Rethinking atomic packing and cluster formation in metallic liquids and glasses

In traditional view,atomic packing is random in glasses made of metallic elements with non-directional interactions as the glass-forming liquid needs to be excited to remain in liquid state before being cooled sufficiently fast to a glass. Locally ordered packing however is possible if certain conditions are favorable,such as a strong bonding between elements,or low configuration energy of a cluster of atoms as suggested by Frank. In alloy systems made of different metallic elements,we show that Frank’s criterion alone does not necessarily lead to certain specific local ordered packing or cluster formation such as icosahedral packing. In this context,we revisit the issue of atomic packing and cluster formation,and show that an alloy system with fairly random liquid configuration could be sufficient to produce a variety of noticeable locally ordered packing with low energy,albeit largely statistical in nature. Therefore,we emphasize the importance of the system parameters such as the atomic size,alloy concentration,and interaction potential in their collective contribution to local atomic packing.     

Ni-Hf非晶合金及其团簇式成分解析

Ni-Hf是一类重要的二元非晶形成体系,但其非晶形成能力较弱,最佳非晶成分尚不明确.本文利用团簇加连接原子模型,首先解析了二元深共晶点Ni65Hf35对应共晶相Ni7Hf3和Ni10Hf7的团簇结构,获得了可代表各自结构特征的主团簇;随后依据非晶合金的通用团簇式[团簇]（连接原子）1或3,设计出若干可能具有高非晶形成能力的合金成分.实验结果表明,急冷甩带后只有Ni71.43Hf28.57和Ni68.75Hf31.25两个成分点形成完全非晶,其中后者成分由团簇成分式[Ni-Ni7Hf5]Ni3描述,并具有最高的热稳定性（877K）和最低的液相线温度Tl（1482K）,被认为是Ni-Hf系中高非晶形成能力的成分点,其位于Ni65Hf35共晶点的富Ni侧,靠近共晶相Ni7Hf3.     

Absence of thermodynamic phase transition in a model glass former

The glass transition can be viewed simply as the point at which the viscosity of a structurally disordered liquid reaches a universal threshold value. But this is an operational definition that circumvents fundamental issues, such as whether the glass transition is a purely dynamical phenomenon. If so, ergodicity gets broken (the system becomes confined to some part of its phase space), but the thermodynamic properties of the liquid remain unchanged across the transition, provided they are determined as thermodynamic equilibrium averages over the whole phase space. The opposite view claims that an underlying thermodynamic phase transition is responsible for the pronounced slow-down in the dynamics at the liquid-glass boundary. Such a phase transition would trigger the dynamic standstill, and then be masked by it. Here we perform Monte Carlo simulations of a two-dimensional system of polydisperse hard disks far within its glassy phase. The approach allows for non-local moves in a way that preserves micro-reversibility. We find no evidence for a thermodynamic phase transition up to very high densities; the glass is thus indistinguishable from the liquid on purely thermodynamic grounds.     

Liquid fragility——A key togoing deep into materials of glassy states

“Liquid fragility“ is a concept that has been widely used in the investigation on the glass community,though it was presented less than two decades ago. The concept enables the comparison between the glass-forming liquids with different dynamic characters by using a general criterion, in which the temperature scale is reduced by the glass transition temperature. In order to illuminate the significance of the concept in the fields of the glass transition,structural relaxation process and the structure of supercooled liquids, the accomplished progress and the faced challenges are summarized from different aspects such as on the correlation between dynamics and thermodynamic characters of condensed matters, on the energy landscape, on the nonexponential relaxation and on the theoretical model of microstructure and medium-range order. The tendency of investigation in “liquid fragility“ is also evaluated.     

Roles of Co element in Fe-based bulk metallic glasses utilizing industrial FeB alloy as raw material

A series of Fe-based bulk metallic glasses were fabricated by a conventional copper mold casting method using a kind of Fe-B industrial raw alloy. It is found that Fe-B-Y-Nb bulk metallic glass with 3 at% of Co addition possesses the best glass forming ability, thermal stability, hardness, magnetic property and anti-corrosion property. The hardness test result indicates a synchronically trend with glass-forming ability parameters. The excellent glass-forming ability and a combination of good mechanical and functional properties suggest that the alloys in this work might be good candidates for commercial use.     

