Anomalous packing state in Ce-Ga-Cu bulk metallic glasses

Formation mechanism of bulk metallic glasses (BMGs) with high glass-forming ability (GFA) has been a long-standing subject in the field of solid state physics. To highlight the GFA-associated local atomic structure, element-specific positron annihilation spectroscopy was conducted for recently discovered ternary Ce₇₀Ga_xCu_30-x (x = 6–13; at.%) BMGs. We succeeded in identifying the packing structure under the condition of ambient pressure, in which Ce atoms are concentrated more than that in Ce crystal. This anomalous glassy state is most efficiently formed at the Ga concentration of ∼10%, where the best glass-forming ability (GFA) is experimentally observed. First-principles computer simulation results suggest that this anomalous packing structure is associated with Ce-4f electron delocalization in the Ce-Ga-Cu BMGs. The findings provide unambiguous evidence for the relationship between the packing-efficient local structure and the GFA.     

The density and packing fraction of binary metallic glasses

The present work develops a physical model of metallic glass structure that gives a reasonable estimate of density. The efficient cluster-packing model is used as a starting point, and is refined by a high-fidelity estimate of the size of structure-forming clusters and cluster–cluster separations. These are predicted as continuous functions of composition and relative atom radii. Predicted densities are all are within ±10% of measured densities for 200 binary metallic glasses, representing a precision in cluster–cluster separations of ±3%. New structural insights from this work include the importance of acknowledging the unique cluster topologies to estimate cluster–cluster separations; an improved ability to estimate the higher packing efficiency of unlike atoms in the first coordination shell of atomic clusters; and an improved estimate of metalloid–metalloid separations. The unusual, bilinear influence of composition on density in Fe–B glasses is explained by considering the sizes of β and γ sites in different metallic glass structures. Global atom packing fractions derived from measured densities range from about 0.62 to 0.76, and the most stable binary glasses all have packing fractions in excess of 0.70, supporting the idea that atom packing efficiency influences glass stability.     

Misch metal based metallic glasses

In this paper, a glass-forming range of metallic glasses based on Ce-rich misch metal (Mm) was pinpointed in Mm-Al-Co composition map by melt spinning. The thermal analysis indicated that the wide supercooled liquid region (above 60 K) can be found out in a large composition range in Mm-Al-Co system. The investigation of the glass-forming ability (GFA) in this system indicated a glassy composition with a larger supercooled liquid region wouldn’t be the glassy former with higher GFA. The reduced glass transition temperature is a better indicator to explore metallic glasses with high GFA. Furthermore, the mechanical properties of Mm₆₅Al₁₀Co₂₅ bulk glassy samples were evaluated in a compressive measurement. The obvious advantages of the Mm-based BMGs with high GFA, good mechanical properties and low material cost make these BMGs hopeful to be applied in the future.     

Pressure effects on metallic glasses

The effects of normal stress/superimposed pressure on the flow/fracture of two different zirconium-based glasses and a hafnium-based bulk metallic glass (BMG) tested at Case Western Reserve University is presented. These results are compared with the pressure sensitivity of the bulk modulus and shear modulus of amorphous metals in the manner previously conducted by Spitzig and Richmond for crystalline metals. Although few experimental results of this nature exist, the data obtained to date reveal minimal normal stress/pressure sensitivity for the BMG examined in this study, consistent with the small pressure dependence of the shear modulus. Comparison with various yield/fracture criteria is also provided.     

