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.     

Minor addition induced enhancement of strength of Mg-based bulk metallic glass

We report that the fracture strength of Mg-based bulk metallic glasses (BMGs) can be dramatically enhanced up to 1.10 GPa by minor Gd addition. The Poisson’s ratio v of the BMG also decreases to 0.261 close to that of brittle oxide glasses when 1 at.% Gd was added. Such significant enhancement in strength which approaches the theoretical strength value and dramatically decrease in the Poisson’s ratio are attributed to the structural change of the BMGs induced by the Gd minor addition.     

The influence of Ag substitution for Cu on glass-forming ability and thermal properties of Mg-based bulk metallic glasses

A group of pseudo-ternary Mg–(Cu–Ag)–Dy bulk metallic glasses (BMGs) was developed by copper mold casting. The glass-forming ability (GFA) is significantly improved by the coexistence of similar elements of Ag and Cu. The critical diameter for glass formation increases from 10 mm for ternary Mg_56.5Cu₃₂Dy_11.5 alloy to 18 mm for pseudo-ternary Mg_56.5Cu₂₇Ag₅Dy_11.5 alloy. Thermal stability, crystallization and melting behaviors of the Mg-based BMGs were evaluated. The decrease of Gibbs free energy difference between undercooled liquid and crystalline phases caused by similar element substitution with optimal amount can be responsible for the increase in GFA of the resulting alloys.     

The effects of microalloying on thermal stability,mechanical property and corrosion resistance of Mg-based bulk metallic glasses

(Mg₆₅Cu₂₀Y₁₀Zn₅)₉₈ M₂ (M = Ti, Cr) bulk metallic glasses with diameter of 3 mm were prepared by copper mold casting. The effects of Ti or Cr on thermal stability, mechanical property and corrosion resistance were systematically investigated. It's shown that the Glass forming ability of the Mg₆₅Cu₂₀Y₁₀Zn₅ bulk metallic glasses slightly decreases with the addition of the elements. Whereas, the strength and corrosion resistance significantly increase. The superior corrosion resistance of the amorphous sample containing Ti or Cr is presented for the forming of homogeneous passive layer with highly protective oxides.     

Bulk metallic glass formation in Cu–Zr–Ti ternary system

The formation of bulk metallic glasses (BMG) in the Cu-rich Cu–Zr–Ti ternary system is studied by using the ‘e/a-variant line criterion’. Three such lines, (Cu₉Zr₄)_1−xTi_x, (Cu_61.8Zr_38.2)_1−xTi_x and (Cu₅₆Zr₄₄)_1−xTi_x, are defined in the Cu–Zr–Ti system by linking three binary compositions Cu₉Zr₄, Cu_61.8Zr_38.2 and Cu₅₆Zr₄₄ to the third element Ti. The binary compositions Cu₉Zr₄, Cu_61.8Zr_38.2 and Cu₅₆Zr₄₄ correspond to specific Cu–Zr binary clusters. BMGs are obtained by copper mould suction casting method with Ti contents of 7.5–15 at.%, 7.5–12.5 at.% and 5–12 at.%, respectively along the (Cu₉Zr₄)_1−x Ti_x, (Cu_61.8Zr_38.2)_1−xTi_x and (Cu₅₆Zr₄₄)_1−xTi_x lines. The BMGs on each composition line manifest decreased thermal stabilities and glass forming abilities (GFAs) with increasing Ti contents. The maximum GFA appears at Cu₆₄Zr_28.5Ti_7.5, with characteristic thermal parameters of T_g = 736 K, T_x = 769 K, T_g/T_l = 0.627 and γ = 0.403, which are all superior to those reported for the known Cu₆₀Zr₃₀Ti₁₀ BMG.     

Hf–Cu–Ni–Al bulk metallic glasses: Optimization of glass-forming ability and plasticity

In the Hf–Cu–Ni–Al quaternary system, the Hf₅₁Cu_27.75Ni_9.25Al₁₂ bulk metallic glass (BMG) exhibits the best combination of the large glass-forming ability (GFA) and compressive plasticity. Minor substitution of Nb for Hf in Hf–Cu–Ni–Al BMGs degrades not only the GFA but also plasticity, while the substitution of Ta does not have an appreciable effect on both properties. For the investigated Hf-based BMGs, the shear modulus G is a more sensitive indicator to correlate with their plasticity than the Poisson′s ratio. Meanwhile, the correlation between the G and the glass transition temperature T_g for the Hf-based BMGs can be expressed as G = 9.9 + 576 (T_g/V_m)[1 ? 4/9(T/T_g)^2/3].     

