Crystallization embrittlement of Ni-Si-B alloys

Changes in the mechanical properties, and in particular the loss of ductility, have been examined during the crystallization of a series of Ni-Si-B amorphous alloys and an Ni-Fe-Si-B alloy. In all cases crystallization takes place by the formation of large numbers of small crystals and at some stage during the crystallization process embrittlement occurs. The geometrical characteristics of the crystal phases, and the mechanical properties of the crystalline and amorphous components are assessed in an attempt to define the factors leading to embrittlement. Sensitivity to embrittlement does not seem to be related to the particular phase nature, geometry nor distribution of the crystals formed. Rather, the mechanical properties of the crystals seem to be the determining factors. Hard and brittle crystals lead to alloy embrittlement; soft, ductile crystals can lead to enhanced toughness of the alloys.     

Rapid solidification processes and perspective part II

The number of alloys that have been produced by rapid solidification processing is immense. This article concentrates on the crystalline and amorphous alloys of commercial interest (e.g. aluminium, nickel and iron-base crystalline alloys and transition metal base glasses). The main structural features of these alloys are described in terms of processing conditions and some typical properties of crystalline materials are given.     

Crystallization behavior of Ge-doped eutectic Sb70Te30 films in optical disks

We report laser-induced crystallization behavior of binary Sb-Te and ternary Ge-doped eutectic Sb70Te30 thin film samples in a typical quadrilayer stack as used in phase-change optical disk data storage. Several experiments have been conducted on a two-laser static tester in which one laser operating in pulse mode writes crystalline marks on amorphous film or amorphous marks on crystalline film, while the second laser operating at low-power cw mode simultaneously monitors the progress of the crystalline or amorphous mark formation in real time in terms of the reflectivity variation. The results of this study show that the crystallization kinetics of this class of film is strongly growth dominant, which is significantly different from the crystallization kinetics of stochiometric Ge-Sb-Te compositions. In Sb-Te and Ge-doped eutectic Sb70Te30 thin-film samples, the crystallization behavior of the two forms of amorphous states, namely, as-deposited amorphous state and melt-quenched amorphous state, remains approximately same. We have also presented experiments showing the effect of the variation of the Sb/Te ratio and Ge doping on the crystallization behavior of these films.     

Cooling rate dependent as-cast microstructure and mechanical properties of Zr-based metallic glasses

In this article, wedge chill casting is used to provide a variation of the cooling rate for the study of cooling rate-dependent as-cast microstructure and mechanical properties of Zr-based metallic glasses with and without yttrium doping. In-situ formed crystalline phases and crystallization sequence are realized in the context of the cooling rate. Macro-hardness is studied via the Brinell ball indentation and the contact morphology is discussed on the basis of microstructure analysis.     

Kinetics of crystallization of a Fe-based multicomponent amorphous alloy

The Fe-based multicomponent amorphous alloys (also referred to as metallic glasses) are known to exhibit soft magnetic properties and, it makes them important for many technological applications. However, metallic glasses are in a thermodynamically metastable state and in case of high temperature operating conditions, the thermally activated crystallization would be detrimental to their magnetic properties. The study of crystallization kinetics of metallic glasses gives useful insight about its thermal stability. In the present work, crystallization study of Fe₆₇Co₁₈B₁₄Si₁ (2605CO) metallic glass has been carried out using differential scanning calorimetry (DSC) technique. Mössbauer study has also been undertaken to know the phases formed during the crystallization process. The alloy shows two-stage crystallization. The activation energy has been derived using the Kissinger method. It is found to be equal to 220 kJ/mol and 349 kJ/mol for the first and second crystallization peaks, respectively. The Mössbauer study indicates the formation of α-(Fe, Co) and (Fe, Co)₃B phases in the alloy.     

Formation and Characterization of Corrosion-Resistant Amorphous Coatings by Thermal Spraying*

Since it was first discovered that amorphous metals could be formed by rapid quenching from the liquid phase, these so-called metallic glasses have attracted an increasing amount of attention due to their unique properties. Forming amorphous metals by quenching from the liquid phase requires cooling rates on the order of 10⁵-10⁶ K/s. In order to achieve such cooling rates, one of the dimensions of the samples must be sufficiently small. For instance, a very thin ribbon enables amorphous alloys to be produced by the well known melt-spinning process. An alternative: approach is the use of thermal spraying to produce amorphous coatings. The first commercial product exploiting this approach was developed in 1984 and the subject has received increasing attention since this first development.

