Mechanical properties of bulk metallic glasses

The mechanical properties of bulk metallic glasses, including their superior strength and hardness, and excellent corrosion and wear resistance, combined with their general inability to undergo homogeneous plastic deformation have been a subject of fascination for scientists and engineers. The scientific interest stems from the unconventional deformation and failure initiation mechanisms in this class of materials in which the typical carriers of plastic flow (dislocations) are absent. Metallic glasses undergo highly localized, heterogeneous deformation by formation of shear bands, a particular mode of deformation of interest for certain applications, but which also causes them to fail catastrophically due to uninhibited shear band propagation. Varying degrees of brittle and plastic failure creating intricate fracture patterns are observed in metallic glasses, quite different from those observed in crystalline solids. The tension–compression anisotropy, strain-rate sensitivity, thermal stability, stress-induced crystallization and polyamorphism transformations, are some of the attributes that have sparked engineering studies on bulk metallic glasses. Understanding of the glass-forming ability and the deformation and failure mechanisms of bulk metallic glasses, has given insight into alloy compositions and intrinsically-forming or extrinsically-added reinforcement phases for creating composite structures, to attain the combination of high strength, tensile ductility, and fracture toughness needed for use in advanced structural applications. The relative ease of fabricating metallic glasses into bulk forms, combined with their unique mechanical properties, has made these materials attractive options for possible applications in aerospace, naval, sports equipment, luxury goods, armor and anti-armor systems, electronic packaging, and biomedical devices.     

Defects activation in CoFe-based metallic glasses during creep deformation

Maxwell-Voigt model with two Kelvin units and one Maxwell unit was utilized to analyze the microalloying effect of Cu on the creep behavior of CoFe-based metallic glasses at different loading rates. The defect activation during creep deformation was detected by the relaxation time spectrum based on this model. The defect, with respect to a short relaxation time in relaxation spectra, intends to be activated at a quasi-static loading mode in the alloy with 0.5 at.% Cu addition. With further increasing loading rates,more defects with a large size were provoked activated at both hard and soft regions in the Cu-containing sample. A softening with the reduction of elastic modulus and hardness about 10 % and 15 %, respectively,was also observed in the Cu-doped sample. It is consistent with the pronounced viscoplastic deformation of this alloy along with the decrease of viscosity. Our work provides a microscopic insight into structural evolution during creep deformation in a Cu-doped metallic glass, which might help for understanding the plastic deformation of metallic glasses upon a minor addition.     

Explosive Welding of metallic glasses onto metals

Different metallic glasses of 40–45 μm thickness were explosively cladded onto metals. The explosive data (amount of explosive and detonation velocity) were changed systematically in a wide range in order to obtain different collision parameters. It was found, that under conditions of explosive welding metallic glasses undergo plastic deformation in different manners. Up to a collision point velocity of v_k = 2300 m/s metglas reveals only a shear band deformation mode, leading to many fractures at the plane interphase between metallic glass and metal. At higher velocities a homogeneous type of deformation is observed. The interphase is of wavy nature then.     

Comparison of compressive deformation and fracture behaviors of Zr- and Ti-based metallic glasses with notches

Compressive tests on the Zr- and Ti-based metallic glasses with different notches were investigated to compare their shear fracture mechanism and plastic deformation abilities. It is found that the plasticity of the two metallic glasses can be improved by installing two semicircular symmetrical notches even for the Ti-based metallic glass which has nearly zero compressive plasticity. The enhanced plasticity may be ascribed to the easy initiation of shear bands (SBs) around the notches, and the consequent blocking effect of notches on the propagation of shear bands according to the large-scale stress gradient. Additionally, based on a theoretical model originated from the concept of critical steady shear displacement (CSSD), compared with the sizes of smooth regions on the fracture surface, the plasticity difference of the two different metallic glasses was analyzed quantitatively. The current findings might provide an approach to understand and estimate the difference in the plastic deformation abilities on diverse metallic glasses, as well as the ones with large-scale stress gradient.     

Inverse ductile–brittle transition in metallic glasses?

