Shear band evolution during large plastic deformation of brittle and ductile metallic glasses

Four monolithic metallic glasses (MMGs) with different plasticities varying from brittle to ductile behavior under unconstrained loading were subjected to small punch testing. All specimens undergo large plastic deformation with multiple cobweb-like shear bands under these conditions. The process of shear band evolution was carefully controlled and investigated. Plasticity of MMGs is characterized by equivalent plastic strain ε* (product of shear band density and critical shear offset). Thus, this article provides an experimental basis for a better understanding of the shear band evolution during plastic deformation of MMGs.     

Structural relaxation and rejuvenation in a metallic glass induced by shot-peening

The metallic glass Pd₄₀Cu₃₀Ni₁₀P₂₀ in as-cast or pre-annealed states was shot-peened successively at room temperature or at 77 K. The structural state of the glass was characterized by the relaxation spectrum measured in a differential scanning calorimeter. Mechanically induced relaxation of the as-cast glass and mechanically induced rejuvenation of pre-annealed samples are both more evident at 77 K than at 298 K, enabling deductions about the underlying mechanisms. The relaxation spectrum of the glass as a function of temperature displays two broad maxima, which occurring at the higher temperature is attributed to the part of the free-volume distribution associated with flow defects. In samples shot-peened at 77 K, the stored energy after deformation can be as high as 20% of the cold work. Shot-peening simultaneously generates flow defects within shear bands and destroys them in the matrix between bands: whose effect dominates is principally dependent on the initial state of relaxation of the glass. Shot-peening of partially crystallized samples appears capable of breaking up and dispersing crystallites without inducing any further significant crystallization.     

Intrinsic factor controlling the deformation and ductile-to-brittle transition of metallic glasses

The energetic driving force and resistance for shearing and cracking in metallic glasses (MGs) are quantitatively evaluated. A universal thermodynamic criterion is proposed for better understanding the intrinsic correlations between fracture toughness and Poisson’s ratio, the competitions between various deformation modes and the ductile-to-brittle transition in MGs and other materials. A new cooperation parameter δ is also introduced to depict quantitatively the relative propensity of shearing versus cracking. This work could provide insights into the long-standing issues of deformation mechanisms of glassy materials, and be helpful in searching for ductile and tough MGs.     

Effect of sample size on ductility of metallic glass

With decreasing sample dimension, the compressive plastic strain of a Zr-based metallic glass increases from near zero to as high as 80% without failure. This indicates that macroscopically brittle or ductile deformation behaviour can occur in chemically and structurally identical metallic glass. A concept of critical shear offset is proposed to explain the strong size effect on the enhanced plasticity of metallic glass by taking the shear fracture energy density into account. This finding provides new understanding on the principle that for metallic glass ‘smaller is more ductile’, even on the macroscopic scale.     

Effect of strain rate on the compressive behaviour of a Zr56Al10.9Ni4.6Cu27.8Nb0.7 bulk metallic glass

We report the study of the effect of strain rate on the compressive behaviour of a Zr₅₆Al_10.9Ni_4.6Cu_27.8Nb_0.7 bulk metallic glass. The results indicated that both the strength and plasticity of the glass increase with increasing the strain rate up to 10^?5 s^?1, above which the strength and plasticity start to decrease. The enhanced mechanical properties under a strain rate of 10^?5 s^?1 are due to the emission/propagation rate of the shear bands being consistent with the strain rate.     

On the plasticity event in metallic glass

Based on a systematic molecular dynamics analysis, this study reveals that plastic deformation of metallic glass is not through a uniform configuration change but via many localized plasticity events. These events are manifested by the atomic clusters of high kinetic energy and high strain rate, emerging even in the elastic deformation regime. The life of such a plasticity event is on the order of 10^?12?s, during which the distribution of kinetic energy follows a power law. The study shows that yielding in metallic glass occurs at the sudden surge point of the number of plasticity events. In the steady plastic deformation regime, the continuous nucleation and annihilation of the plastic events lead to a steady flow stress and stabilized total potential energy.     