Microstructures of Mg–Zn–Nd alloy including small quasicrystalline grains

Two kinds of Mg-rich and low neodymium Mg–Zn–Nd alloys including icosahedral quasicrystal phase (I-phase) were prepared under conventional casting conditions. The microstructures and phases of Mg–Zn–Nd quasicrystal alloys were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). The results indicate that the I-phase in Mg–Zn–Nd quasicrystal alloy is a simple icosahedral quasicrystal with quasi-lattice of a_R = 0.525 nm. It has been proved that the as-cast Mg_70.8Zn₂₈Nd_1.2 quasicrystal alloy mainly consisted of I-phase and Mg₇Zn₃ matrix phase. While the as-cast Mg_70.5Zn_28.5Nd₁ alloy mainly consisted of I-phase, Mg₇Zn₃ matrix phase, dendrite α-Mg phase and a new rod-like hexagonal phase.     

Kinetics of crystallization process of Mg–Cu–Gd based bulk metallic glasses

The crystallization behavior of Mg₆₁Cu₂₈Gd₁₁ and (Mg₆₁Cu₂₈Gd₁₁)₉₈Cd₂ bulk metallic glasses was studied using DSC in the mode of continuous and isothermal heating, and its crystallization process and microstructure were confirmed by XRD and TEM. In continuous heating, the activation energies of glass transition, onset and peak crystallization were determined by the Kissinger method, which yields 110 ± 12, 77 ± 9 and 79 ± 10 kJ/mol, respectively, for Mg₆₁Cu₂₈Gd₁₁ glassy alloy, and 144 ± 10, 126 ± 6 and 131 ± 5 kJ/mol, respectively, for (Mg₆₁Cu₂₈Gd₁₁)₉₈Cd₂ glassy alloy. The isothermal kinetics was modeled by the Johnson-Mehl-Avrami equation. The Avrami exponent of the base alloy was in the range from 1.98 to 2.56 (± 0.01), which indicated a decreasing nucleation rate and a diffusion-controlled growth. For Cd-added glassy alloy, the Avrami exponent was in the range from 3.26 to 4.08, which indicated an increasing nucleation rate. The activation energies in isothermal process were calculated to be 88 ± 2 and 132 ± 2 kJ/mol, respectively, for the base and Cd-added glassy alloys. It was found that Mg₂Cu phase was the primary phase in the initial crystallization and the strong affinity between Cd and Mg/Gd tended to impose resistance to the formation of Mg₂Cu phase and thus improves the thermal stability.     

Glass formation and crystallization behavior in Mg65Cu25Y10−xGdx (x=0, 5 and 10) alloys

The glass forming ability and crystallization behavior of Mg₆₅Cu₂₅Y_10−xGd_x (x=0, 5 and 10) alloys have been investigated. The glass forming ability (GFA) is significantly improved when Y in Mg₆₅Cu₂₅Y₁₀ is substituted with Gd. Ternary Mg₆₅Cu₂₅Gd₁₀ bulk metallic glass (BMG) with diameter of at least 8 mm was successfully fabricated by conventional Cu-mold casting method in air atmosphere. Mg₂(Y, Gd) is the first competing crystalline phase against the glass formation in the Mg₆₅Cu₂₅Y_10−xGd_x (x=0, 5) alloys, while Mg₂Cu and Cu₂Gd are the competing crystalline phases in the Mg₆₅Cu₂₅Gd₁₀ alloy. Therefore, the suppression of the formation of Mg₂(Y, Gd) during cooling from the liquid improves the GFA significantly.     

Formation and properties of RE55Al25Co20 (RE = Y,Ce, La,Pr, Nd,Gd, Tb,Dy, Ho and Er) bulk metallic glasses

We report that a series of ternary RE₅₅Al₂₅Co₂₀ (RE = Y, Ce, La, Pr, Nd, Gd, Tb, Dy, Ho and Er) alloys can be readily cast into bulk glasses by a conventional casting method. The characteristics and properties of these new bulk metallic glasses (BMGs) are studied and compared. Due to the chemical comparability and well-regulated variety in atomic size, properties and elastic constants of these rare earth elements, the RE₅₅Al₂₅Co₂₀ BMGs could be regarded as a model system to investigate the glass-forming ability, thermal stability, glass transition, crystallization behavior, liquid fragility, elastic and mechanical properties as well as their relationships. An attempt is made to highlight commonality and contrasts of the effects of various factors on the metallic glasses formation and properties.     

