Zirconium-based Bulk Metallic Glass Research Articles

Shear yield and flow behavior of a zirconium-based bulk metallic glass

A series of plate-impact combined compression-and-shear experiments are conducted to investigate the effect of normal stress and hydrostatic pressure on the shear yield and flow behavior of a zirconium-based bulk metallic glass (BMG), Zr 41.25Ti 13.75Ni 10Cu 12.5Be 22.5. The experiments involve skew impact of a thick tungsten carbide (WC) flyer with a stationary WC target plate that sandwiches a thin BMG specimen. The experimental results are combined with the results of a previous study by Lewandowski and Lowhaphandu (2002), where the yield and fracture behavior of the same composition BMG at hydrostatic pressures up to 1 GPa were analyzed using a simple Mohr–Coulomb type model, to understand the effects of normal stress and hydrostatic pressure on the yield and flow behavior of the BMG. The results indicate that, within experimental errors, the normal stress as well as the hydrostatic pressure up to 8.8 GPa have a negligible influence on the shear yield and flow behavior of the BMG.

Effect of temperature on the mechanical behaviour of a Ni-free zirconium-based bulk metallic glass

It is well known that nickel-free zirconium-based bulk metallic glasses are of interest for their possible good biocompatibility properties. In this framework, a Zr–Nb–Cu–Al–Fe bulk metallic glass was elaborated under the form of 2 mm diameter rods. The amorphous structure was confirmed by XRD and DSC and the associated reduced index were measured. The mechanical properties of the glass were investigated at both room and high temperature. At room temperature, the glass exhibits some macroscopic plasticity. The rheology at high temperature was studied by both strain rate jump tests and tests at constant strain rate. From these tests, it was difficult to identify an experimental window in which the amorphous structure could be maintained and in which viscoplastic forming could be carried out under low flow stresses. In the super-cooled liquid region, this behaviour is discussed in relation with the resistance to crystallization of the investigated glass and the nature of the formed crystallites was studied by XRD. The mechanical properties of this glass are also compared to results previously obtained in the case of other zirconium-based bulk metallic glasses.

Increased Toughness of Zirconium-Based Bulk Metallic Glasses Tested under Mixed Mode Conditions

The effects of mixed mode loading on the fracture behavior of Zr-based bulk metallic glasses (BMGs) (Vitreloy I and Vitreloy 106) were investigated. Mixed mode I/II and mixed mode I/III fracture conditions were tested using both notched and fatigue-precracked specimens. Fully amorphous samples exhibited tremendous increases in fracture energy with the application of mixed mode loading, while partially crystalline samples exhibited more modest increases. A comparison to the behavior of other material systems (e.g., polymers, ceramics, crystalline metals, and composites) illustrates the tremendous increase in fracture energy exhibited by these BMGs under mixed mode loading conditions.

Spall strength of a zirconium-based bulk metallic glass under shock-induced compression-and-shear loading

We present results of a series of plate-impact experiments conducted to understand spall threshold in a zirconium-based bulk metallic glass (BMG), Zr 41.25Ti 13.75Ni 10Cu 12.5Be 22.5, following normal shock-induced compression and combined compression-and-shear loading. The experiments were conducted using a 82.5 mm bore single-stage gas-gun. A multi-beam VALYN VISAR was used to measure the particle–velocity at the free-surface of the target plate. For the normal shock-compression experiments, the impact velocities were chosen to span the elastic to the elastic–plastic range of the BMG during impact; the spall strength was inferred, at different levels of shock-induced compression, from the measured particle–velocity history of the free-surface of the target plate. For the combined compression-and-shear experiments the shock-induced normal stress was kept constant at ∼5 GPa (i.e. below the HEL for the BMG), while the projectile skew angle was varied from 6° to 24°. These skew angles are expected to result in a maximum shear strains of up to 3.18%. Under normal impact, at impact stress levels below the HEL, the spall strength of the BMG was found to decrease with increasing levels of the impact stress. However, at impact stress levels above the HEL the spall strength is observed to remain constant with increasing impact stress at ∼2.3 GPa. In the case of the combined compression-and-shear loading, with increasing levels of shear strain (at a constant shock-compression level below the HEL), the spall strength of the BMG was found to initially decrease, increase dramatically in the shear strain range of 2–2.4%, and then fall again as the shear strain is increased from 2.4% to 3.18%.