低密度玻璃化液体中力学模量与脆性的关联研究

利用低密度玻璃化液体的径向分布函数,导出过冷液体脆性与玻璃化温度、体模量和剪切模量的比值及与柏松比之间简单明确的关系.此结论定性地与实验结果一致.并为各种玻璃化液体的研究,提供有效的途径.     

Ce - Al - Cu(贫Cu部分)的相关系及其金属玻璃的形成机制

采用X射线衍射(XRD)、差示量热(DSC)分析方法研究了Ce-Al-Cu(贫Cu部分)合金相关系及金属玻璃的形成.从Ce -A1- Cu三元系合金在熔铸态时的相关系组成图发现,Ce - Al - Cu金属玻璃形成的成分范围主要集中在CeAl+ Ce3Al+ Ce2 AlCu2,Ce3 Al+ Ce2AlCu2+ Ce...     

The hidden disintegration of cluster heterogeneity in Fe-based glass-forming alloy melt

The evolution of liquid metal at high temperature is known much less than their solid states. This is partially due to that the message concerning clusters, metastable phase or heterogeneity in liquid is usually too slight to be traced. Here, we shed some light on the nature of structural evolution of Fe-based glass-forming alloy during overheating process by the investigation of high-temperature melt viscometry and first principles simulations. It was found that a structural transition around 1400 ℃ occurred in the melts of initial homogeneous ingot, heterogeneous ingot and amorphous ribbons jointly, and was confirmed by the results from differential scanning calorimeter(DSC), and ab initio molecular dynamics(AIMD). Combining these results with Fe-Si-B ternary phase diagram and the melting characteristics of Fe-B compounds, it is safe to conclude that the disintegration of Fe_2B type clusters to Fe_3B-type clusters leads to the observed transition. These results offer a significant reference for the preparation and property control of Fe-based amorphous alloys.     

High entropy state of orientationally-disordered clusters in Zr-based bulk metallic glass

A phase transformation of a metastable Zr2Ni intermetallic crystalline phase to vitrify was studied with molecular dynamics (MD) simulations based on a plastic crystal model (PCM), which utilizes the orientational disorder of molecules of plastic crystals in a class of soft matter. The simulation results show ed that computational operations in MD-PCM for the random rotati ons of the clusters in the crystalline phase around their center of gravity and subsequent annealing lead to form a dense random pa cking structure from the metastable Zr2Ni phase. These randomly-rotated clusters provide high degrees of freedom in terms of atomic positions, which results in enhancing the glass-forming ability of the alloy. The critical fraction of the number of rotated cl usters for forming the glassy phase ( fc^R) is evaluated to be 0.75 ? 0.80. Thus, the critical fraction of the number of un-rotated clusters (fc^U=1?fc^R) is close to the critical concentration of site percolation for metallic materials. The mechanism of the metastable Zr2Ni phase to vitrify with increasing f^R was analyzed with a concept of communal entropy in the free volume theory with solid- and liquid-like cells proposed by Cohen and Grest. The MD-PCM for the metastable Zr2Ni phase suggests that Zr-based bulk metallic glass (BMG) can be regarded as an alloy in a high rotation entropy state of cl usters and that glass transition takes place by percolation of th e nuclei of a liquid-like glassy phase.     

Glass-forming ability,phase formation and mechanical properties of glass-forming Cu-Hf-Zr alloys

The influence of Hf additions on the glass-forming ability(GFA),phase formation and mechanical properties of Cu_(50)Hf_xZr_(50-x)(x=2,5,10,20 at.%)alloys has been systematically investigated.We report on a distinct correlation between phase formation and GFA of Cu_(50)Zr_(50)-based alloys.Increasing additions of Hf reduce the thermal stability of the high-temperature B2Cu(Hf,Zr)phase,while the thermal stability of the corresponding undercooled melt is enhanced.The GFA of these alloy series gradually raises up to 10 at.%Hf,whereas at 20 at.%Hf,the GFA is drastically lowered,since the B2Cu(Hf,Zr)phase becomes unstable and the precipitation of the lowtemperature equilibrium phases is favoured.This interrelation determines the microstructure and results in the formation of Cu-Hf-Zr-based bulk metallic glass composites.These composites not only show appreciable macroscopic plastic strain,but also high yield strength.     

