Studies on crystallization behaviour and mechanical properties of Al-Ni-La metallic glasses

Alloy ingots with nominal composition, Al_92−x Ni₈La _x (x = 4 to 6) and Al_94−x Ni₆La _x (x = 6, 7), were prepared by induction melting in a purified Ar atmosphere. Each ingot was inductively re-melted and rapidly solidified ribbons were obtained by ejecting the melt onto a rotating copper wheel in an argon atmosphere. The crystallization behaviour of melt-spun amorphous ribbon was investigated by means of differential scanning calorimetry (DSC), X-ray diffractometry and transmission electron microscopy. DSC showed that Al₈₆Ni₈La₆ alloy undergoes a three-stage and rest of the alloys undergo a two-stage crystallization process upon heating. The phases responsible for each stage of crystallization were identified. During the first crystallization stage fcc-Al precipitates for low La-containing alloys and for higher La-containing alloys a bcc metastable phase precipitates. The second crystallization stage is due to formation of intermetallic compounds along with fcc-Al. Microhardness of all the ribbons was examined at different temperatures and correlated with structural evolutions. Precipitation strengthening of nano-size fcc-Al is responsible for maximum hardness in these annealed alloys.     

锆基金属玻璃超声振动下的塑性流动行为

由于应力软化和表面快速扩散效应,超声振动可以用于金属玻璃微成型.然而,超声振动下的结构重排及其对金属玻璃力学响应机制的影响仍不清楚.本工作采用纳米压痕方法研究了超声振动能量为140 J的Zr(35)Ti(30)Cu(8.25)Be(26.75)金属玻璃的塑性流动行为.我们采用Kelvin和Maxwell-Voigt模型...     

低温弛豫对Mge5 Cu25 Gd10大块非晶合金热稳定性及力学行为影响

Mg65 Cu25 Gd10非晶合金的热稳定性关系到其作为结构材料的实用性及发展前景。利用差示扫描量热法（DSC）研究了预先低温弛豫处理后Mg65 Cu25 Gd10非晶的特征转变温度和晶化激活能，分析了低温弛豫对其热稳定性的影响，通过Kissinger方程计算其晶化激活能、频率因子、反应速率系数进一步说明此非晶的晶化过程。同时，通过比较其力学性能的变化，发现非晶压缩性能受非晶稳定性影响不大，抗压强度下降很小。但其断裂方式及断面微观特征有明显变化。     

Plasticity and rejuvenation of aged metallic glasses by ultrasonic vibrations

Metallic glasses(MGs)possess exceptional properties,but their properties consistently deteriorate over time,thereby resulting in increased complexity in processing.It thus poses a formidable challenge to the forming of long-term aged MGs.Here,we report ultrasonic vibration(UV)loading can lead to large plas-ticity and strong rejuvenation in significantly aged MGs within 1 s.A large UV-induced plasticity(UVIP)of 80％height reduction can be achieved in LaNiAl MG samples aged at 85％of its glass transition tem-perature(0.85Tg)for a duration of up to 1 month.The energy threshold required for UVIP monotonously increases with aging time.After the UV loading process,the aged samples show strong rejuvenation,with the relaxation enthalpy even surpassing that of as-cast samples.These findings suggest that UV loading is an effective technique for forming and rejuvenating aged MGs simultaneously,providing an alterna-tive avenue to explore the interplay between the property and microstructures as well as expanding the application prospects of MGs.     

Kinetics of crystallization of a Fe74Co10B16 glass

Differential scanning calorimetry (DSC) technique has been employed to study the crystallization kinetics of a Fe₇₄Co₁₀B₁₆ glass. Crystallization of Fe₇₄Co₁₀B₁₆ glass is known to occur in two distinct stages. It was possible to separate out the isothermal kinetics of both the crystallization stages through a thermal treatment scheme in the DSC. The crystallization processes are interpreted in the light of the kinetic data obtained.     

