Phase separation in Cu46Zr47−xAl7Gdx metallic glasses

The influence of Gd addition on phase separation of rapidly quenched Cu₄₆Zr_47?xAl₇Gd_x metallic glasses (x = 2,5,7) was studied. For low Gd content (x = 2), a homogeneous glass is obtained for the as-quenched state. Annealing leads to cluster formation by nucleation and growth prior to crystallization. For the Gd contents x = 5 and 7, early stages of spinodal decomposition are observed in the as-quenched glasses. Further annealing increases the amplitude of the compositional fluctuations prior to crystallization. Atom probe tomography gives evidence of the presence of Gd-enriched clusters of 2–5 nm size for the Cu₄₆Zr₄₂Al₇Gd₅ glass. The structure formation as a function of the Gd content is essentially determined by the composition dependence of the miscibility gap of the metastable undercooled melt. Early stages of spinodal decomposition or almost homogeneous glassy states are obtained if the critical temperature of liquid–liquid phase separation is close or below to the glass transition temperature.     

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.     

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.     

A Ni-free Zr-based bulk metallic glass with remarkable plasticity

The effect of Nb and Pd combination on the glass forming ability (GFA) and mechanical properties of Zr₅₃Cu₃₀Nb_xPd_9?xAl₈ (x = 3.5–6.0) bulk metallic glasses (BMGs) were systematically investigated by X-ray diffractometry (XRD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and compression test. TEM observation revealed that a nanocrystalline phase embeds in the amorphous matrix of the as-cast Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ alloy. A tiny nano-crystalline phase (with size about 5–20 nm) embedded uniformly in the amorphous matrix of the Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ alloy was observed and identified to be the tetragonal structured NbPd₃ phase based on the analyses of nano beam electron diffraction. According to the results of thermal analyses, the composition of Zr₅₃Cu₃₀Nb₅Pd₄Al₈ and Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ present the optimum GFA as well as thermal stability in the Zr₅₃Cu₃₀Nb_xPd_9?xAl₈ (x = 3.5–6.0) alloy system. In addition, the result of compression test shows that the yield strength significantly increases from 1700 MPa (Zr₅₃Cu₃₀Nb₅Pd₄Al₈) to 1900 MPa (Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈). A remarkable compression plastic strain (11.2%) occurs at Zr₅₃Cu₃₀Nb_4.5Pd_4.5Al₈ BMG rod with 2 mm in diameter. This significant increase in plasticity is presumably due to the restriction on shear banding by the nano-size second phase.     

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.     

Ni–(Zr/Hf)–(Nb/Ta)–Al bulk metallic glasses with high thermal stabilities

Thermal stability is a critical consideration in the application of metallic glasses as hydrogen separation material. The development of new Ni-based bulk metallic glasses (BMGs) with enhanced thermal stability is desirable. The present work investigated the alloying effects of refractory metals Hf and Ta on the Ni₆₀Zr₂₀Nb₁₅Al₅ bulk metallic glass. Two serial alloys, namely, Ni₆₀Zr_20 ? xHf_xNb₁₅Al₅ (x = 0～20 at.%) and Ni₆₀Zr₂₀Ta_yNb_15 ? yAl₅ (y = 0～15 at.%), were investigated in the present work. The addition of Hf or Ta was revealed to be effective in improving the thermal stability of the basic alloy, while the glass-forming ability of the alloy is slightly reduced resulting from the addition of Hf or Ta. The possible mechanism of these alloying effects is discussed.     

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.     

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.     

Role of crystalline precipitates on the mechanical properties of (Cu0.50Zr0.50)100−xAlx (x = 4, 5, 7) bulk metallic glasses

The mechanical behaviour upon compression of (Cu_0.50Zr_0.50)_100?xAl_x (x = 4, 5, 7) rods with 2 and 3 mm diameter was systematically studied and compared with the literature data. Fully amorphous bulk metallic glasses (x = 5, 7) show little permanent deformation and reproducible yield stress values. The remarkable fracture strain, observed for some apparently X-ray diffraction amorphous samples (x = 4), was found to be due to significant amounts (at least 20%) of the B2-CuZr crystalline phase. The effect of possible flaws on the external surface of the rods was evaluated by mechanical testing of either as cast and machined samples.     

Glass formation in La-based La–Al–Ni–Cu–(Co) alloys by Bridgman solidification and their glass forming ability

Six La-based La–Al–Ni–Cu–(Co) alloys were subjected to a systematical study of glass formation by Bridgman unidirectional solidification at growth velocities between 0.1 and 4.82 mm/s at a temperature gradient of 15 K/mm. With increased Cu content the critical growth velocity for glass formation in La₅₅Al₂₅Cu_20−xNi_x (x=0–20) alloys shows a steep minimum at 10 at.% Cu, indicating the largest glass forming ability for the La₅₅Al₂₅Ni₁₀Cu₁₀ alloy. However, replacement of Ni by Co leads to a further decrease in the critical cooling rate for the La₅₅Al₂₅Ni₅Cu₁₀Co₅ alloy. Critical cooling rates for glass formation in the present alloys were also obtained through a study of their melting and solidification behaviours by thermal analytical measurement. The effect of alloying addition and the significance of reduced glass transition temperature for the glass forming ability of these alloys is discussed.     