There is evidence that metallic glasses can show increased plasticity as the temperature is lowered. This behaviour is the opposite to what would be expected from phenomena such as the ductile–brittle transition in conventional alloys. Data collected for the plasticity of different metallic–glass compositions tested at room temperature and below, and at strain rates from rate 10^?5 to 10³ s^?1, are reviewed. The analogous effects of low temperature and high strain rate, as observed in conventional alloys, are examined for metallic glasses. The relevant plastic flow in metallic glasses is inhomogeneous, sharply localised in thin shear bands. The enhanced plasticity at lower temperature is attributed principally to a transition from shear on a single dominant band to shear on multiple bands. The origins of this transition and its links to shear bands operating ‘hot’ or ‘cold’ are explored. The stress drop on a shear band after initial yielding is found to be a useful parameter for analysing mechanical behaviour. Schematic failure mode maps are proposed for metallic glasses under compression and tension. Outstanding issues are identified, and design rules are considered for metallic glasses of improved plasticity.     

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.     

Atomistic simulation and modeling of localized shear deformation in metallic glasses

Since the end of 1980s, bulk metallic glasses became available for various multi-component alloys. Because bulk metallic glasses are applicable to structural materials, their mechanical properties have become a matter of great interest in these decades. A characteristic feature of plastic deformation of metallic glasses at the ambient temperature is the localized shear deformation. Since we have no appropriate experimental technique, unlike crystalline matter, to approach microscopic deformation process in amorphous materials, we have to rely on computer simulation studies by use of atomistic models to reveal the microscopic deformation processes. In this article, we review atomistic simulation studies of deformation processes in metallic glasses, i.e., local shear transformation (LST), structural characterization of the local shear transformation zones (STZs), deformation-induced softening, shear band formation and its development, by use of elemental and metal-metal alloy models. We also review representative microscopic models so far proposed for the deformation mechanism: early dislocation model, Spaepen’s free-volume model, Argons’s STZ model and recent two-state STZ models by Langer et al.     

Formation of solute atmospheres around dislocations by excess vacancies

Long range solute atmospheres tend to form around dislocations by diffusion. They scatter phonons and can be studied through their effect on the lattice thermal conductivity. Previous studies indicate that these atmospheres are formed after plastic deformation in Cu-Al but not in Cu-Ge alloys. Ordinary diffusion at room temperature is too slow to permit atmosphere formation, but excess vacancies originally produced during plastic deformation can enhance diffusion. The vacancies diffuse towards the dislocation, where they are annihilated, and their presence causes solute diffusion and hence atmosphere formation. The atmospheres attain equilibrium over a range R, and R² can be expressed in terms of a time-integrated diffusion coefficient due to excess vacancies until the excess is exhausted. This range R is independent of the vacancy jump rate, and depends only on the initial vacancy concentration and the dislocation density. The values of R thus calculated are much smaller than those observed. However, R is increased if there is enhancement of the solute diffusion relative to the solvent diffusion and also if dislocations do not act as perfect vacancy sinks. Estimates of R are given for Cu-Al, Cu-Ge and Al-Mg.     

Plastic deformation behaviors of Ni- and Zr-based bulk metallic glasses subjected to nanoindentation

Plastic deformation behaviors of Ni₄₂Ti₂₀Zr_21.5Al₈Cu₅Si_3.5 and Zr₅₁Ti₅Ni₁₀Cu₂₅Al₉ bulk metallic glasses at room temperature were studied by nanoindentation testing and atomic force microscopy under equivalent indentation experimental conditions. The different chemical composition of these two bulk metallic glasses produced variant tendencies for displacement serrated flow to occur during the loading process. The nanoindentation strain rate was calculated as a function of indentation displacement in order to verify the occurrence of displacement serrated flow at different loading rates. Atomic force microscopy revealed decreasing numbers of discrete shear bands around the indentation sites as loading rates increased from 0.025 to 2.5 mNs^− 1. Variations in plastic deformation behaviors between Ni and Zr-based glasses materials can be explained by the different metastable microstructures and thermal stabilities of the two materials. The mechanism governing plastic deformation of these metallic glasses was analyzed in terms of an established model of the shear transformation zone.     

The structure and mechanical properties of bulk Zr50Ti16.5Cu15Ni18.5 metallic glasses

The structure and mechanical properties of bulk Zr₅₀Ti_16.5Cu₁₅Ni_18.5 metallic glasses were studied by X-ray diffraction, scanning and transmission electron microscopy and by measurements of mechanical properties. The transition from heterogeneous to homogeneous deformation was found to occur at 575 K. The crystallization was found to begin in the testing part of the sample earlier than in the part placed in a holder during the mechanical testing. The crystallization leads to the formation of the quasicrystalline phase with a ₀ = 2.54 Å, hexagonal phase ZrTi(Ni,Cu) and hexagonal phase (Zr₆CoAl₂ type). The difference of the structure in different parts of the sample has been observed and analyzed. The deformation does not effect on the crystallization processes occurring during the tensile testing at this temperature and the crystallization proceeds owing to the self-heating of the samples during the phase transition. The correlation between mechanical properties, structure and fracture surfaces is discussed.     