Internal friction of hydrogen-doped metallic glasses of high glass forming ability

Seven multi-component metallic glasses of high glass forming ability (four Zr-based, a Mg-based, a Pd-based and a La-based glasses) have been hydrogenized electrolytically and internal friction has been measured at temperatures between 80 and 400 K. Hydrogen damping has been observed in every alloy; the internal friction peak is quite broad, where the peak value increases and then decreases and the peak temperature decreases with increasing hydrogen content. Compared with the results of the hydrogen damping in binary metallic glasses so far reported, the peak height versus peak temperature relation is generally shifted to higher temperature side in multi-component glasses, the origin of which has been discussed.     

Balancing benefits of strength,plasticity and glass-forming ability in Co-based metallic glasses

Development of advanced metals materials with ultrahigh strength,large plasticity and high thermosta-bility is one of the most attractive aims for materials researchers.Co-based bulk metallic glasses(BMGs)with the highest strength(up to 6 GPa)and special strength(up to 650 Nm/g)among all of metals mate-rials so far we known have received extensive attentions.In this paper,a family of Co-Ta-B-Si BMGs with high glass-transition temperature(above 870 K),large compressive plasticity(up to 6.4％)and high strength(above 5.5 GPa),and high glass-forming ability(the critical diameter,Dc:up to 4 mm)was devel-oped by accurately tuning metalloid element contents of Si and B in the parental alloy of Co55Ta10B35.The changes of glass formation and plasticity caused by the adjustment of the constituent metalloid elements were evaluated by the combination of experimental and calculated results.The reason for the significant improvement of plastic deformation is revealed by the analysis of the self-organization behaviors of high-density shear bands.     

Mechanical property and structural changes by thermal cycling in phase-separated metallic glasses

Nondestructive cryogenically thermal cycling has been a simple but effective treatment to enhance mechanical properties of glassy materials.However,how the structural heterogeneities on nanometer scales are affected by thermal cycling is still an issue.Here,we report the response of spatial hetero-geneities in three selected Ti41Zr25Be28Fe6,Zr56Co14Cu14Al16 and Zr42Y14Co22Al22(at.％)metallic glasses(MGs)with different compositions to the thermal cycling,which show significantly different structure and properties after the same treatments and could be ascribed to the joint contribution of relaxation and rejuvenation induced by thermal cycling.The rejuvenation is initially prevailed in a Zr-Y-containing MG,whereas the relaxation is dominant in a Cu-Co-containing MG,both eventually entering into a dynamic equilibrium state.By employing nanometer-scale structural models,the intrinsic correlation between the spatial heterogeneity and thermal cycling is proposed.The discovery could provide the fundamental understanding of the role of spatial heterogeneity in influencing the macroscopic properties of MGs via thermal cycling and help design high-performance glassy materials by tailoring their atomic structures with suitable thermal treatments.     

Thin film metallic glasses: Unique properties and potential applications

A new group of thin film metallic glasses (TFMGs) have been reported to exhibit properties different from conventional crystalline metal films, though their bulk forms are already well-known for high strength and toughness, large elastic limits, and excellent corrosion and wear resistance because of their amorphous structure. In recent decades, bulk metallic glasses have gained a great deal of interest due to substantial improvements in specimen sizes. In contrast, much less attention has been devoted to TFMGs, despite the fact that they have many properties and characteristics, which are not readily achievable with other types of metallic or oxide films. Nevertheless, TFMGs have been progressively used for engineering applications and, thus, deserve to be recognized in the field of thin film coatings. This article will thus discuss both properties and applications of TFMGs including a review of solid-state amorphization upon annealing, the glass-forming ability improvement due to thin film deposition, and mechanical properties, including residual stress, hardness and microcompression, adhesion, and wear resistance. Potential applications and simulations will also be discussed.     

