Glass formability of W-based alloys through thermodynamic modeling: W–Fe–Hf–Pd–Ta and W–Fe–Si–C

Computational thermodynamics, based on the CALculation of PHAse Diagram (CALPHAD) method, can be an efficient way to predict phase stabilities in multi-component engineering materials. By calculating the stability of the liquid phase at low temperatures, this method could be a useful and cost-effective tool for the design of bulk metallic glasses. Based on the thermodynamic modeling of the constituent binary and ternary systems of W with Fe, Hf, Pd, Ta, Si, or C, thermodynamic databases are built to search for W-based metallic glasses in these alloying systems. Modeling of intermetallic phases combines input from first-principles total energy calculations and predictions of finite temperature properties from the Debye–Grüneisen model. Several plausible W-rich glass-forming alloys are identified in the W–Fe–Si–C quaternary system.     

The impact of fragility on the calorimetric glass transition in bulk metallic glasses

The glass transition of bulk metallic glasses with various fragilities as well as strong oxide glasses is studied using differential scanning calorimetry (DSC). It is found that the liquid fragility determined from equilibrium viscosity measurements is very well correlated with the scaled maximum slope of the DSC heat flow during the glass transition. We compare the correlation found in this work and those correlations with fragility from previous studies on other classes of glass-formers and find that the slope, which describes the curvature of the enthalpy on a reduced temperature scale, is a quantity better correlated with fragility, as it reflects the timescale of the non-equilibrium relaxation and the distribution of relaxation times in the glassy state. The present findings are supported by a recent theoretical report for calculated enthalpy curves with different fragilities from a model of selenium using the enthalpy landscape approach.     

Kinetics,Thermodynamics, and Structure of Bulk Metallic Glass Forming Liquids

Bulk metallic glass forming melts are viscous liquids compared with pure metals and conventional alloys. They show intermediate kinetic fragility and low thermodynamic driving force for crystallization, leading to sluggish crystallization kinetics, leaving time for good glass forming ability and bulk casting thickness. We relate the kinetics to the thermodynamics of the supercooled liquid using the Adam–Gibbs equation. The kinetic fragility is also connected to the structural changes in the liquid and can be quantitatively linked to the robustness of medium-range order in the supercooled liquid with increasing temperature. Liquid–liquid transitions from fragile behavior at high temperature to strong behavior at low temperature in the supercooled liquid and in the vicinity of the glass transition emerge as a common phenomenon.     

Thermodynamics and structural relaxation in Ce-based bulk metallic glass-forming liquids

Thermodynamics, kinetics and structural relaxation of Ce-based bulk metallic glass-forming liquid were investigated in the glass transition region by calorimetric measurements. The differences in thermodynamic functions were calculated between the supercooled liquid and crystalline state of the Ce-based alloys. Structural relaxation was studied by heating rate dependence of glass transition temperature. In terms of fragility parameter m, the Ce-based alloys were stronger liquid than other metallic glass-forming liquids. The correlation of the excellent glass-forming ability of Ce-based alloys with the thermodynamic property (Gibbs free energy) and the kinetic property (m) was discussed. The structural relaxation from glass state to the equilibrium supercooled state was well described by Tool-Narayanaswamy-Moynihan (TNM) model using the parameters derived from the calorimetric measurements.     

Development and characterization of Ca–Mg–Zn–Cu bulk metallic glasses

A number of ternary Ca–Mg–Zn and Ca–Mg–Cu and quaternary Ca–Mg–Zn–Cu bulk metallic glasses were produced using recently developed specific criteria. Their glass forming ability was correlated to the alloy chemistry, melting temperature, and driving force for crystallization of super-cooled liquid. A structural assessment using the efficient cluster packing model was also applied and showed a good ability to represent these glasses. Glass transition temperature, crystallization temperature and heat of crystallization were also determined for the produced alloys. Compression tests were conducted on a quaternary alloy at room temperature and in the temperature range of super-cooled liquid.     

