Deformation behavior of Zr55.9Cu18.6Ta8Al7.5Ni10 bulk metallic glass matrix composite in the supercooled liquid region

A Zr_55.9Cu_18.6Ta₈Al_7.5Ni₁₀ bulk metallic glass (BMG) composite with an amorphous matrix reinforced by micro-scale particles of Ta-rich solid solution was prepared by copper-mold casting. Isothermal compression tests of the BMG composite were carried out in the range from glass transition temperature (∼673 K) to onset crystallization temperature (∼769 K) determined by differential scanning calorimetry (DSC). The compressive deformation behavior of the BMG composite in the supercooled region was investigated at strain rates ranging from 1 × 10⁻³ s⁻¹ to 8 × 10⁻² s⁻¹. It was found that both the strain rate and test temperature significantly affect the stress–strain behavior of the BMG composite in the supercooled liquid region. The alloy exhibited Newtonian behavior at low strain rates but became non-Newtonian at high strain rates. The largest compressive strain of 0.8 was achieved at a strain rate of 1 × 10⁻³ s⁻¹ at 713 K. The strain rate change method was employed to obtain the strain rate sensitivity (m). The deformation mechanism was discussed in terms of the transition state theory based on the free volume.     

Deformation behavior and indentation size effect of Au49Ag5.5Pd2.3Cu26.9Si16.3 bulk metallic glass at elevated temperatures

Instrumented nanoindentation was conducted on an Au₄₉Ag_5.5Pd_2.3Cu_26.9Si_16.3 bulk metallic glass from room temperature to supercooled liquid region. It was found that the hardness decreases as the depth of the indentation increases at a modest loading rate (e.g. ～0.5 mN s^?1), which is known as the indentation size effect (ISE). The transition from inhomogeneous to homogeneous flow was clearly observed at the glass transition temperature. However, the deformation behavior of the metallic glass in the supercooled liquid region showed strong loading rate dependence. The deformation mode changed from homogeneous to inhomogeneous, and even exhibited a reverse indentation size effect when the loading rate was sufficiently high (i.e., ≥10 mN s^?1 in the study). The different deformation behaviors and indentation size effects at various temperatures and loading rates are discussed in terms of free volume theory.     

Indentation creep of an Fe-based bulk metallic glass

In this study, the room temperature creep behavior of Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂ bulk metallic glass was investigated using nanoindentation technique with the maximum applied load ranging from 1 mN to 100 mN under different loading rates (0.01–2.5 mN s^?1). The creep stress exponent was derived from the recorded displacement–holding time curve. It was found that the stress exponent increases rapidly from 2.87 to 6.37 with increasing indentation size, i.e. exhibiting a positive indentation size dependence. Furthermore, as the indentation loading rate increases from 0.01 mN s^?1 to 2.5 mN s^?1, the stress exponent decreases gradually from 4.93 to 0.94. The deformation mechanism causing the nanoindentation creep is discussed in the light of the “shear transformation zone” (STZ) which provides qualitative explanation for the observed plasticity in amorphous alloy.     

Viscous flow and microforming of a Zr-base bulk metallic glass

The thermal plastic deformation behavior of the (Zr₅₃Cu₃₀Ni₉Al₈)_99.5Si_0.5 bulk metallic glass (BMG) was studied by means of thermo mechanical analysis (TMA) and high-temperature compression test in the supercooled liquid region. Using the result obtained from TMA, the deformation behavior for (Zr₅₃Cu₃₀Ni₉Al₈)_99.5Si_0.5 BMG rod was investigated by compression test at different strain rates (5 × 10^?3～5 × 10^?1 s^?1) and temperatures in the supercooled liquid region, specifically at 733 K, 738 K, 743 K, 748 K, and 753 K. The value of flow stress with a constant strain rate of 5 × 10^?3 s^?1 decreases with increasing temperature and reaches a relatively low flow stress of 36 MPa at the test temperature of 753 K. Conversely, the value of flow stress increases with the strain rate in compression. In this paper, an imprinted hologram pattern with 0.6 μm depth was demonstrated which showed extremely good microforming ability of this (Zr₅₃Cu₃₀Ni₉Al₈)_99.5Si_0.5 BMG in the supercooled liquid region.     