Comparison of mechanical behavior between bulk and ribbon Cu-based metallic glasses

As-cast bulk and as-spun ribbon Cu₆₀Zr₃₀Ti₁₀ metallic glasses were characterized using differential-scanning calorimetry and instrumented nanoindentation. Two alloys show a significant difference in the amount of free volume, which is attributed to the difference in a cooling rate, while exhibiting a similar serrated plastic flow. Atomic-force-microscopy observations demonstrate the pile-ups containing shear bands around the indents in both alloys. The as-cast bulk alloy has higher hardness and elastic modulus than the as-spun ribbon alloy. The difference in the strengths of two alloys may be related to the different amount of free volume. The strength seems to be more sensitive to a cooling rate during solidification than the plastic-flow behavior in the Cu₆₀Zr₃₀Ti₁₀.     

Designing metallic glasses with optimal combinations of glass-forming ability and mechanical properties

It is a long-standing challenge to search for metallic glasses(MGs)with optimal combinations of glassforming ability(GFA),strength and toughness in the vast compositional space.By taking into account both recently developed ellipse criterion and temperature-based GFA criterion,here we established quantitative correlations among compositions,elastic constants,GFA and mechanical properties of MGs,which enable to predict the GFA,fracture strength and fracture surface simultaneously in advance once the compositions of MGs are determined.Experimental data confirm the validity of this approach in prediction.Finally,a strategy for designing MGs with optimal combinations of strength,toughness and GFA is proposed,which allows for high-throughput discovering glass formers with excellent mechanical properties.     

In situ TEM study of pulse-enhanced plasticity of monatomic metallic glasses

The electropulsing process can be used to tailor the microstructure and deformability of metallic glasses(MGs).Here,we report the microstructural origin of enhanced electroplasticity of monatomic Ta MG nanowires.Under electromechanical loading,the Ta MG nanowire exhibits improved ductility and ob-vious necking behavior.By evaluating the dynamic structural evolution via in situ diffraction,it is found that the atomic mobility in flow units of Ta MG can be improved significantly under the stimulation of pulse current,mainly through the athermal electron-atom interaction,which results in the fast annihila-tion of flow units and,thereby,fast structural relaxation.These structural evolution processes can help to eliminate the formation of the obvious shear band.These findings provide insight into the origin of elec-troplasticity in amorphous materials,which is of scientific and technological significance for the design and processing of a variety of MGs.     

Comparison of compressive deformation and fracture behaviors of Zr- and Ti-based metallic glasses with notches

Compressive tests on the Zr- and Ti-based metallic glasses with different notches were investigated to compare their shear fracture mechanism and plastic deformation abilities. It is found that the plasticity of the two metallic glasses can be improved by installing two semicircular symmetrical notches even for the Ti-based metallic glass which has nearly zero compressive plasticity. The enhanced plasticity may be ascribed to the easy initiation of shear bands (SBs) around the notches, and the consequent blocking effect of notches on the propagation of shear bands according to the large-scale stress gradient. Additionally, based on a theoretical model originated from the concept of critical steady shear displacement (CSSD), compared with the sizes of smooth regions on the fracture surface, the plasticity difference of the two different metallic glasses was analyzed quantitatively. The current findings might provide an approach to understand and estimate the difference in the plastic deformation abilities on diverse metallic glasses, as well as the ones with large-scale stress gradient.     

High voltage electron microscope irradiation and observations in metallic glasses

High voltage electron microscope (hvem) has been extensively used to produce radiation damage and to study the characteristics of defects so produced in crystalline solids. To understand the defect production in metallic glasses and to evaluate the influence of such defects on physical properties like crystallisation temperature etc., high voltage microscopy and subsequentin situ heating and observation has been extremely useful technique. This paper gives a qualitative overview of such work performed in metallic glasses. In particular results obtained on a nickel based metallic glass using ahvem and an electron accelerator are presented. The advantages and limitations ofhvem irradiation are highlighted.     

