Bulk amorphous metal—An emerging engineering material

During the last two decades, researchers have developed families of metal alloys that exhibit exceptional resistance to crystallization in the undercooled liquid state. Upon cooling, these alloys readily form glass or vitrify to form bulk amorphous alloys or bulk metallic glasses. The stability of the undercooled molten alloys with respect to crystallization has enabled studies of liquid thermodynamics, rheology, atomic diffusion, and the glass transition previously not possible in metallic systems. Bulk amorphous alloys exhibit very high strength, specific strength, and elastic strain limit, along with unusual combinations of other engineering properties. These factors, taken together, suggest that bulk amorphous metals will become widely used engineering materials during the next decade.     

Formation and crystallization of bulk Pd_(82)Si_(18) amorphous alloys

1　INTRODUCTIONMetallicglassisregardedasastatethatisofdis orderunlikecrystalalloyswith periodicatomstruc ture.Soitshowsexcellentcapabilitiesofsoftmag netism ,mechanics ,corrosionresistance ,etc .How ever ,mostofamorphousalloyswereproducedbyus ingrapidsolidificationmethodssuchassplatquench ing ,meltspinning ,andsoon ,withcharacteristiccoolingratesof 10 4 10 6 K/s .Becauseamorphousal loysarepreparedwithsilk ,powderandribbon ,itisgreatlylimitedinengineeringapplication .Recently ,severalbulk…     

Structure of amorphous metallic alloys and the conditions of amorphization

The results of x-ray study of amorphous metal alloys are compared with the structural parameters of intermediate phases determined for equilibrium state diagrams and primary products of crystallization upon warming of amorphous alloys and crystal phases (formed at “subcritical” cooling rates) of the alloys susceptible to amorphization. It is found that susceptibility of the alloys to amorphization depends on the specific chemical interaction of the components which is revealed in the formation of the intermediate crystal phases. This conclusion is proved by the results of studying solid-phase reaction of amorphization. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 10, pp. 3–10, October, 2000. The paper is based on the lecture delivered by the author at the annual meeting of the Russian Metallurgists Association on November 24, 1996 (Moscow).     

Structure of Amorphous Metallic Alloys and the Conditions of Amorphization

The results of x-ray study of amorphous metal alloys are compared with the structural parameters of intermediate phases determined for equilibrium state diagrams and primary products of crystallization upon warming of amorphous alloys and crystal phases (formed at subcritical cooling rates) of the alloys susceptible to amorphization. It is found that susceptibility of the alloys to amorphization depends on the specific chemical interaction of the components which is revealed in the formation of the intermediate crystal phases. This conclusion is proved by the results of studying solid-phase reaction of amorphization.     

Thermodynamic properties of supercooled Fe-B liquids—A theoretical and experimental study

The two-level model, recommended at the Ringberg 95 workshop, is applied to extrapolate the thermodynamic properties of liquid Fe-B alloys to large undercooling and to analyze the crystallization of glassy Fe₈₅B₁₅ alloys obtained by melt spinning. The new method yields practically the same phase diagram as the SGTE database but a superior result when evaluating the heat capacity, entropy, and crystallization heat at large undercooling. These properties are compared in the low-temperature range (700 to 800 K) with the experimental data obtained for an Fe₈₅B₁₅ metallic glass by scanning calorimetry. A good agreement between experiments and calculations is obtained when the observed magnetic transition at 530 K is taken into account.     

Thermodynamic properties of supercooled Fe-B liquids—A theoretical and experimental study

The two-level model, recommended at the Ringberg 95 workshop, is applied to extrapolate the thermodynamic properties of liquid Fe-B alloys to large undercooling and to analyze the crystallization of glassy Fe₈₅B₁₅ alloys obtained by melt spinning. The new method yields practically the same phase diagram as the SGTE database but a superior result when evaluating the heat capacity, entropy, and crystallization heat at large undercooling. These properties are compared in the low-temperature range (700 to 800 K) with the experimental data obtained for an Fe₈₅B₁₅ metallic glass by scanning calorimetry. A good agreement between experiments and calculations is obtained when the observed magnetic transition at 530 K is taken into account.     