A bridge from monotectic alloys to liquid-phase-separated bulk metallic glasses: Design,microstructure and phase evolution

The Zr–Ce–La system is characterized by a miscibility gap and a monotectic reaction. It separates into Zr-rich and CeLa-rich liquids upon cooling through the gap. Based on this system, a new Zr–Ce–La–Al–Co monotectic system was created to synthesize liquid-phase-separated bulk metallic glasses (LPS-BMGs) by copper mold casting. A systematical investigation was performed for the effects of the relative atomic ratios of Zr:CeLa, Co:Al and Ce:La on the microstructure features and chemical compositions of the two coexistent phases. Dual atom pairs with positive heat of mixing (Zr–Ce: +12 kJ mol^?1 and Zr–La: +13 kJ mol^?1) are originally adopted to develop such LPS-BMGs. A series of in situ formed LPS-BMGs with a critical thickness of 2.5 mm has been successfully synthesized. By combining the kinetics of liquid–liquid phase separation with the formation of metallic glasses, the mechanisms of phase formation and the microstructure evolution in the rapidly cooled alloys are discussed in detail. Furthermore, a thermodynamic model is proposed for LPS-BMG design, attempting to build a bridge from monotectic/immiscible (M/I) alloys to LPS-BMGs. This work not only provides opportunities for new insights into the synthesis of LPS-BMGs and their properties but also opens new perspectives for processing and research of M/I alloys.     

Relating Dynamic Properties to Atomic Structure in Metallic Glasses

Atomic packing in metallic glasses is not completely random but displays various degrees of structural ordering. While it is believed that local structures profoundly affect the properties of glasses, a fundamental understanding of the structure–property relationship has been lacking. In this article, we provide a microscopic picture to uncover the intricate interplay between structural defects and dynamic properties of metallic glasses, from the perspective of computational modeling. Computational methodologies for such realistic modeling are introduced. Exploiting the concept of quasi-equivalent cluster packing, we quantify the structural ordering of a prototype metallic glass during its formation process, with a new focus on geometric measures of subatomic “voids.” Atomic sites connected with the voids are found to be crucial in terms of understanding the dynamic, including vibrational and atomic transport, properties. Normal mode analysis is performed to reveal the structural origin of the anomalous boson peak (BP) in the vibration spectrum of the glass, and its correlation with atomic packing cavities. Through transition-state search on the energy landscape of the system, such structural disorder is found to be a facilitating factor for atomic diffusion, with diffusion energy barriers and diffusion pathways significantly varying with the degree of structural relaxation/ordering. The implications of structural defects for the mechanical properties of metallic glasses are also discussed.     

Hidden order in the fracture surface morphology of metallic glasses

We report the fractal nature of dimple structures on the fracture surface of various metallic glasses (MGs) with significantly different mechanical properties. The analyzed fractal dimension increments of MGs lie in a narrow range of 0.6–0.8. The results indicate that the MGs with marked differences in plasticity and toughness may have a unified local softening mechanism and similar nonlinear plastic behavior in front of the crack tip during fracture. We present a physical picture for the dimple structure formation from the plastic zone in front of the crack tip. The evolution of the plastic zone from the interaction of the shear transformation zones is theoretically modeled as a stochastic process far from thermodynamic equilibrium, and the model can capture the formation and fractal nature of the dimple structures on the fracture surface of metallic glasses.     

Estimation of Gibbs free energy difference in Pd-based bulk metallic glasses

A new thermodynamic expression for Gibbs free energy difference AG between the under-cooled liquid and the corresponding crystals of bulk metallic glasses was derived. The newly proposed expression always gives results in fairly good agreement with experimental values over entire temperature range between the fusion temperature Tm and the glass transition temperature Tg of Pd40Ni40P20, Pd40Cu30Ni10P20 and Pd43Cu27Ni10P20, which possess different heat capacities. However, the TS and KN expressions cannot always provide results in good agreement with the experimental values. In addition, the deviations between the experimental values and the AG calculated by the proposed expression at Tg are smaller than those given by other expressions for all the bulk metallic glasses studied.     

