Electrochemical behavior of a Ti-based bulk metallic glass

In this study, potentiodynamic experiments were conducted with a Ti-based BMG alloy with a nominal composition of Ti_43.3Zr_21.7Ni_7.5Be_27.5 [atomic percent (at.%)], commonly known as LM-010. Electrochemical characterization was performed in a phosphate-buffered saline (PBS) electrolyte at 37 °C with a physiologically relevant dissolved oxygen content. This BMG exhibited passive behavior at the open-circuit potential with a low mean corrosion-penetration rate. A susceptibility to localized corrosion was observed but is not a concern at the open-circuit potentials. The resistance of the LM-010 alloy to localized corrosion was statistically equivalent to, or better than, all of the BMG materials and the 316L stainless steel for which direct statistical comparisons were possible. Microscopic examination revealed that the samples predominantly exhibited many scattered, small pits (diameter ⩽100 μm) in addition to several larger pits. Based upon the pit morphology and comparisons with the literature, it appears that localized corrosion initiated at clusters of inhomogeneities within the amorphous matrix.     

Electron microscopy of bulk metallic glass machining chips

Machined Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ bulk metallic glass (BMG) chips were characterized using high resolution electron microscopy. Compared to conventional processing techniques, machining produces very high heating/cooling, and deformation rates. It is therefore of interest to compare structural changes in machining chips with those produced by conventional processing. Large (～1 μm) crystalline grain, residual amorphous region, and phase separation in the amorphous–crystalline transition region were detected in bright field TEM images. Three equilibrium phases, Zr₂Cu, ZrAl₂, and Zr₂Ni, which have been identified in samples undergoing conventional annealing, were revealed from selected area electron diffraction patterns of the chips. High magnification TEM micrographs showed nanocrystallites, about 10 nm in size, in the amorphous–crystalline transition region.     

Modeling and measurement of residual stresses in a bulk metallic glass plate

The recent advent of multi-component alloys with exceptional glass forming ability has allowed the processing of large metallic specimens with amorphous structure. The possibility of formation of thermal tempering stresses during the processing of these bulk metallic glass (BMG) specimens was investigated using two models: (i) instant freezing model, and (ii) viscoelastic model. The first one assumed a sudden transition between liquid and elastic solid at the glass transition temperature. The second model considered the equilibrium viscosity of BMG. Both models yielded similar results although from vastly different approaches. It was shown that convective cooling of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 plates with high heat transfer coefficients could potentially generate significant compressive stresses on the surfaces balanced with mid-plane tension. The crack compliance (slitting) method was then employed to measure the stress profiles in a BMG plate that was cast in a copper mold. These profiles were roughly parabolic suggesting that thermal tempering was indeed the dominant residual stress generation mechanism. However, the magnitude of the measured stresses (with peak values of only about 1.5% of the yield strength) was significantly lower than the modeling predictions. Possible reasons for this discrepancy are described in relation to the actual casting process and material properties. The extremely low residual stresses measured in these BMG specimens, combined with their high strength and toughness, serve to further increase the advantages of BMGs over their crystalline metal counterparts.     

Superplastic flow of a Mg-based bulk metallic glass in the supercooled liquid region

The tensile flow behavior of Mg₆₅Cu₂₅Y₁₀ bulk metallic glass was investigated over a range of strain rates (10⁻³–10⁻¹/s) and deformation temperatures (150–170 °C) in the supercooled liquid region. In this region, the relationship between peak flow stress, strain rate and absolute deformation temperature was described adequately by the classic Sellars–Tegart constitutive relationship. There was also a good correlation between the Zener–Hollomon parameter, Z, and the flow characteristics of the material such as the transition from Newtonian to non-Newtonian flow and maximum achievable tensile elongation; the latter was used for determining the optimum conditions for superplastic flow in the material.     

Enhanced glass-forming ability of a Zr-based bulk metallic glass with yttrium doping

A significant enhancement in glass formation in a newly developed Zr₅₁Cu_20.7Ni₁₂Al_16.3 alloy has been achieved by yttrium doping. With just 0.5 at.% yttrium doping, the critical diameter of the as-cast alloys for glass formation has been increased from 3 mm to at least 10 mm. In the undoped, large-sized alloys, massive oxygen stabilized crystalline phases are observed but disappear in yttrium doped alloys. Very small amounts of stable α-Y₂O₃ phases found in the yttrium doped alloys, and their negligible effect on the metallic glasses’ properties, provide a superior solution to achieve metallic glasses with a high glass formability.     

