Synthesis of copper-based bulk metallic glasses in the ternary Cu–Mg–Ca system

This paper reports the discovery of novel copper-based bulk metallic glasses free of group IV transition metals (Zr, Hf and Ti) in the Ca–Cu–Mg ternary system. Alloys of compositions ranging from Cu-33–55 at.%, Mg-18–36 at.% and Ca-18–36 at.%, located far from eutectic reactions, were found to exhibit high glass-forming ability (up to 8 mm using conventional copper mold casting), high hardness (up to 328H_V) and low densities (2.9–4.0 g/cm³).     

Micro-forming of Zr65Al10Ni10Cu15 metallic glasses under superplastic condition

Micro-formability behavior of the Zr₆₅Al₁₀Ni₁₀Cu₁₅ alloy sheet fabricated by using squeeze casting method, which exhibited Newtonian behavior in the supercooled liquid region, was examined by finite element methods and experiments. In micro-forming simulation on a single V-groove, micro-formability index increased with time and nearly completed die-filling was achieved in 100 s when deformed under a constant pressure of 52.6 MPa at 696 K. In micro-forming on the die with multiple V-grooves in array along its width, the degree of die-filling was predicted to depend on the V-groove position. The V-grooves near the center of the die exhibited the high degree filling ratio of 1 but it was only 0.1 near the free end. This trend was confirmed to agree with the experimental results. According to the simulation result, higher friction coefficient and longer loading time could improve the die-filling ability near the free end.     

Casting effect on softening of metallic glasses

Cooling rate-induced softening in Y-based metallic glasses were verified in this work. By varying the speed of the roller in the melt-spinning process, it is found the hardness of the ribbon decrease with the cooling rate. Nanoindentation test also indicated that the ribbon at a higher cooling rate shows higher deformation energy. Contrary to crystalline materials, whose hardness usually increases with the cooling rate, the softening of metallic glasses is attributed to high concentration of defects frozen by fast cooling, and thus more structural relaxation during deformation.     

Nanoindentation analysis of the mechanical behavior of Zr-based metallic glasses with Sn, Ta and W additions

Zr₅₇Cu₂₀Al₁₀Ti₈Ni₅ and modified composition by adding Sn, W or Ta are studied using standard mechanical test and nanoindentation. Addition of refractory elements with a Sn micro-addition increases clearly the Young's modulus and the hardness of basic BMG. However, Sn reduces plasticity. Moreover these experiments allowed, in confine plasticity conditions, estimating an apparent activation volume associated to a plastic deformation (≈150 Å³).     

Intrinsic versus extrinsic effects on serrated flow of bulk metallic glasses

In this work, we systematically investigate the serrated flow behavior in the compression of bulk metallic glasses by varying the intrinsic composition and various extrinsic material and experimental factors including the sample size, the strain rate, and the testing machine stiffness. We find that the serrated flow, characterized by the amplitude of load serrations, can be suppressed for the higher Young's modulus, larger sample size, higher strain rate, and larger testing machine stiffness, respectively, and that it could completely disappear at certain critical strain rates. Meanwhile, the shape of serrated flow, which tends to become more “blunt”, manifests as the increasing ratio of the duration time to the awaiting time of the serrated events. The dependence of the serrated flow on these various factors is interpreted from the stick-slip dynamics of a single dominant shear band in compression process and can be condensed into a unified theoretical parameter k/k_cr, where k is a parameter dependent on the Young's modulus, the sample size and the machine stiffness, and k_cr is expressed as a function of temperature and testing strain rate. The implication of the stick-slip shear band dynamics together with the tuning of these material factors and test parameters will lead to the design of ductile BMGs.     