Interface thermodynamics of nano-sized crystalline,amorphous and liquid metallic systems

Expressions for the Gibbs energies of interfaces occurring in particular for solid and/or liquid/amorphous metals or alloys in contact with each other have been developed. To consider its energetics, an amorphous alloy has been modelled as a mixture of the undercooled liquid metal components near to the glass transition temperature making use of the enthalpy of melting, the entropy of melting and the temperature-dependent contribution of the heat capacity of the undercooled melt. Gibbs surface and interface energies have been obtained on the basis of the “macroscopic atom” Miedema model, where the entropy contributions of alloys have been derived applying a recently developed formalism. The Gibbs energy of a crystalline interface phase has been formulated. The molar fractions of the components of the alloy at the surfaces have been determined by minimising the surface energy. These results provide a thermodynamic basis for unusual phenomena observed in nano-sized systems. The formalism has been applied to calculate the thermodynamic stability of interface phases in a nano-sized, multi-layered system of iron and zirconium and to explain the aluminium-induced crystallisation of amorphous silicon and the layer exchange occurring in bi-layers of crystalline aluminium and amorphous silicon.     

Kinetics and formation mechanism of amorphous Fe52Nb48 alloy powder fabricated by mechanical alloying

A single phase amorphous Fe₅₂Nb₄₈ alloy has been synthesized through a solid state interdiffusion of pure polycrystalline Fe and Nb powders at room temperature, using a high-energy ball-milling technique. The mechanisms of metallic glass formation and competing crystallization processes in the mechanically deformed composite powders have been investigated by means of X-ray diffraction, Mössbauer spectroscopy, differential thermal analysis, scanning electron microscopy and transmission electron microscopy. The numerous intimate layered composite particles of the diffusion couples that formed during the first and intermediate stages of milling time (0–56 ks), are intermixed to form amorphous phase(s) upon heating to about 625 K by so-called thermally assisted solid state amorphization, TASSA. The amorphization heat of formation for binary system via the TASSA, ΔH_a, was measured directly as a function of the milling time. Comparable with the TASSA, homogeneous amorphous alloys were fabricated directly without heating the composite multilayered particles upon milling these particles for longer milling time (86 ks–144 ks). The amorphization reaction here is attributed to the mechanical driven solid state amorphization. This single amorphous phase transforms into an order phase (μ phase) upon heating at 1088 K (crystallization temperature, T_x) with enthalpy change of crystallization, ΔH_x, of −8.3 kJ mol⁻¹.     

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.     

Glassy materials: thermodynamic and kinetic quantities

Modern technology makes it possible to obtain different kinds of glassy material. In the as-prepared state most of these show relaxation effects, glass transition and crystallization under heat treatment, with very similar phenomenological peculiarities. The thermodynamic treatment of glassy materials is complicated by the fact that the glassy state is not always a metastable state but is sometimes an unstable state. Evaluation of the entropy of the glass, with respect to the corresponding stable crystalline phase at the same temperature and pressure, from heat capacity measurements is discussed. The “ideal glass” concept is introduced to calculate the Gibbs free energy of formation of the glass. The residual entropy is presented as a measure of the glass deviation from ideal behaviour. The treatment is extended to alloy glass. The thermodynamic and kinetic parameters which control the crystallization process are reviewed. Polymorphous and primary crystallization are considered, assuming, either homogeneous or inhomogeneous nucleation, followed by interface-controlled crystal growth.     

High-Entropy Metallic Glasses

The high-entropy alloys are defined as solid-solution alloys containing five or more than five principal elements in equal or near-equal atomic percent. The concept of high mixing entropy introduces a new way for developing advanced metallic materials with unique physical and mechanical properties that cannot be achieved by the conventional microalloying approach based on only a single base element. The metallic glass (MG) is the metallic alloy rapidly quenched from the liquid state, and at room temperature it still shows an amorphous liquid-like structure. Bulk MGs represent a particular class of amorphous alloys usually with three or more than three components but based on a single principal element such as Zr, Cu, Ce, and Fe. These materials are very attractive for applications because of their excellent mechanical properties such as ultrahigh (near theoretical) strength, wear resistance, and hardness, and physical properties such as soft magnetic properties. In this article, we review the formation and properties of a series of high-mixing-entropy bulk MGs based on multiple major elements. It is found that the strategy and route for development of the high-entropy alloys can be applied to the development of the MGs with excellent glass-forming ability. The high-mixing-entropy bulk MGs are then loosely defined as metallic glassy alloys containing five or more than five elements in equal or near-equal atomic percent, which have relatively high mixing entropy compared with the conventional MGs based on a single principal element. The formation mechanism, especially the role of the mixing entropy in the formation of the high-entropy MGs, is discussed. The unique physical, mechanical, chemical, and biomedical properties of the high-entropy MGs in comparison with the conventional metallic alloys are introduced. We show that the high-mixing-entropy MGs, along the formation idea and strategy of the high-entropy alloys and based on multiple major elements, might provide a novel approach in search for new MG-forming systems with significances in scientific studies and potential applications.     