The purpose of the present work is to explore the conditions necessary for the formation of combustion flamesprayed amorphous coatings from NiMoFeB and NiCrFeSiB alloys. A technique based on differential thermal analysis has been developed to characterize and quantify the fraction of amorphous phase present in the partially amorphous coatings. It has been shown for the current two alloys that the amorphous content of coatings can be varied through control of the maximum substrate temperature, the initial powder condition and the powder size. These results will be compared to existing knowledge regarding the formation of amorphous alloys and, specifically, thermally-sprayed coatings.     

Phase formation under non-equilibrium processing conditions: rapid solidification processing and mechanical alloying

Rapid solidification processing (RSP) of metallic alloys, involving solidification of liquid metals at very high rates, results in the formation of a variety of metastable phases such as supersaturated solid solutions, crystalline intermetallic compounds, quasicrystalline phases, and metallic glasses. Additionally, significant refinement of the grain sizes and segregation patterns also occurs. Mechanical alloying (MA), another powerful non-equilibrium processing technique, utilizes repeated cold welding, fracturing, and rewelding of powder particles in a high-energy ball mill. MA also results in the formation of metastable phases and microstructural refinement similar to what happens during RSP. Consequently, comparisons are frequently made between the phases produced by RSP and MA and the general understanding is that they both result in similar metastable effects. A detailed analysis of the metastable phases produced by RSP and MA is made in the present work, and it is shown that even though the effects may appear similar, the mechanisms of formation and the composition ranges in which particular phases form are quite different. These two methods also have some unique features and produce different phases. The differences have been ascribed to the fact that RSP involves solidification from the melt while MA is a completely solid-state process that is not restricted by the phase diagram.     

高强度Zr基大块非晶合金的研究进展

Zr基大块非晶合金具有极大的非晶形成能力,能在较低的冷却速率(＜103K/s)下用铜模铸造法制备.Zr基大块非晶合金的强度高达1.8GPa,具有高耐腐蚀性和优异的加工成形性能,在实际工程中有着广阔的应用前景.综述了Zr基大块非晶合金的制备方法、性能特点及主要应用,介绍了国内外在该领域的主要研究进展和研究动态.     

High resolution electron microscopy of alloys

High resolution electron microscopy (HREM) has emerged as a very powerful tool for probing the structure of metals and alloys. It has not only helped in unravelling the structure of materials which have been at the forefront of novel materials development such as quasicrystalline phases and high temperature superconducting compounds, but also is fast becoming a technique for solving some outstanding issues in the case of the commercial alloys thereby helping alloy development. In addition to the determination of the structures of phases, this tool is used for obtaining a first hand information of the arrangement of atoms around the various types of crystallographic defects and interphase interfaces. This mode of microscopy allows direct observation of orientation relationships between two phases across interfaces. HREM can be used for the direct examination of the prenucleation process. Initial stages of nucleation can also be studied readily in amorphous alloys, precipitation hardening alloys like maraging steels and in those systems where the formation of the omega phase occurs. This presentation describes some results of HREM studies on various alloys, commercial as well as alloys of scientific interest, where some of the aforementioned aspects have been examined. The specific examples cited pertain to metallic glasses, NiTi shape memory alloys, Ni-Mo, Zr-Nb and Ti-Al alloys.     

Symbiotic crystal-glass alloys via dynamic chemical partitioning

The design of high performance structural materials is always pursuing combinations of excellent yet often mutually exclusive properties such as mechanical strength, ductility and thermal stability. Although crystal-glass composite alloys provide better ductility compared to fully amorphous alloys, their thermal stability is poor, due to heterogeneous nucleation at the crystal-glass interface. Here we present a new strategy to develop thermally stable, ultrastrong and deformable crystal-glass nanocomposites through a thermodynamically guided alloy design approach, which mimics the mutual stabilization principle known from symbiotic ecosystems. We realized this in form of a model Cr-Co-Ni (crystalline)/Ti-Zr-Nb-Hf-Cr-Co-Ni (amorphous) laminate composite alloy. The symbiotic alloy has an ultrahigh compressive yield strength of 3.6 GPa and large homogeneous deformation of ∼15% strain at ambient temperature, values which surpass those of conventional metallic glasses and nanolaminate alloys. Furthermore, the alloy exhibits ∼200 K higher crystallization temperature (T_X > 973 K) compared to that of the original TiZrNbHf-based amorphous phase. The elemental partitioning among adjacent amorphous and crystalline phases leads to their mutual thermodynamic and mechanical stabilization, opening up a new symbiotic approach for stable, strong and ductile materials.     

