Cu–Zr–Al (Ti) bulk metallic glasses: Cluster selection rules and glass formation

The present paper investigates the bulk metallic glass formation in Cu–Zr–Al and Cu–Zr–Ti ternary systems by using the cluster line criterion. The binary basic clusters are first selected for constructing cluster lines. Three cluster selection rules are proposed: topologically dense packing, chemical short-range order and composition distance to deep eutectics. Three Cu–Zr clusters Cu₈Zr₅, Cu₆Zr₅ and Cu₅Zr₆ are selected according to the three rules. The bulk metallic glass-forming ranges in these two systems are determined. The thermal characteristic parameters of the Cu–Zr–Al BMGs on every composition line increase with increasing Al content and decrease with increasing Ti content in the Cu–Zr–Ti glass-forming range. The optimum bulk amorphous compositions in the Cu–Zr–Al system are Cu_58.1Zr_35.9Al₆ and Cu_39.7Zr_47.1Al_13.2, respectively, located in Cu₈Zr₅–Al and Cu₅Zr₆–Al cluster lines. In the Cu–Zr–Ti system, the Cu₆₄Zr_28.5Ti_7.5 composition with the largest T_g/T_l is located in Cu₉Zr₄–Ti cluster line and the Cu_57.6Zr_32.4Ti₁₀ composition with the largest glass-forming ability is located in Cu₆₄Zr₃₆–Ti line.     

Effect of elements with positive enthalpy of mixing on mechanical properties of bulk metallic glasses

Samples of (Cu₄₆Zr₄₆Al₈)_100?xZ_x metallic glass forming alloys with diameters 2–6 mm were prepared by injection casting. The effect of minor amounts of elements Z = Gd, Co and Re with positive enthalpy of mixing within the Gd–Zr, Cu–Co and Cu–Re terminal systems was compared. The addition of Gd up to x = 2 slightly enhances the glass forming ability, Co reduces the critical diameter of bulk metallic glass formation, whereas even for small fractions of Re bulk samples were crystalline, but only amorphous splats can be prepared. Both Gd and Co diminish the crystallization temperature T_x with respect to the Cu₄₆Zr₄₆Al₈ master alloy, but in Re-bearing splats T_x is increased. Alloying with optimum amounts of Gd and Co up to x = 2 leads to plastic deformability of rods, 2 and 3 mm in diameter, in comparison with the brittle Cu₄₆Zr₄₆Al₈ bulk metallic glass.     

Formation of bulk metallic glasses in Cu45Zr48−xAl7REx (RE = La,Ce, Nd,Gd and 0 ≤ x ≤ 5 at.%)

We report a series of bulk metallic glass-forming alloys of compositions (Cu₄₅Zr_48−xAl₇RE_x, RE = La, Ce, Nd, Gd and 0 ≤ x ≤ 5 at.%). By using a conventional copper mold sucking method, alloys with diameters ranging from 5 to 10 mm can be readily solidified into an amorphous structure without detectable crystallites. The best glass-forming ability is obtained for the alloys Cu₄₅Zr₄₆Al₇RE₂. Possible effects of RE addition on the glass-forming ability are discussed. In addition, the compositional effect on mechanical properties of Zr–Cu–Al–Gd alloys is presented.     

Thermal stability of B2 CuZr phase,microstructural evolution and martensitic transformation in Cu–Zr–Ti alloys

The effects of minor Ti addition on the thermal stability of B2 CuZr phase, the microstructure and the martensitic transformation (MT) in Cu₅₀Zr_50−xTi_x (x = 0, 2.5, 7.5 and 10 at%) alloys were investigated. It was found that the crystallization products, i.e. Cu₁₀Zr₇ and Cu(Ti, Zr)₂, of Cu–Zr–Ti amorphous alloys transform to B2 CuZr phase at high temperatures. The corresponding eutectoid transformation temperature gradually increases with increasing Ti content, implying the decrease of the thermodynamic stability of the B2 CuZr phase. The microstructures of Cu–Zr–Ti martensitic alloys were proven to contain B2 CuZr, CuZr martensite, Cu₁₀Zr₇, Cu(Ti, Zr)₂, and ZrTiCu₂ crystals. Dilatometric measurements reveal that the MT temperature reduces with increasing Ti content, which would be of electronic nature. With increasing thermal cycles, the MT temperature gradually decreases while the reverse MT temperature increases, which results from the enhancement of the dislocation density, the partial decomposition of the equiatomic CuZr crystals and the partially reversible MT.     