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.     

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.     

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.     

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.     

Microstructural tailoring and improvement of mechanical properties in CuZr-based bulk metallic glass composites

A strategy for homogenizing the B2 CuZr phase in CuZr-based bulk metallic glass (BMG) composites based on inoculation is presented. The sizes and distribution of B2 CuZr particles can be effectively homogenized by Ta additions in rapidly solidified Cu₄₇Zr_48?xAl₅Ta_x (0 ≤ x ≤ 1, at.%) alloys. Mechanisms of the homogenizing effect were investigated by analyzing the microstructures of the composites as well as the nucleation and growth processes of the B2 CuZr phase. Mechanical properties of the alloys were significantly improved by the uniform B2 CuZr particles under both compression and tension. The inhibition on the propagation of shear bands and cracks by the ductile crystals and deformation-induced martensitic transformation of the B2 phase was proved to account for the superior tensile properties. Fracture mechanisms were proposed to correlate the tensile fracture behaviors to microstructural features of the alloys. Furthermore, the tensile plastic strain was quantitatively modeled by using the empirical microstructural element body approach as well as percolation theory. This study has important implications for the development and structural applications of high-performance BMG composites.     

Mg–Y–Cu bulk metallic glass prepared by mechanical alloying and vacuum hot-pressing

We have studied the amorphization behavior of Mg_85−xY₁₅Cu_x (x=20–40) alloy powders synthesized by mechanical alloying technique. The as-milled powders were mainly amorphous after 10 h of milling. The thermal stability of these Mg_85−xY₁₅Cu_x glassy powders was investigated by differential scanning calorimeter (DSC). The ranges of Tg, T_x and ΔT_x are around 430–459, 467–497, and 30–46 K, respectively. The Mg₄₉Y₁₅Cu₃₆ glassy powders exhibit the largest supercooled region of 46 K. The amorphization behavior of Mg₆₁Y₁₅Cu₂₄ was examined in details. Amorphous phases gradually became dominant after 7.5 h of milling and fully amorphous powders formed at the end of milling. The thermal stability of Mg₆₁Y₁₅Cu₂₄ glassy powders was similar to that of melt-spun Mg₆₀Y₁₅Cu₂₅ amorphous alloys. Mg₆₁Y₁₅Cu₂₄ bulk metallic glass with homogeneously embedded nanocrystalline precipitates was successfully prepared by vacuum hot pressing. It was found that the applied pressure during consolidation could enhance the thermal stability and prolong the existence of amorphous phase inside Mg₆₁Y₁₅Cu₂₄ powders.     

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.     

A comparative study on characteristic relaxation in Ce65Al10Cu20Co5 and Zr46.75Ti8.25Cu7.5Ni10Be27.5 bulk metallic glasses and supercooled liquids

The relaxation behaviors of Ce₆₅Al₁₀Cu₂₀Co₅ and Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 (Vit4) bulk glass-forming supercooled liquids were investigated by frequency-dependent heat capacity measurements on a temperature-modulated differential scanning calorimetry (TMDSC) apparatus. The relaxation behaviors of the investigated samples were well described by Kohlrausch–Williams–Watts (KWW) function. The temperature dependences of characteristic relaxation time of the glass-forming liquids were fitted by Vogel–Fulcher–Tamman (VFT) equation. The derived fragility index m shows that Ce₆₅Al₁₀Cu₂₀Co₅ (m = 31) liquid is a stronger liquid than Vit4 (m = 42). The sizes of cooperatively rearranging region (CRR) for Ce₆₅Al₁₀Cu₂₀Co₅ and Vit4 liquids were estimated to be 1.6 and 1.4 nm, respectively.     

Plastic deformation in nanostructured bulk glass composites during nanoindentation

Nanoindentation experiments of a Ti₄₅Zr₁₆Be₂₀Cu₁₀Ni₉ bulk metallic glass and partially vitrified nano-composite metallic glass matrix have been performed under a constant maximum load of 10 mN and constant loading rate of 0.08 mN s^?1 with the aim of comparative study of their micro-plastic deformation behavior. Remarkable difference in deformation behavior was found in load–displacement curves of nanoindentation and pile-up morphologies around the indents. The difference in shear banding behavior has been attributed to the presence of nanosized icosahedral particles in amorphous matrix.     

Effects of La content on the glass transition and crystallization process of Al–Ni–La amorphous alloys

Effects of La content on the glass transition and crystallization process of Al_94−xNi₆La_x (x = 3–9) amorphous alloys were investigated by X-ray diffraction and differential scanning calorimeter. The results show that the thermal stability increases with increasing the La content. The crystallization changes from a two-stage process without glass transition at x = 3–6 to a three-stage one with obvious glass transition at x = 7–9. The first crystallization process results in precipitation of single fcc-Al at x = 3–5, fcc-Al plus metastable phase(s) at x = 6 and 7, and single metastable phase at x = 8 and 9. The first crystallization process at x = 4 and 5 is the growth of quenched-in nuclei, whereas that at x = 6, 7 and 9 is the diffusion-controlled growth with a decreasing, constant and increasing nucleation rate, respectively. The activation energy for the first crystallization process is larger in the eutectic reaction than that in the primary reaction, and is the highest when the number of the products is the most.