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.     

Elastic properties of some bulk metallic glasses

The relationships between the elastic moduli, glass forming ability and response to deformation of bulk metallic glasses are investigated. Five bulk metallic glasses are prepared from high purity elements via suction casting. The results confirm that there exists a correlation between energy absorbed to failure during compression testing and the bulk to shear modulus ratio. This finding is developed such that it corresponds only to the elastic component of energy absorption, and that the bulk modulus dominates this. Plastic deformation appears to be favored by a reduced shear modulus, although it shows greater dependence on structural features that are frozen in during the glass transition, and so may well be dependent on the liquid fragility.     

Competition of time and spatial scales in polymer glassy dynamics: Rejuvenation and confinement effects

We use molecular-dynamics (MD) simulations and an original lateral contact experiment to explore the influence of mechanical history on polymer mechanical behavior and segmental mobility. Two typical glassy polymers are considered: bulk acrylate (experiments) and atactic polystyrene (aPS) in a bulk and in thin films (simulations). Stress-strain behavior has been investigated both experimentally for sheared, 50 μm thick, acrylate films and by MD simulations of an aPS in a bulk for two different strain rates in a closed extension-recompression loops. Cyclic shear strains applied in the plastic regime were found experimentally to induce a progressive transition of the mechanical response of the polymer glass toward a steady state which is characterized by a strong reduction of the apparent - non linear - shear modulus. The dynamics of the polymer glass in this yielded state was subsequently analyzed from a measurement of the time dependent linear viscoelastic properties at various imposed frequencies. Immediately after the cyclic plastic deformation, mechanical “rejuvenation” of the polymer is evidenced by a drop in the storage modulus and an increase in the loss modulus, as compared to the initial values recorded before plastic deformation. A progressive recovery of the viscoelastic properties is also measured as a function of time as a result of the enhanced aging rate of the system. This experimentally observed mechanical rejuvenation of polymer has been for the first time connected to the drastic increase in the simulated segmental mobility. A simulated distribution of relaxation times shows a shift to shorter times of the α and β relaxation processes which is consistent with the observed experimental changes in the viscoelastic modulus after rejuvenation. Finally, we present our first findings on the thickness- and substrate-dependence of the simulated glass transition temperature for thin aPS films. We observe the decrease of the glass transition temperature with film thickness, but for extremely thin (less than 2 nm) films.     

Influence of structural relaxation on atomic mobility in a Zr41.2Ti13.8Cu125Ni10.0Be22.5 (Vit1) bulk metallic glass

The effect of an annealing at a temperature above or below the glass transition temperature in a Zr_41.2Ti_13.8Cu₁₂₅Ni_10.0Be_22.5 bulk metallic glass was investigated using dynamic mechanical analysis. Structural relaxation influences both the storage modulus (elastic component) and the loss modulus (viscoelastic component). Kinetics can be captured by a stretched exponential relaxation function. Experimental results are correctly described using a physical model based on the concept of defects for the mechanical response of amorphous materials and especially for the characteristic time relative to atomic mobility.     

First-principles study of the binary Ni60Ta40 metallic glass: The atomic structure and elastic properties

Ni–Ta bulk metallic glass (BMG) with compositions around Ni₆₀Ta₄₀ is a newly found binary BMG with high glass forming ability and extraordinary mechanical strength. Using ab initio molecular dynamics, the local atomic structure, elastic properties and electronic structures of Ni₆₀Ta₄₀ glass have been explored. The pair-correlation functions, coordination numbers, and chemical compositions of the most abundant local clusters have been analyzed. We demonstrated the existence of icosahedral Ni₇Ta₆ clusters as the major Ni-centered clusters, while the most popular Ta-centered cluster is Ta₇Ni₈. These findings agree with our previous cluster model of Ni–Ta binary BMG. The elastic moduli of Ni₆₀Ta₄₀ glass were also computed and the experimental Young's modulus is well reproduced. Analysis of electronic structures further revealed that the interaction between d electrons of Ni and Ta atoms is responsible for the experimentally observed ultrahigh mechanical strength for the Ni–Ta BMGs.     

Extended elastic model for flow in metallic glasses

We report that both shear and bulk moduli, not only shear modulus, are critical parameters involved in both homogeneous and inhomogeneous flows in metallic glass. The flow activation energy (?F) of various glasses when scaled with average molar volume V_m, which is defined as flow activation energy density ρ_E = ?F/V_m, can be expressed as: . The extended elastic model is suggestive for understanding the glass transition and deformation in metallic glasses.     

