Inhomogeneous structure and glass-forming ability in Zr-based bulk metallic glasses

Recently, a series of quaternary Zr-based bulk metallic glasses (BMGs), i.e., Zr₅₃Cu_18.7Ni₁₂Al_16.3, Zr_51.9Cu_23.3Ni_10.5Al_14.3 and Zr_50.7Cu₂₈Ni₉Al_12.3, have been developed and their glass-forming ability (GFA) increases with Cu concentration. In this work, atomic structures of the three BMGs were rebuilt by reverse Monte Carlo simulations based on the reduced pair distribution functions measured by high energy X-ray diffraction. The results show that a certain amount of substitution of short Zr-Cu, Cu-Cu pairs with long Zr-Zr and Zr-Al pairs enhances the local denser packing of Kasper polyhedral centered by Zr and Al atoms. A cell sub-divided method is proposed to describe the fluctuation of local number density which is found to follow the normal distribution function. The largest possible free volume in the three alloys is found to approaches to 3.8 Å. For the three alloys, the enhancement of GFA with increasing Cu content is due to the increase in the fluctuation degree of local density instead of the dense packing.     

Thermal stability and glass-forming ability of new Ti-based bulk metallic glasses

New Ti-based metallic glass (Ti₅₃Cu₁₅Ni_18.5Al₇Si₃M₃B_0.5 (M = Sc, Hf, Ta, Nb)) alloys were prepared by melt spinning and copper mold casting. The effects of foreign atoms on the thermal stability and the glass-forming ability were investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and differential thermal analysis (DTA). The results show that the values of T_rgs are 0.619, 0.609, 0.606, 0.597 K and ΔT_xs are 58, 54, 85, 78 K, respectively. The foreign atom’s size factor (∥R_F − R_Ti∣/R_Ti − 0.12∣) and mixing heat factor (∑x_iH_iF/∑x_iW_i) are proposed to predict the glass-forming ability and thermal stability, respectively.     

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.     

Tantalum based bulk metallic glasses

Ta-based bulk metallic glasses with high strength (2.7 GPa) and hardness (9.7 GPa), high elastic modulus (170 GPa) and high density (12.98 g/mm³) were developed. The best glass forming ability so far for a Ta-Ni-Co system reaches a critical diameter of 2 mm by the copper mold casting method. It shows an exceptionally high glass transition temperature of 983 K and a high crystallization temperature up to 1023 K. The unique mechanical and physical properties make them a promising high strength material.     

Rare earth based bulk metallic glasses

Recently, the rare earth based bulk metallic glasses (REBMGs) have attracted increasing interest due to their unique properties and potential applications as functional glassy materials. These REBMGs display many fascinating properties such as heavy fermion behavior, thermoplastic properties near room temperature, excellent magnetocaloric effect, hard magnetism, and polyamorphism, all of which are of interest not only for basic research but also for metallurgy and technology. These characteristics and properties are ascribed to the unique electronic, magnetic and atomic structures of the REBMGs. In this review paper, the fabrication, glass-forming ability, polyamorphism, elastic, thermal, and physical properties are summarized and discussed. Owing to the unique electronic structure of rare earth elements, the electric and magnetic properties of the REBMGs are especially addressed. The works have implications for seeking novel metallic glasses with controllable properties and for understanding the nature of glass formation. The development of REBMGs as functional materials might promote and extend the commercial applications of metallic glasses.     

Enhanced glass-forming ability of a Zr-based bulk metallic glass with yttrium doping

A significant enhancement in glass formation in a newly developed Zr₅₁Cu_20.7Ni₁₂Al_16.3 alloy has been achieved by yttrium doping. With just 0.5 at.% yttrium doping, the critical diameter of the as-cast alloys for glass formation has been increased from 3 mm to at least 10 mm. In the undoped, large-sized alloys, massive oxygen stabilized crystalline phases are observed but disappear in yttrium doped alloys. Very small amounts of stable α-Y₂O₃ phases found in the yttrium doped alloys, and their negligible effect on the metallic glasses’ properties, provide a superior solution to achieve metallic glasses with a high glass formability.     

Prediction of glass-forming ability for metallic systems

The relative glass-forming ability (GFA) of metallic alloys is considered in terms of a parameter $\text{ΔT}{}^1\text{= (T}{}_{\mathrm{<mtext>liq</mtext>}}{}^{\mathrm{<mtext>mix</mtext>}}\text{? T}{}_{\mathrm{<mtext>liq</mtext>}}\text{)/T}{}_{\mathrm{<mtext>liq</mtext>}}{}^{\mathrm{mix}}$, which represents the departure of the alloy liquids temperature, T_liq, from that of the simple rule of mixtures liquids temperature, T_liq^mix. For values of $\text{ΔT}{}^1\text{> 0.20}$ a metallic system is likely to form a glass by melt-quenching in useful thicknesses (i.e. > 20 μm) at a cooling rate of 10⁵?10⁷ K s^?1. Hence, a rapid assessment of the GFA of novel compositions may in general be obtained simply from a knowledge of the melting points of the pure components and the liquidust emperatures of the alloys.     