Phase formation and thermal stability in Cu–Zr–Ti(Al) metallic glasses

In the present study we investigate the phase formation and the thermal stability of Cu₅₀Zr_50 ? xTi_x (0 ≤ x ≤ 10) and (Cu_0.5Zr_0.5)_100 ? xAl_x glass-forming alloys. Parameters indicating the glass-forming ability (GFA) are calculated from isochronal and isothermal calorimetric experiments. A high Ti content in the Cu–Zr–Ti alloys causes the precipitation of a metastable ternary Laves phase (C15), which does not form in Cu–Zr–Al. Accompanied with it is a significant drop in the activation energy of crystallization. Also the supercooled liquid region (ΔT_x = T_x ? T_g), the reduced glass transition temperature (T_rg = T_g/T_liq), and the γ parameter (γ = T_x/(T_g + T_liq)) (T_x: crystallization temperature, T_g: glass transition temperature and T_liq: liquidus temperature) are sensitive to the change in the crystallization sequence. The fragility values calculated are believed to overestimate the GFA of the investigated alloys. Careful selection of the alloy composition enables the targeted precipitation of different crystalline phases.     

Thermal stability and crystallization of Zr–Al–Cu–Ni based amorphous alloy added with boron and silicon

The amorphous Zr_65−x−yAl_7.5 Cu_17.5Ni₁₀Si_xB_y alloy ribbons, x=1–4 and y=1–2, with 0.1 mm thickness were prepared by melt spinning. The thermal properties and microstructure development during the annealing of amorphous alloys were investigated by the combination of differential thermal analysis, differential scanning calorimetry, X-ray diffractometry, and TEM. Both of the glass transition temperature and the crystallization temperature for Zr_65−x−yAl_7.5 Cu_17.5Ni₁₀Si_xB_y alloys increases with the silicon and boron additions and reaches 674 and 754 K, respectively for Zr₆₀Al_7.5 Cu_17.5Ni₁₀Si₄B₁ alloy. The highest T_rg (0.62) and γ value (0.43) occurred at the Zr₆₀Al_7.5Cu_17.5Ni₁₀Si₄B₁ alloy. In addition, the Zr₆₀Al_7.5Cu_17.5Ni₁₀Si₄B₁ alloy was revealed to have the highest activation energy of crystallization (about 370 kJ/mol as determined by the Kissinger plot). This value is about 20% higher than the activation energy of crystallization for the Zr₆₅Al_7.5Cu_17.5Ni₁₀ based alloy (314 kJ/mol). In parallel, the alloy 4Si1B also performs a longer incubation time at higher isothermally annealing temperature. All of the evidence implies that Zr₆₀Al_7.5 Cu_17.5Ni₁₀Si₄B₁ alloy exhibits the highest thermal stability among those alloys in this study. The crystallization behavior for the alloy 4Si1B isothermally annealed at the supercooled temperature region for different time has also been examined by TEM and discussed.     