Electromechanical behaviour of nanoporous metallic glass

Understanding the electrical and mechanical behaviour of nanoporous materials is critical for their use in energy applications. A palladium-rich nanoporous film, 500 nm thick with pore size ranging from 10 to 50 nm, was obtained by electrochemical dealloying of a Ni–Pd–P–B metallic glass. Nanomechanical and electrical properties were measured simultaneously, as a function of depth, for the nanoporous structure as well as the unaltered metallic glass substrate. The elastic modulus for the nanoporous structure was found to be 22 GPa compared to 131 GPa for the metallic glass substrate. The ratio of moduli scales with the square of the relative density in agreement with linear elasticity models for cellular materials. The electrical resistivity of the nanoporous layer was found to be 2.2 times higher compared to the metallic glass substrate, which was attributed to the tortuosity of current path in cellular structures.     

Nanocrystallization induced by quasi-static fracture of metallic glasses at room temperature

Nanoparticles on the fracture surfaces of Co- and Fe-based metallic glasses during quasi-static compression at room temperature have been observed using a high-resolution scanning electron microscope. In terms of the differential scanning calorimeter, those nanoparticles were identified to be a result of nanocrystallization induced by the rapid fracture. Finally, the nanocrystallization behavior was evaluated by taking into account the super-high crack propagation rate and high elastic energy, which contributed to the local temperature rise up to the onset of crystallization, T _x.     

The kinetics of infralow-frequency viscoelastic internal friction induced by irreversible structural relaxation of a metallic glass

This letter presents the results of internal-friction measurements of Co 70 Fe 5 Si 15 B10 metallic glass at temperatures 400K h T h 700K and frequencies 0.01 Hz h f h 0.05Hz. It is shown that Maxwellian viscoelastic damping is predominant in this case. The nature of this damping is determined by irreversible structural relaxation. The kinetic relaxation law is derived.     

Plastic stability of bimetals composed of Ti-based metallic glass composite and pure titanium

The plastic stability of bimetals composed of a Ti-based metallic glass composite (MGC) and a pure titanium (PT) under tension has been investigated. The results reveal that the plastic stability of the MGC/PT bimetal is strongly dependent on the thickness fraction of the MGC: when the thickness fraction of the MGC decreases to a critical value, a homogeneous plastic deformation occurs in the MGC/PT bimetals, i.e. the strain localization and necking of the MGC is effectively suppressed. This unique phenomenon is explained using the normalized strain-hardening rate, and the quantitative relationship between the normalized strain-hardening rate of the MGC/PT bimetals and the thickness fraction of the MGC is thereby established.     

On the glass transition temperature and the elastic properties in Zr-based bulk metallic glasses

The temperature dependence of the elastic moduli was estimated from ultrasound time of flight measurements performed on bulk metallic glasses of composition Zr_{63? x} Cu₂₄Al _x Ni₁₀Co₃. Using the corresponding values at the glass transition temperature, the local atomic strain was determined. The obtained values for the critical atomic strain calculated for 8 at%?<?x?<?15 at% are close to the predicted universal criterion derived from a topological model, but may also reflect the difference in the chemical interaction that are not accounted by a topological approach.     

Equivalent and effective strains during severe plastic deformation (SPD)

ABSTRACT

This paper considers the characteristics of severe plastic deformation (SPD) to estimate its efficacy and to compare different processing techniques. In contrast to effective strains by von Mises and Hencky, the rotation component of the strain rate is included in the analysis as a mode of deformation, which ranges from simple shear to pure shear. Distortions of material elements during a uniform plane plastic flow are calculated using a kinematic approach. For the fixed deformation mode, the current state is defined by the accumulated shears that are identical to the von Mises effective shear strains. In specific cases of pure shear and simple shear, the accumulated shears match the specific distortions of round or square elements prescribed by Hencky or von Mises approach, respectively. The mode of deformation is important to the structural effects of SPD. In general, two separated characteristics, accumulated shear and the coefficient of deformation mode, are necessary to describe strains during SPD.     

Low-temperature mechanical properties of Ce68Al10Cu20Co2 bulk metallic glass

Although it is well known that traditional metals and alloys will become brittle at low temperatures, the effect of low temperature on the mechanical properties of bulk metallic glasses (BMGs) is yet to be fully understood. In this research, the mechanical properties of Ce₆₈Al₁₀Cu₂₀Co₂ BMG are investigated at low temperatures. It is found that the yield strength of the Ce-based BMG increases significantly with the decrease of temperature. The elastic moduli of the BMG also increase monotonically with the drop of temperature, indicating the continuous stiffening of the BMG, while both Poisson's ratio and global plasticity decline at low temperatures. It is considered that the stiffer atomic bonds of the Ce-based BMG at low temperatures result in the increase of strength, and the higher energy required for nucleation of shear bands also leads to the increase in yield strength.     

