Amorphous phase growth by isothermal annealing-induced interdiffusion reactions in mechanically deformed Ni/Ti multilayered composites

The growth of amorphous phase and the formation of competing intermetallic compounds in mechanically deformed Ni/Ti multilayered composites have been investigated by X-ray diffraction, scanning electron microscopy, transmission electron micrography, and magnetic measurement. Experimental observations in the Ni/Ti system have been performed, supporting a transient nucleation model for solid-state amorphization. The amorphous layers in the Ni/Ti multilayered composites continue to grow until they have attained a temperature-dependent critical thickness or a temperature-dependent critical annealing time. The relationships between the critical amorphous layer thickness and temperature, between the critical annealing time and temperature, and between the annealing conditions (temperature, time) and amorphization transformation, have been deduced and compared with the experimental results. The quantitative relationships can give a direction for the bulk amorphization in the Ni/Ti multilayered composites.     

Solid state amorphization reactions in Ni/Ti multilayer composites prepared by cold rolling

Solid state amorphizations in mechanically deformed Ni/Ti multilayer composites have been investigated by differential scanning calorimetry and X-ray diffraction. The growth of amorphous alloy in our Ni/Ti composites was facilitated by the large number of grain boundaries and relatively large degree of disorder induced in the metal layers by the cold rolling. The formation of different products of solid-state reaction in the Ni/Ti composites has been elucidated in terms of magnetic and enthalpy analysis. It is thought that the solid state amorphization occurs first during heating, followed by the formation of intermetallic compounds through direct solid-state reaction of elemental nickel and titanium and by the crystallization of the amorphous alloy already formed by the solid state amorphization reaction. The values of the activation energy for interdiffusion and interdiffusion coefficient for the formation of amorphous alloy in the Ni/Ti composites have also been calculated.     

On eliminating deposition-induced amorphization of interfaces in refractory metal multilayer systems

Multilayer thin films with refractory metal components are susceptible to solid state amorphization. It is an objective to minimize this interfacial reaction in order to ensure smooth layering and compositionally abrupt interfaces. These features are desirable for maximum reflectivity in X-ray optic applications such as projection lithography as well as neutron supermirror applications. This goal is achievable when the deposition process is optimized to thermalize the sputtered neutrals. Low working gas pressures (below 5 m Torr) and large source-to-substrate distances (more than 12 cm) typically provide the proper conditions to form dense sputter deposits without adatom-induced, interfacial amorphization. The interfaces of Mo/Si and Ni/Ti multilayers are examined with high resolution electron microscopy for samples sputter deposited under thermalized conditions. It is shown to be possible to produce crystalline, refractory metal layers in the as-deposited structure without amorphous interfacial regions.     

电镀Ni-Mo非晶态合金的晶化与强化

用透射电镜研究了电镀 Ni-Mo 非晶态合金晶化过程中形貌和相结构的变化。发现晶化初期除了在非晶基体上析出 Ni 的固溶体外,还先后出现两个亚稳相,一个为面心立方的,a=1.11nm;另一个为六角相,a=0.30,c=0.47nm。最后稳定相是 Ni 和 Ni_3Mo。两相有固定取向关系和良好共格关系。这样,可以从固溶,弥散和析出相几种强化机制上来理解电镀 Ni-Mo 非晶态合金加热过程中的强化现象。     

Solid-state amorphization of Ni/Zr bilayer through diffusion-limited-reaction observed by molecular-dynamics simulation

Amorphization in the Ni/Zr bilayer upon isothermal annealing is observed by a molecular-dynamics simulation using an n-body potential. The disordering process is found to initiate and extend from a preset disordered interfacial layer through atomic diffusion and alloying between Ni and Zr layers. Furthermore, it is clearly evidenced by our simulation that the growing process of amorphous phase follows a t^1/2 time dependence kinetics, featuring a typical diffusion-limited growth with asymmetric speeds in two opposite directions. All these results suggest that the observed amorphization is, in fact, a result of diffusion-limited-reaction.     

Solid-state amorphization behaviour in Ni/Zr multilayers with oxygen contamination

The solid-state amorphization by the interdiffusion reaction in sputter-deposited Ni-Zr multilayer films with oxygen contamination has been investigated by differential scanning calorimetry and X-ray diffractometry. Through X-ray photoelectron spectroscopy analysis, it was found that the multilayer films were contaminated with oxygen during deposition in a low-vacuum system (10^–5 torr), and the concentration was modulated having the maximum in zirconium-rich regions. The kinetics of amorphization reactions has been examined by non-isothermal and isothermal annealing. Oxygen introduced into the sample during sample preparation and annealing treatments appears to affect the kinetics of the amorphization reaction associated with variation of the activation energy for interdiffusion in the amorphous layer and a critical thickness of the amorphous layer. The origins of abnormal behaviour in forming intermetallic compound as well as amorphous phase, are discussed in the context of the oxygen incorporation.     