Nature‐Inspired Design and Application of Lipidic Lyotropic Liquid Crystals

Amphiphilic lipids aggregate in aqueous solution into a variety of structural arrangements. Among the plethora of ordered structures that have been reported, many have also been observed in nature. In addition, due to their unique morphologies, the hydrophilic and hydrophobic domains, very high internal interfacial surface area, and the multitude of possible order?order transitions depending on environmental changes, very promising applications have been developed for these systems in recent years. These include crystallization in inverse bicontinuous cubic phases for membrane protein structure determination, generation of advanced materials, sustained release of bioactive molecules, and control of chemical reactions. The outstanding diverse functionalities of lyotropic liquid crystalline phases found in nature and industry are closely related to the topology, including how their nanoscopic domains are organized. This leads to notable examples of correlation between structure and macroscopic properties, which is itself central to the performance of materials in general. The physical origin of the formation of the known classes of lipidic lyotropic liquid crystalline phases, their structure, and their occurrence in nature are described, and their application in materials science and engineering, biology, medical, and pharmaceutical products, and food science and technology are exemplified.     

大块非晶合金Zr55Cu30Al10Ni5的电子结构特征及电击穿行为

测定了大块非晶合金Zr55Cu30Al10Ni5晶化前后的费米能级和各元素的电子结合能,研究了非品合金的电子结构特征和电击穿行为．测试并讨论了非晶材料场发射能力和耐电压强度的关系．结果表明,对于Zr基合金,非品态比品态合金具有更大的功函数．比较了Zr55Cu30Al10Ni5合金非晶态与晶态的耐电压强度数值,发现非晶态合金的耐电压强度数值比较分散,品化合金的耐电压强度相对比较集中．耐电压强度平均值表明,Zr基合金非晶态具有更好的耐电压能力．     

Formation and magnetic properties of Fe-B-Si metallic glasses

The equilibrium solidification behaviour of Fe-rich Fe-B-Si alloys has been characterized in terms of primary and secondary crystallization, and correlated to metallic glass formation and properties. A eutectic trough extends from the binary eutectic on the Fe-B edge of the ternary composition diagram toward the binary eutectic on the Fe-Si edge. Metallic glass forming compositions closely follow the path of the eutectic trough, extending from the Fe-B edge to compositions containing approximately 20 at% Si. The relative ease of glass formation and the thermal stability of the glassy state are maximized at compositions along the eutectic trough. The magnetic coercive forces of both as-cast and annealed metallic glasses are minimized along the eutectic trough.     

Atomistic simulation and modeling of localized shear deformation in metallic glasses

Since the end of 1980s, bulk metallic glasses became available for various multi-component alloys. Because bulk metallic glasses are applicable to structural materials, their mechanical properties have become a matter of great interest in these decades. A characteristic feature of plastic deformation of metallic glasses at the ambient temperature is the localized shear deformation. Since we have no appropriate experimental technique, unlike crystalline matter, to approach microscopic deformation process in amorphous materials, we have to rely on computer simulation studies by use of atomistic models to reveal the microscopic deformation processes. In this article, we review atomistic simulation studies of deformation processes in metallic glasses, i.e., local shear transformation (LST), structural characterization of the local shear transformation zones (STZs), deformation-induced softening, shear band formation and its development, by use of elemental and metal-metal alloy models. We also review representative microscopic models so far proposed for the deformation mechanism: early dislocation model, Spaepen’s free-volume model, Argons’s STZ model and recent two-state STZ models by Langer et al.     

