Thermodynamical properties of Zr-based bulk metallic glasses

The temperature dependence of Gibb's free energy difference (ΔG), entropy difference (ΔS) and enthalpy difference (ΔH) between the undercooled melt and the corresponding equilibrium solid phases of bulk metallic glass (BMG) forming melts has been proved to be very useful in the study of their thermodynamical behavior. The present study is made by calculating ΔG, ΔS and ΔH in the entire temperature range T_m (melting temperature) to T_g (glass transition temperature) for three Zr-based samples of BMGs: Zr₅₇Cu_15.4Ni_12.6Al₁₀Nb₅, Zr_41.2Ti_13.8Ni₁₀Cu_12.5Be_22.5 and Zr_58.5Cu_15.6Ni_12.8Al_10.3Nb_2.8. The study is made on the basis of Taylor's series expansion and a comparative study is also performed between the present result and the result obtained in the framework of expansions proposed by earlier workers, and also with the experimental results. An attempt has also been made to study the glass forming ability for BMGs.     

Atomic structures of Zr-based metallic glasses

The atomic structures of Zr-Ni and Zr-Ti-Al-Cu-Ni metallic glasses were investigated by using classical molecular dynamic (MD), reverse Monte Carlo (RMC), ab initio MD (AIMD) simulations and high resolution transmission electron microscopy (HRTEM) techniques. We focused on the short-range order (SRO) and medium-range order (MRO) in the glassy structure. It is shown that there are icosahedral, FCC-and BCC-type SROs in the Zr-based metallic glasses. A structural model, characterized by imperfect ordered packing (IOP), was proposed based on the MD simulation and confirmed by the HRTEM observation. Furthermore, the evolution from IOP to nanocrystal during the crystallization of metallic glasses was also explored. It is found that the growth from IOP to nanocrystal proceeds through three distinct stages: the formation of quasi-ordered structure with one-dimensional (1D) periodicity, then 2D periodicity, and finally the formation of 3D nanocrystals. It is also noted that these three growth steps are crosslinked. Supported by the National Natural Science Foundation of China (Grant Nos. 50431030 and 50471097), the National Basic Research Program of China (Grant No. 2007CB613901), and the Programme of Introducing Talents of Discipline to Universities (Grant No. B07003)     

LaGa-based bulk metallic glasses

We report the formation of La Ga-based bulk metallic glasses. Ternary La–Ga–Cu glassy rods of 2–3 mm in diameter can be easily formed in a wide composition range by the conventional copper mold casting method. With minor addition of extra elements such as Co, Ni, Fe, Nb, Y, and Zr, the critical diameter of the full glassy rods of the La–Ga–Cu matrix can be markedly enhanced to at least 5 mm. The characteristics and properties of these new La Ga-based bulk metallic glasses with excellent glass formation ability and low glass transition temperature are model systems for fundamental issues investigation and could have some potential applications in micromachining field.     

Dual self-organised shear banding behaviours and enhanced ductility in phase separating Zr-based bulk metallic glasses

The multiplication and interaction of self-organised shear bands often transform to a stick-slip behaviour of a major shear band along the primary shear plane, and ultimately the major shear band becomes runaway and terminates the plasticity of bulk metallic glasses (BMGs). Here, we examined the deformation behaviours of the nanoscale phase-separating Zr_65–xCu₂₅Al₁₀Fe_x (x = 5 and 7.5 at.%) BMGs. The formation of multi-step phase separation, being mainly governed by nucleation and growth, results in the microstructural inhomogeneity on a wide range of length-scales and leads to obviously macroscopic and repeatable ductility. The good deformability can be attributed to two mechanisms for stabilizing shear banding process, i.e. the mutual interaction of multiple shear bands away from the major shear band and the delaying slip-to-failure of dense fine shear bands around the major shear band, both of which show a self-organised criticality yet with different power-law exponents. The two mechanisms could come into effect in the intermediate (stable) and later plastic deformation regime, respectively. Our findings provide a possibility to enhance the shear banding stability over the whole plastic deformation through a proper design of microstructure heterogeneities.     

Compressive fracture characteristics of Zr-based bulk metallic glass

The compressive fracture characteristics of Zr-based bulk metallic glass under uniaxial compression tests are studied.The zigzag rheological behavior is observed in the compression stress-strain curves of amorphous alloys.At room temperature the uniaxial compression fracture takes place along the plane which is at a 45-degree angle to the direction of the compressive stress.The microstructure of a typical fracture pattern is the vein network.A unique,finger-like vein pattern is found to exist at the fractur...     