Fatigue crack growth behavior of a Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 bulk metallic glass-forming alloy

Fatigue crack growth behavior was studied for a Zr_58.5Cu_15.6Ni_12.8Al_10.3Nb_2.8 bulk metallic glass in ambient air, demonstrating a fatigue threshold of ΔK_TH = 1.4 MPa√m and a Paris law exponent of 1.7. A nearly stress intensity-independent crack growth regime occurred at 2.5 × 10^?8 m cycle^–1, suggesting an environmental influence of ambient air on the fatigue crack growth, as has been observed for Zr–Ti–Ni–Cu–Be bulk metallic glasses. However, this environmental fatigue effect was shifted to 25× higher growth rates due to the different chemistry.     

Effect of the nanoindentation rate on the shear band formation in an Au-based bulk metallic glass

This study investigated the nanoindentation behavior of Au₄₉Ag_5.5Pd_2.3Cu_26.9Si_16.3 bulk metallic glass samples at loading rates ranging from 0.03 to 300 mN s⁻¹. Notable shear band pop-in events were observed. The pop-in size was observed to increase linearly with the load and decreased exponentially with the strain rate. A free-volume mechanism was proposed for interpreting these observations quantitatively. The results and analyses also shed light on the shear band nucleation and evolution processes in bulk metallic glasses.     

Viscous flow behavior and thermal properties of bulk amorphous Mg58Cu31Y11 alloy

The viscous flow behavior of the Mg₅₈Cu₃₁Y₁₁ bulk amorphous rods in the supercooled viscous region is investigated using differential scanning calorimetry (DSC) and thermomechanical analyzer (TMA). Below the glass transition temperature, T_g, a linear thermal expansion coefficient of 3 ± 1 × 10⁻⁶ m/m K was obtained. In contrast, significant viscous deformation occurred as a result of a compressive load above T_g. The onset, steady state, and finish temperatures for viscous flow, determined by TMA, are slightly different from the glass transition and crystallization temperatures measured by DSC. The appropriate working temperature for microforming as determined by the steady state viscous flow temperature is about 460–474 K. The effective viscosity within this temperature range is estimated to be about 10⁷–10⁹ Pa s, and it increases with increasing applied stress. The onset, steady state, and finish temperatures all decrease with increasing applied stress, suggesting accelerated crystallization in the present Mg₅₈Cu₃₁Y₁₁ under stress.     

A constitutive theory for metallic glasses at high homologous temperatures

The elastic–viscoplastic constitutive theory of Anand and Su [Anand L, Su C. J Mech Phys Solids 2005;53:1362] for metallic glasses has been extended to the high homologous temperature regime. The constitutive equations appearing in the theory have been specialized to model the response of metallic glasses in the temperature range 0.7ϑ_g ⪅ ϑ ⪅ ϑ_g and strain rate range [10⁻⁵, 10⁻²] s⁻¹. The material parameters appearing in the theory have been estimated for the metallic glass Pd₄₀Ni₄₀P₂₀ from the experimental data of De Hey et al. [De Hey P, Sietsma J, Van Den Beukel A. Acta Mater 1998;46:5873]. The model is shown to capture the major features of the stress–strain response, and the evolution of an order-parameter for this metallic glass. In particular, the phenomena of stress overshoot and strain softening in monotonic experiments at a given strain rate and temperature, as well as strain rate history effects in experiments involving strain rate increments and decrements, are shown to be nicely reproduced by the model.     