Tin-modified gold-based bulk metallic glasses

Tin was selected as a modifying element in low-gold-content (50 at.%) bulk metallic glasses (BMGs) aiming at developing alloys with cost-effective performance. New gold-based Au–Sn–Cu–Si alloys were fabricated by injection-casting into a copper mold. The as-cast BMG Au₅₀Sn₆Cu₂₆Si₁₈ with 18.6-karat gold and a diameter of 1 mm possessed a lower glass transition temperature (T _g) of 82°C (355 K), a lower liquid temperature of 330°C (603 K), and a super-cooled liquid region of 31°C. The viscosity range of this BMG Au₅₀Sn₆Cu₂₆Si₁₈ was from 10⁸ to 10⁹ Pa s measured at a low applied stress of 13 kPa. To compare the viscosity with different applied stresses, its viscosity clearly increased with applied stress below T _g but not so obvious above T _g. The low viscosity of this BMG Au₅₀Sn₆Cu₂₆Si₁₈ at around 102°C, which is very close to the boiling temperature of water (100°C), rendered easy thermal–mechanical deformation in a boiling water-bath by hand-pressing and tweezers-bending. Such a deformation capability in boiling water is beneficial to the further applications in various fields.     

Recent progress in metallic glasses in Taiwan

The recent research and development on metallic glasses in Taiwan over the past decade is reviewed in this paper. The major focus was to develop tougher bulk metallic glasses (BMGs), bulk metallic glass composites (BMGCs), and thin film metallic glasses (TFMGs), mostly in Zr and Mg based systems. Due to the Taiwan industry characteristics, metallic glasses are favored in the application for micro-electro-mechanical systems (MEMS), including micro- or nano-imprinting for optoelectronic devices and hologram patterns.     

金属玻璃中取决于加载速率的非均匀蠕变行为

大块金属玻璃中的蠕变变形可分为均匀流动和非均匀流动.为了理解两种蠕变变形的转化条件,使用纳米压痕试验和分子动力学模拟研究加载速率对Ti40Zr10Cu47Sn3(摩尔分数,％)大块金属玻璃室温蠕变行为的影响.结果发现,在低加载速率下,加载阶段出现很多的锯齿流动,导致蠕变阶段出现非均匀的锯齿流动;而当加载速率足够高时,蠕...     

The Soret effect in bulk metallic glasses

Compositional inhomogeneity induced by the Soret effect was studied in two Zr-based bulk metallic glasses (BMGs): Zr₅₀Cu₅₀ and Zr₅₀Cu₄₀Al₁₀ (at%), and one Cu-based BMG: Cu₆₀Zr₃₀Ti₁₀ (at%), all of which were prepared by rapid solidification. The concentration of Cu increases from the surface to the interior, while the concentrations of Zr, Ti and Al decrease. The magnitude of the Soret effect is found to be highly dependent on the sample size and interactions between the diffusing atoms in bulk metallic glasses. For the Zr₅₀Cu₅₀ alloy, a large sample size favors the Soret effect, because of the longer diffusion time it affords compared to a small sample. Further, the additions of Al and Ti in the Zr–Cu BMGs reduce the magnitude of the Soret effect by the formation of short-range order and/or inter-atomic clusters.     

Bulk metallic glasses for biomedical applications

The selection criteria for biomaterials include the material’s properties and biocompatibility, and the ability to fabricate the desired shapes. Bulk metallic glasses (BMGs) are relative newcomers in the field of biomaterials but they exhibit an excellent combination of properties and processing capabilities desired for versatile implant applications. To further evaluate the suitability of BMGs for biomedical applications, we analyzed the biological responses they elicited in vitro and in vivo. The BMGs promoted cell adhesion and growth in vitro and induced improved foreign body responses in vivo suggesting their potential use as biomaterials. Because of the BMGs’ flexible chemistry, atomic structure, and surface topography, they offer a unique opportunity to fabricate complex implants and devices with a desirable biological response from a material with superior properties over currently used metallic biomaterials.     