Misch metal based metallic glasses

In this paper, a glass-forming range of metallic glasses based on Ce-rich misch metal (Mm) was pinpointed in Mm-Al-Co composition map by melt spinning. The thermal analysis indicated that the wide supercooled liquid region (above 60 K) can be found out in a large composition range in Mm-Al-Co system. The investigation of the glass-forming ability (GFA) in this system indicated a glassy composition with a larger supercooled liquid region wouldn’t be the glassy former with higher GFA. The reduced glass transition temperature is a better indicator to explore metallic glasses with high GFA. Furthermore, the mechanical properties of Mm₆₅Al₁₀Co₂₅ bulk glassy samples were evaluated in a compressive measurement. The obvious advantages of the Mm-based BMGs with high GFA, good mechanical properties and low material cost make these BMGs hopeful to be applied in the future.     

Room-temperature flow in a metallic glass - Strain-rate dependence of shear-band behavior

The rate dependence of serrated flow in amorphous Al_86.8Ni_3.7Y_9.5 has been investigated by nanoindentation. Three samples, containing different initial amounts and distributions of free volume, were used: as quenched, cold rolled and annealed below the crystallization temperature. When the cold-rolled sample is indented at low rates, no new shear bands form, and stable, time-dependent, flow takes place at pre-existing shear bands, well below the glass transition temperature. Aside from instrumental resolution, factors that likely affect serrated flow include the magnitude of the yield drop and the shear-band propagation velocity. The trends we observe are compared with calculations based on the free-volume theory.     

Synthesis and characterization of amorphous Zr-based alloys with Ta and W additions

A reference Zr₅₇Cu₂₀Al₁₀Ni₈Ti₅ bulk metallic glass as well as a number of alloys obtained by addition of refractory elements Ta or W (combined with Sn) to the main Zr–Cu–Al–Ni system were elaborated by mould casting, twin roll casting and planar flow casting. The chemical compositions were chosen according to empirical rules, and as combinations of the binary eutectics for strongly interacting elements, taking into account the corresponding enthalpies of mixing. Optical microscopy, X-ray diffractometry, differential thermal analysis, differential scanning calorimetry, scanning electron microscopy and tensile mechanical testing were used to characterize the fully and partially obtained amorphous materials. Correlation of glass forming ability, thermodynamic parameters, crystallization behaviour and mechanical properties with chemical composition and production technology is discussed.     

Molecular dynamics of shear transformation zones in metallic glasses

Molecular dynamics simulations have been employed to investigate the dynamics of atomic rearrangements in Ni₅₀Zr₅₀ amorphous alloys sheared at different pressures. Local atomic dynamics has been characterized in terms of number of displacing atoms as well as of apparent activation volume and energy. The dimensionality of rearranging atom clusters has been also evaluated. Numerical findings, although not conclusive, indicate that atomic rearrangements are probably triggered by a free volume redistribution. Highly localized and cooperative, such plastic rearrangements occur in regions tending to a two-dimensional occupancy of space.     

Preparation and properties of quaternary CoMoPB bulk metallic glasses

A new series of Co_80−xMo_xP₁₄B₆ (x = 7, 9, and 11 at%) bulk glassy alloys were successfully prepared by a combination method of fluxing treatment and J-quenching technique. The glass-forming ability (GFA) of the obtained Co-based alloys is sensitive to the Mo content substituted for Co, and the maximum attainable diameter for a fully amorphous state can reach 4.5 mm at x = 9. The compressive tests show that the obtained Co-based BMGs exhibit a compressive strength of 3.3–3.9 GPa, but nearly zero compressive plasticity. The new Co-based BMGs possess good soft magnetic properties, and their saturated magnetization values decrease from 47 emu/g (0.45 T) to 14 emu/g (0.14 T) with increasing the content of the Co substitute from 7 at% to 11 at%, which may be attributed to the anti-ferromagnetic coupling between the Mo and Co atoms. Because of their good GFA, high Co content, few constituting elements, and relatively high strength, the obtained Co-based BMGs (especially Co₇₁Mo₉P₁₄B₆ BMG) can be considered promising as starting alloys to develop the new Co-based BMGs for the advanced structural and functional applications.     

