Characteristic features of slip bands in submicron single-crystal gold component produced by fatigue

In order to investigate the fatigue behavior of submicron metal components, a resonant fatigue experiment is conducted using a single-crystal gold specimen that possesses a test section with a submicron width. Crystallographic slip bands appear on the test section surface due to fatigue when the resonant frequency of the specimen decreases abruptly. These slip bands form due to activation of a slip system with the maximum resolved shear stress amplitude. The critical value for slip band formation is evaluated to be over 150 MPa, which is over six times larger than that of persistent slip bands (PSBs) in the bulk counterpart. Cross-sectional field-emission scanning electron microscopy observations reveal that extrusions/intrusions that are ～15 nm wide form on the surface at slip bands. Although the slip bands have similar morphologies to those of PSBs, they are much narrower (bulk width is larger than 1 μm). The high fatigue strength may be attributed to the narrow slips required. Strain localization at the slip bands is revealed by cyclic deformation in the surface-polished specimen after fatigue. This suggests that there may be a certain fatigue understructure at the slip bands.     

Surface oxidation of nickel metal as studied by X-Ray photoelectron spectroscopy

X-ray photoelectron spectroscopy has been used to study the oxidation of polycrystalline nickel metal. The results indicate that the oxidation process takes place in three stages; associative adsorption of molecular oxygen, followed by the combination of oxygen atoms with surface nickel atoms and, ultimately, the formation of bulk oxide. At room temperature only the first two stages can be detected. For exposures below 1 L the O 1s photoelectron spectrum is considered to be characteristic of an associatively adsorbed oxygen species, but for exposures above this value evidence for the formation of a monolayer of NiO is suggested by the development of an O 1s peak at 529.9 eV. Incorporation of oxygen into the nickel lattice is observed at temperatures >500°K. The activation energy for this place-exchange process was estimated at 1.80±0.06 eV.     

Materials applications of photoelectron emission microscopy

Photoelectron emission microscopy (PEEM) is a versatile technique that can image a variety of materials including metals, semiconductors and even insulators. Under favorable conditions the most advanced aberration corrected instruments have a spatial resolution approaching 2 nm. Although PEEM cannot compete with transmission or scanning electron microscopies for ultimate resolution, the technique is much gentler and has the unique advantage of imaging structure as well as electronic and magnetic states on the nanoscale. Since the image contrast is derived from spatial variations in electron photoemission intensity, PEEM is ideal for interrogating both static and dynamic electronic properties of complex nanostructured materials. Here, we review the key principles and contrast mechanisms of PEEM and briefly summarize materials applications of PEEM.     

Size dependence of the plane-strain response of single-crystal Al to indentation by diamond wedges

A single-crystal Al specimen was indented at room temperature with a series of prismatic diamond indenters. The indenters had triangular or wedge-shaped cross-sections and were relatively long, thus inducing a two-dimensional (2-D) plane-strain response. Different included wedge angles were used. For each wedge indenter, instrumented indentations were carried out at varying loads. Analysis of the response from each wedge indentation experiment clearly revealed the presence of a size effect, manifested in an increasing projected contact pressure as the contact dimensions decreased from micron to submicron scales. When indentation responses from 2-D wedge indenters with different included angles were compared, the data showed evidence that this indentation size effect depended on the included wedge angle. This dependence was analyzed and compared to existing models of indentation based on the concept of geometrically necessary dislocations. While the model prediction of the dependence of the characteristic length on the included wedge angle appears to be consistent with current experiments, the model estimate of the magnitude of the characteristic length significantly exceeds the value deduced from experimentation. The present results offer new experimental evidence of size-dependent indentation response in plane-strain geometries.     

Electronic structure of perylene on Au studied by ultraviolet photoelectron spectroscopy and density functional theory

The electronic structure of perylene was analyzed by using ultraviolet photoelectron spectroscopy to introduce the energy level alignment of perylene/Au interface. The energy level alignment was studied by the onset of the highest occupied molecular orbital level and the shift of the vacuum level of the perylene layer, which was deposited on Au surface by stages. The measured onset of the highest occupied molecular orbital energy level was 1.0 eV from the Fermi level of Au, and the vacuum level was shifted 0.2 eV toward higher binding energy side with additional perylene layer. Furthermore, the density functional theory calculation was performed to identify the valence band spectrum of perylene film. The good agreement between the experimental and theoretical valence band spectrum allows us to assign each peak of the valence band spectrum, which was obtained from the perylene/Au film. The representative molecular orbital shapes, which composed the valence band of perylene, are presented in this report.     

Homogenization of twin-roll cast AA8079 aluminum alloy studied by in-situ TEM

AA8079 is a commonly used stock material for manufacturing thin packaging foils. The primary alloying elements Fe and Si can form binary and tertiary intermetallics. In-situ TEM simulating homogenization annealing process of the as-cast material was used to analyze the real-time changes of the shape, type, and distribution of these particles. They affect the mechanical properties of the final product and susceptibility of the material to the formation of pinholes and other macroscopic defects. Another set of as-cast samples were annealed in a regime simulating industrial treatment in combination with measurements of resistivity to validate the results of the in-situ experiment. The results show clear temperature intervals of recovery, matrix desaturation, and phase transformations occurring in several stages: spheroidization of the original particles above 450 °C, nucleation of new particles at 475 °C, particles coarsening above 525 °C, and an entire dissolution of the original particles above 550 °C.     

