Zr–Cu–Ni–Al bulk metallic glasses with superhigh glass-forming ability

Zr–Cu–Ni–Al quaternary amorphous alloy compositions with varying glass-forming ability are developed by an efficient method of proportional mixing of binary eutectics. The critical diameter of the glassy sample is improved from 6 mm for Zr₅₃Cu_18.7Ni₁₂Al_16.3 to 14 mm for Zr_50.7Cu₂₈Ni₉Al_12.3 by straightforwardly adjusting the eutectic unit’s coefficients. The drastic improvement in GFA is attributed to balancing the chemical affinities of the Zr, Cu, Ni and Al components in the melt prior to solidification which makes the precipitation of competing crystalline phases more difficult. As the glass-forming ability increases, the concentration of Cu in the alloys exhibits a same trend. Based on synchrotron radiation high-energy X-ray diffraction analysis and Miracle’s structural model, it is envisioned that the substitution of additional Cu atoms for Zr atoms in the investigated alloys stabilizes the efficient cluster packing structure of the amorphous alloys, leading to the pronounced increase in their glass-forming ability.     

Effect of addition of Zn and Al elements on glass-forming ability and thermal stability of Mg-Cu-Y bulk metallic glasses

After substituting partial Cu and Mg with Zn or Al elements for Mg65Cu25Y10 alloy, respectively, the metallic glass plate samples with thickness of 2-3 mm were prepared by water-quenching, their respective glass-forming ability and thermal stability were studied by using differential thermal analysis （DTA） and X-ray diffraction （XRD）. Using Kissinger equation, the activation energies of crystallization of these metallic glasses heated with a constant rate were calculated. The results show that Al element is greatly harmful to the glass-forming ability of Mg-Cu-Y alloys and cannot acquire bulk amorphous alloys; nevertheless, the effect of Zn element addition is indeterminate for various components. The magnitudes of thermal stability are also revealed.     

Criteria for tensile plasticity in Cu–Zr–Al bulk metallic glasses

(Cu_0.5Zr_0.5)_100?xAl_x (x = 5, 6, 8) bulk metallic glasses (BMGs) were deformed in tension. Besides ductility up to 0.5%, the material shows work-hardening behaviour. Both effects are attributed to the deformation-induced precipitation of B2 CuZr nanocrystals and the formation of twins in the nanocrystals larger than 20 nm. The precipitation of the nanocrystals alters the stress field in the matrix and is expected to retard shear band propagation, which in turn allows stresses in the nanocrystals to rise. This stress build-up is more severe in the larger grains and might be responsible for the subsequent twinning. Both deformation-induced nanocrystallization and twinning consume energy and avoid crack formation and with it premature failure.     

First-principles prediction of the glass-forming ability in Zr–Ni binary metallic glasses

In the present study, the atomistic approach, developed previously by hybridizing both internal energies and atomic-scale defect structures, is extended to predict the trend of glass forming ability (GFA) from the Zr–Cu binary alloy system with good GFA to the Zr–Ni binary system with relatively poor GFA. The predicted composition dependence of the GFA in the Zr–Ni alloy system is consistent with those obtained from experimental results, indicating the validity of the proposed atomistic approach in both of the alloy systems. The different GFAs in the Zr–Cu and Zr–Ni systems have been puzzled by the metallic glass community for quite a long time, and our study provides the physical reasons for the different glass forming behaviors of the two alloy systems in terms of both atomic configurations and relative stability of their intermetallic phases.     

Glass forming ability and mechanical properties of Nb-containing Cu–Zr–Al based bulk metallic glasses

Mechanical properties of (Cu₅₀Zr₄₃Al₇)_100-xNb_x (x=0,1,3,6,9) bulk metallic glasses rods with a diameter of 2.5～mm prepared by suction casting method were studied. The results of uniaxial compression tests at room temperture show that the best mechanical properties of 2.8% and 1.98 GPa for plastic strain and fracture strength, respectively, in the sample with x=3. Microstructure, fracture surface and shear bands of the samples were observed by SEM and XRD methods.     