Metallic Glass Formation by Ion Mixing and Calculation of Glass-Forming-Ability from Inter-Atomic Potential in the Binary Metal Systems

In this article, a brief summary of the up-to-date progress of metallic glass formation by ion mixing of metallic multilayers in the binary metal systems is first presented. Secondly, thermodynamic modeling of metallic glass formation is developed with special consideration of the interfacial free energy of the multilayers. Thirdly, results of molecular dynamics simulations for some representative systems are presented to show the calculation of the glass-forming ability directly from the inter-atomic potential of the binary metal systems.     

The relationship between fragility, configurational entropy and the potential energy landscape of glass-forming liquids

Glass is a microscopically disordered, solid form of matter that results when a fluid is cooled or compressed in such a manner that it does not crystallize. Almost all types of materials are capable of glass formation, including polymers, metal alloys and molten salts. Given such diversity, general principles by which different glass-forming materials can be systematically classified are invaluable. One such principle is the classification of glass-formers according to their fragility. Fragility measures the rapidity with which a liquid's properties (such as viscosity) change as the glassy state is approached. Although the relationship between the fragility, configurational entropy and features of the energy landscape (the complicated dependence of energy on configuration) of a glass-former have been analysed previously, a detailed understanding of the origins of fragility is lacking. Here I use simulations to analyse the relationship between fragility and quantitative measures of the energy landscape for a model liquid whose fragility depends on its bulk density. The results reveal that fragility depends on changes in the vibrational properties of individual energy minima in addition to their total number and spread in energy. A thermodynamic expression for fragility is derived, which is in quantitative agreement with kinetic fragilities obtained from the liquid's diffusivity.     

超导涡旋系统的非平衡态相变

利用三维各向异性XY模型和电阻分流结动力学,研究了较强钉扎势下第二类层状超导体涡旋系统的非平衡态相变.结果发现,平衡态的涡旋玻璃相可在电流驱动下转变为运动玻璃相.在低温下系统经历了三次相变:随电流的增加,系统首先从运动涡旋玻璃相转变为运动Bragg玻璃相,再转变为运动Smectic相,最后融化为运动液体相.     

热致型液晶聚合物的玻璃态结构及形成热力学和动力学

利用热台偏光显微镜、差示扫描量热法(DSC)、广角X-射线衍射(WAXD)等分析测试手段,对聚1,4-苯二氧乙酸-1,8-二氧代辛撑基二苯酚酯的织态结构及相态进行了研究。结果表明,该聚合物在经过一次加热冷却循环后。生成了液晶玻璃态。对相转变的转变熵值的分析表明,上述结论是正确的。并在热力学与动力学共同作用的基础上,讨论了热致型液晶聚合物液晶玻璃态的形成,提出了固化温度大于结晶温度是液晶玻璃态产生的必要条件。     

Correlation between the glass-forming ability and activation energy of crystallization for Zr75−x Ni25Al x metallic glasses

The thermal stability and the kinetics of glass transition and crystallization for Zr_75?x Ni₂₅Al _x (x = 8–15) metallic glasses were investigated using differential scanning calorimetry (DSC) under continuous heating conditions. The apparent activation energy of glass transition rises monotonously with the Al content increasing; the activation energy of crystallization increases with Al changing from 8at% to 15at%, and then decreases with Al further up to 24at%, which exhibits a good correlation to the thermal stability and the glass-forming ability (GFA). The Zr₆₀Ni₂₅Al₁₅ metallic glass with the largest supercooled liquid region and GFA possesses the highest activation energy of crystallization. The relation between the thermal stability, GFA and activation energy of crystallization was discussed in terms of the primary precipitated phases.     

ZrCuAlMo大块非晶的制备及热稳定性研究

通过铜模吸铸法制备（Zr47Cu44Al9）100-xMox（x=0,1,2,3）大块非晶合金.利用X射线衍射、差热分析等,研究分析添加Mo对Zr47Cu44Al9合金非晶形成能力及热稳定性的影响.结果表明,Mo的添加量为2%时,合金具有最大的非晶形成能力,纯非晶试样的临界尺寸从4mm增大到6mm以上,8mm的样品只有微弱的衍射峰.Mo提高非晶形成能力的原因主要是抑制引起异质形核的CuZr相的析出与长大.利用Kissinger方法计算得到的其玻璃转变激活能Eg为278.57kJmol-1、晶化激活能Ex为392.46kJmol-1、晶化峰值的激活能Ep为376.97kJmol-1.与原合金相比,晶化激活能与晶化峰值激活能的数据都增大,说明Mo的添加不仅提高了原合金的非晶形成能力,同时也增强了合金的热稳定性.     