Tensile properties of ZrCu-based bulk metallic glasses at ambient and cryogenic temperatures

The tensile behaviors of a series of (Zr_47.5Cu_47.5Al₅)_1 ? x(Zr₈₀Nb₂₀)_x (x = 0, 0.05, 0.10, 0.15) bulk metallic glasses were studied at ambient and cryogenic (77 K) temperatures. It is found that the tensile strength of the alloys increases as the temperature decreases from 298 K to 77 K. The maximum enhancement is 15.7%, and the toughness of these alloys does not deteriorate at low temperatures. We demonstrate that the higher energy required to raise the temperature in the shear bands from the cryogenic temperature to glass transition temperature is the origin of the tensile strength enhancement at low temperatures.     

Lutetium and thulium based rare earth bulk metallic glasses

We report the formation of lutetium and thulium based bulk metallic glasses based on the correlations between the thermodynamic, kinetic, elastic and other properties of metallic glasses. The two novel rare earth based bulk metallic glasses (REBMGs) exhibit excellent glass formation ability, high elastic moduli, considerable smaller Poisson’s ratio, high thermal stability, and even higher mechanical strength than that of typical high strength Zr-based BMGs. The reasons for the properties of the Lu- and Tm-based and other REBMGs are discussed.     

Effect of microalloying on the glass-forming ability of Cu60Zr30Ti10 bulk metallic glass

One atomic percent of Sn and Si each was added (replacing Zr) to the bulk-glass-forming Cu₆₀Zr₃₀Ti₁₀ alloy. Sn improves the glass-forming ability (GFA) of the alloy, while Si triggers nanocrystalline phase formation in the glassy matrix, as resolved by high-resolution transmission electron microscopy. The observed variation does not originate from atomic size and/or heats of mixing effects. The results described here indicate, rather, that Sn improves GFA because it reduces the liquidus temperature and shifts the composition toward the off-eutectic reaction during melting.     

Stability of Ni-based bulk metallic glasses

Several ternary (Ni_xNb_ySn_z) refractory alloy glasses (RAGs) were studied at elevated temperatures in order to assess the stability of the amorphous state, i.e. devitrification, and to identify subsequent phase transformations in these materials. differential scanning calorimetry (DSC) experiments indicated a complex phase transformation sequence with several distinct crystallization and melting events being recorded above the glass transition temperature, T_g. Below T_g the RAG samples were studied with an in situ environmental X-ray furnace facility, which allowed step-wise isothermal ramping experiments commencing at a temperature below the reduced temperature of T/T_g ≈ 0.80. Distinct crystalline phases were observed when T/T_g ≈ 0.84 for ternary RAG alloys, while similar experiments on Zr-based Vit 106 glass alloys did not reveal any apparent phase separation until T/T_g ≈ 0.96. The phase separation kinetics followed an Arrhenius type of relationship with Ni₃Sn, and Nb₂O₅ being the principle crystalline precipitates.     

High mixing entropy bulk metallic glasses

Bulk metallic glasses (BMGs) are usually based on a single principal element such as Zr, Cu, Mg and Fe. In this work, we report the formation of a series of high mixing entropy BMGs based on multiple major elements, which have unique characteristics of excellent glass-forming ability and mechanical properties compared with conventional BMGs. The high mixing entropy BMGs based on multiple major elements might be of significance in scientific studies, potential applications, and providing a novel approach in search for new metallic glass-forming systems.     

Elastic properties of some bulk metallic glasses

The relationships between the elastic moduli, glass forming ability and response to deformation of bulk metallic glasses are investigated. Five bulk metallic glasses are prepared from high purity elements via suction casting. The results confirm that there exists a correlation between energy absorbed to failure during compression testing and the bulk to shear modulus ratio. This finding is developed such that it corresponds only to the elastic component of energy absorption, and that the bulk modulus dominates this. Plastic deformation appears to be favored by a reduced shear modulus, although it shows greater dependence on structural features that are frozen in during the glass transition, and so may well be dependent on the liquid fragility.     