Variations in the mechanical properties of glass induced by high-pressure phase change

The elastic property of As₂Se₃ glass has been determined under high pressure from the data of the sound wave velocities and density. The sound velocities in As₂Se₃ glass have been measured under hydrostatic pressure up to 20 kb by means of ultrasonic interferometry. Sound velocities for both the longitudinal and shear mode, bulk modulus and shear modulus were found to increase linearly with pressure up to 8 kb. Above 9 kb the pressure derivatives diminished. The acoustic Grüneisen parameter shows an abrupt decrease at about 9 kb.Comparisons were made among the observed values of the volume ratio V/V₀, the isothermal bulk modulus K_T and the pressure derivative K′_T, and predictions based upon the Murnaghan equation or the lattice theory equation of state for solids. It is concluded that the abrupt changes in the parameters are probably due to a phase transition in glass at 9–10 kb.The clear resolution of the bulk modulus on both sides of the transition enables one to compute all the mechanical properties of the high-pressure phase, using the law of corresponding states. The decrease of K′_T from 7.86 to 6 in the high-pressure phase change is consistent with the change of density from 4.60 to 4.75.A modified equation of state has been proposed to describe the elastic property of glass. This equation includes a variable parameter C which reflects the volume change in glass due to bond angle change with pressure. By this equation the pressure dependence of C can also explain the elastic anomaly of silica glass as well as the variation of elastic constants of As₂Se₃ glass across the phase transition pressure.     

Distinguish bonding characteristic in metallic glasses by correlations

By statistically analyzing 48 kinds of metallic glasses, we report clear correlations between the dimensionless ratio of glass transition temperature/Debye temperature (T_g/Θ_D) and density (ρ), and between Young's modulus or shear modulus and T_g/ρ, for the glasses consisting of only metal elements, while the metallic glasses alloyed with metalloid elements exhibit distinct deviation from the correlations. It is suggested that the alloying of metalloid elements would show covalent-like bonding characteristics in metallic glass, and the found correlations can be used to distinguish different bonding characteristics in metallic glasses.     

Tantalum based bulk metallic glasses

Ta-based bulk metallic glasses with high strength (2.7 GPa) and hardness (9.7 GPa), high elastic modulus (170 GPa) and high density (12.98 g/mm³) were developed. The best glass forming ability so far for a Ta-Ni-Co system reaches a critical diameter of 2 mm by the copper mold casting method. It shows an exceptionally high glass transition temperature of 983 K and a high crystallization temperature up to 1023 K. The unique mechanical and physical properties make them a promising high strength material.     

Characterization of the free volume in a Zr45.0Cu39.3Al7.0Ag8.7 bulk metallic glass by reverse Monte Carlo simulation and density measurements

The absolute contents of free volume in the as-cast and annealed Zr_45.0Cu_39.3Al_7.0Ag_8.7 bulk metallic glass (BMG) samples were quantified by density measurements and reverse Monte Carlo (RMC) simulation using the total structural factors F(Q) determined by X-ray diffraction (XRD) experiments as fitting constraints. The densities of the as-cast, annealed and crystallized samples measured by Archimedes method are 7.319, 7.327 and 7.342 g/cm³ (precision ± 0.003 g/cm³), respectively. A new approach was used in the RMC simulation to define the free volume as the difference between the volume of the Voronoi polyhedron and the volume of the Wigner-Seitz cell of the constituent atoms. Two types of initial configurations were constructed: (1) in configuration A, all types of potential atomic pairs are allowed; (2) in configuration B, Al-Al and Ag-Ag atomic pairs are excluded. The contents of free volume in the as-cast and annealed samples were found to be 0.59% and 0.39% (configuration A), 0.55% and 0.34% (configuration B), respectively (precision ± 0.03%). The probability distribution of the as-calculated free volume can be well-fitted by the equation proposed by Turnbull and Cohen. Finally, it is shown that the contents of free volume determined by density measurements and RMC are comparable, while the discrepancy of the results is discussed.     