Bio-corrosion study on zirconium-based bulk-metallic glasses

Bio-corrosion behaviors of Zr 55Al 10Cu 30Ni 5 and (Zr 55Al 10Cu 30Ni 5) 99Y 1 (at.%) BMGs in a physiologically relevant environment were investigated and compared to those of standard, crystalline biomaterials. The effect of 1 at.% yttrium addition on the corrosion resistance of the Zr 55Al 10Cu 30Ni 5 BMG was determined. Cyclic-anodic polarization tests were performed at 37 °C in a phosphate-buffered saline (PBS) electrolyte aerated with a 4 vol.% O 2/N 2 gas mixture. Surface morphologies after electrochemical studies were observed by the scanning-electron microscopy (SEM). Compositions of corrosion products were investigated by the energy dispersive X-ray (EDX) to identify the corrosion mechanism. It was found that (Zr 55Al 10Cu 30Ni 5) 99Y 1 BMGs exhibited a superior bio-corrosion resistance with a corrosion-penetration rate (CPR) comparable to those standard, crystalline biomaterials, while Zr 55Al 10Cu 30Ni 5 BMGs exhibited a lower bio-corrosion resistance. Possible mechanisms for the beneficial effect of the Y addition are proposed that the structure of the passive film was changed, and/or the formation of the passive film was accelerated.

Dynamic compression of a zirconium-based bulk metallic glass confined by a stainless steel sleeve

Anvil-on-rod impact experiments were performed on zirconium-based bulk metallic glass rods with stainless steel confinement sleeves. High-speed photography and velocity interferometry were used to obtain qualitative and quantitative information about the transient deformation and failure response of the impacted specimens. Fracture surfaces of the recovered, impacted specimens were analyzed using scanning electron microscopy. Numerical simulations were also used to elucidate the deformation and failure response and better understand the effects of the altered stress states caused by the confinement sleeve.

Interactions between high-temperature deformation and crystallization in zirconium-based bulk metallic glasses

High-temperature deformation of a ZrTiCuNiBe bulk metallic glass (BMG) is investigated by compression tests in the supercooled liquid region. When the temperature is decreased or strain rate increased, the amorphous alloy exhibits the usual Newtonian/non-Newtonian transition behaviour. Using specific heat treatments, partially crystallized alloys are produced, the associated microstructures characterized and the volume fractions of the crystal measured. The interaction between high-temperature deformation and crystallization is investigated by appropriate mechanical testing. According to these measurements, partial crystallization is responsible for a significant increase in flow stress and the promotion of non-Newtonian behaviour. Deformation does not significantly change the volume fraction, composition or size of the crystal. The flow-stress increase with crystallization is analyzed under different hypotheses. We conclude that the flow-stress increase cannot be interpreted through a compositional change in the residual amorphous matrix, either by reinforcement due to hard crystallites or by connections between crystals. It appears that the effect is due to the nanometric size of the crystals alone.

Zirconium based bulk metallic glass / tungsten fibre composite — fabrication and characterization

The aim of the present work was to fabricate and characterize a composite consisting of Zr based bulk metallic glass as the matrix and W fibres as the reinforcement. This kind of composite because of its very high impact toughness has got widespread applications including in some strategic areas. The glass forming Zr52Ti6Al10Cu18Ni14 (at%) alloy was selected for this purpose because of its high glass forming ability and tungsten was selected as the reinforcing medium because of its high melting point, non reactivity with the liquid phase and high strength. The composite was fabricated in a unique way where a preform of W wires was made and the glass forming alloy was vitrified with this preform as reinforcement using the copper mold casting technique The composite produced was characterized using optical microscopy and EPMA studies. Compression testing was done to evaluate the mechanical properties of the composite.