Ni-based Ni-Fe-B-Si-Ta bulk metallic glasses

The glass-forming ability and properties of Ni-based Ni-Fe-B-Si-Ta bulk metallic glasses are explored in this work. The alloy compositions are determined by using a combination of the cluster line approach, the multi-alloying strategy and the substitutions of similar elements. Bulk metallic glasses with diameters of 3 mm take shape at compositions formulated under the clus- ter-plus-glue-atom model [M9B]B~[(Ni1-xFex)7.71(Si0.66Ta0.34)1.29B]B0.94=(Ni1-xFex)70.5B17.7Si7.8Ta4, x=0.35–0.45, where the bracketed part is the cluster and the unbracketed part is the glue atoms. These alloys exhibit good magnetic properties. The maximum Is is found in the (Ni0.55Fe0.45)70.5B17.7Si7.8Ta4 alloy which reaches 0.51 T, with its Hc as low as 8.5 A/m. Interestingly, these alloys display dual glass transitions at (Ni0.65Fe0.35)70.5B17.7Si7.8Ta4, (Ni0.60Fe0.4)70.5B17.7Si7.8Ta4 and (Ni0.55Fe0.45)70.5B17.7- Si7.8Ta4 as unveiled by Temperature-Modulated Differential Scanning Calorimetry.     

Developing aluminum-based bulk metallic glasses

This paper details a systematic investigation of the formation of Al-based bulk metallic glasses, expanding on an earlier brief report [Scripta Mater. 61 (2009) p.423]. We discuss an approach for designing and predicting the best glass-forming composition in the Al–TM–RE systems, based on the atomic cluster packing model for the internal structure of the glass. The effects of additional elements in quaternary and quinary systems on the glass-forming ability and thermal stability of the glasses are also discussed. Three new compositions, Al₈₆Ni₆Y_4.5Co₂La_1.5, Al₈₆Ni₇Y₅Co₁La₁ and Al₈₆Ni₇Y_4.5Co₁La_1.5, are capable of forming fully glassy rods of 1 mm in diameter; their glass transition and other thermal properties are systematically characterized.     

金属玻璃的热塑性成型

金属玻璃在其过冷液相区内表现出随着温度升高黏度逐渐降低的特性,因此可以对其进行热塑性加工.该性质颠覆了传统金属的加工成型方式,使得其在远低于传统金属材料加工的温度和应力作用下可以按照人们的要求进行成型.因此,一些具有低玻璃转变温度的金属玻璃又被称作金属塑料.另外,由于金属玻璃是一种无序结构材料,不存在位错、晶界等晶体缺陷,且热膨胀系数小,在热塑性成型中具有优异的尺寸精度,因此被认为是理想的微成型材料,有广阔的应用前景.本文系统介绍了金属玻璃的热塑性成型性质及其应用,从热塑性成型的基本概念出发,阐述了金属玻璃热塑性成型能力的评估指标、热塑性成型技术、热塑性微成型及其理论、热塑性微成型的应用等,对认识金属玻璃的热塑性及扩展其应用有重要的意义.     

Effect of overheating-induced minor addition on Zr-based metallic glasses

Melt treatment is well known to have an important influence on the properties of metallic glasses(MGs). However,for the MGs quenched from different melt temperatures with a quartz tube, the underlying physical origin responsible for the variation of properties remains poorly understood. In the present work, we systematically studied the influence of melt treatment on the thermal properties of a Zr₅₀Cu₃₆Al₁₄ glass-forming alloy and unveiled the microscopic origins...     

Isomechanical groups in bulk metallic glasses

Isomechanical groups are, as defined by Frost and Ashby [H.J. Frost and M.F. Ashby, Deformation–Mechanism Maps, Pergamon Press, Oxford, 1982], separate classes of materials that exhibit similar deformation and transport properties when normalised by an appropriate parameter. Fundamentally, this separation results from significant differences in material structure and bonding. Here, such an analysis is applied to 40 bulk metallic glasses (BMGs) grouped into three classes according to their Poisson's ratio, which is known to be an indicator of intrinsic toughness. Through rigorous statistical analysis, it is found that isomechanical groups are present and that they may result from (1) variation in the tendency for directional bonding, and (2) how liquid-like the structure is, which may be characterised by a quantification of local volumetric strain. These results suggest that, although experimentally observed properties from BMGs in different isomechanical groups are all typically considered within the same framework, differences in atomic packing and inter-atomic bonding mean that they should in fact be treated separately. These fundamental differences in bonding and structure may explain the known large variation in the tendency for toughness and plasticity in BMGs.     

