In-situ studies on phase transformations under electron irradiation in a high voltage electron microscope

High voltage electron microscopy (HVEM), using electron energies adequate for causing displacements of atoms from lattice sites, is a very effective technique for studying mechanisms of solid state phase transformations and for charting the path of phase evolution in real time. This has been demonstrated in studies on chemical ordering in nickel-molybdenum alloys and on theβ → Ω displacement ordering in zirconium-niobium alloys. The enhanced diffusivity due to electron irradiation makes it possible to explore a sequence of phase evolution at low enough temperatures where even some first-order transformations are driven by free energy (G) instabilities with respect to the relevant order parameter (η). Specific issues addressed in these studies are reviewed in this paper.     

Diffusion in metallic glasses

Rutherfordα-particles backscattering technique was employed for measurements of diffusion rates in metallic glasses. Effects of relaxation, crystallization and plastic deformation on diffusion rates were also investigated. It has been observed that the diffusion rates of a metallic solute are of the same orders of magnitude in both metal-metal and metal-metalloid glasses. A higher diffusivity is likely if there is a large difference between melting points of the solute and matrix. Relaxation has no effect on diffusion, however, diffusivity increases on crystallization. An increase in diffusivity is also observed on plastic deformation of metallic glass.     

电子束照射下钼纳米微粒的形成

使用高分辨透射电子显微镜在室温台上通过电子束照射ＭｏＯ３,制备钼及钼的亚氧化物钠米微粒,实验发现微米级ＭｏＯ３颗粒强度为１０＾２１ｅ／ｃｍ＾２．ｓ的电子束照射下转变为纳米级亚氧化钼,经过电子束的进一步照射后,亚氧化钼转变为钼。这种现象可能是由于电子束的激发作用和通过撞击效应而使原子发生错排引起氧原子分离造成的。     

高熵非晶合金耐腐蚀性能研究进展

高熵非晶合金是近年来发展起来的一种新型合金材料,因其兼具高熵合金和非晶合金优异的力学性能、耐腐蚀性能、磁性能等功能特性,引发了众多学者的广泛关注。本文简述了高熵非晶合金的含义与特点,介绍了高熵非晶材料的制备方法及组织与性能;归纳了该类材料的耐蚀机理与耐腐蚀性能的最新研究成果;展望了采用机器学习助力设计高熵非晶合金的新范式,并指出探究工况环境下的腐蚀失效机制、完善高熵非晶合金微观耐蚀机理与优化相关制备工艺是该材料广泛应用的前提条件。针对高熵非晶合金的开发及其耐腐蚀性开展的应用基础研究,将为我国海洋事业的“远洋化、深海化”提供先进的技术支撑和材料保障。     

Mechanical behaviours of workhardening and worksoftening bulk metallic glasses

Abstract

The workhardening Cu_47·5Zr_47·5Al₅ and the worksoftening Zr_52·5Cu_17·9Ni_14·6Al_10·0Ti_5·0 bulk metallic glasses before and after precompression deformation were characterised for thermal and mechanical behaviours. The predeformation introduces excessive free volume in both glasses. Cu_47·5Zr_47·5Al₅ and Zr_52·5Cu_17·9Ni_14·6Al_10·0Ti_5·0 exhibit substantial workhardening and worksoftening behaviours respectively. For Cu_47·5Zr_47·5Al₅, the precompression has a negligible effect on serrations in the plastic flow during nanoindentation, which is related to the hardening of a shear band, while for Zr_52·5Cu_17·9Ni_14·6Al_10·0Ti_5·0, the precompression moderates serrations in the plastic flow during nanoindentation, which is associated with the softening of a shear band. Strengthening from mechanically induced nanocrystallites at shear bands is responsible for the workhardening of Cu_47·5Zr_47·5Al₅, which overwhelms softening due to the introduction of excessive free volume.     

