The S-D transition and high-pressure phase stability of transition metals

Abstract

The recent discovery of a solid-solid phase change in shock-compressed Mo and the theoretical interpretation suggest valence d-electron density as a major influence on structural stability. The relationship of this experimental result to the transition metal structures and alloy phase diagrams will be discussed. Specific predictions will be presented for the locations and slopes of transition metal and transition metal alloy phase boundaries.     

The importance of the activation volume for the description of the molecular dynamics of glass-forming liquids

High pressure dielectric measurements were carried out on hydrogen bonded d-glucose and two van der Waals peracetyl saccharides, i.e. α pentaacetyl glucose and α octaacetyl maltose. In this study we found that after removing H bonds, the molecular dynamics of both modified saccharides is very sensitive to pressure, as reflected by a large value of the pressure coefficient of the glass transition temperature, equal to 270 K GPa(-1) and 280 K GPa(-1) for α pentaacetyl glucose and α octaacetyl maltose, respectively. On the other hand, dT(g)/dP for d-glucose is much lower, equal to 67 K GPa(-1). Our result confirms the general rule that the hydrogen bonding glass-forming liquids exhibit much lower values of dT(g)/dP compared to the van der Waals systems. Additionally, on the basis of results reported herein and also recent literature data for polyalcohols, we point out that the activation volume correlates fairly well with the molecular volume in the case of hydrogen bonding liquids. On the other hand, much larger values of the activation volumes at T(g) with respect to the molecular volumes were found for both peracetyl saccharides.     

非晶合金及合金液体的局域五次对称性

简要回顾了从20世纪30年代至今,有关非晶合金及合金液体的局域结构五次对称性的实验、理论和模拟研究.在简单液体的早期研究中,人们已经意识到五次对称性在简单液体的无序结构、过冷和晶化等起着重要作用,二十面体短程序作为五次对称性的典型代表受到了广泛关注.自从Frank提出简单液体中二十面体短程序的结构单元,大量的理论和实验研究已经明确在简单液体、合金液体和金属玻璃中存在局域五次对称性,并且建立了局域五次对称性与合金液体复杂动力学行为、玻璃转变、液体-液体相变以及非晶合金的形变等统一的定量描述和物理图像,表明了局域五次对称性作为结构参量的简单、普遍和有效性.     

Melting point trends in intermetallic alloys

We have examined the melting points of approximately 500 intermetallic binary alloys. We attempted to correlate the melting point behavior of the binary (1:1) alloys with a number of elemental variables including electron number, atomic size, orbital radii, electronegativity, etc. We find that a “Vegards's Law” of melting points works very well for predicting the melting points of binary transition metal alloys, i.e. the melting point of the alloy correlates with the linear average of the elementary melting points. However, this “law” works only moderately well for alloys involving simple metals. In addition, we find that transition metal alloys tend to have melting points below the averaged elemental melting points. This finding is in sharp contrast to simple metal alloys where the opposite trend is observed and it is indicative of fundamental differences between transition metal and simple metal binding. Finally, we have attempted to correlate deviations from a Vegard's law of melting with elemental variables. We found no strong correlation with elemental variables (or the heats of formation of the alloy in question) with the possible exception being a correlation with elemental volume changes upon alloying. The consequences of this correlation upon alloy design and metallic alloy formation are briefly discussed.     

金属玻璃液体中的强脆转变现象

强脆转变是玻璃形成液体在从低温到高温升温过程中由强性液体转变为脆性液体的现象,反之从高温到低温冷却过程即为脆强转变.由于其意味着液体的结构发生了某种快速、非连续的变化,强脆转变现象成为异常动力学的典型代表.自1999年《Nature》杂志首次报道了水的强-脆转变现象之后,液体的强脆转变现象就作为凝聚态物理和材料科学领域中的前沿和热点问题被广泛关注.越来越多的研究表明,强脆转变现象在金属玻璃形成液体中普遍存在.为阐明金属玻璃强-脆转变现象对于深入理解玻璃转变本质、探讨液固遗传微观结构特征、揭示晶化过程相互竞争规律、提高玻璃形成能力、促进金属玻璃制备和处理工艺标准化等方面的重要意义,综合评述了强脆转变现象在金属玻璃形成液体中的普遍性、特殊性、定量表征、热力学表现以及结构起源等研究领域的最新进展,并指出了该领域今后的发展方向.     

Crystallographic texture and microstructure evolution during hot compression of Ti–6Al–4V–0.1B alloy in the (α + β)-regime

Microstructure and texture are known to undergo drastic modifications due to trace hypoeutectic boron addition (~0.1 wt.%) for various titanium alloys e.g. Ti–6Al–4V. The deformation behaviour of such an alloy Ti–6Al–4V–0.1B is investigated in the (α?+?β) phase field and compared against that of the base alloy Ti–6Al–4V studied under selfsame conditions. The deformation microstructures for the two alloys display bending and kinking of α lamellae in near α and softening via globularization of α lamella in near β phase regimes, respectively. The transition temperature at which pure slip based deformation changes to softening is lower for the boron added alloy. The presence of TiB particles is largely held attributable for the early softening of Ti–6Al–4V–0.1B alloy. The compression texture of both the alloys carry signature of pure α phase defamation at lower temperature and α→β→α phase transformation near the β transus temperature. Texture is influenced by a complex interplay of the deformation and transformation processes in the intermediate temperature range. The contribution from phase transformation is prominent for Ti–6Al–4V–0.1B alloy at comparatively lower temperature.     