Transient regimes during high-temperature deformation of a bulk metallic glass: A free volume approach

The homogeneous deformation of a zirconium-based bulk metallic glass is investigated in the glass transition range. Compression and stress-relaxation tests have been conducted. The stress–strain curves are modeled in the framework of the free volume theory, including transient phenomena (overshoot and undershoot). This approach allows several physical parameters (activation volume, flow defect creation and relaxation coefficient) to be determined from a mechanical experiment. This model is able to rationalize the dependency of stress overshoot on relaxation time. It is shown that, due to the relationship between flow defect concentration and free volume model, it is impossible to determine the equilibrium flow defect concentration. However, the relative variation of flow defect is always the same, and all the model parameters depend on the equilibrium flow defect concentration. The methodology presented in this paper should, in the future, allow the consistency of the free volume model to be assessed.

High-Pressure Equation of the State of a Zirconium-Based Bulk Metallic Glass

The high stress Us-Up Hugoniot equation of state (EOS) of a zirconium-based bulk metallic glass (BMG, Zr57Nb5Cu15.4Ni12.6Al10) was determined using plate impact experiments on disk-shaped samples of 10-mm diameter and 2-mm thickness. The National Institute for Materials Science (NIMS) two-stage light-gas gun was used for the high stress measurements (∼26 to 123 GPa), and the Georgia Institute of Technology (GT) single-stage gas gun was used for the lower stress measurements (∼5 to 26 GPa). The NIMS experiments were instrumented with streak photography and used the inclined mirror (IM) method to simultaneously measure shock velocity and free surface velocity. The GT experiments used polyvinylidene fluoride (PVDF) stress gages and velocity interferometry (VISAR) to simultaneously measure the shock velocity, free surface velocity, and stress. Results from the streak camera records and PVDF gages + VISAR traces, as well as impedance matching calculations, were used to generate the Us-Up Hugoniot EOS for the BMG over a wide range of stresses. The Us-Up data show evidence of a low pressure phase, a transition to a mixed phase region at ∼26 GPa, followed by transition at ∼67 GPa to a high-pressure phase of bulk modulus of 288 GPa.

Reciprocating wear mechanisms in a Zr-based bulk metallic glass

The dry sliding friction coefficient μ and the wear volume loss W, in a zirconium-based bulk metallic glass (BMG) under high-frequency (50 Hz) reciprocating conditions, were investigated with the objective of assessing the influence of free volume and crystallization on the wear behavior of amorphous metals. The BMG samples were annealed either below the glass transition temperature Tg to induce structural relaxation and hence reduce the free volume that controls plasticity through shear-band formation or above Tg to crystallize the amorphous BMG prior to wear testing. Results show that the wear behavior of both the as-cast and relaxed glasses was dominated by the oxidation of the surface layers. A sharp transition in the contact electrical resistance complemented by a marked increase in μ was noted. This was attributed to the formation of a thick tribo film with high oxygen concentration and its subsequent delamination. The μ values, before as well as after the transition, in the relaxed glasses were similar to those for the as-cast alloy. However, a gradual decrease in W with annealing temperature was observed. A good correlation between W and nanohardness was noted, implying that the intrinsic hardness in the BMGs controlled the wear rate.

Spall strength and Hugoniot elastic limit of a zirconium-based bulk metallic glass under planar shock compression

Results are presented on the shock response of a zirconium-based bulk metallic glass (BMG), Zr41.25Ti13.75Ni10Cu12.5Be22.5, subjected to planar impact loading. An 82.5-mm bore single-stage gas-gun facility at Case Western Reserve University, Cleveland, OH, was used to conduct the shock experiments. The particle velocity profiles, measured at the back (free) surface of the target plate by using the velocity interferometer system for any reflector (VISAR), were analyzed to (i) better understand the structure of shock waves in BMG subjected to planar shock compression, (ii) estimate residual spall strength of the BMG after different levels of shock compression, and (iii) obtain the Hugoniot elastic limit (HEL) of the material. The spall strength was found to decrease moderately with increasing levels of the applied normal impact stress. The spall strength at a shock-induced stress of 4.4 GPa was 3.5 GPa while the spall strengths at shock-induced stresses of 5.1, 6.0, and 7.0 GPa were 2.72, 2.35, and 2.33 GPa, respectively. The HEL was estimated to be 6.15 GPa.

