Heavy rare earth based bulk metallic glasses with high thermal stability

We report a family of ternary Gd–Al–Co alloys based on the heavy gadolinium rare earth element can be readily cast into fully glassy rod by a conventional casting method. It is found that the bulk metallic glasses (BMGs) have much high thermal stability (i.e. high glass transition temperature and crystallization temperature) and high moduli compared with those of other known rare-earth based BMGs. It is confirmed that, in addition to the strong chemical interaction among the components, the high bulk modulus of the base component in the BMGs is dominantly responsible for the high thermal stability. The thermal stability is correlated with the bulk modulus of the base element in the rare earth based BMGs.     

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.     

Strengthening bulk metallic glasses with minor alloying additions

Cu47Ti34Zr11Ni8, （Cu47Ti34Zr11Ni8）99Si and （Cu47Ti34Zr11Ni8）99Al bulk metallic glass were prepared by copper mold casting method, and the thermal stability, mechanical properties and microstructures of them were studied. With minor alloying of Si and Al additions, the glass transition temperature （Tg）, crystallization temperature （Txl） and temperature interval of supercooled liquid region △Tx （=Txl-Tg） and reduced glass transition temperature （Trg） were proved to be changed from 672 K, 734 K, 62 K, 0.575 to 691 K, 752 K, 61 K, 0.592 and to 681 K, 729 K, 48 K, 0.590, respectively. The results indicate that the glass-forming ability （GFA） are improved with minor alloying additions. And the bulk glasses also exhibits high three point-bending flexural strength. Because of the additions of Si and Al, three point-bending flexural strength and flexural modulus of the bulk glass change from 2 350 MPa, 102 GPa to 3 260 MPa, 102 GPa and 2 970 MPa, 108GPa respectively. The obvious strengthening is due to the appearance of the medium-range ordered regions with a size of 2-5 nm under the high-resolution TEM image. The reason that the mixed amorphous and nanocrystalline phases caused by minor alloying of Si and Al additions, is that Si or Al is the third kind of elements, which are different from other constituting elements, and there are a strong bonding and atoms size effects between constituting elements, which cause the glass-forming ability （GFA） and the bulk metallic glasses strength improving.     

New ternary Ni–Ta–Sn bulk metallic glasses

A new ternary Ni-base bulk metallic glass (BMG) system, Ni–Ta–Sn, was explored. There exists a wide BMG forming regime, in at.%, 33<Ta<38 and 2<Sn<9 with cast BMG rods of at least 1 mm diameter. The crystallization temperature being among the highest in Ni-based BMGs, 762–671 °C decreases with Sn content. These BMGs have wide super-cooled liquid region 57–61 K. They show high hardness, typically HV∼1000, and a compressive fracture strength 856–1192 MPa. The elastic strain limit is 0.0237–0.0266. The potentiodynamic studies in a solution of 12 M HCl show a passivation current density about 0.18 A m⁻² for Ni₅₈Ta₃₆Sn₆ BMG in the anodic region revealing a good corrosion resistance.     

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.     

Formation and mechanical properties of bulk Cu-Ti-Zr-Ni metallic glasses with high glass forming ability

Bulk amorphous Cu52.5Ti30Zr11.5Ni6 and Cu53.1Ti31.4Zr9.5Ni6 alloys with a high glass forming ability can be quenched into single amorphous rods with a diameter of 5 mm, and exhibit a high fracture strength of 2 212 MPa and 2 184 MPa under compressive condition, respectively. The stress—strain curves show nearly 2% elastic strain limit, yet display no appreciable macroscopic plastic deformation prior to the catastrophic fracture due to highly localized shear bands. The present work shows clearly evidence of molten droplets besides well-developed vein patterns typical of bulk metallic glasses on the fracture surface, suggesting that localized melting induced by adiabatic heating may occur during the final failure event.     