高温金属熔体黏度突变探索

高温金属熔体的黏度是衡量液态金属动力学性质的一个重要指标,是高温金属熔体的基本物理性能之一.熔体的黏度在表征脆性系数、金属玻璃形成能力的大小和液-液相变现象方面起关键性作用.本文在介绍高温金属熔体黏度测量方法的基础上,综合评述了单质、二元和多元合金黏度随温度的变化规律和黏度突变特征,分析了黏度突变研究的物理意义,并指出高温金属熔体黏度今后研究的发展方向。     

Ab initio calculation of the Gilbert damping parameter via the linear response formalism

A Kubo-Greenwood-like equation for the Gilbert damping parameter α is presented that is based on the linear response formalism. Its implementation using the fully relativistic Korringa-Kohn-Rostoker band structure method in combination with coherent potential approximation alloy theory allows it to be applied to a wide range of situations. This is demonstrated with results obtained for the bcc alloy system Fe(1-x)Co(x) as well as for a series of alloys of Permalloy with 5d transition metals. To account for the thermal displacements of atoms as a scattering mechanism, an alloy-analogy model is introduced. The corresponding calculations for Ni correctly describe the rapid change of α when small amounts of substitutional Cu are introduced.     

二十面体团簇的遗传:一个与快凝Cu56Zr44合金玻璃形成能力有关的动力学参数

采用分子动力学方法模拟研究了液态Cu₅₆Zr₄₄合金在不同冷速γ与压力P下的快速凝固过程, 并通过基于Honeycutt-Andersen键型指数的扩展团簇类型指数法对其微结构演变特性进行了分析. 结果表明: 快凝玻璃合金的局域原子组态主要是(12 12/1551)规则二十面体、以及 (12 8/1551 2/1541 2/1431)与(12 2/1441 8/1551 2/1661) 缺陷二十面体. 通过原子轨迹的逆向跟踪分析发现: 从过冷液体中遗传下来的二十面体对快凝合金的玻璃形成能力(GFA)具有重要影响, 不仅其可遗传分数F_i =N_{300 K←Tg}ⁱ/N_Tg 与GFA密切相关, 而且其遗传起始温度(T_onset)与合金约化玻璃转变温度T_rg = T_g/T_m也存在很好的对应关系.     

The formation of α at triple junctions of parent β phase in titanium alloys

We have examined the formation of α phase at grain boundary triple junctions of parent β in a metastable β titanium alloy with orientation imaging microscopy based on electron backscattered diffraction (EBSD). As in the case of α formed at grain boundaries of parent β grains, α at a triple junction also forms with the Burgers orientation relationship with one of the three neighbouring β grains. The experimental results are analyzed in terms of the deviation of the 36 possible α variants that can form at a triple junction from the Burgers orientation relationship with neighbouring grains.     

Medium-Range Order Structure and Fragility of Superheated Melts of Amorphous CuHf Alloys

The structural factors of amorphous CuHf alloys at different temperatures are determined by using a high temperature x-ray diffractometer. It is found that not only the short-range order structure but also the medium-range order structure exists in amorphous CuHf alloys. The dynamic viscosities of CuHf alloy melts are measured by a torsional oscillation viscometer. The fragility of superheated melts of CuHf alloys is calculated based on the viscosity data. The experimental results show that the glass-forming ability of the CuHf alloys is closely related to the fragility of their superheated melt. The relationship between the medium-range order structures and the fragility of superheated melts has also been established in amorphous CuHf alloys. In contrast to the fragility of supercooled liquids, the fragility of superheated liquids promises a better approach to reflecting the dynamics of glass forming liquids.     

Specific features of exoelectronic emission in amorphous metal alloys

This paper reports on a study of the specific features of photo-and thermostimulated exoelectronic emission in amorphous metal alloys of Fe₆₄Co₂₁B₁₅ composition. The temperature dependences of the exoelectronic emission spectrum were established to adequately reflect the two-stage nature of the transition of an amorphous alloy to the crystalline state. The exoelectronic emission spectrum is sensitive to variations in the thermal treatment to which the alloy is subjected. Thermal treatment of an amorphous metal alloy gives rise to an increase in the exoelectron emission intensity. The growth in the exoelectron emission intensity was found to be the highest for alloys in the initial stage of crystallization.     