Magnetic properties of Sm-based bulk metallic glasses

We perform detailed investigation on the magnetization and specific heat of Sm-based ternary bulk metallic glasses with different Co contents at low temperature. A low temperature cluster spin-glass phase below T_f ∼25 K is evidenced for all samples. This cluster spin-glass behavior is ascribed to competition among the multi-fold magnetic interactions and the intrinsic structural inhomogeneity. The magnetic relaxation behavior of the spin-glass phase can be well described using the stretched exponential dynamics. The magnetic hysteresis under field-cooling condition and spin dynamics further demonstrate the low temperature cluster spin-glass behavior. It is revealed that the Co atoms play an important role in modulating the physical properties of the present Sm-based ternary metallic glasses.     

超高强块体非晶合金的研究进展

研制具有极限力学性能的金属材料一直是材料研究人员的梦想.超高强块体非晶合金是一类具有极高断裂强度(4 GPa)、高热稳定性(玻璃化转变温度通常高于800 K)和高硬度(通常高于12 GPa)的新型先进金属材料,其代表合金材料Co-Ta-B的断裂强度可达6 GPa,为目前公开报道的块体金属材料的强度记录值.本文系统地综述了该类超高强度块体非晶合金的组分、热学性能、弹性模量及力学性能,阐述了该类材料的研发历程;以弹性模量为联系桥梁,阐明了该类超高强块体非晶合金材料各物理性能的关联性,并揭示了其高强度、高硬度的价键本质.相关内容对于材料工作者了解该类超高强度金属材料的性能和特点,并推进该类材料在航空航天先进制造、超持久部件、机械加工等领域的实际应用有着重要意义.     

Extraordinary plasticity of ductile bulk metallic glasses

Shear bands generally initiate strain softening and result in low ductility of metallic glasses. In this Letter, we report high-resolution electron microscope observations of shear bands in a ductile metallic glass. Strain softening caused by localized shearing was found to be effectively prevented by nanocrystallization that is in situ produced by plastic flow within the shear bands, leading to large plasticity and strain hardening. These atomic-scale observations not only well explain the extraordinary plasticity that was recently observed in some bulk metallic glasses, but also reveal a novel deformation mechanism that can effectively improve the ductility of monolithic metallic glasses.     

High-frequency dynamics in metallic glasses

Using inelastic x-ray scattering we studied the collective dynamics of the glassy alloy Ni33Zr67 in the first pseudo-Brillouin-zone, an energy-momentum region still unexplored in metallic glasses. We determine key properties such as the momentum transfer dependence of the sound velocity and of the acoustic damping, discussing the results in the general context of recently proposed pictures for acoustic dynamics in glasses. Specifically, we demonstrate the existence in this strong glass of well defined (in the Ioffe-Regel sense) acoustic-like excitations well above the boson peak energy.     

Three-point bending fracture characteristics of bulk metallic glasses

This paper presents the SEM micrographs for the three-point bending fracture surfaces of Zr-based, Ce-based and Mg-based bulk metallic glasses (BMGs), which show the dimple structures in the three kinds of BMGs. The shapes of the giant plastic deformation domain on the fracture surface are similar but the sizes are different. The fracture toughness KC and the dimple structure size of the Zr-based BMG are both the largest, and those of the Mg-based BMG are the smallest. The fracture toughness KC and the dimple structure size of the Ce-based BMG are between those of the Zr-based and the Mg-based BMG. Through analyzing the data of different fracture toughnesses of the BMGs, we find that the plastic zone width follows w = (KC/σY)2/(6π).     

Tribological characterisation of Zr-based bulk metallic glass in simulated physiological media

Due to their excellent wear resistant properties and high strength, as well as a low Young's modulus, Zr-based bulk metallic glasses (BMGs) are potentially suitable biomaterials for low-friction arthroplasty. The wear characteristics of the Zr_60.14Cu_22.31Fe_4.85Al_9.7Ag₃ bulk amorphous alloy against ultra-high-molecular-weight polyethylene (UHMWPE) compared to a CoCrMo/UHMWPE combination were investigated in two different wear screening test devices, reciprocating and unidirectional. Hank's solution and sterile calf bovine serum were selected as the lubricant fluid media. It was found that different fluid media had insignificant effect on polyethylene wear against BMG counterfaces. The wear behaviour obtained on both test devices demonstrated that Zr-based BMG achieved UHMWPE counterface wear rates superior to conventional cast CoCrMo alloy, where the wear rate of UHMWPE is decreased by over 20 times. The tribological performance of these joints is superior to that of conventional metal-on-polymer designs. Contact angle measurements suggested that the advantage of BMG over a CoCrMo alloy counterface is attributed to its highly hydrophilic surfaces.     