Shear bands evolution in bulk metallic glass with extended plasticity

Shear band development and evolution of their spacing under bending in a Zr-based bulk metallic glass with extended plasticity has been monitored as a function of bending angle from the onset of plastic deformation to fracture. We find that sliding of existing shear bands is an important mechanism accounting for the plastic deformation of the plastic bulk metallic glass. The results are beneficial to understanding deformation in metallic glasses.     

Enhanced plasticity of a Zr50Cu48Al2 bulk metallic glass

In this work, a Zr₅₀Cu₄₈Al₂ bulk metallic glass (BMG) with improved glass forming ability and mechanical properties was synthesized by adding a small amount of Al to a Zr-Cu binary glass-forming alloy. The as-cast Zr₅₀Cu₄₈Al₂ glassy rod does not exhibit macroscopic failure under compression at room temperature indicating the excellent plasticity of the BMG. The deformation behavior of the BMG was studied intensively by examining the true stress-strain curve and the surface morphology of the post-deformation sample. The effect of minor Al addition on the mechanical properties including the fracture stress, the modulus, and the kinetic characteristics of the BMG was also investigated. The enhanced plasticity of the BMG is considered to be related to its reduced fragility parameter m value.     

Deformation behavior of a Ni59Zr20Ti16Si2Sn3 metallic glass matrix composite reinforced by copper synthesized by warm extrusion of powders

A metallic glass matrix composite (MGMC) reinforced by copper short fibers has been prepared by warm extrusion of powders, and its deformation behavior at room temperature and in the supercooled liquid region of the metallic glass has been investigated. A mixture of Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ metallic glass powders and copper powders is extruded in the supercooled liquid region of the metallic glass with an extrusion ratio of 5. The volume fraction of the copper phase is 0.2. After extrusion, initially spherical powders are elongated along the extrusion direction; no pores are visible. The MGMC shows a high failure strength of around 1.85 GPa, slightly lower than that of the as-cast Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ metallic glass, under uniaxial compression. However, due to the crack bridging mechanism produced by the randomly distributed copper short fibers, the MGMC does not catastrophically fail by a single shear band propagating across the whole monolithic sample. In the supercooled liquid region of the metallic glass, the MGMC shows large elongation to failure but fails by cavitation due to the preexisting Ni-based crystalline powders.     

A study of mechanical homogeneity in as-cast bulk metallic glass by nanoindentation

A surface softening effect induced during copper-mould suction casting of bulk metallic glass is investigated as a function of rod diameter and glass fragility index, m, by nanoindentation. A reduction in hardness and reduced modulus at the rod surface is found to be favoured in small diameter castings and in fragile systems, respectively resulting from limited in-situ annealing and from a greater diversity of metastable atomic environments in the potential energy landscape of fragile glasses. Enhanced propensity for shear transformation zone nucleation in the low moduli surface is explained in terms of reduced atomic connectivity arising from a reduction in local co-ordination number and a lowering of the shear modulus. Finally, the structure and mechanical diversity that is possible in as-cast bulk metallic glass rods is explored through a relative quantification of shear modulus and plastic zone size across the whole as-cast state and in a single rod. These findings illustrate the sensitivity of bulk metallic glass to preparation, especially in respect of thermal history, potentially making replication of mechanical data between researchers problematic.     

Minor addition induced enhancement of strength of Mg-based bulk metallic glass

We report that the fracture strength of Mg-based bulk metallic glasses (BMGs) can be dramatically enhanced up to 1.10 GPa by minor Gd addition. The Poisson’s ratio v of the BMG also decreases to 0.261 close to that of brittle oxide glasses when 1 at.% Gd was added. Such significant enhancement in strength which approaches the theoretical strength value and dramatically decrease in the Poisson’s ratio are attributed to the structural change of the BMGs induced by the Gd minor addition.     