Studies on crystallization behaviour and mechanical properties of Al-Ni-La metallic glasses

Alloy ingots with nominal composition, Al_92−x Ni₈La _x (x = 4 to 6) and Al_94−x Ni₆La _x (x = 6, 7), were prepared by induction melting in a purified Ar atmosphere. Each ingot was inductively re-melted and rapidly solidified ribbons were obtained by ejecting the melt onto a rotating copper wheel in an argon atmosphere. The crystallization behaviour of melt-spun amorphous ribbon was investigated by means of differential scanning calorimetry (DSC), X-ray diffractometry and transmission electron microscopy. DSC showed that Al₈₆Ni₈La₆ alloy undergoes a three-stage and rest of the alloys undergo a two-stage crystallization process upon heating. The phases responsible for each stage of crystallization were identified. During the first crystallization stage fcc-Al precipitates for low La-containing alloys and for higher La-containing alloys a bcc metastable phase precipitates. The second crystallization stage is due to formation of intermetallic compounds along with fcc-Al. Microhardness of all the ribbons was examined at different temperatures and correlated with structural evolutions. Precipitation strengthening of nano-size fcc-Al is responsible for maximum hardness in these annealed alloys.     

Effect of metalloids on crystallization and magnetic behaviour of FeCoSiB based metallic glass

A series of amorphous iron-cobalt alloys with varying metalloid, boron and silicon contents were studied for their thermal stability and magnetic behaviour. The crystallization temperature and thermal stability increased with the silicon content. Good soft magnetic properties were observed for the materials with nominal composition, (Fe_0.79Co_0.21)77Si_12.2B_10.8. The magnetic properties were further improved by annealing.     

Thin film metallic glasses in optoelectronic,magnetic, and electronic applications: A recent update

Thin film metallic glass (TFMG) is a new class of metallic thin film with unique characteristics, including smooth surface, absence of grain boundaries, second-order glass transition, annealing-induced amorphization, soft magnetic properties, and high thermal stability. Hence, with these properties, TFMGs are found very useful and promising in many areas, ranging from structural, biomedical to electrical components. This review provides an update on future challenges and opportunities associated with the further development of TFMG.     

Corrosion performances in simulated body fluids and cytotoxicity evaluation of Fe-based bulk metallic glasses

The aim of this work is to investigate the corrosion resistance and biocompatibility of three kinds of Fe based bulk metallic glasses (BMGs), Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂ (BMG1), (Fe₄₄Cr₅Co₅Mo₁₃Mn₁₁C₁₆B₆)₉₈Y₂ (BMG2), and Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂ (BMG3) by electrochemical measurements and indirect contact cytotoxicity assays, respectively. In comparison with 316 L SS biomedical steel, Fe based BMGs show better corrosion resistance in both simulated body fluids (Hank's solution and artificial saliva). The OCP curves show that the passive film on the Fe based BMG surfaces is quite stable, like 316 L SS. The corrosion current densities obtained from the anodic polarization curves from the lowest to highest are as follows: BMG3 < BMG1 < BMG2 < 316 L SS. The EIS analysis indicates that the Fe Based BMGs have larger polarization resistance value than that of 316 L SS except for BMG2 in artificial saliva. The pitting corrosion potentials of Fe based BMGs are much higher than that of the 316 L SS, resulting in very few ions releasing into the electrolytes while a significant amount of Ni and Fe ions release was found for 316 L SS under the same condition. The indirect cytotoxicity results suggest that all three Fe based BMG extracts have no cytotoxicity to L929 and NIH3T3 cells. All these results demonstrate that Fe based BMGs will open up a new path for the biomedical applications, especially in dental implantology.     