Amorphization reaction during mechanical alloying: influence of the milling conditions

Amorphous Ni-Zr powders have been prepared by mechanical alloying of elemental crystalline powders. The glass-forming range has been determined in detail at different milling intensities. Depending on the milling conditions, at least partial crystallization of the formerly amorphous material can occur from 66 to 75 at% Ni, due to a temperature rise during milling at high intensity. In comparison with isothermal annealing experiments at various temperatures on completely amorphous powder, a relation between milling temperature and milling time is shown. This confirms the similarity of the amorphization process during mechanical alloying with the solid-state interdiffusion reaction in alternating crystalline multilayers.     

Kinetics of Ni-Mo electrodeposition from Ni-rich citrate baths

The kinetics of Ni-Mo alloy electrodeposition on steel substrates from an aqueous citrate-ammonia complex baths has been investigated by means of steady-state polarisation measurements in a system with a rotating disc electrode (RDE). Partial current densities for discharge of Ni(II) and Mo(VI) ions and hydrogen evolution as a function of molybdate concentration in the bath, cathode potentials and the rate of mass transport were determined. It has been shown that - under all investigated conditions - Ni-Mo alloy deposition is more favourable than pure nickel and the cathodic process is strongly influenced by the Mo(VI) content in the solution. The Ni(II) electroreduction rate initially increases, as the cathode potential shifts towards more negative values and the concentration of molybdate grows in the solution. However, for the highest examined MoO₄^2 − content, a considerable decrease in the rate of the process is subsequently observed at certain limit potentials, the values of which depend on molybdate concentration and hydrodynamic conditions. This effect, related to the formation of intermediate molybdenum oxides (characterised by very low overvoltage for hydrogen evolution), becomes less pronounced when the RDE rotation speed is increased. Hydrogen evolution is strongly associated with molybdenum deposition. An increase of the molybdate ions concentration in the bath, as well as an increase in the rate of mass transport, leads to an increase in Mo content in deposits and to the reduction of current efficiency. The Ni-Mo coatings electrodeposited from the designed bath (with the current efficiency of about 70%) containing about 30 wt.% Mo, are characterised by a shiny-grey appearance and good adhesion to the steel substrate. They are characterised by column growth and amorphous microstructure with randomly distributed nanocrystallites of the MoNi₄ intermetallic phase.     

Long-Range Ordered Amorphous Atomic Chains as Building Blocks of a Superconducting Quasi-One-Dimensional Crystal

Crystalline and amorphous structures are two of the most common solid-state phases. Crystals having orientational and periodic translation symmetries are usually both short-range and long-range ordered, while amorphous materials have no long-range order. Short-range ordered but long-range disordered materials are generally categorized into amorphous phases. In contrast to the extensively studied crystalline and amorphous phases, the combination of short-range disordered and long-range ordered structures at the atomic level is extremely rare and so far has only been reported for solvated fullerenes under compression. Here, a report on the creation and investigation of a superconducting quasi-1D material with long-range ordered amorphous building blocks is presented. Using a diamond anvil cell, monocrystalline (TaSe₄)₂I is compressed and a system is created where the TaSe₄ atomic chains are in amorphous state without breaking the orientational and periodic translation symmetries of the chain lattice. Strikingly, along with the amorphization of the atomic chains, the insulating (TaSe₄)₂I becomes a superconductor. The data provide critical insight into a new phase of solid-state materials. The findings demonstrate a first ever case where superconductivity is hosted by a lattice with periodic but amorphous constituent atomic chains.     

冷轧 Ni/Ti 多层中的固态非晶化反应——Ⅱ.恒速升温退火处理(英文)

通过重复冷轧 Ni/Ti 包覆粉末制备出 Ni/Ti 多层。使用 XRD,SEM,TEM,DSC 和磁性分析方法研究了该多层由恒速升温退火导致的固态非晶化反应。恒速升温退火时多层中首先形成非晶相,当非晶相层厚超过一临界值后,元素间将通过互扩散形成金属间化合物 Ni_3Ti 和 NiTi,最后已形成的非晶相晶化成金属间化合物 Ni_3Ti 和 Ti_2Ni。首次利用扩散控制的层长大机制和瞬时形核模型,计算了恒速升温退火处理时多层中非晶层长大厚度和金属间化合物的形成温度。     