Free volume model: High-temperature deformation of a Zr-based bulk metallic glass

The homogeneous deformation of a zirconium-based bulk metallic glass is investigated in the glass transition region. Compression tests at different temperatures and strain rates have been conducted. The mechanical behavior is analyzed in the framework of the free volume model, taking into account the dependence of the flow defect concentration on deformation. The activation volume is evaluated and allows one to gather the viscosity data (for the different strain rates and temperatures) on a unique master curve. It is also shown that, due to the relation between flow defect concentration and free volume, it is not possible to deduce the equilibrium flow defect concentration directly from mechanical measurements. However, if this parameter is arbitrarily chosen, mechanical measurements give access to the other parameters of the model, these parameters for the alloy under investigation being of the same order of magnitude as those for other metallic glasses.

High-Strain-Rate Dynamic Mechanical Properties of a W-Reinforced Zr-Based Bulk Metallic Glass Composite

AbstractThe structural/mechanical behavior of zirconium-based (Vitreloy106) bulk metallic glass reinforced with tungsten particles is evaluated using elastic and plastic property measurements via static and dynamic compression tests performed on rod shaped samples. Anvil-on-rod impact tests combined with high-speed digital photography and velocity interferometry are used to obtain qualitative and quantitative information about the transient deformation and failure response of the composites. The deformation and failure mechanisms of recovered impacted specimens are also characterized and correlated with their structure and tungsten phase distribution. The results of these experiments and initial attempts at validating constitutive equations based on homogeneous/inhomogeneous plastic/viscous flow in glassy and glassy-crystalline composites will be presented.

Fracture and deformation of bulk metallic glasses and their composites

Bulk metallic glasses represent an exciting new class of materials with a wide range of potential applications. Plastic deformation of metallic glasses occurs by the formation of shear bands; however, the effect of stress state and the extent of local structural rearrangement during flow are under active investigation. In this work, we review our investigations of the mode II deformation and fracture behavior of a zirconium-based bulk metallic glass. The mode II fracture toughness was found to be ∼75 MPa√m, ∼4–5 times larger than the mode I toughness. This may be explained by the sensitivity of flow to normal stresses. The fracture and fatigue behavior of a BMG composite utilizing a ductile reinforcement phase has also been examined and compared with that of the monolithic alloy. The second phase blocks the propagation of shear bands and distributes the plastic deformation, resulting in extensive stable crack growth at stress intensities double the intrinsic toughness of the unreinforced glass. The composite also exhibits an improved fatigue endurance limit. Implications for future composite microstructures are discussed.

Overheating threshold and its effect on time–temperature-transformation diagrams of zirconium based bulk metallic glasses

A pronounced effect of overheating is observed on the crystallization behavior for the three zirconium-based bulk metallic glasses: Zr41.2Ti13.8Cu12.5Ni10Be22.5, Zr57Cu15.4Ni12.6Al10Nb5, and Zr52.5Cu17.9Ni14.6Al10Ti5. A threshold overheating temperature is found for each of the three alloys, above which there is a drastic increase in the undercooling level and the crystallization times. Time–temperature-transformation (TTT) diagrams were measured for the three alloys by overheating above their respective threshold temperatures. The TTT curves for Zr41.2Ti13.8Cu12.5Ni10Be22.5 and Zr57Cu15.4Ni12.6Al10Nb5 are very similar in shape and scale with their respective glass transition temperatures, suggesting that system-specific properties do not play a crucial role in defining crystallization kinetics in these alloys. The critical cooling rates to vitrify the alloys as determined from the TTT curves are about 2 K/s for Zr41.2Ti13.8Cu12.5Ni10Be22.5 and 10 K/s for Zr57Cu15.4Ni12.6Al10Nb5. The measurements were conducted in a high-vacuum electrostatic levitator.