Mo¨ssbauer study of iron sulfides doped with 3d-transition metals

It is found by Mo¨ssbauer measurements on M_0.025Fe_0.975S (M=Sc, Ti, V, Cr, Mn, Co, Ni, Cu) that the 3d-transition metal impurities profoundly affect both the crystallographic and spin rotation transitions of iron sulfide.It is noteworthy that both V_0.025Fe_0.975S and Co_0.025Fe_0.975S have Morin transition temperatures T_M which are distinctly different from that of FeS; furthermore, the directions of changes of T_M are opposite for V_0.025Fe_0.975S and Co_0.025Fe_0.975S.A vanadium impurity of 2.5% of the metal atoms in the iron sulfide makes the crystallographic transition take place rapidly in a narrow temperature region of about 15 K, while the α transition in FeS takes place over a wide temperature range of about 200 K.It is also found that the α transition for V_0.025Fe_0.975S has a hysteresis width of 5 K.     

Selection of α variants during microstructural evolution in α/β titanium alloys

The solid-state β→β?+?α transformation in titanium alloys leads to complex microstructures with feature spanning across a range of length scales. In order to develop a better understanding of the microstructural evolution process, a detailed characterization of the crystallography of α laths formed from the β phase in a candidate α/β Ti alloy, Timetal 550, has been carried out. Specifically, the influence of the orientation relationship (OR) between the grain boundary α (GB α) and the adjacent β grains on the microstructural evolution has been investigated in this alloy employing orientation imaging microscopy (OIM) studies in a high-resolution SEM. The results indicate that the colony microstructure (clustering of α laths belonging to the same variant) tends to develop in the β grain that exhibits the Burgers OR with the GB α allotriomorph, whereas the basketweave microstructure (clustering of multiple variants) develops in the adjacent β grain. Additionally, the multiple variants of α laths forming the basketweave microstructure appear to be related by certain selection criteria.     

The relationship between kinetic and thermodynamic fragilities in metallic glass-forming liquids

The correlation between the temperature dependence of the kinetic and thermodynamic properties of a series of metallic glass-forming liquids is investigated using the concept of fragility. The results indicate a correlation between the kinetic fragility and thermodynamic fragility in these liquids. The correlation depends critically on the approach used to evaluate the thermodynamic fragility. Two distinct correlation lines are found for the metal–metalloid and for the all-metallic-constituents glass-forming liquids. For the same thermodynamic fragility the metal–metalloid liquids exhibit a distinctively larger kinetic fragility than the pure-metallic liquids. From the evaluation of the Gibbs free-energy difference between the undercooled liquid and the crystalline phase mixture, a correlation between the kinetic fragility and the driving force for nucleation is found, showing that for glass formation in metallic alloys the thermodynamic and kinetic contributions act together.     

Ti,Nb, Al及其二元合金状态方程的第一原理计算及其应用

金属状态方程对于探究金属及合金原子的相互作用中起到了至关重要的作用.本文使用第一原理计算了Ti, Nb, Al及其二元合金能量与原子间距关系(E-r关系),并使用得到的E-r关系计算了金属及合金的体弹性模量,结果与实验值吻合的很好.计算结果表明,使用不同的赝势,计算不同金属表现出不同的适用性;二元合金的E-r曲线处于对应合金化元素曲线之间;并且合金的E-r曲线会更靠近合金内含量较高的元素的E-r曲线;并发现合金E-r关系可通过组成合金的纯金属的E-r关系计算获得,且用该方法计算材料的体弹性模量与实验值非常符合.     

Final state core level binding energy shifts in binary alloys

The final state or relaxation contribution to core level binding energy shift is calculated for some noble metal/transition metal alloy systems, using the pseudopotential linear response method developed previously for pure metals. The core hole perturbation is described by an ab-initio pseudopotential. A basic parameter of the method is the homogeneous electron gas density for which, in the present case, a mean value between those corresponding to each pure metal is adopted. The final state contribution is generally found to be important and sometimes dominant. From the experimental core level shifts and the calculated final state shifts, some considerations about initial state contribution are given.     

T g in Confined Systems: A Free Volume Approach

Abstract

In glass-forming materials confined in films or pores of small dimension, the glass transition temperature T _g is depressed or increased depending on the nature of the interactions at the interface. It is shown that these effects, observed in polymers and simple liquids, can be interpreted in the framework of the free volume model. Similar effects of confinement on the glass and melting transitions are stressed; this permits prediction of the variations of the cooperative molecular motions α with temperature.     

Fe基α相合金的冲击相变及其对层裂行为的影响研究

利用正向加载和逆向加载相结合的实验方法,实验研究了从低压到高压三种不同压力范围内Fe基α相合金(Fe-85.03wt%,Mn-7.63wt%,Ni-7.01wt%,Al-0.3wt%)的冲击相变和卸载逆相变历程及对应加载状态的层裂行为特征.给出了Fe基α相合金含冲击相变和卸载逆相变的加卸载Hugoniot线,发现冲击相变阈值、逆相变阈值和冲击加载压力共同决定其冲击加卸载历程,冲击相变强烈影响其层裂行为,导致样品发生了"异常"层裂.利用获得的冲击加卸载历程从应力波相互作用的角度解 关键词： α相合金')" href="#">Fe基α相合金 相变 逆相变 层裂