Molecular dynamics simulation of amorphous segregation in Ag-Rh alloys

Molecular dynamics simulation was carried out to investigate the liquid and amorphous microstructures of binary Agx-Rh(100-x) (x = 25, 50, 75 in atom fraction) alloys. Segregation feature of homogeneous interatomic binding of Ag-Rh liquid was found and probed, which can be retained into amorphous solids upon rapid cooling. Homogeneous binding may occur when the difference in the elemental atomic sizes is less than 10%. The icosahedra in liquid before the formation of amorphous state exist in a stable state and the network formed by 1551-clusters in molten alloys would inhibit the crystallization and diffusion of atoms. A higher degree of 1551-clusters will be favorable to form metallic glasses.     

Electron-irradiation-induced nano-crystallization in quasicrystal-forming Zr-based metallic glass

Phase selection in electron-irradiation-induced crystallization and crystal-to-amorphous-to-crystal (C–A–C) transition at 298 K in quasicrystal-forming Zr–Pt metallic glass alloys were investigated. Two types of f.c.c. nano-crystalline precipitates were formed in amorphous Zr₈₀Pt₂₀ and Zr_66.7Pt_33.3 alloys under electron irradiation; such unique nano-crystalline structures were not observed during thermal annealing. It was inferred that unique phase selection in electron-irradiation-induced crystallization and thermal crystallization can be explained by the large negative chemical mixing enthalpy (ΔH_chem) in Zr–Pd and Zr–Pt alloys.     

液淬工艺和微量元素对非晶态Fe—B—Si合金热稳定性的影响

研究了液淬工艺参数和添加微量稀土元素Ｃｅ对非昌态Ｆｅ－Ｂ－Ｓｉ合金热稳定性的影响,结果表明,制备非晶条带时的液淬速率愈高,在低温退火时合金的结构弛豫过程愈难进行。而非晶晶化与液淬条件之间不存在直接的联系。Ｃｅ的适量添加,在氏温阶延迟了结构弛豫过程中非晶条带延－脆转变的发生;在高温阶段使合金开始晶化的温度降低,且民条带中Ｃｅ的含量成正比。     

Thermodynamic modeling and composition design for the formation of Zr–Ti–Cu–Ni–Al high entropy bulk metallic glasses

This work explores the idea of predicting metallic glass forming composition in a multi component alloy for which an equilibrium phase diagram is yet to be deciphered. Deep eutectic regions in a quaternary alloy (Zr–Ti–Cu–Ni) have been extrapolated to the quinary Zr–Ti–Cu–Ni–Al system for designing a potential bulk glass forming composition. P_HSS parameter which is the product of mixing enthalpy, mismatch entropy and configurational entropy of an alloy, has been utilized for thermodynamic modeling. P_HSS values are computed through substitution of Al into the each of the fifteen quaternary eutectics that have been reported in the literature in the Zr–Ti–Cu–Ni system. A good correlation of P_HSS range between modeled alloys and established glass formers indicates the subtle efficacy of this method for high entropy amorphous alloy design through a rationale thermodynamic approach.     

Environmental properties of Zr-based metallic glasses and nanocrystalline alloys

Zr-Cu-Ni-Al belongs to the best glass forming systems known; these glasses are suitable as precursor material for nanocrystalline alloys. For an application as hydrogen storage materials for example it is of great interest to know more about these metastable materials in regard to their environmental properties. Corrosion as studied by a salt spray test or anodic polarization in aqueous solutions exhibit a rather high sensitivity with no significant differences between the amorphous and nanocrystalline state. Hydrogen charging was performed electrochemically in a glycerine-phosphoric acid electrolyte. In Zr-Cu-Ni-Al alloys absorption kinetics and storage capacity were found to be very similar for the amorphous and the nanocrystalline phase. In the nanocrystalline alloy consisting mainly of a fcc (big cube) phase with a NiTi₂ type structure a hydrogen induced amorphization was observed. Oxidation of metastable Zr-based materials in air was studied below the glass transition temperature at 360°C by thermogravimetry. Oxidation resistance was found to improve very significantly from the amorphous to the nanocrystalline microstructure. The scales formed on both materials consist mainly of columnar ZrO₂ with diameter in the nanometer range; Probably including the other metals as a nanocrystalline solid solution.     