Consolidation and mechanical properties of Cu46Zr42Al7Y5 metallic glass by spark plasma sintering

Cu₄₆Zr₄₂Al₇Y₅ metallic glass with nearly 100% relative density was obtained by spark plasma sintering (SPS) with a diameter of 15 mm, which was larger than the largest size of 10 mm for the as-cast specimen. The fracture strength of the sintered specimen reached 2044 MPa, which was 15% higher than that of the as-cast Cu₄₆Zr₄₂Al₇Y₅ glassy specimen. The densification and compressive properties of the sintered specimens were related to sintering temperature. Structural changes of the specimens sintered at various sintering temperatures resulted in the difference of macro-mechanical properties.     

Synthesis and characterization of bulk amorphous Zr65Cu18Ni9Al8 and [Zr0.65Cu0.18Ni0.09Al0.08]98Er2 alloys

Bulk metallic glasses (BMGs), especially Zr-based BMGs, have attracted lot of attention of materials scientists because of their very attractive physical, thermal and mechanical properties and a few unique applications. In the present study, Zr₆₅Cu₁₈Ni₉Al₈ alloy was designed according to the criterion of conduction electron/atom (e/a ratio) ∼1.395 and average atomic size of alloy (R_a) ∼0.1498 nm. Addition of 2 at.% Er was carried out in the base alloy to investigate its effect on thermal and mechanical properties. Characterization of alloys was performed using the techniques of XRD, DSC, and SEM/EDS. Mechanical properties like Vicker’s microhardness, nanohardness, elastic modulus, density and fracture strength were measured. Average shear angle was found to be ∼35 ± 1° for base alloy and about 31 ± 1° for alloy containing 2 at.% Er. Wide supercooled liquid regions of 129 K and 119 K were found for the base alloy and the alloy containing 2 at.% Er.     

Investigation of viscosity and crystallization in supercooled-liquid region of Zr-based glassy alloys

Using viscosity measurement method and in-situ heating synchrotron radiation, the viscosity of the (Zr_0.55Al_0.1Ni_0.05Cu_0.3)_100 ? xY_x (x = 0, 0.5, 1, 2) bulk metallic glasses (BMGs) in their supercooled liquid regions (SLRs) and the in-situ heating nucleation were investigated, respectively. In the SLR, the (Zr_0.55Al_0.1Ni_0.05Cu_0.3)₉₉Y₁ metallic glass which shows distinct plastic strain in compression exhibits higher viscosity than the other three BMGs, however their Poisson's ratios are almost the same. The synchrotron diffraction results show that crystallization happened in the SLR of the (Zr_0.55Al_0.1Ni_0.05Cu_0.3)₉₉Y₁ glassy alloy, which could be the reason for the higher viscosity and larger plastic strain in compression compared to the other three alloys. The fracture surfaces of the glassy alloys were observed and analyzed.     

Formation and thermal stability of Pd-based bulk metallic glasses

The glass-forming ability and thermal stability of bulk glassy Pd₇₉Cu₆Si₁₀P₅ alloy were studied by substitution of Cu with Ag and with Au + Ag from 0 to 6 at.%. The results indicated that the small addition of Ag strongly affects the thermal stability and glass-forming ability of the Pd₇₉Cu₆Si₁₀P₅ alloy. The alloy doped with 4 at.% Ag (Pd₇₉Cu₂Ag₄Si₁₀P₅) exhibits the largest glass-forming ability among the Pd₇₉Cu_6−xAg_xSi₁₀P₅ (x = 0–6 at.%) alloys. The critical diameter for glass formation of this alloy reaches as large as 7 mm by copper mold casting. On the other hand, the multi-addition of Au + Ag does not increase the glass forming ability though the Ag and Au are similar in atomic size. The largest glass forming ability is obtained at 1 at.% Au + 2 at.% Ag among the Pd₇₉Cu_6−x−yAu_xAg_ySi₁₀P₅ (x = 1–4 at.%, y = 1–3 at.%) alloys. The critical diameter of this alloy is 5 mm by copper mold casting.     

Lightweight Ti–Zr–Be–Al bulk metallic glasses with improved glass-forming ability and compressive plasticity

A series of lightweight Ti–Zr–Be–Al bulk metallic glasses (BMGs) have been developed through the addition of Al to Ti–Zr–Be ternary glassy alloy. By replacing Be with Al, the critical size of the glassy rod has been increased from 5 mm for Ti₄₁Zr₂₅Be₃₄ alloy to 7 mm for Ti₄₁Zr₂₅Be₂₉Al₅ alloy, while the yield strength of Ti₄₁Zr₂₅Be_34 ? xAl_x (x = 2–10) has been greatly enhanced, resulting in a significant increase of the specific strength which is defined as yield strength/density. Among these newly developed Ti–Zr–Be–Al BMGs, Ti₄₁Zr₂₅Be₂₆Al₈ glassy alloy exhibits a high specific strength of 4.33 × 10⁵ Nm/kg and a very large compressive plastic strain of 47.0%, which are much larger than those (3.69 × 10⁵ Nm/kg and 2.9%, respectively) for Ti₄₁Zr₂₅Be₃₄ glassy alloy. The present results show that Al is an effective alloying element for improving the glass-forming ability (GFA) and mechanical properties of Ti-Zr-Be glassy alloy.     