Zr–(Cu,Ag)–Al bulk metallic glasses

In this paper, we report the formation of a series Zr–(Cu,Ag)–Al bulk metallic glasses (BMGs) with diameters at least 20 mm and demonstrate the formation of about 25 g amorphous metallic ingots in a wide Zr–(Cu,Ag)–Al composition range using a conventional arc-melting machine. The origin of high glass-forming ability (GFA) of the Zr–(Cu,Ag)–Al alloy system has been investigated from the structural, thermodynamic and kinetic points of view. The high GFA of the Zr–(Cu,Ag)–Al system is attributed to denser local atomic packing and the smaller difference in Gibbs free energy between amorphous and crystalline phases. The thermal, mechanical and corrosion properties, as well as elastic constants for the newly developed Zr–(Cu,Ag)–Al BMGs, are also presented. These newly developed Ni-free Zr–(Cu,Ag)–Al BMGs exhibit excellent combined properties: strong GFA, high strength, high compressive plasticity, cheap and non-toxic raw materials and biocompatible property, as compared with other BMGs, leading to their potential industrial applications.     

Corrosion behavior of Zr–Cu–Ni–Al bulk metallic glasses in chloride medium

Polarization and passivation behavior of three Zr-based BMGs, i.e. Zr_58.3Al_14.6Ni_8.3Cu_18.8, Zr₅₈Al₁₆Ni₁₁Cu₁₅ and Zr_57.5Al_17.5Ni_13.8Cu_11.3 were investigated in 3% NaCl aqueous solution. Electrochemical investigations were carried out by potentiodynamic polarization method at room temperature. The corroded sample surfaces were examined using scanning electron microscope having energy dispersive spectroscopy (EDS) attachment. The results of the present investigation revealed that Zr₅₈Al₁₆Ni₁₁Cu₁₅ and Zr_57.5Al_17.5Ni_13.8Cu_11.3 BMGs having relatively larger supercooled liquid region (ΔT_x) and pitting overpotential (η_pit) values exhibit low corrosion current density (i_corr) and corrosion penetration rate (CPR) values.     

Correlation between mechanical behavior and glass forming ability of Zr–Cu metallic glasses

A series of Zr–Cu glassy ribbons were fabricated, and the compositional dependence of microhardness in the as-quenched and annealed state was systematically investigated. The as-quenched microhardness exhibits a positive deviation from linearity at the compositions which correspond to local maximum in glass forming ability (GFA). Upon annealing, the microhardness change is relatively smaller for the high GFA compositions. High atomic packing density at the special compositions should be responsible for the microhardness behavior.     

Thermal stability,magnetic and mechanical properties of Fe–Dy–B–Nb bulk metallic glasses with high glass-forming ability

The effects of Dy addition on the thermal stability, glass-forming ability (GFA), magnetic and mechanical properties of quaternary (Fe_0.76−xDy_xB_0.24)₉₆Nb₄ (x = 0–0.07) bulk metallic glasses (BMGs) were investigated. Increasing Dy content from x = 0 to 0.05 extended the supercooled liquid region up to 112 K, allowing the fabrication by copper mold casting of BMGs rods with 5.5 mm in diameter. The high GFA was found to be related to the structure of primary crystalline phase. For the x = 0.05 alloy, the competitive formation process of the complex Fe₂₃B₆ and Dy₂Fe₁₄B phases enabled to obtain the largest GFA value. Moreover, the Fe–Dy–B–Nb BMGs exhibited good soft-magnetic properties, i.e., high saturation magnetization of 1.18–0.56 T and low coercive force of 1.9–21.6 A/m. In addition, the glassy alloy rods also showed high compressive fracture strengths of 4400–4150 MPa and high Vickers hardness of 1110–1090 kg/mm².     