The fatigue behavior of a zirconium-based bulk metallic glass in vacuum and air

Early fatigue studies on bulk metallic glasses (BMGs) have indicated fatigue strengths lower than anticipated. It was suspected that environmental effects degraded the fatigue life. In the present study, a zirconium-based BMG, BMG-11 (Zr–10Al–5Ti–17.9Cu–14.6Ni, at.%), was tested in vacuum and air. This comparison was made to define any detrimental environmental effects due to water vapor on the fatigue lifetime of BMG-11. The limited results obtained so far suggest that water vapor in air does not affect the lifetime. Moreover, the fatigue endurance limit for BMG-11 in air was found to be quite high, 907 MPa, and comparable to, or better than, those of many conventional high-strength alloys.

Electrochemical Reactivity of Zirconium-Based Bulk Metallic Glasses

ABSTRACTAmorphous Zr-(Ti)-(Nb)-Al-Cu-Ni alloy samples were prepared by melt-spinning and copper mould casting in an argon atmosphere and characterized regarding their microstructure and thermal behavior. Their anodization behavior in aqueous environments with pH= 0.5 – 13 was studied by electrochemical polarization techniques in combination with surface analytical investigations, i.e. SEM/EDX, AES. In chloride-containing solutions the macroscopic corrosion resistance of bulk amorphous alloys is affected by the presence of heterogeneities, such as concentrated cluster zones of selected components or crystalline defects. Pitting phenomena are studied in neutral and acidic chloride solutions and as a result a local corrosion mechanism is proposed. The cathodic reactivity of alloy samples at different microstructural states and after pre-etching in fluoride solutions was investigated. After pre-etching melt-spun amorphous samples exhibit a significant increase in surface reactivity as expressed by a drastic increase in electrochemical capacities and in cathodic current densities as well as by a significant reduction of overpotentials for the hydrogen reduction reaction. The hydrogen sorption behavior was studied on samples galvanostatically charged at various cathodic current densities by means of XRD, DSC, TEM and thermal desorption analysis TDA. At room temperature Zr-based alloys absorb hydrogen up to H/M=1.65 mainly by interstitial solution of hydrogen atoms in the amorphous structure. The effect of absorbed hydrogen on the thermal stability and the crystallization behavior is described.

Fatigue behavior of Zr52.5Al10Ti5Cu17.9Ni14.6 bulk metallic glass

In the present study, fatigue tests were conducted on a zirconium-based bulk metallic glass (BMG), BMG-11 (Zr–10Al–5Ti–17.9Cu–14.6Ni, atomic percent), in air and vacuum to elucidate the possible environmental effects. In air, the fatigue endurance limit and the fatigue ratio were found to be 907 MPa and 0.53, respectively. These values are better than many conventional high-strength crystalline alloys. Unexpectedly, the fatigue lifetimes in vacuum were found to be lower than in air. Additional testing indicated that dissociation of residual water vapor to atomic hydrogen in the vacuum via a hot-tungsten-filament ionization gauge, and subsequent hydrogen embrittlement of the BMG-11, could have been a factor causing the lower fatigue lifetimes observed in vacuum.

Localized corrosion behavior of a zirconium-based bulk metallic glass relative to its crystalline state

To date, few detailed corrosion studies of the new bulk metallic glasses (BMGs) have been presented. In the present work, the aqueous electrochemical corrosion properties of BMG-11, 52.5Zr–17.9Cu–14.6Ni–5.0Ti–10.0Al (atomic percent), were investigated. Cyclic-anodic-polarization tests were conducted on amorphous and crystalline specimens in a 0.6 M NaCl solution (simulated seawater) and on amorphous specimens in a 0.05 M Na2SO4 solution (simulated moisture condensation, as related to ongoing fatigue experiments in humid air), all at room temperature. In the NaCl solution, both amorphous and crystalline materials were found to exhibit passive behavior with low corrosion rates (15 μm/year or less). However, susceptibilities to pitting corrosion were observed. The amorphous material was found to be more resistant to the onset of pitting corrosion under natural corrosion conditions. In the 0.05 M Na2SO4 solution, the amorphous BMG-11 was found to exhibit passive behavior with a very low corrosion rate (0.4 μm/year), and to be immune to pitting corrosion. Furthermore, when the protective passive film was removed by scratching with a diamond stylus, it was found to quickly reform. This result suggested that a corrosion influence on the fatigue properties of BMG-11 in humid air would be minimal.