Liquid–liquid demixing and microstructure of Co–Cu–Zr alloys with low Zr content

Liquid–liquid phase separation and its effect on the microstructure has been investigated along the quasi-binary line (Co₄₀Cu₆₀)_100−xZr_x with x = 2, 4, 6, 9 and additionally for (Co₅₀Cu₅₀)₉₄Zr₆ and (Co₆₀Cu₄₀)₉₄Zr₆. The elemental distributions and the microstructures were analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy for samples that were (i) processed by thermal cycling in alumina crucibles at 10, 20 and 30 K/min with simultaneous differential thermal analysis, (ii) rapidly quenched by single-roller melt spinning and (iii) quenched after having been electromagnetically levitated at various temperatures. The metastable miscibility gap of the binary Co–Cu system with phase separation into Co- and Cu-rich liquids transforms into a stable miscibility gap for Zr contents 3 < x < 7.5 with separation into Co/Zr-rich and Cu-rich liquids. In contrast to the binary Co–Cu system where the Cu-rich liquid phase always surrounds the Co-rich phase, the Zr addition modifies the surface tension energies and/or wetting behavior in a peculiar way so that the Co/Zr-rich phase always encloses the Cu-rich liquid phase concerning the ternary Co–Cu–Zr system in that compositional range. The macrosegregation morphologies of the liquid phase separation that built up via Ostwald ripening, gravity-driven convection, collision, coalescence and wetting effects proceed on a very short time scale and even samples that have been prepared by rapid quenching techniques still exhibit phase separated regions in the micrometer regime.     

Quenched-in quasicrystal medium-range order and pair distribution function study on Zr55Cu35Al10 bulk metallic glass

The quasicrystals phase was found in a crystallized Zr₅₅Cu₃₅Al₁₀ bulk metallic glass by X-ray analyses. Pair-distribution-function (PDF) studies on as-cast and partially crystallized states were performed by neutron scattering measurements. The PDF study shows that a medium-range order exists in the as-cast metallic glass. The investigation on the partial crystallization confirms that the local structures of the metallic glass consist of a quenched-in icosahedral medium-range order, which contribute to the quasicrystallization by reheating.     

Fe–C micro-alloying effect on properties of Zr53Al11.6Ni11.7Cu23.7 bulk metallic glass

(Zr₅₃Al_11.6Ni_11.7Cu_23.7)_1–x(Fe_77.1C_22.9)_x (x=0−2.2, at.%) bulk metallic glasses (BMGs) were prepared by copper mold suction casting method. Their glass forming ability and physical and chemical properties were systematically investigated. The glass forming ability is firstly improved with increasing x, and then decreased when x exceeds 0.44 at.%. Both glass transition temperature and crystallization temperature are increased, while the supercooled liquid region is narrowed, with Fe–C micro-alloying. The hardness, yielding and fracture strength, and plasticity firstly increase and then decrease when x reaches up to 1.32 at.%. The plasticity of the BMG (x=1.32 at.%) is six times that of the Fe-free and C-free BMG. In addition, by the Fe–C micro-alloying, the corrosion potential is slightly decreased, while the corrosion current density increases. The pitting corrosion becomes increasingly serious with the increase of Fe and C content.     

Molecular dynamic simulation study of the structural anisotropy in Cu50Zr50 and Cu64.5Zr35.5 metallic glasses induced by static uniaxial loading within the elastic regime

The structural anisotropy in both Cu₅₀Zr₅₀ and Cu_64.5Zr_35.5 metallic glasses induced by various static uniaxial loads within the elastic regime was studied by molecular dynamic simulations. Constant tensile and compressive loads from zero up to 200 MPa below the flow stress were applied in the simulation. The degree of anisotropy was characterized using a second order contact fabric tensor. It is found that the degree of anisotropy within the elastic regime increases with the applied load following an exponential growth function. The most part of the structural anisotropy can be attributed to the Zr–Cu atomic pairs in Cu_64.5Zr_35.5 and Zr–Cu, Cu–Cu atomic pairs in Cu₅₀Zr₅₀. The evolution of the structural anisotropy associated with the Zr–Zr and Cu–Cu pairs exhibits complex behavior, which implies that the deformation mechanism and the dynamics of Cu₅₀Zr₅₀ and Cu_64.5Zr_35.5 metallic glasses under compressive loads may be quite different from those under tensile loads.     

