical response of multi-layered G-11 woven glass/epoxy laminates with cracks and temperature-dependent properties under tension at liquid helium temperature (4 K). The composite material is assumed to be in generalized plane strain. Cracks are located in the fill fiber bundles and are assumed to span the width of the fill fiber bundles. Finite element model is used to study the influence of residual thermal stresses on the mechanical behavior of multi-layered woven laminates with cracks. Numerical calculations are carried out, and the Young's modulus and stress distributions near the crack tip are shown graphically.     

Tension behavior of metallic glass coating on Cu foil

The tensile behavior of the monolithic ZrCu thin film metallic glass and the ZrCu/Cu multilayer coating on pure Cu foil is systematically examined. The extracted tensile modulus and strength of the 1 μm films are in good agreement with the theoretical rule of mixture prediction. The extracted 2 μm film data are lower, but can be corrected back by considering the actual intact cross-sectional area during the tensile loading. The current results reveal that the ZrCu/Cu multilayer coating exhibit much better tensile performance than the monolithic ZrCu coating despite of the brittle deformation. To obtain ductile amorphous/crystalline multilayer coating, two principles are suggested in this paper.