Quantitatively defining interconnecting-zone in metallic glasses

The interconnecting-zone(I-zone) concept is important for understanding the nature of metallic glasses, especially the thermal-physical property change around the glass transition. The analyses of the pair-distribution function measured at 15 K using high intensity neutron source on the as-cast amorphous state and its crystallized counterpart of a Zr–Cu–Al alloy provided solid evidence on the I-zone concept. Together with the ab-initio molecular-dynamics simulations, it enables us to quantitatively describe this I-zone concept. The nearest atom pairs in the I-zone were also analyzed.     

The impact of fragility on the calorimetric glass transition in bulk metallic glasses

The glass transition of bulk metallic glasses with various fragilities as well as strong oxide glasses is studied using differential scanning calorimetry (DSC). It is found that the liquid fragility determined from equilibrium viscosity measurements is very well correlated with the scaled maximum slope of the DSC heat flow during the glass transition. We compare the correlation found in this work and those correlations with fragility from previous studies on other classes of glass-formers and find that the slope, which describes the curvature of the enthalpy on a reduced temperature scale, is a quantity better correlated with fragility, as it reflects the timescale of the non-equilibrium relaxation and the distribution of relaxation times in the glassy state. The present findings are supported by a recent theoretical report for calculated enthalpy curves with different fragilities from a model of selenium using the enthalpy landscape approach.     

Thermal stability and magnetic properties of Gd–Fe–Al bulk amorphous alloys

Gd₆₅Fe₂₀Al₁₅, Gd₆₅Fe₁₅Al₂₀ and Gd₇₀Fe₁₅Al₁₅ bulk amorphous alloys were produced by copper mold casting method with the maximum diameters of 2, 1 and 1 mm, respectively. The crystallization temperature (T_x) and melting temperature (T_m) of the Gd₆₅Fe₂₀Al₁₅ bulk amorphous alloy are 808 and 943 K, respectively. Accordingly, the temperature interval of T_m and T_x, ΔT_m (=T_m − T_x), is as small as 135 K and the reduced crystallization temperature (T_x/T_m) is as high as 0.86. The small ΔT_m and high T_x/T_m values are presumed to be the origin for the achievement of the high amorphous-forming ability of the Gd–Fe–Al bulk amorphous alloy. The Gd₆₅Fe₂₀Al₁₅, Gd₆₅Fe₁₅Al₂₀ and Gd₇₀Fe₁₅Al₁₅ bulk amorphous cylinders with a diameter of 1 mm exhibit superparamagnetism at room temperature, while the amorphous ribbon shows the paramagnetism at room temperature. Finally, the mechanical properties of Gd₆₅Fe₂₀Al₁₅ bulk amorphous alloys are investigated.     

Direct synchrotron x-ray measurements of local strain fields in elastically and plastically bent metallic glasses

In situ high-energy synchrotron X-ray diffraction was conducted on elastically and plastically bent bulk metallic glass (BMG) thin plates, from which distinct local elastic strain fields were mapped spatially. These directly measured residual strain fields can be nicely interpreted by our stress analysis, and also validate a previously proposed indirect residual-stress-measurement method by relating nanoindentation hardness to residual stresses. Local shear strain variations on the cross sections of these thin plates were found in the plastically bent BMG, which however cannot be determined from the indirect indentation method. This study has important implications in designing and manipulating internal strain fields in BMGs for the purpose of ductility enhancement.     

Variability in the yield strength of a metallic glass at micron and submicron length scales

This work presents an in-depth investigation of the dispersion in the compressive yield strength of a Zr-based bulk metallic glass as the specimen dimensions decrease from the micron to the submicron length scale. While the investigated alloy belongs to one of the so-called “ductile” glass families, the degree of variation in strength exhibited in the submicron size range approaches that of conventional brittle ceramics. Using a three-parameter Weibull analysis, however, we find that the engineering reliability of the glass is independent of specimen size. The glass exhibits a size-independent failure-free stress of 1825 MPa, below which yielding is not expected. The three-parameter Weibull modulus is also size independent. The dispersion in yield strengths above the failure-free stress is due to the change in the sampling volume for specimens 1 μm in diameter or larger, while interactions between defects and the surface actually limit the breadth of the strength distribution observed in specimens 200–330 nm in diameter relative to what may be expected for that volume. Using an instability criterion for a penny-shaped crack under shear with interfacial friction, a relationship between yield strength and the corresponding defect size is established, and the defect concentration is estimated.     