扩散焊连接Zr-Ti-Ni-Cu-Be块体非晶合金

采用超塑性扩散焊方法,对2 mm厚的锆基大块非晶合金Zr44Ti11Ni10Cu10Be25（vit1b）进行了焊接试验,获得了无界面缺陷的焊接接头,并且试样没有晶化。研究了非晶合金的扩散焊接工艺,根据试验结果提出了该合金优化的焊接工艺参数,探讨了非晶合金的扩散连接机理。研究发现,连接过程中,非晶合金发生了结构驰豫;接头质量与试样变形率的关系较为密切,只有当变形率达到一定值时才可以形成焊接接头。     

摩擦焊焊接块体非晶合金

在合理的工艺条件下,采用摩擦焊成功地实现了块体非晶合金Zr65Cu12.5Al7.5Ni15的连接;根据试验结果,探讨了焊接接头保持非晶的条件,即:被焊块体非晶合金处于过冷液态,且该过冷液体具有较好的热稳定性、超塑性和粘滞性,同时接触面的温度应始终低于该非晶合金的晶化温度.     

The fracture toughness of bulk metallic glasses

Stiffness, strength, and toughness are the three primary attributes of a material, in terms of its mechanical properties. Bulk metallic glasses (BMGs) are known to exhibit elastic moduli at a fraction lower than crystalline alloys and have extraordinary strength. However, the reported values of fracture toughness of BMGs are highly variable; some BMGs such as the Zr-based ones have toughness values that are comparable to some high strength steels and titanium alloys, whereas there are also BMGs that are almost as brittle as silicate glasses. Invariably, monolithic BMGs exhibit no or low crack growth resistance and tend to become brittle upon structural relaxation. Despite its critical importance for the use of BMGs as structural materials, the fracture toughness of BMGs is relatively poorly understood. In this paper, we review the available literature to summarize the current understanding of the mechanics and micromechanisms of BMG toughness and highlight the needs for future research in this important area.     

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.     

The superplastic forming of bulk metallic glasses

Superplastic forming (SPF) is introduced in this article as a net-shape processing method for bulk metallic glasses (BMGs), commercially known as Liquidmetal® alloys. This method decouples fast cooling and forming of the BMG. Forming takes place in the supercooled liquid region, where the BMG exists as a highly viscous liquid and increases its fluidity with increasing temperature. The SPF method is very similar to techniques used for processing thermoplastics. In this work, a simple flow law is used to quantify the forming ability and to estimate both the potential and the limitations of the SPF method. This process is especially well suited to replicate small features and thin sections with high aspect ratios, which makes this process appropriate for microelectromechanical systems, nano- and microtechnology, jewelry, medical and optical applications, and data storage.     

Biocompatibility of Ni-free Zr-based bulk metallic glasses

Three Ni-free Zr-based BMGs with composition of Zr₆₀Nb₅Cu₂₀Fe₅Al₁₀, Zr₆₀Nb₅Cu_22.5Pd₅Al_7.5, Zr₆₀Ti₆Cu₁₉Fe₅Al₁₀ were fabricated by suck copper-mould casting. All the BMGs prepared exhibit good glassy forming ability and wide supercooled liquid region ranging from 38 to 99 K. These BMGs also show good mechanical properties under static compression with yield strength of over 1350 MPa, Young's modulus of 70–80 GPa, and plastic strain of 3.6–9.5%. Friction and wear tests revealed that the BMGs exhibit much better wear resistance than the medical alloy Ti6Al4V, although BMGs have a higher friction coefficient. In addition, the in vitro test indicated that the BMGs have a similar or even better cell viability and proliferation activity as compared with Ti6Al4V. Finally, the in vivo evaluation of the BMGs was carried out by the implantation of BMG samples into white rabbits. It is shown that the BMG implants performed as well as the Ti alloy, demonstrating that the Ni-free Zr-based BMGs developed in this work are promising in medical applications.     