Structural relaxations in Fe-Co-Si-B amorphous alloys

The structural relaxation involved in the thermal treatment of Fe-Co-Si-B metallic glasses and the crystallization process as a function of Co content have been investigated. Magnetic measurements and the Mössbauer temperature scanning method for measuring the Curie temperature were used. One stage of structural relaxation above 570 K in metallic glasses with composition Fe_78–x Co _x Si₉B₁₃ was observed. This stage is characterized by a change of the chemical short-range ordering. The crystallization process detected by the magnetic measurements proceeds in two and in some cases in three steps.     

Plastic deformation of Zr-based bulk metallic glasses under nanoindentation

The plastic deformation of two Zr-based bulk metallic glasses (BMGs), Zr₅₅Cu₃₀Ni₅Al₁₀ and Zr_69.5Cu₁₂Ni₁₁Al_7.5, was investigated by instrumented nanoindentation over a broad range of loading rates. It was found that the plastic flow of the two BMGs exhibited conspicuous serrations at low loading rates. The serrations, however, became less prominent as the rate of indentation increased. AFM showed a significant pile-up of materials around the indents, indicating that a highly localized plastic deformation occurred under nanoindentation. The mechanism governing the plastic deformation in BMGs was tentatively discussed in terms of the mode of strain-induced free volume.     

Kinetics of crystallization of a Fe-based multicomponent amorphous alloy

The Fe-based multicomponent amorphous alloys (also referred to as metallic glasses) are known to exhibit soft magnetic properties and, it makes them important for many technological applications. However, metallic glasses are in a thermodynamically metastable state and in case of high temperature operating conditions, the thermally activated crystallization would be detrimental to their magnetic properties. The study of crystallization kinetics of metallic glasses gives useful insight about its thermal stability. In the present work, crystallization study of Fe₆₇Co₁₈B₁₄Si₁ (2605CO) metallic glass has been carried out using differential scanning calorimetry (DSC) technique. Mössbauer study has also been undertaken to know the phases formed during the crystallization process. The alloy shows two-stage crystallization. The activation energy has been derived using the Kissinger method. It is found to be equal to 220 kJ/mol and 349 kJ/mol for the first and second crystallization peaks, respectively. The Mössbauer study indicates the formation of α-(Fe, Co) and (Fe, Co)₃B phases in the alloy.     

Fabrication of Cu rich bulk metallic glass composites via solidification method

Abstract

Cu based bulk metallic glasses and composites with tiny crystalline phases embedded in metallic glass matrix have been successfully fabricated by solidification technique in the present work. The formation of crystalline phases and structure inhomogeneity in bulk metallic glasses was characterised. Al is used as the minor alloying element to partly substitute Cu element in 61Cu–34Zr–5Ti. The results show that quarternary 60Cu–34Zr–5Ti–1Al alloy exhibits monoamorphous feature, and 56Cu–34Zr–5Ti–5Al alloy has a few crystalline peaks superimposed on a broad diffraction peak, suggesting that a composite structure forms in certain solidification conditions. To further identify the microstructure of the as cast rod, all samples were characterised by scanning electron microscopy (SEM). Small size phases are found in 2 mm diameter 56Cu–34Zr–5Ti–5Al rod, which has larger plastic deformation. The composition of those crystalline phases is also investigated. All results indicate that the presence of certain phases in metallic matrix benefits the mechanical properties of the as cast bulk metallic glasses.     

Tensile ductility and necking of metallic glass

Metallic glasses have a very high strength, hardness and elastic limit. However, they rarely show tensile ductility at room temperature and are considered quasi-brittle materials. Although these amorphous metals are capable of shear flow, severe plastic instability sets in at the onset of plastic deformation, which seems to be exclusively localized in extremely narrow shear bands approximately 10 nm in thickness. Using in situ tensile tests in a transmission electron microscope, we demonstrate radically different deformation behaviour for monolithic metallic-glass samples with dimensions of the order of 100 nm. Large tensile ductility in the range of 23-45% was observed, including significant uniform elongation and extensive necking or stable growth of the shear offset. This large plasticity in small-volume metallic-glass samples did not result from the branching/deflection of shear bands or nanocrystallization. These observations suggest that metallic glasses can plastically deform in a manner similar to their crystalline counterparts, via homogeneous and inhomogeneous flow without catastrophic failure. The sample-size effect discovered has implications for the application of metallic glasses in thin films and micro-devices, as well as for understanding the fundamental mechanical response of amorphous metals.     