The thermal stability and activation energy of crystallization of (Cu61.8Zr38.2)1−xAlx bulk metallic glasses

The present paper reports on the thermal stability and activation energy of crystallization of bulk metallic glasses (BMGs) (Cu_61.8Zr_38.2)_1−xAl_x. The (Cu_61.8Zr_38.2)_1−xAl_x composition series, prepared by copper mould suction casting into bars with a diameter of 3 mm, form BMGs with an e/a range of 1.24–1.3. These BMGs manifest increased thermal stability with increased e/a ratios. The activation energies (ΔE) of crystallization as derived from thermal analysis at different heating rates follow a similar tendency to that of the thermal stability, indicating stronger short-range ordering with increasing e/a ratios. The optimum BMG Cu_58.1Zr_35.9Al₆ exhibits the highest thermal stability and the largest ΔE.     

Preparation and mechanical properties of a bulk icosahedral quasicrystalline Ti45Zr35Ni17Cu3 alloy

A bulk Ti₄₅Zr₃₅Ni₁₇Cu₃ alloy, which consisted of the icosahedral quasicrystalline phase, was prepared by mechanical alloying(MA) and subsequent pulse discharge sintering. Ti₄₅Zr₃₅Ni₁₇Cu₃ amorphous powders (with particle size <50 μm) were obtained after mechanical alloying for more than 150 h from the mixture of the elemental powder. The transformation temperature range from amorphous phase to the quasicrystalline phase was from 400 K to 900 K. The mechanical properties of the bulk quasicrystalline alloy have been examined at room temperature. The Vickers hardness and compressive fracture strength were 620 ± 40 and 1030 ± 60 MPa, respectively. The bulk quasicrystalline alloy exhibited the elastic deformation by the compressive test. The fracture mode was brittle cleavage fracture.     

Morphological control of MgxZn1−xO layers grown on Ga:ZnO/glass substrates for photovoltaics

Mg_xZn_1?xO has been used in various photovoltaic cells because its energy bandgap can be tailored by controlling the Mg composition in this ternary compound. The Mg_xZn_1?xO layers with different surface morphologies including two-dimensional (2-D) films and one-dimensional (1-D) nanostructures are preferred for conventional p–n junction solar cells and polymer–inorganic hybrid solar cells, respectively. The Mg_xZn_1?xO layers are sequentially grown on Ga-doped ZnO (GZO) transparent conductive electrode using metalorganic chemical vapor deposition (MOCVD). The effect of the buffer layers on Mg_xZn_1?xO surface morphology is investigated. It is observed that Mg_xZn_1?xO deposited at ～500 °C on a low-temperature (～250 °C) ZnO buffer layer is in the form of 2-D dense and smooth films, whereas, on a high-temperature (～520 °C) ZnO buffer layer is in the form of 1-D nanostructures. Based on the structure characterization results, a growth mechanism in terms of nucleation and texturing is proposed to explain the buffer layer effect.     

Alkaline earth zinc borate glasses doped with Cu2+ ions studied by EPR,optical and IR techniques

Copper ions incorporated into alkaline earth zinc borate glasses 10RO + 30ZnO + 60B₂O₃ (R = Mg, Ca and Sr) and 10SrO + (30 ? x)ZnO + 60B₂O₃ + xCuO (x = 0, 0.1, 0.3, 0.5, and 0.7 wt.%) were characterized by electron paramagnetic resonance (EPR), optical absorption and FTIR techniques. The EPR spectra of all the glass samples exhibit resonance signals characteristic of Cu²⁺ ions. The values of spin-Hamiltonian parameters indicate that the Cu²⁺ ions in alkaline earth zinc borate glasses were present in octahedral sites with tetragonal distortion. The spin concentration (N) participating in resonance was calculated as a function of temperature for strontium zinc borate (SrZB) glass sample containing 0.7 wt.% of Cu²⁺ ions and the activation energy was calculated. From the EPR data, the paramagnetic susceptibility (χ) was calculated at different temperatures and the Curie constant was evaluated from the 1/χ-T graph. The optical absorption spectra of these samples show only one absorption band. The optical band gap energies (E_g) and Urbach energy (ΔE) are calculated from their ultraviolet edges. The FTIR studies show different stretching and bending vibrations of alkaline earth zinc borate glasses.