Zr-Co(Cu)-Al bulk metallic glasses with optimal glass-forming ability and their compressive properties

The formation of bulk metallic glasses (BMGs) in the ternary Zr₅₆Co₂₈Al₁₆ and quaternary Zr₅₆Co_28–xCu_xAl₁₆ (x=2, 4, 5, 6, 7, mole fraction, %) glassy alloys was investigated via the copper mold suction casting method. The main purpose of this work was to locate the optimal BMG-forming composition for the quaternary ZrCo(Cu)Al alloys and to improve the plasticity of the parent alloy. The X-ray diffractometry (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) were used to investigate the glassy alloys structure and their glass forming ability (GFA). In addition, the compression test, microhardness, nano-indentation and scanning electron microscopy (SEM) were utilized to discuss the possible mechanisms involved in the enhanced plasticity achievement. The highest GFA among Cu-containing alloys was found for the Zr₅₆Co₂₂Cu₆Al₁₆ alloy, which was similar to that of the base alloy. Furthermore, the plasticity of the base alloy increased significantly from 3.3% to 6% for the Zr₅₆Co₂₂Cu₆Al₁₆ BMG. The variations in the plasticity and GFA of the alloys were discussed by considering the positive heat of mixing within Cu and Co elements.     

Formation of amorphous phase with crystalline globules in Fe–Cu–Si–B and Fe–Cu–Zr–B immiscible alloys

The microstructure of rapidly solidified melt-spun ribbons of (Fe_0.75M_0.10B_0.15)_100−xCu_x (M = Si, Zr) alloys was investigated focusing on amorphous-phase formation and the solidification structure. In this study, Fe–Cu–Si–B and Fe–Cu–Zr–B alloys were designed to show amorphous-phase formation and liquid-phase separation simultaneously. Amorphous-phase formation was confirmed in both Fe–Cu–Si–B and Fe–Cu–Zr–B alloys. Minor exceptions in a combination map of mixing enthalpy and quaternary predicted phase diagram are acceptable range for designing a quaternary Fe–Cu-based alloy system that shows liquid-phase separation in Fe-based and Cu-based liquids and the formation of an Fe-based amorphous phase.     

Formation and properties of Ti-based Ti–Zr–Cu–Fe–Sn–Si bulk metallic glasses with different (Ti + Zr)/Cu ratios for biomedical application

Ti-based Ti–Zr–Cu–Fe–Sn–Si bulk metallic glasses (BMGs) free from highly toxic elements Ni and Be were developed as promising biomaterials. The influence of (Ti + Zr)/Cu ratio on glass-formation, thermal stability, mechanical properties, bio-corrosion resistance, surface wettability and biocompatibility were investigated. In the present Ti-based BMG system, the Ti₄₇Zr_7.5Cu₄₀Fe_2.5Sn₂Si₁ glassy alloy exhibited the highest glass forming ability (GFA) corresponding to the largest supercooled liquid region, and a glassy rod with a critical diameter of 3 mm was prepared by copper-mold casting. The Ti-based BMGs possess high compressive strength of 2014–2185 MPa and microhardness of 606–613 Hv. Young's modulus of the Ti₄₇Zr_7.5Cu₄₀Fe_2.5Sn₂Si₁ glassy alloy was about 100 GPa, which is slightly lower than that of Ti–6Al–4V alloy. The Ti₄₇Zr_7.5Cu₄₀Fe_2.5Sn₂Si₁ glassy alloy with high GFA exhibited high bio-corrosion resistance, and good surface hydrophilia and cytocompatibility. The mechanisms for glass formation as well as the effect of (Ti + Zr)/Cu ratio on bio-corrosion behavior and biocompatibility are discussed.     

