Influence of cyclic loading on the onset of failure in a Zr-based bulk metallic glass

Samples of Zr_62.5Cu_22.5Fe₅Al₁₀ bulk metallic glass are subjected to uniaxial compression. Comparison of tests in monotonic loading and cyclic loading (repeated loading to the onset of plasticity, then unloading) shows that the compressive plasticity of the glass is drastically reduced under cyclic loading. It is argued that this effect arises from stress reversal accelerating the concentration of shear on a dominant shear band. The link with compression–compression fatigue results, and the consequences for low-cycle fatigue of metallic glasses are considered.     

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.     

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.     

Improved Room Temperature Plasticity of Zr41.2Ti13.8Cu12.5Ni10Be22.5 Bulk Metallic Glass by Channel‐Die Compression

In this work, the effect of channel‐die compression (CDC) on the mechanical behavior of the Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass is analyzed. The results indicate that CDC can be successfully used as a pre‐deformation process to effectively enhance the room temperature plastic strain ability of metallic glasses. The origin of the improved mechanical properties is most likely due to the creation during CDC of a heterogenous microstructure consisting of hard and soft regions able to hinder the rapid propagation of shear bands.     

Composition design and mechanical properties of ternary Cu–Zr–Ti bulk metallic glasses

The new compositions of ternary Cu–Zr–Ti bulk metallic glasses are predicted by integrating calculation of vacancy formation energy, mixing enthalpy and configuration entropy of the alloys based on thermodynamics of glass formers. The monolithic amorphous rods of 3 mm diameter have been successfully fabricated, and characterized by X-ray diffractometry, differential scanning calorimetry, scanning electronic microscopy, transmission electronic microscopy and compression tests. The results show that the designed alloys possess good glass forming ability and excellent mechanical properties. The mechanical properties of the samples can be effectively improved by regulating their composition. The monolithic amorphous rod of Cu₅₀Zr₄₄Ti₆ exhibits a high fracture strength of 1855 MPa and excellent plastic deformation up to ∼7.4%. The formation and propagation of shear bands in samples are also investigated. The enhancement of plastic deformation is mainly contributed to multiplication and intersection of shear bands.     

Depth dependent hardness variation in Ni–P amorphous film under nanoindentation

Abstract

The deformation behaviour and the depth dependent hardness variation in Ni–P amorphous alloy were investigated by nanoindentation. It was found that in addition to circular shear bands around the indent and on the indent surface, which have been previously observed, straight shear bands on the indent surface were also formed during nanoindentation. The indentation depth dependent hardness in the metallic glass is not modelled by the conventional dislocation based strain gradient theory for crystalline materials; instead it can be well described by a function of the inverse square root of the indentation depth. The structure evolution beneath the indentor is proposed to be the probable cause for the length scale dependent properties in metallic glass.     

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.     

Use of Nanoindentation,Finite Element Simulations,and a Combined Experimental/Numerical Approach to Characterize Elastic Moduli of Individual Porous Silica Particles

This article describes the use of a combination of experimental nanoindentation and finite element numerical simulations to indirectly determine the elastic modulus of individual porous, micron-sized silica (SiO₂) particles. Two independent nanoindentation experiments on individual silica particles were employed, one with a Berkovich pyramidal nanoindenter tip, the other with a flat punch nanoindenter tip. In both cases, 3D finite element simulations were used to generate nanoindenter load–displacement curves for comparison with the corresponding experimental data, using the elastic modulus of the particle as a curve-fitting parameter. The resulting indirectly determined modulus values from the two independent experiments were found to be in good agreement, and were considerably lower than the published values for bulk or particulate solid silica. The results are also consistent with previously reported modulus values for nanoindentation of porous thin film SiO₂. Based on a review of the literature, the authors believe that this is the first article to report on the use of nanoindentation and numerical simulations in a combined experimental/numerical approach to determine the elastic modulus of individual porous silica particles.     

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.     

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.     

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.     

New features of the low temperature ductile shear failure observed in bulk amorphous alloys

Fractographic studies of ductile shear failure under the uniaxial compression for rod–like samples of the Zr_41.2Ti_13.8Ni₁₀Cu_12.5Be_22.5 and Cu₅₀Zr₃₅Ti₈Hf₅Ni₂ bulk amorphous alloys at temperatures 300 and 77 K are presented. The mechanisms of shear deformation and failure appeared to have characteristics in common with other amorphous alloys prepared in the form of thin ribbons. However, there were a number of new fractographic features observed due to the bulk character of the samples and to the large supercooled liquid region of these alloys.     

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.     

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.     