Influence of minor aluminum concentration changes in zirconium-based bulk metallic glasses on the elastic,anelastic, and plastic properties

The Poisson’s ratio of Zr-based bulk metallic glasses in the system Zr_63?xCu₂₄Al_xNi₁₀Co₃ was found to exhibit a non-monotonous behavior as a function of x when measured with ultrasound by the pulse–echo technique. In addition, from wave propagation velocity measurements at different frequencies, i.e. f = 2.25 MHz and f = 10 MHz, a composition-dependent anelastic behavior as a function of x is found, exhibiting a similar non-monotonous behavior. In this work we further investigated the plastic deformation and the creep properties of this glass system in compression tests and by nanoindentation. The plastic strain and the measured creep deformation show correlations with the Poisson’s ratio. We then discuss the anelastic behavior observed while measuring the sound-wave propagation velocity in the frame of the thermoelastic damping and the bond reorientation as proposed by Egami. Finally we discuss these effects with regard to X-ray diffraction analysis.     

Strain rate-dependent compressive deformation behavior of Nd-based bulk metallic glass

Compressive deformation behavior of the Nd₆₀Fe₂₀Co₁₀Al₁₀ bulk metallic glass was characterized over a wide strain rate range (6.0×10⁻⁴ to 1.0×10³ s⁻¹) at room temperature. Fracture stress was found to increase and fracture strain decrease with increasing applied strain rate. Serrated flow and a large number of shear bands were observed at the quasi-static strain rate (6.0×10⁻⁴ s⁻¹). The results suggest that the appearance of a large number of shear bands is probably associated with flow serration observed during compression; and both shear banding and flow serration are a strain accommodation and stress relaxation process. At dynamic strain rates (1.0×10³ s⁻¹), the rate of shear band nucleation is not sufficient to accommodate the applied strain rate and thus causes an early fracture of the test sample. The fracture behavior of the Nd₆₀Fe₂₀Co₁₀Al₁₀ bulk metallic glass is sensitive to strain rate.     

Effect of annealing on microstructure and mechanical property of a Ti–Zr–Cu–Pd bulk metallic glass

The crystallization behavior and mechanical properties of Ti₄₀Zr₁₀Cu₃₆Pd₁₄ bulk metallic glass were investigated. Nano-composites consisting of glassy matrix are formed in the alloys annealed at 693 and 723 K. After annealing at 823 K, the residual glass is completely crystallized. High yield strength of over 2100 MPa and distinct plastic strain of 0.8% are obtained in the nano-composite alloy annealed at 693 K. The compressive strength decreases for the alloys annealed at higher temperatures of 723 and 823 K, accompanying with the progress of crystallization. The alloy rods both in as-cast and annealed (at 693 K for 10 min) states show the fracture surfaces consisting of vein pattern. On the other hand, the fracture morphology of the alloys annealed at 723 and 823 K changes to brittle character with flat facets.     

Stick–slip behavior of serrated flow during inhomogeneous deformation of bulk metallic glasses

Accurate compression tests with a piezoelectric load cell and an acquisition rate of up to 10 kHz were performed on a Zr-based bulk metallic glass in the temperature range 210–320 K at a strain rate of 10^?3 s^?1. Information about the stress drop magnitude and the associated size of shear displacements as a function of temperature and strain provides detailed insights into the shear band characteristics, which can be described by a stick–slip process. The average shear slip displacement is on average about 1–2 μm, irrespective of temperature, whereas the associated slip time (or stress drop time) increases from ～1 ms at 320 K to ～0.4 s at 213 K, yielding values on the deformation kinetics and the shear viscosity. Scanning electron microscopy investigations on shear surfaces and in situ acoustic emission measurements provide further understanding into the complex multistep shear slip process.     