Characterization of plasticity-induced structural changes in a Zr-based bulk metallic glass using positron annihilation spectroscopy

Changes in the free volume distribution in Zr_58.5Cu_15.6Ni_12.8Al_10.3Nb_2.8 bulk metallic glass with inhomogeneous plastic deformation and annealing were examined using positron annihilation spectroscopy. Results indicate that the size distribution of open volume sites is at least bimodal prior to deformation. The size and concentration of the larger sites, identified as flow defects, changes with processing. The size of the flow defects initially increases with deformation. More extensive deformation shifts the distribution, with a third group of much larger sites forming at the expense of flow defects. This suggests that a critical strain is required for the growth of nanovoids observed elsewhere by HRTEM. Although these observations suggest the presence of three open volume size groups, further analysis indicates that all groups have a similar distribution of chemical species around them as evidenced by the same line shape parameter. This may be due to the disordered structure of the glass and the positron affinity to particular atoms surrounding the open volume regions.     

Highly processable Mg65Cu25Tb10 bulk metallic glass

Mg₆₅Cu₂₅Tb₁₀ alloy was cast into glassy rod of 5-mm in diameter in air and argon atmosphere by normal copper mold casting method. The glass-forming ability, ignition and oxygen resistances of the alloy were investigated by X-ray diffraction, differential scanning calorimeter and acoustic measurement. The glass-forming alloy is fund to show high ignition and oxygen resistances due to the Tb addition. The beneficial effects of Tb on the glass-forming ability and ignition resistance are discussed.     

Superplasticity and superplastic forming ability of a Zr-Ti-Ni-Cu-Be bulk metallic glass in the supercooled liquid region

The superplastic deformation behavior and superplastic forming ability of the Zr_41.25Ti_13.75Ni₁₀Cu_12.5Be_22.5 (at.%) bulk metallic glass (BMG) in the supercooled liquid region were investigated. The isothermal tensile results indicate that the BMG exhibits a Newtonian behavior at low strain rates but a non-Newtonian behavior at high-strain rates in the initial deformation stage. The maximum elongation reaches as high as 1624% at 656 K, and nanocrystallization was found to occur during the deformation process. Based on the analysis on tensile deformation, a gear-like micropart is successfully die-forged via a superplastic forging process, demonstrating that the BMG has excellent workability in the supercooled liquid region.     

Influence of high temperature deformation on the fracture behavior of a bulk Zr-based metallic glass

Nanocrystalline/amorphous matrix composites obtained by isothermal compression at high temperatures and low strain rates were characterized using transmission electron microscopy. To study the influence of high temperature deformation on the fracture behavior and room temperature plasticity, compression tests with a constant strain rate of 1 × 10⁻⁴ s⁻¹ were applied to the deformed samples. Fracture features of as-cast alloy and deformed samples were analyzed using scanning electron microscopy. Compared with the as-cast alloy, the room temperature plasticity of deformed sample is not destroyed both in the range of 370-395 °C at 1 × 10⁻³ s⁻¹ and at 395 °C in 1 × 10⁻² to 1 × 10⁻³ s⁻¹, and deteriorated at higher temperatures and lower strain rates. Corresponding to the TEM images, the homogenously dispersed nanocrystals with small size contribute to the compressive plasticity, and the aggregated large nanoparticles destroy the plasticity of the sample after high temperature deformation.     

The correlation between the pressure sensitivity and the fragility/glass transition temperature in bulk metallic glasses

The correlations between the pressure sensitivity and the fragility/glass transition temperature have been addressed in various bulk metallic glasses in the present work. The results demonstrate that the pressure sensitivity of bulk metallic glasses is closely related to both the fragility index (m) and the glass transition temperature (T_g). The physical origin of the correlations has been discussed from their disordered structure, which is determined by the glass transition behavior and the glass transition temperature.     

Thermal stability and crystallization phenomena of low cost Ti-based bulk metallic glass

The thermal stability, kinetics of crystallization, and glass forming ability of a Ti₄₈Ni₃₂Cu₈Si₈Sn₄ bulk amorphous alloy have been studied by differential scanning calorimetry using both isothermal and non-isothermal experiments. The activation energy, frequency factor, and reaction rate for the crystallization cascade were determined via the Kissinger method. X-ray diffractometry and transmission electron microscopy studies revealed that crystallization starts with the primary precipitation of Ti(Ni,Cu), followed by the nucleation of Cu₃Ti from the amorphous precursor. The kinetics of nucleation of the primary crystalline phase was also investigated using the Johnson–Mehl-Avrami method and the Avrami exponent, n, was determined. This new alloy possesses a significantly larger supercooled liquid region than any other non beryllium- or non rare earth – containing titanium-based bulk metallic glass to date.     