Structural relaxation and related viscous flow of Zr–Cu–Al-based bulk glasses produced from the melts with different glass-forming ability

The relaxation ability of Zr₄₆(Cu_4/5Ag_1/5)₄₆Al₈ and Zr₄₆Cu₄₆Al₈ bulk glasses displaying different glass-forming ability (GFA) in the liquid state has been studied by measurements of the shear modulus, creep and stress relaxation. It has been found that the four-component alloy with higher GFA always displays higher relaxation ability in the glassy state. This fact does not seem to correlate with the amount of the frozen-in free volume as dependent on the GFA. It has been argued that the relaxation centers responsible for structural relaxation and related viscous flow are similar to dumbbell interstitials in simple metals. Such an approach precisely describes the kinetics of shear modulus relaxation and reasonably agrees with a phenomenological interpretation of structural relaxation-induced viscoelastic phenomena in metallic glasses.     

The effects of hydrogen on viscoelastic relaxation in Zr–Ti–Ni–Cu–Be bulk metallic glasses: implications for hydrogen embrittlement

The effects of hydrogen on the viscoelastic relaxation behavior of a Zr–Ti–Ni–Cu–Be bulk metallic glass have been investigated in an attempt to elucidate hydrogen-affected flow and fracture behavior. Dynamic mechanical testing was performed to study relaxation behavior near the glass transition temperature. Relaxation time constants were increased in the presence of hydrogen with a concomitant increase of thermal activation energy. In addition, the glass transition temperature was increased and crystallization kinetics retarded in the presence of hydrogen leading to enhanced thermal stability. Positron annihilation spectroscopy was employed to study the interaction of hydrogen and open-volume regions. While hydrogen charging was found to decrease the open-volume regions in the amorphous phase, an increase in free volume was observed in the crystalline counterpart. The amorphous phase was found to have a greater hydrogen absorption capacity compared to its crystalline counterpart. Relaxation behavior, crystallization kinetics and the interaction of hydrogen with the amorphous microstructure are discussed. Finally, the effects of retarded relaxation processes on fracture resistance are considered.     

Effects of rare-earth elements on the glass-forming ability and mechanical properties of Cu46Zr47-xA17Mx （M = Ce, Pr, Tb, and Gd） bulk metallic glasses

Cu₄₆Zr_47−x Al₇M _x (M = Ce, Pr, Tb, and Gd) bulk metallic glassy (BMG) alloys were prepared by copper-mold vacuum suction casting. The effects of rare-earth elements on the glass-forming ability (GFA), thermal stability, and mechanical properties of Cu₄₆Zr_47−x Al₇M _x were investigated. The GFA of Cu₄₆Zr_47−x Al₇M _x (M = Ce, Pr) alloys is dependent on the content of Ce and Pr, and the optimal content is 4 at.%. Cu₄₆Zr_47−x Al₇Tb _x (x = 2, 4, and 5) amorphous alloys with a diameter of 5 mm can be prepared. The GFA of Cu₄₆Zr_47−x Al₇Gd _x (x = 2, 4, and 5) increases with increasing Gd. T _x and T _p of all decrease. T _g is dependent on the rare-earth element and its content. ΔT _x for most of these alloys decreases except the Cu₄₆Zr₄₂Al₇Gd₅ alloy. The activation energies ΔE _g, ΔE _x, and ΔE _p for the Cu₄₆Zr₄₂Al₇Gd₅ BMG alloy with Kissinger equations are 340.7, 211.3, and 211.3 kJ/mol, respectively. These values with Ozawa equations are 334.8, 210.3, and 210.3 kJ/mol, respectively. The Cu₄₆Zr₄₅Al₇Tb₂ alloy presents the highest microhardness, Hv 590, while the Cu₄₆Zr₄₃Al₇Pr₄ alloy presents the least, Hv 479. The compressive strength (σ _c.f.) of the Cu₄₆Zr₄₃Al₇Gd₄ BMG alloy is higher than that of the Cu₄₆Zr₄₃Al₇Tb₄ BMG alloy.     