Studies of shear band velocity using spatially and temporally resolved measurements of strain during quasistatic compression of a bulk metallic glass

We have made measurements of the temporal and spatial features of the evolution of strain during the serrated flow of Pd₄₀Ni₄₀P₂₀ bulk metallic glass tested under quasistatic, room temperature, uniaxial compression. Strain and load data were acquired at rates of up to 400 kHz using strain gages affixed to all four sides of the specimen and a piezoelectric load cell located near the specimen. Calculation of the displacement rate requires an assumption about the nature of the shear displacement. If one assumes that the entire shear plane displaces simultaneously, the displacement rate is approximately 0.002 m s^–1. If instead one assumes that the displacement occurs as a localized propagating front, the velocity of the front is approximately 2.8 m s^?1. In either case, the velocity is orders of magnitude less than the shear wave speed (～2000 m s^?1). The significance of these measurements for estimates of heating in shear bands is discussed.     

La-based bulk metallic glasses with critical diameter up to 30 mm

We report composition optimization, thermal and physical properties of new La-based bulk metallic glasses with high glass forming ability (GFA) based on a ternary La₆₂Al₁₄Cu₂₄ alloy. By refining the (Cu, Ag)/(Ni, Co) and La/(Cu, Ag) ratios in the La–Al–(Cu,Ag)–(Ni, Co) pseudo-quaternary alloy, the formation of 30 mm diameter of La₆₅Al₁₄(Cu_5/6Ag_1/6)₁₁(Ni_1/2Co_1/2)₁₀ bulk metallic glass (BMG) alloy is achieved using water quenching. The origin of the high GFA was investigated from the kinetic, structural and thermodynamic points of view, and was found to be due to the smaller difference in Gibbs free-energy between the amorphous and crystalline phases in the pseudo-quaternary alloy. These alloys exhibit low glass transition temperatures, below 430 K, and relatively wide supercooled liquid regions of 40–60 K. Mechanical tests on these alloys show a fracture strength of 650 GPa, Vicker’s hardness 200 kg mm⁻², Young’s modulus 35 GPa, shear modulus 13 GPa and Poisson ratio 0.356. The La-based BMGs are useful for both scientific and engineering applications.     

Single-crystal plasticity of the complex metallic alloy phase β-Al–Mg

We have determined the macroscopic plastic deformation behaviour of high-quality single-crystalline samples of the complex metallic alloy β-Al–Mg. Uniaxial deformation experiments at a constant strain rate of 10⁻⁴ s⁻¹ were performed at temperatures between 200 and 375 °C. The material exhibits ductile behaviour down to temperatures of 225 °C. At this temperature an upper yield stress of 780 MPa was observed, which is a very high value compared to commercial Al–Mg alloys. The upper yield point is followed by an almost constant flow stress level up to strains of about 6%. Stress-relaxation tests and temperature changes were carried out in order to determine the thermodynamic activation parameters of the deformation process.     

Formation,microstructure and properties of long-period order structure reinforced Mg-based bulk metallic glass composites

In situ Mg–Cu–Y–Zn bulk metallic glass (BMG) matrix composites, in which Mg solid solution flakes of 0.5–1 μm thickness and 2–10 μm length are dispersed, have been prepared by copper mold casting. The Mg flakes are characterized as a long-period order structure (LOS), i.e. periodic arrays of six close-packed planes distorted from the ideal hexagonal lattice of 6H-type. The formation mechanism of LOS is interpreted as the precipitation of the leading phase of the eutectic reaction above the glass transition temperature. In comparison with monolithic Mg-based BMG alloys, the composites with an LOS exhibit significant improvement in mechanical properties, e.g. a compressive plastic strain of ∼18% and ultimate strength of ∼1.2 GPa, have been measured in Mg₈₁Cu_9.3Y_4.7Zn₅ alloy. It is suggested that the enhancement of the mechanical properties of the composites can be attributed to the generation of multiple shear bands and the deformation of the LOS.     