Transition from a disordered to a crystalline state in II–VI and III–V films

The initial stages of HgCdTe growth on Al₂O₃, GaAs, CdTe, and KCl substrates have been studied by electron diffraction. HgCdTe films were produced by pulsed laser deposition and isothermal vapor phase epitaxy. InGaAs films were grown by isothermal chloride epitaxy on GaAs substrates. In the initial stages of the growth process, we observed a transition from an amorphous to a textured polycrystalline phase and then to a mosaic single-crystal structure. We have calculated the critical size of crystalline grains below which amorphization occurs in II-VI and III-V compounds. The critical grain size agrees with the grain size of the disordered (amorphous) phase that forms in the initial stage of epitaxy. We have determined some characteristics of the heterostructures: critical film thickness below which pseudomorphic growth is possible without misfit dislocation generation, elastic stress in the epitaxial system, surface density of dangling bonds at dislocations, and the critical island radius above which no interfacial misfit dislocations are generated.     

非晶化程度对不同成分Mg-Ni非晶合金电极充放电容量的影响

研究了球磨过程中的非晶化程度对不同成分Mg-Ni非晶合金电化学吸放氢性能的影响。研究结果表明,随着合金成分的不同,非晶化程度的影响不同。当Ni含量低于50％(原子分数)时,合金粉末中的非晶相所占比例越高,即非晶化程度越高时,合金电极的放电容量越大;而当Ni含量高于50％(原子分数)时,非晶合金中存在少量游离态的Ni相,可提高电极的放电量。分析认为这与Ni相的存在改善了合金的吸放氢动力学性能有关。     

干摩擦应变诱导板条马氏体固态非晶化的能量分析及其模型

使用定点离子束切割制样(FIB)并根据透射电镜(TEM)表征，分析了板条马氏体钢干摩擦层内部板条马氏体协调塑性变形、演变为纳米层片结构并发生非晶化的全过程。结果表明，高密度位错缠结和缺陷集中是纳米层片结构的典型特征，这种结构产生的界面在高应变驱动下发生非晶化。这些非晶产物，为进一步细化磨屑和形成表面自润滑层提供结构条件。基于上述实验结果并结合摩擦学和材料学理论建立了干摩擦过程中的非晶化形核模型，计算了发生非晶化的热力条件和能量壁垒。结果表明，根据经典形核理论和晶体向非晶转变的吉布斯自由能壁垒计算公式所建立的干摩擦非晶化形核能量模型，可用于计算发生非晶化必需的临界位错密度值。根据对应的计算结果，可控制摩擦条件用干摩擦应变诱导板条马氏体的固态非晶化。     

Stress and microstructure evolution during growth of magnetron-sputtered low-mobility metal films: Influence of the nucleation conditions

Large tensile stresses (up to 3 GPa) were previously observed in low-mobility metallic Mo_1 − xSi_x films grown on amorphous Si and they were ascribed to the densification strain at the amorphous-crystalline transition occurring at a critical film thickness. Here, we focus on the influence of the nucleation conditions on the subsequent stress build-up in sputter-deposited Mo_0.84Si_0.16 alloy films. For this purpose, growth was initiated on various underlayers, including amorphous layers and crystalline templates with different lattice mismatch, and the stress evolution was measured in situ during growth using the wafer curvature technique. Tensile stress evolutions were observed on amorphous SiO₂ and (111) Ni underlayers, similarly to the stress behaviour found on amorphous Si. For these series, the films were characterized by large in-plane grain size (~ 500 nm). However, on a (110) Mo buffer layer, a different stress behaviour occurred: after an initial tensile rise ascribed to coherence stress, a reversal towards a compressive steady state stress was observed. A change in film microstructure was also noticed, the typical grain size being ~ 30 nm. The origin of the compressive stress source in the metastable Mo_0.84Si_0.16 alloy grown on (110) Mo is discussed based on the stress evolutions measured at varying deposition rates and Ar working pressures, as well as in comparison with stress evolutions in pure Mo films.     

Substantial diffusion of Zr during solid-state amorphization in Ni/Zr multilayers

To examine the proposed model that amorphization is achieved by the motion of one species only, Ni/Zr multilayers have been prepared by sputter deposition at a pressure of 1 Pa. The as-deposited specimen shows the planar and sharp interface between Ni and Zr layers. The amorphous phase is clearly observed at the residual Ni layers and around the void edge in the Zr layer from the cross-sectional TEM images of the samples annealed at 300 C for 1 h. Composition analysis verifies that considerable Zr diffusion occurs through the grain boundary in the Ni layer during the annealing. The activation energy for amorphization is determined to be 1.17 eV per atom through a Kissinger-type analysis. This paper clearly shows the formation of amorphous NiZr alloys when Zr diffusion is comparable to Ni diffusion.     