High-resolution and high-voltage electron microscopy at the University of California,Berkeley

After 20 years of exciting growth in our theoretical and experimental expertise in transmission electron microscopy, the development of new generations of instruments over the past few years has opened up new opportunities for sophisticated studies of microstructure under well-controlled conditions and at near-atomic levels. High-voltage electron microscopy now provides unique capabilities for dynamic studies of bulk specific behaviour during exposure to ‘ real world ’ conditions, e.g., varying environments, deformation, irradiation, or combinations of these, while developments in lattice-imaging techniques afford new insights into structures and mechanisms operating at a near-atomic level. Both these approaches are being explored and exploited at Berkeley in a variety of applications to practical problems in materials science. Several examples are presented in this paper which illustrate the power of high resolution lattice-imaging in studies of interfacial structures, and in its combination with laser optical microdiffraction for studying lattice parameters and compositional changes over small (10 Å) regions. Other examples will underline the importance of imaging techniques and environmental control duringin-situ studies of ceramic materials in the high-voltage electron microscope. Finally, future directions and anticipated developments are briefly discussed.     

Dynamic observation of vortices in superconductors using a holography electron microscope

Individual vortices in superconductors are directly observed with our holography electron microscope. Vortices cannot be viewed as an electron micrograph, but only as a holographic interference micrograph or a defocused micrograph (Lorentz micrograph), since vortices are phase objects to the illumination electron beam. Since the flux itself, and not its replica, is detected, even the dynamic behavior can be observed. The dynamics of vortices in a Nb thin film can be observed in real time when the sample conditions, such as the temperature or the applied magnetic field, change.     

Densities,microhardnesses and electron microscopic studies of As-Se glasses

Densities and microhardnesses of As-Se glasses have been measured over a wide range of composition. Anomalous variations of densities and microhardnesses have been noted. It has been found possible to explain the variation on the basis of chemical ordering in these glasses. Scanning electron microscopy has been used to investigate the nature of etching in various compositions. The chemical stabilities of these glasses towards etchants also seem to support chemical ordering in them. Communication No. 95 from Solid State and Structural Chemistry Unit.     

低温弛豫对Mge5 Cu25 Gd10大块非晶合金热稳定性及力学行为影响

Mg65 Cu25 Gd10非晶合金的热稳定性关系到其作为结构材料的实用性及发展前景。利用差示扫描量热法（DSC）研究了预先低温弛豫处理后Mg65 Cu25 Gd10非晶的特征转变温度和晶化激活能，分析了低温弛豫对其热稳定性的影响，通过Kissinger方程计算其晶化激活能、频率因子、反应速率系数进一步说明此非晶的晶化过程。同时，通过比较其力学性能的变化，发现非晶压缩性能受非晶稳定性影响不大，抗压强度下降很小。但其断裂方式及断面微观特征有明显变化。     

Synthesis and properties of bulk metallic glasses in the ternary Ni–Nb–Zr alloy system

Bulk metallic glasses (BMGs) with high thermal stability, good mechanical properties and high corrosion resistance were synthesized in the Ni–Nb–Zr system. A large bulk glass-forming region with 60 < Ni < 64, 28 < Nb < 38 and 0 < Zr < 9 (in at.%) was found. The critical size for the glass formation is 3 mm. These investigated Ni-based BMGs process high glass transition temperature of about 880–900 K and high on-set crystallization temperature of 915–932 K as well as high compressive fracture strength of approximate 3.0–3.2 GPa along with some compressive plasticity of about 2%. Electrochemical measurements indicate they also exhibit high corrosion resistance, i.e., large passive region above 1.5 V (vs. saturated calomel reference electrode, SCE). The influence of the Zr content on the glass-forming ability (GFA) and corrosion behaviors was carefully studied, indicating that some Zr addition improves the GFA and corrosion resistance.     

Cooling rate effects on the microstructure and phase formation in Zr51Cu20.7Ni12Al16.3 bulk metallic glass

A detailed transmission electron microscope (TEM) study has been conducted to investigate the microstructures of the Zr₅₁Cu_20.7Ni₁₂Al_16.3 metallic glass formed at different cooling rates. It has been found that the most competitive crystalline phase to the amorphous structure is an oxygen-stabilized FCC NiZr₂-type phase, which in turn acts as the leading phase to trigger the formation of other crystalline phases in the slow-cooled alloy.     