Formation,thermal stability and corrosion behavior of glassy Ti45Zr5Cu45Ni5 alloy

Ti-rich Ti₄₅Zr₅Cu₄₅Ni₅ bulk metallic glass with critical diameter reaching 3 mm and supercooled liquid region of 42 K was prepared by copper mold casting. The glass transition temperature and onset temperature of crystallization are 673 K and 715 K, respectively. The glassy Ti–Zr–Cu–Ni alloy is passive in 3 mass% NaCl, 1 N HCl and 1 N H₂SO₄ solutions, although pitting corrosion occurred by anodic polarization at higher potential in the Cl⁻-containing solutions. Corrosion rates of the glassy Ti₄₅Zr₅Cu₄₅Ni₅ alloy are of the order of 10⁻³ mm/year in the NaCl and H₂SO₄ solutions and about 1 × 10⁻² mm/year in the HCl acid.     

Mg–Ni–Gd–Ag bulk metallic glass with improved glass-forming ability and mechanical properties

The improvement of glass-forming ability (GFA) and mechanical properties by using Ag to substitute Mg in the Mg–Ni–Gd bulk metallic glass (BMG) were studied. The Mg₆₉Ni₁₅Gd₁₀Ag₆ bulk metallic glass (BMG) could be cast into glassy rod up to 7 mm. The activation energies of Mg₆₉Ni₁₅Gd₁₀Ag₆ metallic glass were calculated by the Kissinger’s method to be E_g = 2.24 eV, E_x = 1.65 eV, E_p1 = 1.36 eV, E_p2 = 1.59 eV, E_p3 = 1.26 eV and E_p4 = 1.99 eV. The compressive fracture strength and the plastic strain of Mg₆₉Ni₁₅Gd₁₀Ag₆ BMG reached 846 MPa and 0.37% respectively.     

High strength ductile Cu-base metallic glass

Usually, bulk metallic glasses exhibit strength values superior to conventional crystalline alloys, often combined with a large elastic limit and rather low Young's modulus. This combination of properties renders such alloys quite unique when compared to commercial materials. However, the major drawback for engineering applications is their limited room temperature ductility and toughness due to the localized deformation processes linked to shear banding, where high plastic deformation is accumulated in a very narrow region without contributing to macroscopic deformation, work hardening or yielding. In this work we report on a new class of metallic glass in a simple Cu-base alloy. Addition of 5 at.% Al increases the glass-forming ability of binary Cu₅₀Zr₅₀. The resulting Cu_47.5Zr_47.5Al₅ glass exhibits high strength (2265 MPa) together with large room temperature ductility up to 18%. After yielding a strong increase in the flow stress is observed during deformation. The structure of the metallic glass exhibits atomic-scale heterogeneities that enable easy nucleation and continuous multiplication of shear bands. The interaction and intersection of shear bands increases the flow stress of the material with further deformation, leading to a ‘work hardening’-like behavior and yields a continuous rotation of the shear angle up to fracture resulting in a high compressive ductility.     

An in situ bulk Zr58Al9Ni9Cu14Nb10 quasicrystal-glass composite with superior room temperature mechanical properties

An in situ bulk Zr₅₈Al₉Ni₉Cu₁₄Nb₁₀ quasicrystal-glass composite has been fabricated by means of copper mould casting. The microstructure and constituent phases of the alloy composite have been analyzed by using X-ray diffraction, transmission electron microscopy and high-resolution transmission electron microscopy. Icosahedral quasicrystals were found to be the majority phase and the grain size is in half-μm scale. In between the I-phase grains is a glassy phase. Optical microscopy and scanning electron microscopy revealed that the as-cast alloys were pore-free. The microhardness of the composite is about 5.90 ± 0.30 GPa. The room temperature compression stress–true strain curve exhibits a 2% elastic deformation up to failure, and a maximum fracture stress of 1850 MPa at a quasi-static loading rate of 4.4 × 10⁻⁴ s⁻¹. The mechanical property is superior to the early developed quasicrystal alloys, and is comparable to Zr-based bulk metallic glasses and their nanocomposites. The quasicrystal-glass composite exhibits basically a brittle fracture mode at room temperature.