Fe-B-Y-Nb bulk metallic glasses in relation to clusters

Bulk metallic glass formations in the Fe-B-Y-Nb quaternary alloy system were investigated by using the cluster line rule in combination with the minor alloying principle. The Fe-B-Y ternary system was selected as the basic system and the intersections of cluster lines were taken as the basic ternary compositions. The basic compositions were further alloyed with minor amounts of Nb. After 3–5 at.% Nb was added, the basic composition Fe_68.6B_25.7Y_5.7, which was developed from the most densely packed cluster Fe₈B₃, formed 3 mm bulk metallic glasses. These quaternary bulk metallic glasses (Fe_68.6B_25.7Y_5.7)_100−x Nb _x (x = 3–5 at.%) are expressed approximately with a unified simple composition formula: (Fe₈B₃)₁(Y, Nb)₁. The (Fe_68.6B_25.7Y_5.7)₉₇Nb₃ bulk metallic glass has the largest glass forming ability with the following characteristic parameters T _g = 907 K, T _x = 1006 K, T _g/T _l = 0.644, γ = 0.434, and longness t = 22 mm. The combination of the cluster line rule and the minor-alloying principle is a promising new route towards the quantitative composition design of multi-component metallic glasses. Supported by the National Natural Science Foundation of China (Grant Nos. 50671018, 50631010 and 50401020) and the National Basic Research Program of China (Grant No. 2007CB613902)     

Dynamic strength behavior of a Zr-based bulk metallic glass under shock loading

Dynamic strength behavior of Zr51Ti5Ni10Cu25Al9 bulk metallic glass(BMG) up to 66 GPa was investigated in a series of plate impact shock-release and shock-reload experiments.Particle velocity profiles measured at the sample/Li F window interface were used to estimate the shear stress,shear modulus,and yield stress in shocked BMG.Beyond confirming the previously reported strain-softening of shear stress during the shock loading process for BMGs,it is also shown that the softened Zr-BMG still has a high shear modulus and can support large yield stress when released or reloaded from the shocked state,and both the shear modulus and the yield stress appear as strain-hardening behaviors.The work provides a much clearer picture of the strength behavior of BMGs under shock loading,which is useful to comprehensively understand the plastic deformation mechanisms of BMGs.     

In-situ neutron scattering study of crystallization in a Zr-based bulk metallic glass

In this paper, we report the structure, optical, and gas-sensing properties of Co-doped ZnO nanocrystals prepared by a simple solvothermal route. The red-shift of the band-gap edge is attributed to a merging of donor and conduction bands due to Co doping. Photoluminescence (PL) spectra were used to study the dependence of doping on the deep-level emission, which show obvious enhanced blue-green emission after Co doping. Gas sensors were prepared and tested for the detection of C₂H₅OH in air. It is found that the Co-doped nanocrystals have a significantly better sensing performance than pure ZnO, which is comparable to the Au-functionalized ZnO sensors. We provide a possible explanation in terms of the sensing mechanism of the surface reaction process.     

Zr-based bulk metallic glass as a cylinder material for high pressure apparatuses

Zirconium-based bulk metallic glass (Zr-based BMG) has outstanding properties as a cylinder material for piston-cylinder high pressure apparatuses and is especially useful for neutron scattering. The piston-cylinder consisting of a Zr-based BMG cylinder with outer/inner diameters of 8.8/2.5?mm sustains pressures up to 1.81?GPa and ruptured at 2.0?GPa, with pressure values determined by the superconducting temperature of lead. The neutron attenuation of Zr-based BMG is similar to that of TiZr null-scattering alloy and more transparent than that of CuBe alloy. No contamination of sharp Bragg reflections is observed in the neutron diffraction pattern for Zr-based BMG. The magnetic susceptibility of Zr-based BMG is similar to that of CuBe alloy; this leads to a potential application for measurements of magnetic properties under pressure.