Zr–(Cu,Ag)–Al bulk metallic glasses

In this paper, we report the formation of a series Zr–(Cu,Ag)–Al bulk metallic glasses (BMGs) with diameters at least 20 mm and demonstrate the formation of about 25 g amorphous metallic ingots in a wide Zr–(Cu,Ag)–Al composition range using a conventional arc-melting machine. The origin of high glass-forming ability (GFA) of the Zr–(Cu,Ag)–Al alloy system has been investigated from the structural, thermodynamic and kinetic points of view. The high GFA of the Zr–(Cu,Ag)–Al system is attributed to denser local atomic packing and the smaller difference in Gibbs free energy between amorphous and crystalline phases. The thermal, mechanical and corrosion properties, as well as elastic constants for the newly developed Zr–(Cu,Ag)–Al BMGs, are also presented. These newly developed Ni-free Zr–(Cu,Ag)–Al BMGs exhibit excellent combined properties: strong GFA, high strength, high compressive plasticity, cheap and non-toxic raw materials and biocompatible property, as compared with other BMGs, leading to their potential industrial applications.     

Finite element simulation and experimental investigation of forming micro-gear with Zr–Cu–Ni–Al bulk metallic glass

Bulk metallic glasses exhibit some unique physical properties as compared to their corresponding crystalline alloys. Due to the superplasticity by behaving like a Newtonian fluid in their supercooled liquid region, the bulk metallic glasses can be used to make high strength microparts by net-shape forming. In this paper, the compressive tests of Zr–Cu–Ni–Al metallic glass are performed with different strain rates at a temperature of 683 K. According to the experimental results, the forming evolution of a metallic glass micro-gear is simulated using a finite element simulation software DEFORM 3D, and the forming load is predicted at different processing parameters. Meanwhile, the filling stages of bulk metallic glass in the micro-gear mold cavity are investigated by finite element simulation and experiment. The predicted workpiece geometry shows good agreement with experimental result. The forming experiments for micro-gear of Zr–Cu–Ni–Al metallic glass are carried out by hot embossing process, and the amorphous micro-gears are obtained successfully. It is found that the finite element simulation results are in reasonable agreement with the experimental observation.     

Temperature dependence of the crystal–melt interfacial energy of metals

A model to express the dependence of the crystal–melt interfacial energy on the temperature for metals is proposed. The crystal–melt interfacial energies, the homogeneous nucleation undercoolings and the critical cooling rates to form ideal metallic glasses of silver, copper and nickel have been predicted according to the present model and simulated by the molecular dynamics method. The results show that the crystal–melt interfacial energy of metals increases nonlinearly with temperature. Over a wide temperature range from the melting point to the glass transition temperature the predicted results for the crystal–melt interfacial energy, the homogeneous nucleation undercooling and the critical cooling rate to form ideal metallic glasses from the present crystal–melt interfacial energy model are in good agreement with the experimental results reported, as well as the results of molecular dynamics simulations based on different EAM potentials of the metals.     

Production of Fe based bulk metallic glasses using electromagnetic vibrations

Abstract

It is known that cooling rate from the liquid state is an important factor for producing the bulk metallic glasses. However, little work has been conducted on the influence of other factors such as electric and/or magnetic fields. The authors have previously reported that the glass forming ability of Mg–Cu–Y and Fe–Co–B–Si–Nb alloys is enhanced with increasing electromagnetic vibration force. The present study aims to investigate effect of the electromagnetic vibrations on the crystal particles in Fe–Co–B–Si–Nb bulk metallic glasses. As a result, the electromagnetic vibrations were found to act mainly on decreasing the number of crystal nuclei.     