Effect of Nb content on the microstructures and mechanical properties of Zr–Ti–Cu–Be–Nb glass-forming alloys

(Zr₃₅Ti₃₀Be_27.5Cu_7.5)_100?xNb_x (x = 0, 5, 8, 10, 12, 15 at.%) glass-forming alloys were prepared by copper-mould suction casting. The alloys with different Nb contents exhibited different microstructures and mechanical properties. The proper addition of Nb (x = 5, 8 at.%) to the Zr–Ti–Be–Cu system could ensure the formation of mostly amorphous phase. And excessive amount of Nb favored the formation of the bcc β-Zr solid solution. The alloys with Nb contents of 8 at.%, 10 at.%, and 12 at.% displayed the distinguished plasticity of 11.1%, 7.6%, and 11.0%, respectively.     

Composition optimization of the Al–Co–Zr bulk metallic glasses

Composition optimization for locating the composition with the largest glass forming ability in the Al–Co–Zr system is attempted in this investigation. The criteria that we have developed are respectively related to a specific conduction electron concentration, termed the e/a-constant criterion, and to a specific cluster structure, termed the e/a-variant criterion. For this system, the two criteria are incarnated into the composition line with constant e/a=1.5 and the Co₄Zr₉–Al composition line. Bulk metallic glasses are obtained by suction casting for compositions with e/a=1.3–1.5, with their thermal stabilities and glass forming abilities being increased with increasing e/a ratios. The crossing point of the e/a=1.5 line and the Co₄Zr₉–Al line gives the composition Al_23.5Co_23.5Zr₅₃ with the largest GFA (e.g. T_g/T_m=0.637), superior to the reported Al₂₀Co₂₅Zr₅₅ alloy with T_g/T_m=0.621.     

Effect of Al addition on formation and mechanical properties of Mg-Cu-Gd bulk metallic glass

The effect of partial substitution of Al for Cu on the glass forming ability（GFA） and mechanical properties of Mg65Cu25-xAlxGd10 （x=0, 1, 3 and 5, molar fraction, %） alloys were studied by X-ray diffi-actometry（XRD）, differential scanning calorimetry（DSC） and uniaxial compression test. The result reveals that GFA of the alloys changes slightly with increasing x from 0 to 3, and then abruptly decreases with x increasing up to 5. The plasticity can be greatly improved with appropriate substitution of Cu by A1 （3%, molar fraction） in Mg65Cu25Gd10 bulk metallic glass, and the resultant fracture strength, total strain to failure, and plastic strain are 898 MPa, 2.19% and 0.2%, respectively.     

Structural origins for the high plasticity of a Zr–Cu–Ni–Al bulk metallic glass

The structural origins for the high plasticity of a Zr₅₃Cu_18.7Ni₁₂Al_16.3 (at.%) bulk metallic glass are explored. Under plastic flow conditions, in situ synchrotron high-energy X-ray diffraction reveals that the atomic strain saturates to the closest packing in the longitudinal direction of the applied load while atoms yield in the transverse plane. Scanning electron microscopy investigation reveals that global plasticity benefits from abundant shear band multiplication and interactions. Atomic level flows are seen to accompany profuse shear bands. The plasticity enhancement of this metallic glass benefits from such atomic level flows. Atomic level flow facilitates the activation of shear transformation zones that further self-assemble to promote shear band multiplication. On the other hand, it also mitigates the shear band propagation that prevents catastrophic shear band extension.     