玻璃形成液体其内在结构中的二十面体

采用分子动力学模拟(molecular dynamics简称MD)的方法,并利用势能曲面(potential energy landscape简称PEL)理论对玻璃形成液体进行研究.在模拟中,键角分布序参数(An)被首先应用于描述二元玻璃形成液体其内在结构(Inherent Structure简称IS)中的结构序.通过计算An,得出了一般液体和IS液体中六面体、八面体和二十面体结构随温度的变化情况.在IS液体中二十面体序所占比例明显多于一般液体.在IS液体中,六面体、八面体的结构随温度的变化不是很明显,而二十面体结构随着温度的变化呈现出一定的规律.得出了二十面体是IS的主要变化的结构.玻璃转变是一般液体接近IS的过程,同时也是二十面体增多的过程.     

Multiscale structures of Zr-based binary metallic glasses and the correlation with glass forming ability

Thermal behaviors and structures of three Zr-based binary glass formers, Zr50Cu50, Zr64Cu36 and Zr64Ni36, were investigated and compared using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), high energy X-ray diffraction (XRD) and small angle X-ray scattering (SAXS). The high energy XRD results show that the bulk glass former Zr50Cu50 has a denser atomic packing efficiency and reduced mediumrange order than those of marginal glass formers Zr64Cu36 and Zr64Ni36. Based on TEM observations for the samples after heat treatment at 10 K above their crystallization onset temperatures, the number density of crystals for Zr50Cu50 was estimated to be 1023–1024 m?3, which was four-orders higher than that in Zr64Cu36 and Zr64Ni36 metallic glasses. SAXS results indicate that Zr50Cu50 has higher degree of nanoscale inhomogeneities than those in Zr64Cu36 and Zr64Ni36 at as-cast state. The observed multiscale structures are discussed in terms of the phase stability and glass-forming ability of Zr-based binary glass formers.     

Clarifying the glass-transition behaviour of water by comparison with hyperquenched inorganic glasses

The formation of glasses is normal for substances that remain liquid over a wide temperature range (the 'good glassformers') and can be induced for most liquids if cooling is fast enough to bypass crystallization. During reheating but still below the melting point, good glassformers exhibit glass transitions as they abruptly transform into supercooled liquids, whereas other substances transform directly from the glassy to the crystalline state. Whether water exhibits a glass transition before crystallization has been much debated over five decades. For the last 20 years, the existence of a glass transition at 136 K (ref. 3) has been widely accepted, but the transition exhibits qualities difficult to reconcile with our current knowledge of glass transitions. Here we report detailed calorimetric characterizations of hyperquenched inorganic glasses that, when heated, do not crystallize before reaching their glass transition temperatures. We compare our results to the behaviour of glassy water and find that small endothermic effects, such as the one attributed to the glass transition of water, are only a 'shadow' of the real glass transition occurring at higher temperatures, thus substantiating the conclusion that the glass transition of water cannot be probed directly.     

Dynamics of supercooled water in confined geometry

As with most liquids, it is possible to supercool water; this generally involves cooling the liquid below its melting temperature (avoiding crystallization) until it eventually forms a glass. The viscosity and related relaxation times (tau) of glass-forming liquids typically show non-Arrhenius temperature (T) dependencies. Liquids with highly non-Arrhenius behaviour in the supercooled region are termed 'fragile'. In contrast, liquids whose behaviour is close to the Arrhenius law (In tau infinity 1/T) are termed 'strong'. A unique 'fragile-strong' transition around 228 K has been proposed for supercooled water; however, experimental studies of bulk supercooled water in this temperature range are generally hampered because crystallization occurs. Here we use broad-band dielectric spectroscopy to study the relaxation dynamics of supercooled water in a wide temperature range, including the usually inaccessible temperature region. This is possible because the supercooled water is held within a layered vermiculite clay-the geometrical confinement and presence of intercalated sodium ions prevent most of the water from crystallizing. We find a relaxational process with an Arrhenius temperature dependence, consistent with the proposed strong nature of deeply supercooled bulk water. Because water that is less supercooled has been established as highly fragile, our results support the existence of a fragile-strong transition.