Elastic properties of GeSbSe glasses

The results of the measurement of transverse and longitudinal sound velocities on eigth glass compositions of the GeSbSe system are reported and their elastic moduli evaluated. While the velocities, elastic moduli and Debye temperature show variation with composition for Ge_xSb₁₀Se_90?x glasses, they are essentially constant for the glasses with stoichiometric compositions. The dependence of bulk modulus on mean atomic volume has been analysed. Both the mean atomic volume and the type of bonding are found to be effective in determining the composition dependence of bulk modulus.     

Nanoindentation of glass wool fibers

The nanoindentation technique is used to analyze the depth dependence of the hardness and the reduced elastic modulus of bulk glasses and glass wool fibers (4-12 μm in diameter) of calcium aluminosilicate composition. In spite of the fiber geometry and the delicate sample mounting-technique, nanoindentation proves to be a relatively accurate method that provides reproducible data for both hardness (H) and reduced elastic modulus (E_r) of thin glass fibers. It is found that H and E_r are generally lower for the fiber than for the bulk sample. Within a given fiber, both H and E_r are approximately constant with increasing indentation depth. However, both of these parameters decrease with diminishing fiber diameter. This trend is attributed to an increase of the free volume of the fibers with decreasing fiber diameter, i.e. to an increase of the fictive temperature.     

Structure and properties of Ni60(Nb100−xTax)34Sn6 bulk metallic glass alloys

Refractory bulk metallic glasses and bulk metallic glass composites are formed in quaternary Ni-Nb-Ta-Sn alloy system. Alloys of composition Ni₆₀(Nb_100−xTa_x)₃₄Sn₆ (x = 20, 40, 60, 80) alloys were prepared by injection-casting the molten alloys into copper molds. Glassy alloys are formed in the thickness of half mm strips. With thicker strips (e.g., 1 mm), Nb₂O₅ and Ni₃Sn phases and the amorphous phase form an in situ composite. Glass transition temperatures, crystallization temperatures, and ΔT_x, defined as T_x1 − T_g (T_x1: first crystallization temperature, T_g: glass transition temperature) of the alloys increase dramatically with increasing Ta contents. These refractory bulk amorphous alloys exhibit high Young’s modulus (155-170 GPa), shear modulus (56-63 GPa), and estimated yield strength (3-3.6 GPa).     

Atomic dynamics in different regions of the potential energy surface

The atomic dynamics in two (bulk) metallic glasses, Ni₄₀Pd₄₀P₂₀ and Zr₅₅Cu₃₀Al₁₀Ni₅, were investigated by neutron inelastic scattering in different regions of the potential energy landscape, which are reached by slow cooling the bulk glasses and by hyper-quenching the same alloys. The results prove that the atomic dynamics depends also on the fictive temperature, i.e. the region of the potential energy surface, in which the glass is frozen in. Obviously the shapes of the basins or inherent structures are not the same everywhere on the potential energy surface, and the glass with a higher fictive temperature has more low energy modes than has the same glass with a lower fictive temperature. As results from computer simulation have suggested already, on moving to regions of lower mean potential energy (aging), part of theses low energy modes are transferred to the energy region of the calculated Debye cut-off energy. The difference between the vibrational entropies, calculated from the generalized vibrational density-of-states, which have been determined for both fictive temperatures, shows that the contribution from the vibrational entropy to the total entropy change, when moving through the potential energy landscape, is small for the two metallic glasses investigated. Structural relaxation of the hyper-quenched glass removes part of the additional low energy modes, but quantitatively possibly only at the low and perhaps also at the high-energy limit of the density-of-states. The wavelength dependence of the dynamics suggests that the additional low energy modes in the glass with the higher fictive temperature are not dominated by extended but more likely by localized modes.     

Mg based bulk metallic glasses: Glass transition temperature and elastic properties versus toughness