Effects of Zn on the glass forming ability and mechanical properties of MgLi-based bulk metallic glasses

Through the addition of Zn element, Mg-Li-Cu-Zn-(Y, Gd) bulk metallic glasses (BMGs) with diameter of 2 mm have been successfully fabricated by conventional copper mold injection casting method. The X-ray diffractometer (XRD) patterns and differential scanning calorimeter (DSC) traces demonstrated that the as-cast Mg₆₀Li₅Cu₂₀Zn₅(Y, Gd)₁₀ BMGs were fully amorphous phase. By the minor addition of 5 at.% Zn, the supercooled liquid regions (SLR) of Mg₆₀Li₅Cu₂₀Zn₅(Y, Gd)₁₀ BMGs increased 6 K and 3 K respectively compared with that of Zn-free samples, which implied that their glass forming ability (GFA) and thermal stability were improved. It was also proved that the addition of Zn effectively enhanced the alloys’ fracture strength (453 MPa and 456 MPa) and elastic strain (0.75% and 0.92%), which were much higher than that of Mg₆₀Li₅Cu₂₅Y₁₀ BMG (403 MPa, 0.62%) and Mg₆₀Li₅Cu₂₅Gd₁₀ BMG (412 MPa, 0.82%).     

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.     

The slow β-relaxation observed in Ce-based bulk metallic glass-forming supercooled liquid

Dynamic mechanical relaxation measurements are performed on a Ce-based metallic supercooled liquid close to its glass transition temperature T_g. An obvious excess wing is observed both in the temperature and frequency dependent loss modulus curves by the calculation the relaxation time of the α-relaxation in supercooled liquid with the fit by the combination of the Kohlrausch–Williams–Watts and Vogel–Fulcher–Tamman equation. The results indicate that the slow β-relaxation process exists in the metallic liquid and arises from the small-scale translational motions of atoms that are linked in its metastable islands.     

Distinguish bonding characteristic in metallic glasses by correlations

By statistically analyzing 48 kinds of metallic glasses, we report clear correlations between the dimensionless ratio of glass transition temperature/Debye temperature (T_g/Θ_D) and density (ρ), and between Young's modulus or shear modulus and T_g/ρ, for the glasses consisting of only metal elements, while the metallic glasses alloyed with metalloid elements exhibit distinct deviation from the correlations. It is suggested that the alloying of metalloid elements would show covalent-like bonding characteristics in metallic glass, and the found correlations can be used to distinguish different bonding characteristics in metallic glasses.     

Plasticity induced by nanoparticle dispersions in bulk metallic glasses

We show room temperature plasticity in several ZrCu-based bulk metallic glasses (BMG) after dispersions of crystalline nanoparticles were generated prior to mechanical testing. BMGs are heated in synchrotron light in transmission such that annealing can be interrupted at the very first detection of nucleation of the crystallites. Effect of embedded nanocrystals on the mechanical properties of BMG-Composites was investigated by compressive testing. When nanocrystal dispersions were generated, zirconium-copper-based BMGs that initially showed no plastic deformation prior to fracture, exhibited ductile behavior in compression with about 10% deformation.     

Evaluation of glass-forming ability criterion from phase-transformation kinetics

Applying kinetic analysis upon crystallization of metallic glass, a quantitative relation between the critical cooling rate and the onset temperature of crystallization was obtained for glass-forming alloys. Effects of the onset temperature of crystallization, the liquidus temperature and the glass transition temperature on the critical cooling rate were analytically described. Three rules guiding the development of more reliable glass-forming ability criteria are suggested.     

The effect of Sn addition on the glass-forming ability of Cu-Ti-Zr-Ni-Si metallic glass alloys

The effect of Sn substitution for Ni on the glass-forming ability was studied in Cu₄₇Ti₃₃Zr₁₁Ni_8−xSn_xSi₁ (x=0,2,4,6,8) alloys by using thermal analysis and X-ray diffractometry. With increasing x from 0 to 8, the glass transition temperature, T_g, of melt-spun Cu₄₇Ti₃₃Zr₁₁Ni_8−xSn_xSi₁ alloys increased gradually from 720 to 737 K. On the other hand, the crystallization temperature, T_x, increased from 757 K at x=0 to 765 K at x=2, being nearly same with further increase of x. Partial substitution of Ni by Sn in Cu₄₇Ti₃₃Zr₁₁Ni₈Si₁ promotes the glass formation. Both amorphous Cu₄₇Ti₃₃Zr₁₁Ni_8−xSn_xSi₁ alloys prepared by melt spinning and injection casting showed similar crystallization process during continuous heating in DSC. Temperature range of undercooled liquid region exhibits good correlation with the critical diameter for the formation of an amorphous phase in injection casting.