Cytotoxicity of Zr-based bulk metallic glasses

Recently we developed a family of Ni-free Zr-based bulk metallic glasses in the Zr–Cu–Fe–Al system. X-ray diffraction and differential thermal analysis measurements demonstrate its good glass-forming ability, and amorphous rods of up to 13 mm in diameter can be produced for the alloy Zr₅₈Cu₂₂Fe₈Al₁₂. This new glassy system is potentially very interesting for biomedical applications. Thus we have investigated the effect of surface modification (treatment with nitric acid and oxygen plasma) on the alloy's cytotoxicity and compared it with the results for a Ni-bearing Zr-based bulk metallic glass. The surfaces were analyzed by X-ray photoelectron spectroscopy, and cytotoxicity was tested by measuring the viability and metabolic activity of mouse fibroblasts. Our results show that the surfaces of the as-cast glasses consist almost exclusively of zirconium oxide, which yields good biocompatibility. With nitric acid treatment this oxide layer can be stabilized further, to the extent that the cytotoxicity becomes as good as that of the non-toxic negative control (polystyrene).     

Embrittlement of Zr-based bulk metallic glasses

Embrittlement of Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 and Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glasses (BMGs) is studied after annealing at temperatures below and above the glass transition temperature T_g for time scales comparable with structural relaxation and crystallization. The effect of annealing on the bending ductility, the isoconfigurational elastic constants, the structure and the thermal stability is examined. The embrittlement during sub-T_g annealing originates from structural relaxation and can be reversed by subsequently annealing for a short duration above T_g. The embrittlement kinetics correlate with the structural relaxation. However, only a fraction of relaxation time at a given temperature (<T_g) is sufficient to embrittle the BMG significantly. Above T_g, plasticity is retained for annealing far beyond the relaxation time but, instead, embrittlement is caused by crystallization. The magnitude of the decrease in Poisson’s ratio is insufficient to explain the severe embrittlement within the framework of a critical value as previously suggested.     

Mechanical failure and glass transition in metallic glasses

The current majority view on the phenomenon of mechanical failure in metallic glasses appears to be that it is caused by the activity of some structural defects, such as free-volumes or shear transformation zones, and the concentration of such defects is small, only of the order of 1%. However, the recent results compel us to revise this view. Through molecular dynamics simulation it has been shown that mechanical failure is the stress-induced glass transition. According to our theory the concentration of the liquid-like sites (defects) is well over 20% at the glass transition. We suggest that the defect concentration in metallic glasses is actually very high, and percolation of such defects causes atomic avalanche and mechanical failure. In this article we discuss the glass transition, mechanical failure and viscosity from such a point of view.     

Irreversible atomic rearrangements in elastically deformed metallic glasses

The atomistic behaviour of elastically deformed Ni₅₀Zr₅₀ metallic glasses obtained at different quenching rates was studied by molecular dynamics. Deformation induces the irreversible rearrangement of atomic clusters of various chemical composition. The relative amount of Ni and Zr atoms participating in irreversible rearrangements depends on the quenching rate. The rearrangements are related to the potential energy difference between initial and final cluster configurations, connected in turn with volume effects. The role of local structures was investigated by focusing the attention on icosahedral coordination. The numerical findings indicate that icosahedral clusters become involved in rearrangements only after the average strain has overcome a certain value. Therefore, at small elastic strain only small atomic clusters with spatial organization different from the icosahedral one rearrange. The different involvement in rearrangements of different local structures is tentatively related to apparent local strain.     

Self-organized intermittent plastic flow in bulk metallic glasses

Under stress, bulk metallic glasses irreversibly deform through shear banding processes that manifest as serrated flow behavior. These serration events exhibit a shock-and-aftershock, earthquake-like behavior. Statistical analysis shows that the shear avalanches can self-organize to a critical state (SOC). In analogy to the smooth macroscopic-scale crystalline plasticity that arises from the spatio-temporal averages of disruptive earthquake-like events at the nanometer scale, shear avalanches in glassy metals are another model system that can be used to study SOC behavior. With our understanding of SOC behavior, we further demonstrate how to enhance the plasticity of glassy (brittle) materials. It is expected that the findings can be extended to other glassy or brittle materials.