Formation and mechanical properties of Ni-free Zr-based bulk metallic glasses

Glass formation and mechanical properties of Zr–Al–Co–Cu–Ag bulk metallic glasses (BMGs) were investigated. The glass-forming ability (GFA) of Zr₅₅Al₂₀Co₂₀Cu₅ alloy is significantly improved with minor addition of Ag, indicating by the impressive increase of the critical diameter of glass formation from 5 mm for Zr₅₅Al₂₀Co₂₀Cu₅ to 16 mm for (Zr_0.55Al_0.20Co_0.20Cu_0.05)₉₇Ag₃ and (Zr_0.55Al_0.20Co_0.20Cu_0.05)₉₅Ag₅ alloys. The Zr–Al–Co–Cu–Ag BMGs exhibit high compressive strength of 2160–2280 MPa and distinct plasticity of 0.6–2.5%. The Zr-based BMGs with outstanding GFA and mechanical properties as well as low-level cytotoxicity elements are expectative for industrial and biological applications.     

Structural evolution of Cu–Zr metallic glasses under tension

The structural behaviour of Cu₅₀Zr₅₀ and Cu₆₅Zr₃₅ metallic glasses under uniaxial tensile stress was investigated in situ by high-energy X-ray synchrotron diffraction. The components of the elastic strain tensor were determined from both the change of positions of first maximum of the structure factor in reciprocal space as well as from the maxima of the atomic pair correlation function in real space. The atomic scale strain agrees with the macroscopic strain values. The topological and chemical short-range order of the Cu–Zr glasses changes upon loading. The number density of Cu–Zr and Zr–Zr nearest neighbour atomic pair becomes oriented along the loading direction whereas the partial nearest neighbour distances are only weakly influenced.     

High strength Ni–Zr–(Al) nanoeutectic composites with large plasticity

Micrometer-sized γ−Ni dendrite reinforced nanoeutectic matrix composites have been developed in (Ni_0.92Zr_0.08)_100–xAl_x (0 ≤ x ≤ 4) by arc melting. The eutectic matrix is composed of alternate nano-lamellae of intermetallic Ni₅Zr and fcc–Ni solid solution phases. All these composites exhibit very high strength, large compressive plasticity ∼25% and strain-hardening up to 1780 MPa. Al dissolves in γ−Ni(Zr) solid solution phase, decreases its hardness/strength, and increases the volume % of γ−Ni dendrite from 20% (x = 0) to 29% (x = 4). Whereas, refinement of the eutectic lamellae thickness from 275 nm (x = 0) to 160 nm (x = 4) increases the matrix hardness and retains the global strength of the composites. The effect of Al addition on the microstructure formation, volume fraction as well as the length scale of the constituent phases, and mechanical properties, have been discussed using an analytical model.     

Crystallization behavior of the Zr63Al7.5Cu17.5Ni10B2 amorphous alloy during isothermal annealing

The crystallization behavior of Zr₆₃Al_7.5Cu_17.5Ni₁₀B₂ amorphous Alloy was studied by means of scanning differential calorimetry (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM). A single stage transformation of the amorphous phase forming a Zr₂Cu-type crystalline phase was observed. Kinetics for such single stage crystallization was analyzed by means of Johnson–Mehl–Avrami equation and discussed regarding to the value of Avrami exponent. In addition, the cube of crystal size presents a linear relationship with isothermal annealing time. This indicates that the crystallization process of Zr₆₃Al_7.5Cu_17.5Ni₁₀B₂ amorphous alloy belongs to thermally activated process of Arrhenius type. From the HRTEM analysis, small amount of Zr₂Cu-type crystals in the nano-scale dimension (10–20 nm) were observed to precipitate from the amorphous matrix upon the early stage of isothermal annealing the amorphous alloy at the temperature between the glass transition (T_g) and the onset crystallization temperature (T_x).     