In this work, optimal compositions for bulk metallic glasses (BMGs) formation in the ternary Mg-Cu-Nd and Mg-Ni-Nd systems are located at the Mg₅₇Cu₃₄Nd₉ and Mg₆₄Ni₂₁Nd₁₅, respectively, with the critical diameter of 4 mm for the rods fabricated by copper mold casting. As indicated by notch toughness testing, Mg₆₄Ni₂₁Nd₁₅ BMG (K_Q = 5.1 MPa√m) manifests higher toughness with respect to the Mg₅₇Cu₃₄Nd₉ (K_Q = 3.6 MPa√m), even though both BMGs have similar compressive fracture strength of 870-880 MPa. Such an improvement in toughness for Mg BMGs correlates with the reduction of shear modulus and the enhancement of thermal stability to resist to the structural relaxation at room temperature, which is indicated by the elevated glass transition temperature T_g. Under the Mode I loading condition, morphology in fracture surface of the Mg₆₄Ni₂₁Nd₁₅ BMG varies along the crack propagation path. Fractographic evolution of the fracture surface follows the Taylor's meniscus instability criterion. For the Mg-based BMGs, shear modulus scales with the glass transition temperature, and can be expressed as μ = 4.7 + 625T_g/V_m[1-4/9(T/T_g)^2/3]. Meanwhile, correlation between the calorimetric T_g and elastic properties at T_g can be rationalized with Egami's model.     

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.     

Glass formation ability and mechanical properties of Ti40Cu40Zr10Ni10 alloy

Melt-spun ribbon and bulk samples in cylindrical rod form with diameter ranging from 2 mm to 4 mm of Ti₄₀Cu₄₀Zr₁₀Ni₁₀ alloy were prepared by melt-spinning technique and copper mould casting method, respectively. The microstructure, thermal stability and mechanical properties of the bulk samples were investigated. A completely glassy single phase is formed in the 2 mm rod sample. Increasing the diameter of the rod samples resulted in the formation of CuTi crystalline phase in the 3 mm and 4 mm rod samples. The 2 mm single glassy rod sample exhibited a large supercooled liquid region ΔT_x = 58 K and γ = T_x/(T_g + T_l) is 0.390, which indicated that the alloy possessed a good glass-forming ability. The bulk samples also exhibited good mechanical properties. The 2 mm rod sample showed the highest yield strength of about 2086 MPa. The 3 mm rod sample not only showed high yield strength of about 2000 MPa, but also enhanced plastic strain of about 0.71%.     

Effects of Zn addition on the improvement of glass forming ability and plasticity of Mg–Cu–Tb bulk metallic glasses

The effects of Zn addition on the glass forming ability and mechanical properties of Mg₆₅Cu_25?xZn_xTb₁₀ (x = 0, 2.5, 5, and 7.5) have been investigated. We show that small amounts of Zn addition improve the glass forming ability, strength, and ductility of the Mg–Cu–Tb bulk metallic glass. For the best glass forming composition, amorphous rods of Mg₆₅Cu₂₀Zn₅Tb₁₀ with a diameter of at least 7 mm have been prepared by a conventional copper mold casting method. Additionally, this composition exhibits obvious yielding and plastic deformation upon quasi-static compressive loading. The fracture strength, total strain to failure, and the plastic strain of the Mg₆₅Cu₂₀Zn₅Tb₁₀ bulk metallic glass reaches 1025 MPa, 2.05% and 0.15%, respectively. This is significantly superior compared to that exhibited by the original Zn-free Mg–Cu–Tb amorphous alloy.     

Microhardness and abrasive wear resistance of metallic glasses and nanostructured composite materials

The present paper reports on as-cast bulk zirconium-based, lanthanum-based and palladium-based metallic glasses and on melt-spun aluminum-based amorphous ribbons. Their glass forming abilities, nano(quasi)crystallization mechanisms and mechanical properties are discussed through a short review of very recent papers and by the presentation of some experimental results. The tribological aspect such as microhardness and abrasive wear resistance are more particularly investigated for their potential applications technologically useful as coatings. The formation of nanostructured composite materials consisting of nanophase particles embedded in a residual amorphous matrix by appropriate thermal treatments is also studied and the influence of the nanocrystallized volume fraction is evaluated and discussed for both hardness and wear resistance.     

Properties of as-cast and structurally relaxed Zr-based bulk metallic glasses

We have studied the influence of the arc melting procedure of Nb-containing Zr-based bulk metallic glasses (BMGs) on their thermal and mechanical properties. We found that the strength and plastic strain to failure, determined at room temperature, increases by the addition of a few percentage of Nb into the Zr-based BMGs. High-resolution transmission-electron microscopy results show that the addition of 2 at.% Nb introduces the formation of as-cast nanocrystals. At the same time, thermal analyses indicate an increase in glass forming ability with the small addition of Nb. Contrary to the reported results in other amorphous alloys, we found that the plastic strain increases further after heat-induced structural relaxation in the Zr-based BMGs.