Amorphous phase formation and stability in W-Ti-Si metallization materials

The range of composition over which co-sputtered alloys in the W-Ti-Si system are deposited in the amorphous state has been determined. Alloys are found to be crystalline only if they contain less than ∼ 10% Si and this is associated with the presence of terminal solid solutions in the respective binary phase diagrams. The stability of W-Si amorphous alloys was examined using differential scanning calorimetry and X-ray diffraction. The most stable glasses exist in the range 10 to 30% Si and about the WSi₂ composition. There is some correlation between increasing stability and increasing activation energy for crystallization.     

Development of quaternary Fe-based bulk metallic glasses

Fe-based bulk metallic glasses have been designed using several modeling criteria. Quaternary alloys were successfully developed in the Fe-rich compositional region where the only solutes used were Cr, Nb, and B. Furthermore, the exceedingly high level of Fe, exceeding 75 wt.% in some alloys, makes this series of alloys, compositionally very close to conventional steel alloys. Therefore, their production using conventional facilities and materials is more easily attainable than previously developed bulk metallic glasses. The modeling criteria used, simultaneously analyzes the thermodynamics and kinetics of the vitrification behavior in a potential glass-forming alloy. A liquidus model, which determines and ranks the presence of deep eutectics, is used to determine the optimal compositional region. This criterion is cross-checked with an elastic strain model. Alloys compositionally located near a deep eutectic, while simultaneously containing a topology that induces significant elastic strain in a developing crystalline lattice exhibits experimentally good glass forming ability.     

Influence of Small Cr Addition on Thermal Stability and Magnetic Properties of Fe-Co-Zr-Nb-B Glassy Alloys

Fe62Cos-xCrxZr6Nb4B20 （x=0-4 at. pct） metallic glasses show high thermal stability with a maximum supercooled liquid region of about 84.8 K. The addition of 2 at. pct Cr causes the extension of the supercooled liquid region remarkably, leading to the enhancement of thermal stability and glass-forming ability. The crystallization of the Fe-based glassy alloys takes place through a single exothermic reaction, accompanying the precipitation of more than three kinds of crystallized phases such as α-Fe, Fe2Zr and ZrB2. The long-range atomic rearrangements required for the precipitation of the multiple crystalline phases seem to play an important role in the appearance of the large supercooled liquid region through the retardation of the crystallization reactions. The Fe-based alloys exhibit soft ferromagnetic properties. The saturation magnetization decreases with increasing Cr content while the saturated magnetostriction increases as a function of Cr content. There is no distinct change in the saturation magnetization and coercive force with annealing temperature below the crystallization temperature. The devitrification gives rise to a considerable enhancement in both as and He.     

Ductilization and embrittlement during the crystallization of Ni-Ti-B glasses

The fracture responses of three Ni-Ti-B glasses have been studied during the very early stages of crystallization. Comparison of alloys leads to a separation of relaxation and crystallization effects. The tendency toward reductilization in the presence of a small quantity of crystalline particles is directly related to the fine distribution and very small size of these particles which tends to interrupt concentrated shear without causing significant localized failure. Such particles thereby tend to distribute the plastic deformation homogeneously and to compensate the simultaneous relaxation embrittlement.     

Metallic glasses from “alchemy” to pure science: Present and future of design,processing and applications of glassy metals

Metallic glasses, first discovered a half century ago, are currently among the most studied metallic materials. Available in sizes up to several centimeters, with many novel, applicable properties, metallic glasses have also been the focus of research advancing the understanding of liquids and of glasses in general.Metallic glasses (MGs), called also bulk metallic glasses (BMGs) (or glassy metals, amorphous metals, liquid metals) are considered to be the materials of the future. Due to their high strength, metallic glasses have a number of interesting applications, for example as coatings. Metallic glasses can also be corrosion resistant. Metallic glasses, and the crystalline materials derived from them, can have very good resistance to sliding and abrasive wear. Combined with their strength – and now, toughness – this makes them ideal candidates for bio-implants or military applications. Prestigious Journals such as “Nature Materials”, “Nature” frequently publish new findings on these unusual glass materials. Moreover Chinese and Asian scientists have also been showing an interest in the study of metallic glasses.This review paper is far from exhaustive, but tries to cover the areas of interest as it follows: a short history, the local structure of BMGs and the glass forming ability (GFA), BMGs’ properties, the manufacturing and some applications of BMGs and finally, about the future of BMGs as valuable materials.