X-ray microanalysis in the transmission electron microscope

The technique of x-ray microanalysis of thin specimens in the transmission electron microscope is well established. Quantitative analyses can be obtained by converting observed x-ray intensity ratios into weight fraction ratios by incorporating suitable correction factors. Flourescence corrections in thin specimens are always significantly less than in bulk specimens and, except where strong characteristic radiation fluorescence enhancement is predicted in the bulk, can be safety ignored. As analysis probes less than 100 Å diameter have become available, the beam spreading has become an important parameter in determining the spatial resolution for analysis. Present developments are directed at improving the quality of the analyses which can be obtained. This requires the generation of high quality correction factors, particularly forZ<13, rapid and accurate methods of determining sample thickness, so that absorption corrections can be made, and the development of reliable procedures for determining the shape of the activated volume in a thin specimen.     

Effect of ion irradiation in an Al90Fe2Ce8 metallic glass

Here the irradiation effect of an AlFeCe metallic glass under nitrogen ions was investigated. Ion irradiation induced crystallization can be observed in the studied metallic glass. The surface morphologies have been also examined by atomic force microscopy prior to and after ion irradiation. It is found that the surfaces have been smoothened after ion irradiation. The nanohardness and Young’s modulus of metallic glass increase after ion irradiation, which can be attributed to the formation of crystalline phases. The results obtained here clearly suggest that ion irradiation can be a useful method to tailor the microstructure and mechanical properties of metallic glasses.     

金属玻璃切削加载的形变屈服准则与实验验证

以金属玻璃切削过程中的屈服形变为研究对象,引入源于岩土领域的M-C屈服准则来解决传统Treaca准则及Mises准则不能反映金属玻璃的应力敏感性问题。另外鉴于金属玻璃的温度敏感特性以及切削加工时较高的切削温升,将经过温度修正的改进型M-C屈服准则应用于金属玻璃的切削模型之中。切削力实验表明,基于传统屈服准则、不含温度项的M-C屈服准则以及经过温度修正的M-C屈服准则所建立的切削力模型中,后者的解析解与切削力实测值相比误差最小(平均误差8.92%),说明经过温度修正的M-C屈服准则可以较好地反映金属玻璃切削加载的切削力及材料形变过程,为后续金属玻璃切削机理的深入研究奠定了理论基础。     

Quantitative characterization of the composition, thickness and orientation of thin films in the analytical electron microscope

Compositional variations in thin films can introduce lattice-parameter changes and thus create stresses, in addition to the more usual stresses introduced by substrate-film mismatch, differential thermal expansion, etc. Analytical electron microscopy comprising X-ray energy-dispersive spectrometry within a probe-forming field-emission gun scanning transmission electron microscope (STEM) is one of the most powerful methods of composition measurement on the nanometer scale, essential for thin-film analysis. Recently, with the development of improved X-ray collection efficiencies and quantitative computation methods it has proved possible to map out composition variations in thin films with a spatial resolution approaching 1-2 nm. Because the absorption of X-rays is dependent on the film thickness, concurrent composition and film thickness determination is another advantage of X-ray microanalysis, thus correlating thickness and composition variations, either of which may contribute to stresses in the film. Specific phenomena such as segregation to interfaces and boundaries in the film are ideally suited to analysis by X-ray mapping. This approach also permits multiple boundaries to be examined, giving some statistical certainty to the analysis particularly in nano-crystalline materials with grain sizes greater than the film thickness. Boundary segregation is strongly affected by crystallographic misorientation and it is now possible to map out the orientation between many different grains in the (S)TEM.     

Comparison of glass hydration layer thickness measured by transmission electron microscopy and nanoindentation

The thicknesses of the hydration layers on the surfaces of 2 silicate glasses have been assessed using a) a combination of focused ion beam milling and transmission electron microscopy and b) nanoindentation; the two approaches give consistent layer thicknesses. Lighter contrast of the hydrated layers in TEM suggests that the layers have reduced density when compared to the bulk glasses; this is consistent with the reduced near surface modulus and hardness of hydrated glasses observed in nanoindentation.