Remarkable effect of Ce base element purity upon glass forming ability in Ce–Ga–Cu bulk metallic glasses

Raw Ce element materials of eleven different purities are used to prepare bulk metallic glasses with the same nominal composition of Ce₇₀Ga₈Cu₂₂ (at.%). In the high-purity regime of Ce (98.13–99.87wt.%), three distinct peaks are observed in the curve plotting the purity vs. the glassy rod critical diameter (D_c); and with ∼0.11 wt.% decrease in purity, the D_c can increase sharply from 1 to 10 mm. In the relatively low-purity regime of 96.15–98.13 wt.%, the low material purity is found to be beneficial for glass formation; and with a ∼0.61 wt.% decrease in purity, the D_c increases dramatically from 1.5 mm to at least 20 mm. Such a sensitive and systematic purity-dependent glass-forming ability has rarely been reported before in metallic glasses. It is also suggested that the high stability of the competing crystalline phases results from the mixture effect via addition of multiple impurity elements into the matrix glass-forming alloys, and that this addition of impurity elements may be the dominant factor responsible for their intrinsic glass-forming ability of these alloys. The results provide systematic evidence for the strong purity and composition effects that are present in glass formation, and can be used to shed light on scientific research and industrial applications in the field of metallic glasses.     

The effect of oxygen on the microstructural evolution in crystallized Cu–Zr–Al metallic glasses

Cu–45Zr–8Al and Cu–44.1Zr–7.8Al–2Y metallic glass ribbons were prepared by melt spinning and heat treated at two different temperatures followed by microstructural characterization by SEM and TEM. The compositions were selected to better understand the effect that oxygen has in the sequence and morphology of the phases formed during devitrification. A novel microstructure consisting of the Heusler Cu₂ZrAl phase precipitated inside a B2–CuZr matrix that has shape memory properties was produced. It is proposed that this novel microstructure may be used to prepare samples with improved shape memory responses. The known cube–cube relationship between the Heusler and B2 phases also provides direct evidence about the orientation relationship between the martensite and the parent B2 austenite and the results are in contrast to what is reported in the literature. The results also confirm that oxygen dissolved in these alloys leads to the formation of the B2–CuZr phase at lower temperatures than comparable low oxygen alloys.     

Effect of Nb addition on microstructure evolution and nanomechanical properties of a glass-forming Ti–Zr–Si alloy

The glass-forming Ti₇₅Zr₁₀Si₁₅ alloy is regarded as a potential material for implant applications due to its composition of non-toxic, biocompatible elements and some interesting mechanical properties. The effects of partial substitution of 15 at.% Ti by Nb on the microstructure and the mechanical behaviour have been investigated by X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray analysis, transmission electron microscopy and nanoindentation techniques. Copper mold casting and melt-spinning methods have been applied to study the influence of the cooling rate on the properties of both alloys, Ti₇₅Zr₁₀Si₁₅ and Ti₆₀Zr₁₀Nb₁₅Si₁₅. As a result of different cooling rates, significant microstructural variations from multiphase crystalline states in cast rods to nanocomposite structures in ribbons were observed. The limited glass-forming ability (GFA) of the Ti₇₅Zr₁₀Si₁₅ alloy results for melt-spun ribbons mainly in nanocomposite structures with β-type nanocrystals being embedded in a glassy matrix. Addition of Nb increases the glass-forming ability. Raising the overheating temperature of the melt prior to melt-spinning from 1923 K to 2053 K yields for both alloys a higher amorphous phase fraction. The mechanical properties were investigated using compression tests (bulk samples) and the nano-indentation technique. A decrease of hardness (H), ultimate stress and reduced Young's modulus (E_r) is observed for Ti₆₀Zr₁₀Nb₁₅Si₁₅ rods as compared to Ti₇₅Zr₁₀Si₁₅ ones. This is attributed to an increase of the fraction of the β-type phase. The melt-spun ribbons show an interesting combination of very high hardness values (H) and moderate reduced elastic modulus values (E_r). This results in comparatively very high H/E_r ratios of >0.075 which suggests these new materials for applications demanding high wear resistance.