HRTEM study of crystallization of Zr–Cu–Ni metallic glass

Employing differential scanning calorimetry (DSC) and high-resolution transmission electron microscopy (HRTEM), the micromechanism for crystallization of Zr₇₀Cu₂₀Ni₁₀ metallic glass under isothermal annealing conditions has been investigated. It is found that the relationship between the annealing temperature and the peak position, incubation time and ending time in the isothermal annealing DSC traces of Zr₇₀Cu₂₀Ni₁₀ metallic glass obeys a first-order exponential function. However, the time–temperature transformation curves of Zr₇₀Cu₂₀Ni₁₀ metallic glass at different crystallized volume fractions can be well fitted by a second-order exponential function. It is observed that at the initial crystallization stage some ordered atomic clusters precipitate first, acting as nucleation sites and facilitating the subsequent crystallization process, and the crystal growth process mainly proceeds through the atomic depositing on the previously formed crystals. This behavior confirms that the new micromechanism for crystallization of amorphous alloys proposed by Lu and Wang can also be applied to the new series of zirconium based amorphous alloys.     

预退火时间对Pd40Cu30Ni10P20玻璃转变及晶化的影响

采用示差扫描量热(DSC)分析方法,测定了大块非晶合金Pd40Cu30Ni10P20经523 K((Tg-100 K)＜7＜Tg)不同时间(0～64 h)预退火后的玻璃转变温度7g、玻璃转变峰温度TM、起始晶化温度Tx、晶化峰的峰温Tp、晶化焓以及在玻璃转变过程中的比热容增量,并根据Kissinger公式计算了晶化的表观活化能.同时,测量了不同时间预退火后样品的显微硬度.结果表明:在玻璃转变温度以下的预退火处理使Pd40Cu30Ni10P20大块非晶合金的微观原子组态发生变化,从而影响了其随后的玻璃转变行为,但对晶化的影响不大.其显微硬度随预退火时间的延长而逐步增加后趋于稳定.并利用结构弛豫理论分析了预退火对玻璃转变、晶化和显微硬度的影响.     

Gd60Co26Al14大块非晶合金的磁熵变

利用铜模吸铸法制备了Gd60Co26Al14非晶合金.采用示差扫描量热法(DSC),X射线衍射(XRD)和超导量子磁强计(SQUID)研究了其结构与磁热性能.XRD分析表明:铸态Gd60Co26Al14合金是完全的非晶结构;DSC测试显示Gd60Co26Al14合金在加热过程中在571 K发生玻璃化转变,并且出现了两个结晶温度,分别是602 K和642 K.SQUID测试结果表明:Gd60Co26Al14合金出现两个居里温度,分别是82 K和128 K;合金在外磁场5 T下82 K处的磁熵变达到7 J·(kg·K-1).     

Topological instability and glass forming ability of Al–Ni–Sm alloys

The thermal crystallization of Al-based metallic glasses can be described in association with the topological instability λ criterion. In the present work, we report on the crystallization behavior and glass forming ability of Al-rich, Al–Ni–Sm alloys, designed with compositions corresponding to the same topological instability condition of λ ≈ 0.1. Amorphous melt-spun alloys were prepared with the following compositions, varying the ratio of Ni and Sm elements: Al_87.5Ni₄Sm_8.5, Al_83.5Ni₁₀Sm_6.5, Al_80.5Ni_14.5Sm₅ and Al_76.5Ni_20.5Sm₃. The glass forming ability of each alloy composition was evaluated based on the thermal parameters obtained from DSC runs and on X-ray diffraction patterns. Better glass forming ability was observed in compositions whose Sm content was increased and Ni content reduced. Thermal crystallization of the alloys with low Sm content showed only one crystallization peak and no glass transition event. In alloys with higher rare-earth content, a glass transition event was clearly detected before the crystallization event. The results are interpreted considering the different types and proportions of Sm–Al and Ni–Al clusters that can be formed in the alloys along the λ ≈ 0.1 line. They also emphasize the relevance of these different types of clusters in the amorphous phase in defining the stability of the glass and the types of thermal crystallization.