Changes of hardness and electronic work function of Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass on annealing

The hardness and electronic work function (EWF) of a bulk metallic glass, namely Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5, have been studied experimentally, with an emphasis on the effect of heat treatments. The glass was annealed at different time and temperatures, and its hardness and EWF measured using the Rockwell indentation technique and a scanning Kelvin probe system, respectively. It is found that the EWF decreases with annealing time and temperature, whereas the hardness increases. This study shows a close relationship between hardness and EWF, indicating that the EWF could be a sensitive parameter for characterising and investigating the mechanical behaviour of BMG at the electronic level.     

Deformation twins induced by multi-mode deformation in nanocrystalline copper

A multi-mode deformation model is used in a molecular dynamics simulation of nanocrystalline copper. Abundant deformation twin lamellae are developed by shearing the following compression to the elastic limit. Deformation twins (DTs) nucleate through two different mechanisms facilitated by Shockley partial slips. Interactions between DTs and Shockley partials are observed in this simulation.     

HRTEM observation of misfit dislocations at non-faceted Al–Pb interfaces

Al–Pb samples containing 1?at.%?Pb were produced by high-energy ball milling and investigated by high-resolution transmission electron microscopy. The Pb inclusions embedded in an Al matrix exhibit a non-faceted, curved morphology. Dislocations have been found to accommodate the misfit of about 22% at the Al–Pb interfaces. Burgers circuits, drawn around these misfit dislocations, exhibit closing failures of the type a₀ /4〈211〉.     

Microwave-induced heating of a single glassy phase and a two-phase material consisting of a metallic glass and Fe powder

Metallic glasses exhibit low viscosity in a temperature range between the glass transition and the crystallization temperature, a feature that allows successful sintering of glassy powders. Microwave heating, being volumetric, has significant advantages over conventional heating in materials processing, such as substantial energy savings, high heating rates and process cleanliness. In the present study, we investigate the stability of Cu₅₀Zr₄₅Al₅ glassy powders and the formation of a bulk two-component metallic glassy-crystal sample by microwave heating in a single-mode cavity (915 MHz) in an alternating magnetic field.     

Understanding the shear band interaction in metallic glass

Qualitative and quantitative models were proposed to understand the shear band (SB) interaction scenario found in the compressive tests on specimen with two symmetrical semi-circular notches. The so-called ‘work-hardening’ behavior could be ascribed as the stress interaction which was caused by stress fields around the SB tips. Besides, the SB bending was observed along propagation orientation. The quantitative analysis based on traditional shear deformation mechanism could reasonably account for how the SB was bent. It is anticipated that the present work could provide a pathway to understand the deep SB deformation mechanism of metallic glass.     

A structural model for surface-enhanced stabilization in some metallic glass formers

A structural model for surface-enhanced stabilization in some metallic glass formers is proposed. In this model, the alloy surface structure is represented by five-layer Kagomé-net-based lateral ordering. Such surface structure has intrinsic abilities to stabilize icosahedral-like short-range order in the bulk, acting as ‘a cloak of liquidity’. In particular, recent experimental observations of surface-induced lateral ordering and a very high glass forming ability of the liquid alloy Au₄₉Ag_5.5Pd_2.3Cu_26.9Si_16.3 can be united using this structural model. This model may be useful for the interpretation of surface structure of other liquid alloys with a high glass forming ability. In addition, it suggests the possibility of guiding the design of the surface coating of solid containers for the stabilization of undercooled liquids.     

Strain-induced submicrocrystalline grains developed in austenitic stainless steel under severe warm deformation

Strain-induced grain evolution in a 304 type austenitic stainless steel has been studied in multiple compression with the loading direction being changed in each pass. The tests were carried out to total strains above 6 at 873 K (0.5 T _m) at a strain rate of about 10^-3 s^-1. Multiple deformation promotes the rapid formation of many mutually crossing subboundaries because various slip systems operate from pass to pass. The gradual rise in misorientations across dislocation subboundaries with increasing strain finally leads to the evolution of very fine grains with large-angle boundaries. It is concluded that a new grained structure can result from a kind of continuous reaction during deformation, namely continuous dynamic recrystallization. Such deformation-induced grains are characterized by relatively low densities of dislocations, and considerable lattice curvatures developed in their interiors. The latter observations suggest that high elastic distortions are developed in the grain interiors and so such strain-induced grain structures are in a non-equilibrium state.