Shear banding deformation in Cu/Ta nano-multilayers

Nanoscale Cu/Ta multilayers with individual layer thickness ranging from 3 to 70 nm were deformed under nanoindentation at room temperature. Shear bands can be observed only when individual layer thickness is reduced to 9 nm or below, indicating formation of shear bands in the Cu/Ta multilayers is layer thickness dependent. By observing the cross sectional transmission electron microscope images of the indentation fabricated through focused ion beam technique, shear banding deformation causing a unique layer-morphology with prevalent mismatched laminate structure has been reported for the first time. By capturing and analyzing a series of typical indentation-induced deformed microstructures, a new physical mechanism of shear banding behavior in metallic nano-multilayers is suggested.     

Hysteresity effects in 3 mol% yttria-doped zirconia (t'-phase)

Single-crystal and polycrystal samples of 3 mol % Y₂O₃-doped zirconia (t'-phase) were subjected to uniaxial compression tests at 1000°C in order to separate the effects of phase transformation (tetragonal-to-monoclinic) from ferroelastic domain switching. Plastic deformation was observed after an elastic regime, with attributes characteristic of domain switching. X-ray diffraction traces at room and high temperatures before and after the compression test verified that there was, indeed, a variant reorientation within each sample. Deformation bands were observed on single crystals, and Raman spectroscopy revealed that no monoclinic phase was present. These results verify the existence of ferroelastic domain switching phenomenon in this material.     

A simple method for measuring tensile and shear bond strength of ceramic-ceramic and metal-ceramic joining

A simple method for measuring the ceramic-ceramic and metal-ceramic bond strength was presented, by which uniaxial tensile stress normal to the interface or shear stress in the interface can be produced using uniaxial compression load on a cross-bonded sample. Both tensile and shear bond strength were obtained by this testing technique for Ti₃SiC₂–TiO₂ and Ti₃SiC₂–Al₂O₃ composite as well as for glued steel samples, respectively. The novel method provided a solution for determining bond strength in solid (especially brittle) materials, and it is also demonstrated as a useful method for evaluating the tensile and shear strength of various glues. Electronic Publication     

Strain- and strain-rate-dependent mechanical properties and behaviors of Cu<Subscript>3</Subscript>Sn compound using molecular dynamics simulation

This work aims at investigating the mechanical properties and behaviors of orthorhombic Cu₃Sn crystals at room temperature through molecular dynamics (MD) simulation. The focuses are placed on the tensile stress–strain behaviors and properties of the Cu₃Sn single crystal and also their dependence on applied strain and strain rate. An attempt to characterize the deformation evolution of the Cu₃Sn nanostructure during the stress–strain test is also made. In addition, the elastic properties of bulk polycrystalline Cu₃Sn are estimated, as a function of strain rate and applied strain, by using the monocrystal results. The effectiveness of the MD model is demonstrated through comparison with the nanoindentation results and also published theoretical and experimental data. The calculated orthotropic elastic and shear moduli and Poisson’s ratio of Cu₃Sn single crystal reveal not only high anisotropy, but also the great effects of applied strain and strain rate only as the strain rate exceeds a threshold value of about 0.072% ps⁻¹. Specifically, raising the strain rate increases the orthotropic elastic properties and also the ultimate tensile and shear strengths of the nanocrystal, whereas increasing the applied strain reduces them.     

Multiresolution atomistic simulations of dynamic fracture in nanostructured ceramics and glasses

Multimillion atom molecular-dynamics (MD) simulations are performed to investigate dynamic fracture in glasses and nanostructured ceramics. Using multiresolution algorithms, simulations are carried out for up to 70 ps on massively parallel computers. MD results in amorphous silica (a-SiO₂) reveal the formation of nanoscale cavities ahead of the crack tip. With an increase in applied strain, these cavities grow and coalesce and their coalescence with the advancing crack causes fracture in the system. Recent AFM studies of glasses confirm this behavior. The MD value for the critical stress intensity factor of a-SiO₂ is in good agreement with experiments. Molecular dynamics simulations are also performed for nanostructured silicon nitride (n-Si₃N₄). Structural correlations in n-Si₃N₄ reveal that interfacial regions between nanoparticles are amorphous. Under an external strain, nanoscale cavities nucleate and grow in interfacial regions while the crack meanders through these regions. The fracture toughness of n-Si₃N₄ is found to be six times larger than that of crystalline -Si₃N₄. We also investigate the morphology of fracture surfaces. MD results reveal that fracture surfaces of n-Si₃N₄ are characterized by roughness exponents 0.58 below and 0.84 above a certain crossover length, which is of the order of the size of Si₃N₄ nanoparticles. Experiments on a variety of materials reveal this behavior. The final set of simulations deals with the interaction of water with a crack in strained silicon. These simulations couple MD with a quantum-mechanical (QM) method based on the density functional theory (DFT) so that chemical processes are included. For stress intensity factor K=0.4 MPa m^1/2, we find that a decomposed water molecule becomes attached to dangling bonds at the crack or forms a Si-O-Si structure. At K=0.5 MPa m^1/2, water molecules decompose to oxidize Si or break Si-Si bonds.