Static and dynamic crystallization in Mg–Cu–Y bulk metallic glass

The static and dynamic crystallization behavior of Mg₆₅Cu₂₅Y₁₀ bulk metallic glass was investigated by X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. It was found that the kinetics of both anisothermal and isothermal crystallization were adequately represented by the Kissinger and KJMA relations, respectively. The apparent activation energy for crystallization was calculated to be 139 kJ/mol; this value is close to the self diffusion of Mg in both a crystalline and non-crystalline matrix. The Avrami exponent was found to vary from 2.2 to 2.5 with increasing annealing temperature which implies that, at high annealing temperatures, nucleation occurs at a constant rate accompanied by diffusion-controlled growth of spherical grains. Tensile straining in the supercooled liquid region indicated that crystallization is slightly accelerated compared with static crystallization; this phenomenon was found to adversely affect the ductility of the alloy.     

Isochronal crystallization kinetics of Cu60Zr20Ti20 bulk metallic glass

Isochronal crystallization kinetics of Cu₆₀Zr₂₀Ti₂₀ bulk metallic glass has been investigated by differential scanning calorimetry. By means of the Kissinger, Ozawa, Kempen, Matusita and Gao methods, average effective activation energies for the first and second crystallization reactions in Cu₆₀Zr₂₀Ti₂₀ are calculated to be about 375 ± 9 and 312 ± 11 kJ mol⁻¹, respectively, which are smaller than the values deduced from isothermal experiments. Meanwhile, average Avrami exponents, 3.0 ± 0.1 and 3.4 ± 0.2, for two crystallization reactions in isochronal anneals, differ from the value about 2.0 in isothermal anneals. The nonidentity of the Avrami exponents and effective activation energies may be contributed to different crystallization mechanisms and the nature of non-isokinetic between isochronal and isothermal experiments. The values of frequency factor k₀ for the first and second crystallization reactions of Cu₆₀Zr₂₀Ti₂₀ are (1.7 ± 0.3) × 10²⁴ and (7.0 ± 0.8) × 10¹⁸ s⁻¹, respectively, and the large value of k₀ has been discussed in terms of the atomic configuration and interaction.     

Phonon dispersion in bulk metallic glass Zr55Cu30Al10Ni15

The expressions for the phonon frequencies of Zr₅₅Cu₃₀Al₁₀Ni₁₅ bulk metallic glass employing a simple model given by Bhatia and Singh for a hypothetical one-component metallic glass are derived both for longitudinal and transverse modes of excitations. The model assumes a central force, effective between the nearest neighbours, and a volume dependent force. Both types of excitations of phonons are computed for the Zr₅₅Cu₃₀Al₁₀Ni₁₅ bulk metallic glass for the first time both for self-consistent screening of conduction electrons with and without the inclusion of correlation effects. Phonon frequency expressions reproduce the main characteristic features of the dispersion curves. The theoretical results predicted are in a good agreement with available experimental data of different quaternary bulk metallic glasses having same constituents. These are also compared to the theoretical results of a quaternary glass to have an insight of the structural behavior of the glass under consideration.     

Effect of strain rates on the fracture morphologies of Zr-based bulk metallic glasses

The effect of strain rates from 1 × 10⁻⁴ s⁻¹ to 2 × 10³ s⁻¹ on tensile fracture morphologies of Zr_52.5Al₁₀Ni₁₀Cu₁₅Be_12.5, Zr₆₅Al₁₀Ni₁₀Cu₁₅, and Zr_52.5Al₁₀Ni_14.6Cu_17.9Ti₅ bulk amorphous alloys was investigated by scanning electron microscopy. The results show that the tensile fracture morphologies of three compositions of bulk amorphous alloys are dependent on strain rate. At low strain rates, the tensile fracture surface morphology of Zr-based bulk metallic glasses presents cleavage veins. However, the morphology will become microvoid-coalescence dimples when the strain rate is high enough.