电沉积Ni-Mo/ZrO2合金镀层结构及其电化学性能

为提高Ni-Mo合金镀层的电化学性能,在Ni-Mo镀液中添加纳米ZrO2颗粒成功制备了Ni-Mo/Zr02复合镀层,研究了Ni-Mo/ZrO2合金电镀层的结构及其化学活性,并与Ni-Mo、Ni/ZrO2的相关性能进行了对比.研究结果表明:Ni-Mo/ZrO2镀层结构为非晶+纳米晶并混杂有ZrO2纳米颗粒,添加ZrO2纳...     

Solid-state reaction between crystalline nickel and amorphous Fe78Si12B10 powders during mechanical milling

The solid-state reaction between crystalline nickel and pulverized melt-spun amorphous Fe₇₈Si₁₂B₁₀ powders during mechanical milling has been investigated. For the powder mixtures with low nickel content ( 30 at%), the reaction results in the production of a new amorphous phase. For the powder mixture with a high nickel content (up to 50 at%), crystalline (Fe, Ni) and new amorphous phases are obtained. During the milling process, nickel atoms can dissolve in the amorphous matrix by diffusion due to severe deformation. The existence of free volume in the melt-spun amorphous phase may favour the diffusion of nickel atoms. At the same time, the elemental atoms (such as iron) in the amorphous phase may also dissolve into the nickel matrix. The amorphization between the amorphous and crystalline phases is attributed to the asymmetry of interdiffusion of the atoms in different matrices.     

Triggering dynamics of the high-pressure benzene amorphization

Success in designing and tailoring solid-state reactions depends on the knowledge of the mechanisms regulating the reactivity at the microscopic level. In spite of several attempts to rationalize the reactivity of crystals, the question of the existence of a critical distance for a reaction to occur remains unsolved. In this framework, the role of lattice phonons, which continuously tune the relative distance and orientation of the molecules, is still not fully understood. Here, we show that at the onset of the transformation of crystalline benzene to an amorphous hydrogenated carbon the intermolecular C-C distance is always the same (about 2.6 A) once collective motions are taken into account, and it is independent of the pressure and temperature conditions. This conclusion is supported by first-principles molecular-dynamics simulations. This is a clear demonstration of the role of lattice phonons in driving the reactivity in the crystalline phase by fine-tuning of the nearest-neighbour distances. The knowledge of the critical C-C distance can be crucial in planning solid-state reactions at moderate pressure.     

Tailoring the microstructure and surface morphology of metal thin films for nano-electro-mechanical systems applications

Metallic structural components for micro-electro-mechanical/nano-electro-mechanical systems (MEMS/NEMS) are promising alternatives to silicon-based materials since they are electrically conductive, optically reflective and ductile. Polycrystalline mono-metallic films typically exhibit low strength and hardness, high surface roughness, and significant residual stress, making them unusable for NEMS. In this study we demonstrate how to overcome these limitations by co-sputtering Ni-Mo. Detailed investigation of the Ni-Mo system using transmission electron microscopy and high-resolution transmission electron microscopy (TEM/HRTEM), x-ray diffraction (XRD), nanoindentation, and atomic force microscopy (AFM) reveals the presence of an amorphous-nanocrystalline microstructure which exhibits enhanced hardness, metallic conductivity, and sub-nanometer root mean square (RMS) roughness. Uncurled NEMS cantilevers with MHz resonant frequencies and quality factors ranging from 200-900 are fabricated from amorphous Ni-Mo. Using a sub-regular solution model it is shown that the electrical conductivity of Ni-Mo is in excellent agreement with Bhatia's structural model of electrical resistivity in binary alloys. Using a Langevin-type stochastic rate equation the structural evolution of amorphous Ni-Mo is modeled; it is shown that the growth instability due to the competing processes of surface diffusion and self-shadowing is heavily damped out due to the high thermal energies of sputtering, resulting in extremely smooth films.     

Nano-size crystallites in Mo/Si multilayer optics

In this paper, we present an experimental study on the structural and crystalline properties of electron beam evaporated Mo/Si multilayers. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) experiments were used to characterize the crystalline nature of Mo and Si. TEM and XRD show that Si grows amorphous, while Mo clearly crystallizes. The Mo crystallites show no preferred growth orientations and a columnar growth mode, with crystallite sizes limited by the Mo layer thickness. From samples with low Mo content, we show that crystallization of Mo starts directly at the Mo/Si interfaces, with the formation of a Mo₃Si compound. The crystallite lattice strains that develop during Mo-on-Si growth due to lattice mismatches, will add to the macroscopic multilayer stress. By comparing Mo lattice strains derived from XRD with changes in substrate curvatures, as determined by interferometry, we show that compressive and tensile substrate deformations are predominantly caused by the internal crystallite strains.