Strengthening behavior due to cyclic elastic loading in Pd-based metallic glass

Nanoindentation technique was utilized to unravel the strengthening behavior of Pd-based metallic glass under elastic cyclic loading. This was conducted to ultimately evaluate fatigue behavior which is thought to be a major concern for the structural performance of bulk metallic glasses. The event of shear band formation is considered after subjecting the alloy to loading cycles in the nominal elastic regime, followed by a monotonic deep indent. In all experiments, substantial shift in the load needed to initiate a shear band (indicative of plasticity) was noted. Following prolonged cycling, a fatigue limit was observed evident by the saturation in hardening effect. By varying the amplitude of elastic loading, it appears that a critical loading is needed to initiate strengthening in the bulk metallic glass (BMG); i.e. a specific threshold need to be exceeded for the hardening effect to trigger. This was concluded following successive experiments with increasing number of cycles which proves the effect over multiple cycling. Moreover, the effect of higher loading rates becomes more significant as the successive number of cycles increases. For further assessment, comparative experiments were carried out between holding and cycling in the elastic region. These experiments suggested the necessity of cycling to achieve hardening in the alloy, as no hardening effect was observed following holding experiments. In general, strengthening effect was attributed to the possible development of regions of microplasticity due to the actuating force, without being able to detect these regions in the global load-displacement response. Additional tests were conducted to unravel the irreversibility of these cycling/hardening effects. It was found that slight recovery is possible when no resting time was introduced before the deep indent. This suggests that part of the cycling effects can be reversed which can be linked to the instability of the shear transformation zones (STZs).     

Formation and properties of the Zr75−xAlxNi10Cu10Ti5 bulk metallic glasses

Zr-based bulk metallic glasses (BMGs) exhibit interesting mechanical properties since they combine high fracture stress, elastic strain (up to 2%), significant fracture toughness and good corrosion resistance. Quaternary systems with general composition Zr–Ni–Cu–Ti show wide composition ranges in which BMG can be obtained. The addition of the another element to the quaternary alloys often increases the glass forming ability (GFA). The aim of this work was to study the influence of aluminium content on the GFA and on the mechanical properties of the Zr–Ni–Cu–Ti alloys. Multicomponent Zr_75−xAl_xNi₁₀Cu₁₀Ti₅ (x = 15, 20 at%) alloys were produced by melt spinning method obtaining ribbons, and by casting technique into a copper mould, manufacturing rod shape samples with maximum diameter of 2 mm. Supercooled liquid region depends on chemical composition and exceeds 45 °C. Vickers microhardness of studied alloys is comparable to the highest ones for other Zr-based BMG.     

Nanostructure controlling in Zr-based metallic glasses using icosahedral local structure

Glass-nano(quasi)crystal composite materials based on Zr–Al–Ni–Cu metallic glasses have been synthesized by controlling the nucleation and growth rates of the precipitated phase correlated with a unique icosahedral local structure. It is well known that the Zr₆₅Al_7.5Ni₁₀Cu_17.5 metallic glass has a high glass-forming ability (GFA), which enables us to produce the glassy sample with a bulky shape. Controlling a substitution of QC-forming elements such as noble metals with Cu and annealing condition, the nanoscale icosahedral quasicrystalline phase (I-phase) with various grain sizes and nucleus densities can be formed. Moreover, we have succeeded to control the nano-QC phase nucleation by changing the atmosphere pressure during casting, which results in the formation of new bulk metallic glasses (BMGs). These nanoscale structure and nucleation controlling techniques in BMGs bring a significant improvement of mechanical properties such as high strength and good ductility.