铜基块体非晶合金纳米压痕研究

本文利用三种不同的纳米压痕模式研究了两种铜基大块金属玻璃Cu₅₉Zr₃₆Ti₅和Cu₆₁Zr₃₄Ti₅。分别采用了加载速率控制模式、载荷控制模式和循环加载模式。当加载速率不超过5mN/S时,试样的杨氏模量随加载速率而变。Cu₅₉Zr₃₆Ti₅和Cu₆₁Zr₃₄Ti₅的弹性模量均随峰值载荷和加载速率的增加而降低。但峰值载荷和加载速率对硬度影响不大。循环加载使Cu₅₉Zr₃₆Ti₅产生轻微加工硬化,而Cu₆₁Zr₃₄Ti₅则不显示这样的结果。而且,Cu₆₁Zr₃₄Ti₅的硬度和模量都明显高于Cu₅₉Zr₃₆Ti₅。     

Strain rate-dependent deformation in bulk metallic glasses

Metallic glasses have metastable structures. As a result, their plastic deformation is dependent upon structural dynamics. In the present paper, we present data obtained from Zr-base and La-base metallic glasses and discuss the kinetic aspects of plastic deformation, including both homogeneous and heterogeneous deformation. In the case of homogeneous deformation (typically occurring in the supercooled liquid region), Newtonian behavior is not universally observed and usually takes place only at low strain rates. At high strain rates, non-Newtonian behavior is usually observed. It is demonstrated that this non-Newtonian behavior is associated with in situ crystallization of the amorphous structure. In the case of heterogeneous deformation (occurring at room temperature), deformation is controlled by localized shear banding. The plastic deformation of a La-base metallic glass is also investigated using instrumented nanoindentation experiments over a broad range of indentation strain rates. At low rates, the load-displacement curves during indentation exhibit numerous serrations or pop-ins, but these serrations become less prominent as the indentation rate is increased. Using the tip velocity during pop-in as a gauge of serration activity, we find that serrated flow is only significant at indentation strain rates below a certain critical value.     

Structural bulk metallic glasses with different length-scale of constituent phases

Bulk metallic glass composites containing constituent phases with different length-scales are prepared via an in situ method by copper mold casting homogeneous Zr–Ti–Nb–Cu–Ni–Al melts. The phase formation and the microstructure of the composite materials are investigated by X-ray diffraction, optical, scanning and transmission electron microscopy, and microprobe analysis. The composition of the melt as well as the cooling conditions realized during casting determine the type and the morphology of the phases present in the composite. The mechanical properties of composite materials with quasicrystalline or ductile bcc phase reinforcements are tested in uniaxial compression at room temperature, showing that the deformation is controlled by the type of the constituent phases and their morphology. Ductile phase-containing metallic glass composites demonstrate improved work hardening and ductility compared to monolithic metallic glasses. Similar results are obtained for composites with ductile bcc phase dendrites embedded in a nanocrystalline matrix. The improved ductility of the composites is due to the presence of the ductile second phase, which counteracts catastrophic failure by shear localization.     

Fabrication of CuZr(Al) bulk metallic glasses by high pressure torsion

In this paper, pure Cu, Zr and Al metal sheets were alternatively stacked and, then, cold roll-bonded. The overall compositions of the stacks were Cu₇₁Zr_29, Cu₆₂Zr_38, Cu₅₃Zr₄₇ and Cu₅₂Zr₃₈Al₁₀. The bonded materials were subsequently sliced into disks and, then, used for high pressure torsion (HPT) process. After HPT process up to maximum equivalent strain of about 1200 at ambient temperature, bulk materials with 10 mm in diameter and 0.30 mm in thickness were obtained. The phase constitution and microstructural evolutions of the HPTed samples with various rotations ranging from 0.5 to 20 were studied by XRD, FESEM and TEM. Thermal properties of the HPTed samples were analyzed by DSC. It was found that CuZr(Al) BMGs could be fabricated with a wide composition range by the HPT process. Tensile tests were used to measure the mechanical properties of the HPT processed samples. The mechanism of the solid-state amorphization during the HPT process was put forward and discussed.