Preferred clusters in metallic glasses

In this work,we present a feasible scheme based on framework of the sophisticated Voronoi tessellation method in order to evaluate what clusters should be preferred for building blocks in any given metallic glass,by analysing the fivefold-symmetry axes as well as the degree of structural regularity in various clusters.This scheme is well proved by a group of experiments and calculations,which may have broad implications for exploration of obtaining explicit and proper structural pictures,and understanding the structural origin of the unique properties and glass forming ability in these novel amorphous alloys.     

Design of high T g Zr-based metallic glasses using atomistic simulation and experiment

In this paper, we systematically investigate local atomic structures of Zr_100?x Al _x (0???x???72) alloys using molecular dynamics simulations. Radial distribution functions of Zr-Al configurations at 300 K indicate that Zr-Al metallic glasses form only when the Al atomic concentration is larger than 32%. Voronoi polyhedral analysis shows that Zr₄₀Al₆₀ has the highest fraction of ?0,0,12,0? icosahedra around Al atoms, which are characteristic of amorphous microstructures. Variations of thermal expansion coefficient and heat capacity of Zr_100?x Al _x (40???x???72) metallic glasses as a function of temperature from 1100 to 800?K reveal that Zr₄₀Al₆₀ has the highest transition temperature of 1008?K. To confirm the simulation results, Zr-Al metallic glasses were fabricated using co-sputtering deposition; differential scanning calorimetry testing suggests the highest crystallisation-onset temperature of above 920?K is within Zr_100?x Al _x where 43?<?x?<?61. The experimental finding is in a good agreement with the simulation predictions.     

Fluctuation-dissipation ratio in three-dimensional spin glasses

We present an analysis of the data on aging in the three-dimensional Edwards-Anderson spin-glass model with nearest-neighbor interactions, which is well suited for the comparison with a recently developed dynamical mean-field theory. We measure the parameterx(q) describing the violation of the relation among correlation and response functions implied by the fluctuation-dissipation theorem.     

钴和铁熔体短程序的分子动力学模拟

通过分子动力学模拟研究了金属钴和铁熔体从普通液态到过冷液态普通微观结构的性质.所计算两体分布函数与可获得的实验结果基本一致,从普通液态到过冷液态的局域结构中的原子配位数为11和12的分布几率随着温度的降低而变小,而原子的配位数大于12的分布几率反而增加,角分布函数位于55.有一个明显的峰,位于110°有一个宽展的峰,位于150°有一个肩膀,表明金属钴和铁熔体的微观局域结构要比规则的二十面体团簇的结构复杂得多.我们的模拟结果表明金属熔体中的二十面体短程序随着温度的降低而增加,并在过冷液态中而占优势.     

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.     

Superconducting state parameters of binary metallic glasses

Ashcroft’s empty core (EMC) model potential is used to study the superconducting state parameters (SSPs) viz. electron-phonon coupling strength λ, Coulomb pseudopotential μ*, transition temperature T _C , isotope effect exponent αand effective interaction strength N _O V of some binary metallic glasses based on the superconducting (S), conditional superconducting (S’) and non-superconducting (NS) elements of the periodic table. Five local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used for the first time with EMC potential in the present investigation to study the screening influence on the aforesaid properties. The T _C obtained from the H-local field correction function are in excellent agreement with available theoretical or experimental data. In the present computation, the use of the pseudo-alloy-atom model (PAA) was proposed and found successful. Present work results are in qualitative agreement with such earlier reported experimental values which confirm the superconducting phase in all metallic glasses. A strong dependency of the SSPs of the metallic glasses on the valence ‘Z’ is identified.      

Superconducting state parameters of CuCZr100-C metallic glasses

The theoretical investigation of the superconducting state parameters (SSP) viz. electron-phonon coupling strength λ, Coulomb pseudopotential μ*, transition temperature T _c, isotope effect exponent α and effective interaction strength N ₀ V of ten Cu _C Zr_100−C metallic glasses have been reported using Ashcroft’s empty core (EMC) model potential. Three local field correction functions proposed by Hartree (H), Taylor (T) and Ichimaru-Utsumi (IU) are used in the current investigation to study the screening influence on the aforesaid properties. It is observed that the electron-phonon coupling strength λ and the transition temperature T _C are quite sensitive to the selection of the local field correction functions, whereas the Coulomb pseudopotential μ*, isotope effect exponent α and effective interaction strength N ₀ V show weak dependences on local field correction functions. The T _c obtained from IU-local field correction function are found an excellent agreement with available theoretical or experimental data. Also, the present results are found in qualitative agreement with other such earlier reported data, which confirms the superconducting phase in metallic glasses.      

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.     