过热处理时间对Zr-Cu基大块非晶合金力学性能的影响

在一定初始温度下经过不同时间的熔体过热处理,利用铜模吸铸法,制备纯非晶合金Zr_(48)Cu_(36)Ag_8Al_8棒状试样,通过X射线衍射仪(XRD)、差示扫描热分析仪(DSC)、万能力学试验机和场发射扫描电子显微镜(SEM)研究过热处理对其力学性能的影响。结果表明,在一定的处理时间范围内,随着处理时间的增长,Zr_(48)Cu_(36)Ag_8Al_8非晶合金原子排列的混乱度增加,非晶合金的平均自由体积增加,Zr_(48)Cu_(36)Ag_8Al_8非晶合金的变形局域化程度降低,变形能力随之增强,非晶合金的断裂强度和塑性得到了提高。     

Size-dependent cohesive energy,melting temperature,and Debye temperature of spherical metallic nanoparticles

It is necessary to theoretically evaluate the thermodynamic properties of metallic nanoparticles due to the lack of experimental data. Considering the surface effects and crystal structures, a simple theoretical model is developed to study the size dependence of thermodynamic properties of spherical metallic nanoparticles. Based on the model, we have considered Co and Cu nanoparticles for the study of size dependence of cohesive energy, Au and Cu nanoparticles for size dependence of melting temperature, and Cu, Co and Au nanoparticles for size dependence of Debye temperature, respectively. The results show that the size effects on melting temperature, cohesive energy and Debye temperature of the spherical metallic nanoparticles are predominant in the sizes ranging from about 3 nm to 20 nm. The present theoretical predictions are in agreement with available corresponding experimental and computer simulation results for the spherical metallic nanoparticles. The model could be used to determine the thermodynamic properties of other metallic nanoparticles to some extent.     

四种块体非晶合金在过冷液相区内的微成形能力

采用Si模制作的V型槽微压印实验评价了4种具有不同脆度的块体非晶合金(Pd40Cu30Ni10P20,Zr65Cu15Al10Ni10,Cu46Zr42Al7Y5,Zr58.5Cu15.6Al10.3Ni12.8Nb2.8)在过冷液相区内的微成形能力.结果表明,尽管不同的BMG表现出不同的表观粘度,但在相同的应变条件下,各种BMG却呈现出相似的充型面积,这表明它们具有相似的微成形能力.这一结果源于在本实验条件下(成形温度T=1.07Tg,应变率(ε)=2×10-3s-1),各BMG均遵循相同的牛顿流变机制.最后,通过有限元模拟验证了这一结论.     

Influence of the molten quenching temperature on the thermal physical behavior of quenched Zr-based metallic glasses

Thermo-physical behavior of some Zr-based metallic glasses prepared by different molten quenching temperatures was studied by Differential scanning calorimetry (DSC) measurements. The characteristic thermo-physical properties are normally used for evaluating the glass-forming ability (GFA) of metallic glasses. Our results show that the glass transition temperature, crystallization temperature and supercooled liquid region of these metallic glasses increased with increasing the molten quenching temperature. Their glass-forming abilities were discussed in terms of the GFA criterion γ and the reduced glass transition temperature, T_rg, using these thermo-physical properties.     

非晶合金冲击韧性研究现状及展望

非晶合金因其独特的内部结构,具有优异的力学性能。例如:高强度、高硬度、大弹性极限等,是一种先进的结构工程材料。材料的冲击韧性能够反映材料内部的细微缺陷和抗冲击性能,是材料强度和塑性的综合表征,是一种重要的力学指标。本文对非晶合金冲击韧性研究现状进行综述,介绍试样尺寸、服役温度、合金成分、热处理等因素对非晶合金冲击韧性的影响,总结了改善非晶合金冲击韧性的措施,并对今后非晶合金冲击韧性研究值得关注的问题进行展望。