Enthalpy relaxation in Cu46Zr45Al7Y2 and Zr55Cu30Ni5Al10 bulk metallic glasses by differential scanning calorimetry (DSC)

Structural relaxation process in Cu₄₆Zr₄₅Al₇Y₂ and Zr₅₅Cu₃₀Ni₅Al₁₀ bulk metallic glasses during annealing below the glass transition temperature T_g was investigated by differential scanning calorimetry (DSC). The features of enthalpy relaxation are sensitive to both annealing temperature and annealing time. For a given annealing time t_a, the results indicated that the relaxation time t_a decreases with increasing the annealing temperature T_a, in good agreement with results relative to other bulk metallic glasses. Additionally, the enthalpy relaxation behaviour of the bulk metallic glasses appears independent on the cooling rate used before the physical aging experiments, i.e. on the initial as-cast state. The recovered enthalpy evolution of the bulk metallic glasses is well described by the Kohlrausch–Williams–Watts (KWW) exponential relaxation function as ΔH(T_a) = ΔH_eq{1 ? exp[?(t_a/τ)^β]}. Kohlrausch exponent β and enthalpy relaxation time τ are sensitive to the composition of the bulk metallic glasses. Finally, the influence of different heating treatment processes on the enthalpy relaxation in the bulk metallic glasses is presented and shows that this phenomenon is mainly reversible. The structural relaxation behaviour is interpreted by free volume model and quasi-point defects model. Kinetic fragility parameters m in Cu₄₆Zr₄₅Al₇Y₂ and Zr₅₅Cu₃₀Ni₅Al₁₀ bulk metallic glasses are 72 and 69, respectively, indicating therefore that these alloys are intermediate glasses.Crystallization process was also investigated by DSC experiments. According to the Kissinger model, corresponding activation energy is 3.18 eV in Cu₄₆Zr₄₅Al₇Y₂, and 3.19 eV in Zr₅₅Cu₃₀Ni₅Al₁₀, respectively.     

Effect of Ta on glass formation,thermal stability and mechanical properties of a Zr52.25Cu28.5Ni4.75Al9.5Ta5 bulk metallic glass

The effect of Ta on glass-forming ability, crystallization behavior and mechanical properties of Zr_52.25Cu_28.5Ni_4.75Al_9.5Ta₅ bulk metallic glass (BMG) is investigated. The solubility of Ta in the Zr-base BMG alloy depends on the arc melting conditions. 3.2 at.% Ta dissolve in the alloy inducing an increase of about 20 K in both glass transition temperature and crystallization temperature of the BMG. However, Ta does not significantly change the extension of the supercooled liquid region. The remaining Ta particles in the master alloy may induce a composition-segregation layer around the particles upon subsequent casting. This further induces the crystallization of Zr₂Cu that deteriorates the ductility of the samples. The compressive strength and ductility of the as-cast 3 mm diameter Zr_52.25Cu_28.5Ni_4.75Al_9.5Ta₅ samples are improved in comparison with the Zr₅₅Cu₃₀Ni₅Al₁₀ BMG alloy. The fracture plane of the present alloy has an angle of 31–33° with respect to the stress axis, which remarkably deviates from the maximum shear stress plane. The improvement of the mechanical properties and the peculiar fracture feature for the Zr_52.25Cu_28.5Ni_4.75Al_9.5Ta₅ BMG alloy can be attributed to the effect of dispersed Ta particles.     

Formation and properties of Zr48Nb8Cu14Ni12Be18 bulk metallic glass

Zr₄₈Nb₈Cu₁₄Ni₁₂Be₁₈ bulk metallic glass (BMG) with excellent glass-forming ability was prepared by water quenching method. The BMG exhibits high glass transition temperature T_g and onset crystallization temperature T_x, compared with Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 BMG. The crystallization processes, change of elastic constants, and density and hardness in the crystallization process were studied by using X-ray diffraction, differential scanning calorimetry and acoustic method. The shear modulus, Poisson ratio, density and hardness are found to be sensitive to the crystallization process. A striking softening of long-wavelength transverse acoustic phonons in the BMG relative to its crystallized state is observed. The linear expansion coefficient, determined by a dilatometer method, is α_TG=1.04×10⁻⁵ K⁻¹ (300–656 K) for the BMG and α_TC=1.11×10⁻⁵ K⁻¹ (356–890 K) for the crystalline alloy. The Mie potential function and the equation of state of the BMG are determined from the expansion coefficient and acoustic experiments.