First-principles study of 4d solute diffusion in nickel

Diffusion of the 4d transition elements in Ni has been investigated within the five-frequency model framework using migration energy barriers calculated from the first principles. Agreement with counterintuitive experimental/calculated data is observed; atoms in the middle of 4d row have the smallest atomic radii while exhibiting the lowest diffusivity as compared to larger atoms at the beginning and the end of 4d row. We show that 4d solute diffusion is controlled mainly by the size misfit. The larger atoms have higher solute–vacancy binding energies and lower migration barriers. Both were shown to correlate with a displacement of the equilibrium solute position toward the adjacent vacancy. The difference in mechanisms controlling sp- and transition elements diffusion rates in Ni is discussed.     

Radiotracer investigation of diffusion, segregation and wetting phenomena in grain boundaries

The combination of so-called B- and C-type solute grain boundary (GB) diffusion measurements is a unique and reliable tool for the determination of the solute segregation factor for the true dilute limit conditions. A series of such measurements with different solutes in the same high purity Cu material gained comprehensive quantitative information on the solute segregation behaviour. Specially designed radiotracer experiments on bicrystals supplied valuable information on non-linear solute segregation and even a complete solute segregation isotherm could be determined. The strong solute segregation may invoke new effects. The GB wetting transition in the Bi–Cu system was investigated by radiotracer diffusion. A pronounced increase of GB diffusivity was observed in the two-phase (solid + liquid) region of the corresponding phase diagram, with the GB diffusivity of Cu being similar to the diffusion rate in a liquid. Such a GB diffusion enhancement exists even in the single phase (solid solution in Cu) region manifesting the existence of a pre-wetting GB phase transition in this system.     

Extrusion properties of a Zr-based bulk metallic glass

The extrusion behavior of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 metallic glasses in the supercooled liquid region was investigated. Good extrusion formability was observed under low strain rates at temperatures higher than 395 °C. The metallic glasses were fully extruded without crystallization and failure within the range of T = 395-415 °C under strain rates from 5 × 10^− 3 s^− 1 to 5 × 10^− 2 s^− 1, and the deformation behavior of the metallic glasses during the extrusion was found to be in a Newtonian viscous flow mode by a strain rate sensitivity of 1.0.     

Variability of Poisson's Ratio and Enhanced Ductility in Amorphous Metal

Ductile bulk metallic glass of composition 53.0Zr–18.7Cu–12.0Ni–16.3Al (at%) is plastically deformed under uniaxial compression and observed in situ by synchrotron high‐energy X‐ray diffraction. The diffraction patterns reveal the induced atomic strain is orientation dependent. At the onset of plastic deformation, the atomic strain in the compression direction saturates to a close‐nearest‐neighbor distance while atoms relax in the transverse direction. The ever increasing transverse atomic strain expresses in an augmentation of the apparent Poisson's ratio up to ν = 0.5, which is consistent with volume conservation. Contradicting phenomena from linear mechanics, such as the non‐vanishing shear modulus at ν = 0.5 can be explained by the non‐affine character of the deformation, giving rise to characteristics of a localized martensitic phase transformation. The findings explain the often‐reported phenomena such as, the high Poisson's ratio values found in metallic glasses, the partially liquid character of the structure, the free volume increase and the Bauschinger effect.     

Studies of crystallization kinetics of Fe40Ni40P14B6 and Fe80B20 metallic glasses under non-isothermal conditions

A model for the glass crystallization at constant rate heating is presented. Based on the model a technique for determination of the constants involved in the classical equations for the rates of homogeneous nucleation and linear crystal growth is derived. The effect of the heating rate (in the wide range from 2×10-2 to 16 K s-1) on the temperature of crystallization as well as on the average grain size in fully crystallized specimens of Fe40Ni40P14B6 and Fe80B20 metallic glasses has been studied. The values of the interface diffusion coefficient, the rates of nucleation and growth and the volume density of quenched-in nuclei deduced in the present study are in good agreement with those derived from direct observations. It has been confirmed that crystallization of Fe80B20 occurs mainly by the three-dimensional growth of the pre-existing crystallites while the Avrami exponent for the Fe40Ni40P14B6 glass exceeds 4 implying non-steady-state nucleation. It has been demonstrated that the proposed model allows one to generalize the isothermal and non-isothermal kinetic crystallization curves. This revised version was published online in November 2006 with corrections to the Cover Date.