Static and cyclic mechanical behaviours and fracture mechanisms of Zr-based metallic glass at elevated temperatures

To comprehensively understand the quantitative mechanical–thermal properties and fracture mechanisms of Zr-based bulk metallic glass (BMG), by using self-made miniature tensile/compressive device and motor-piezoelectric coupling driven fatigue device, a series of static and dynamic mechanical tests at elevated temperatures of bulk Zr₅₅Cu₃₀Al₁₀Ni₅ BMG with glass transition temperature T_g of 411°C were carried out. Uniaxial tensile and compressive behaviours at temperatures with range from RT to 400°C were experimentally obtained by means of digital speckle correlation analysis, the estimation of Young’s modulus and fracture strengths as a function of the applied temperatures were investigated. The static and cyclic fracture morphologies at various temperatures were obtained to describe the mechanical–thermal fracture mechanisms in detail. The coupling effect of loading types, stress-induced temperature rise and applied temperature on the evolution of cleavage features, dimples, vein patterns and shear softening behaviours were investigated.     

非晶塑性变形机理研究新进展

非晶合金的塑性变形机理一直是材料科学和凝聚态物理研究的热点问题之一.文章简单介绍了近来中国科学院物理研究所在非晶合金塑性机理研究方面的最新进展,介绍了玻璃转变和塑性变形机制之间的关联性及最新的实验证据,以及从非平衡态统计力学角度对非晶塑性变形机制的理解,指出非晶合金的塑性和剪切带的动力学状态密切相关,发现韧性非晶合金在变形过程中可以演化到自组织临界状态.这对认识非晶合金的形成本质,探索具有实际应用价值的非晶合金具有重要意义.     

脆性金属玻璃断面上的奇妙图案

文章简要介绍了脆性块体金属玻璃(简称BMG)断裂面上几种主要的图案花样:河流花样、"韧窝"结构花样和自组装条纹结构花样,并总结了目前对上述各种形貌形成机理的可能的物理解释.对BMG断裂面上形貌的研究可以揭示材料的断裂机理,有助于更深刻地理解材料的力学性能,开发高性能金属玻璃材料,并为工程选材提供安全标准.     

Monte Carlo simulation of dual magnetic phase behavior in bulk metallic glasses

Taking magnetic clusters as magnetic units, the Monte Carlo simulation method was employed to investigate the behavior of dual magnetic phase in bulk metallic glasses (BMGs). The hysteresis loops of a BMG system containing two kinds of magnetic clusters at different temperatures were simulated. The results showed that at high temperatures, only one kind of magnetic cluster is active, however, at low temperatures, both clusters responded to the applied field, which was evidenced by a step observed in the hysteresis loop of a BMG system. The simulated magnetic behavior of the two-cluster BMG system is consistent with the experimental findings of a Nd-based bulk metallic glass with dual magnetic phases.     

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.     

High pressure atomic structure of Zr–Cu metallic glass via EXAFS spectroscopy and molecular dynamics simulations

ABSTRACT

In this work, we use extended X-ray absorption fine structure (EXAFS) data collected using nano-polycrystalline diamond anvil cell to study the atomic arrangement in Zr–Cu metallic glass in high pressure (HP) conditions. To reveal the microscopic details of stress accommodation mechanism, we performed molecular dynamics (MD) simulations of the HP atomic arrangement. By comparing the experimental and the calculated Zr and Cu K-edge EXAFS signal we prove the realistic character of the computer simulations. A detailed geometrical analysis of the simulated atomic configurations shows that with increasing hydrostatic pressure the local structure of Zr–Cu amorphous alloy becomes gradually dominated by Cu-centred icosahedral structural motifs involving fivefold symmetry incompatible with crystalline ordering. The variation of the short-range order is attributed to preferential straining of mechanically soft bonds between zirconium atoms.     

Preparation and characterization of Fe-based bulk metallic glasses in plate form

Amorphous alloys with composition (at%) Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Gd₂ (alloy A) and Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Y₂ (alloy B) were prepared either using pure elements (A and B1) and a commercial AISI430 steel as a base material (B2). When prepared from pure elements both alloys (A and B1) could be cast in plate form with a fixed thickness of 2 mm and variable lengths between 10 and 20 mm by means of copper-mold injection in air atmosphere. In the case of alloy B2, prepared using commercial grade raw materials, rods of 2 mm diameter were obtained.     

Modification of magnetic anisotropy in metallic glasses using high-energy ion beam irradiation

Heavy ion irradiation in the electronic stopping power region induces macroscopic dimensional change in metallic glasses and introduces magnetic anisotropy in some magnetic materials. The present work is on the irradiation study of ferromagnetic metallic glasses, where both dimensional change and modification of magnetic anisotropy are expected. Magnetic anisotropy was measured using Mössbauer spectroscopy of virgin and irradiated Fe₄₀Ni₄₀B₂₀ and Fe₄₀Ni₃₈Mo₄B₁₈ metallic glass ribbons. 90 MeV ¹²⁷I beam was used for the irradiations. Irradiation doses were 5×10¹³ and 7.5×10¹³ ions/cm². The relative intensity ratios D ₂₃ of the second and third lines of the Mössbauer spectra were measured to determine the magnetic anisotropy. The virgin samples of both the materials display in-plane magnetic anisotropy, i.e., the spins are oriented parallel to the ribbon plane. Irradiation is found to cause reduction in magnetic anisotropy. Near-complete randomization of magnetic moments is observed at high irradiation doses. Correlation is found between the residual stresses introduced by ion irradiation and the change in magnetic anisotropy.