Diffusion mechanisms at metallic grain boundaries

Summary As the sizes of electronic devices continue to shrink, understanding key atomic phenomena vital to macroscopic processes become increasingly important. Mass transport along grain boundaries (GBs) is such a key process. We have studied diffusion mechanisms at metallic GBs of Ag and Al with the embedded-atom method, with molecular statics (MS) and molecular dynamics (MD), as well as with massively parallel computers at Oak Ridge National Laboratory and Sandia National Laboratory. Formation and migration energies of interstitials and vacancies at the optimal symmetric tilt GBs of different angles in Ag were first obtained using MS at 0 K. Extensive MD simulations were then carried out for selected Ag GBs using massively parallel computers at different finite temperatures up to the melting point. In this way, we were able to determine the dominant diffusion mechanism within different temperature regimes by comparison of the activation energies from MS results for the identified diffusion processes with the MD simulation results. For the first time, the results of this study on Ag GBs had provided realistic explanation and simulation-based evidence for the discrepancy between the activation energies from the recent low-temperature experiments by Ma and Balluffi and those at high temperatures reported in the literature. Preliminary MD results on Al GB diffusion are in excellent agreement with experiment and on-going work on Al and Al-Cu systems aimed to further understand electromigration phenomena will be briefly discussed.     

Defects activation in CoFe-based metallic glasses during creep deformation

Maxwell-Voigt model with two Kelvin units and one Maxwell unit was utilized to analyze the microalloying effect of Cu on the creep behavior of CoFe-based metallic glasses at different loading rates. The defect activation during creep deformation was detected by the relaxation time spectrum based on this model. The defect, with respect to a short relaxation time in relaxation spectra, intends to be activated at a quasi-static loading mode in the alloy with 0.5 at.% Cu addition. With further increasing loading rates,more defects with a large size were provoked activated at both hard and soft regions in the Cu-containing sample. A softening with the reduction of elastic modulus and hardness about 10 % and 15 %, respectively,was also observed in the Cu-doped sample. It is consistent with the pronounced viscoplastic deformation of this alloy along with the decrease of viscosity. Our work provides a microscopic insight into structural evolution during creep deformation in a Cu-doped metallic glass, which might help for understanding the plastic deformation of metallic glasses upon a minor addition.     

利用界面设计制造巨型金属玻璃

开发具有优良性能的材料一直是人类不懈的追求.如果将尺寸放大到与传统金属相当的水平,金属玻璃将是一种理想的金属材料.为了应对这一挑战,在过去的几十年中,研究学者们已经尝试了多种方法,包括基于热力学的合金开发、3D打印以及基于人工智能学习的合金优化设计新理念.本文提出了一种简便、灵活的界面设计理念来制造直径大于100 mm...     

Composition and size dependent torsion fracture of metallic glasses

The fracture of metallic glasses(MGs)of different compositions and sizes down to micrometers under torsion loading were systematically investigated.Contrary to the flat shear fracture along the circumfer-ential plane as commonly supposed under torsion,we find that the torsion fracture of metallic glasses can deviate from flat shear plane,and the fracture angle is closely dependent on the composition and the size of MG samples.With a conversion method,we show that the torsion fracture of both millimeter-and micrometer-sized MGs can be described by the ellipse fracture criterion as originally proposed for the tension fracture.The deviation from the circumferential shear plane under torsion is further shown to intrinsically relate to the fracture toughness of MGs.The tougher MG tends to have a smaller fracture angle with respect to the maximum shear plane,and vice versa,indicating a correlation between the fracture toughness and pressure/normal stress sensitivity in MGs.Our results provide new insights on the fracture mechanism and are helpful to design and control the deformation and fracture behavior of MGs under torsion loading.     

Compressive deformation behaviors of tungsten fiber reinforced Zr-based metallic glass composites

Subjected to a wide range of strain rates, the compressive deformation and fracture behavior at different temperatures of tungsten fiber reinforced Zr-based metallic glass composites (BMGC) were investigated. Upon increasing temperature, the compressive strength decreases, accompanied with reduced elasticity and enhanced plasticity. In combination with fracture analysis, the effects of temperature, applied strain rate, and the reinforcement of tungsten fiber were explicitly described.