Application of acoustic emission sensing to slip detection in robot grippers

This paper discusses the application of acoustic emission signal analysis as a sensing tool for the detection of contact and slip related motion between a workpiece and an end effector. Some results from past work in this area are first reviewed. Different modes of slip motion (rotational and translational) are then discussed. The ability to distinguish between these slip modes is important since it enables the robot to assess the gripping stability and recover positional control of the workpiece. Experimental results are presented to demonstrate the sensitivity of the detected acoustic emission signal to these different modes of slip motion. The effects of the gripping force on the detected acoustic emission is also discussed.     

3104 铝合金热连轧板的织构

分别用X射线反射法和透射法检测了3104铝合金热连轧板不同厚度的织构及整体织构。结果表明：3104铝合金热连轧工艺使得热轧板的织构不均匀性很小。对于3104铝热连轧板,采用传统的X射线反射法只检测一层的织构基本能反映热轧板的织构特征。     

Orientation dependence of shear banding in face-centered-cubic single crystals

We present crystal plasticity finite element simulations of plane strain compression of α-Brass single crystals with different initial orientations. The aim is to study the fundamentals of mesoscale structure and texture development in face-centered-cubic (fcc) metals with low stacking fault energy (SFE). Shear banding depends on the initial orientation of the crystals. In Copper and Brass-R-oriented crystals which show the largest tendency to form shear bands, an inhomogeneous texture distribution induced by shear banding is observed. To also understand the influence of the micromechanical boundary conditions on shear band formation, simulations on Copper-oriented single crystals with varying sample geometry and loading conditions are performed. We find that shear banding can be understood in terms of a mesoscopic softening mechanism. The predicted local textures and the shear banding patterns agree well with experimental observations in low SFE fcc crystals.     

pH dependence of phosphate sorption on aluminum

The sorption of phosphate ions on aluminum was studied by an in situ radiotracer technique. The kinetics and reversibility of sorption and the influence of the solution pH on phosphate coverage were followed in 0.1 mol dm⁻³ NaCl solution. It was found that phosphate adsorption on aluminum is determined by the surface charge and the protonation state of phosphate in the bulk solution. Despite the high chloride concentration, significant phosphate sorption was identified, indicating strong phosphate binding to the surface. Phosphate sorption increased with pH up to pH=4.7. As the pH of the solution increased further, phosphate desorbed from aluminum. The exchange experiments indicated that the phosphate sorption on the passive aluminum surface is a largely irreversible process. It is assumed that, in near neutral solution, there is an ongoing surface transformation, i.e., oxide film growth, that entraps phosphate ions from the solution.     

Stray grain formation in the seed region of single-crystal turbine blades

Seed crystals are frequently used to provide an off-axial 〈001〉 crystallographic orientation to investment cast single-crystal, nickel-based superalloy turbine blades. However, stray grain defects can form during the melt-back of the seed crystal, requiring the use of a helical grain selector between the seed and the blade to remove them. Using meso-scale numerical simulations, the formation mechanisms of these stray grain defects have been investigated. Also investigated was the influence of the seed’s crystallographic orientation relative to blade axis. The model is first validated by comparison to experimental observations and then by its application to a range of casting situations. The results show that initiation of these defects is difficult to avoid. Instead, the impact of stray grains should be controlled during their growth. For more information, contact P.D. Lee, Department of Materials, Imperial College London, Exhibition Road, London SW7 2BP, U.K.; e-mail p.d.lee@imperial.ac.uk.     

Microstructural path and temperature dependence of recrystallization in commercial aluminum

The isothermal recrystallization of 90% cold-rolled commercial purity aluminum alloy AA1050 was studied by means of quantitative microscopy at four temperatures from 245°C to 280°C. The microstructural properties, V_v, the volume fraction recrystallized, S_v, the interfacial area density separating recrystallized grains from deformed grains and 〈λ〉, the mean recrystallized grain free length, were measured stereologically as a function of time. The kinetics, microstructural path, grain boundary migration rates and temperature dependence of recrystallization were quantified experimentally. Based on analysis of all data and microstructural path modelling, recrystallization was determined to be growth (boundary migration rate) controlled; all nucleation occurred in time periods short compared to the earliest annealing times. The activation energy for grain boundary migration was calculated to be 172–183 kJ/mole suggesting that a solute-limited grain boundary migration rate mechanism was operative in the alloy. The recrystallization microstructural path was found to be isokinetic, i.e. identical at all the annealing temperatures studied. Two stages of recrystallization kinetics were observed; an early transient-like stage characterized by decreasing growth rates and a later stage in which the kinetics approached Avrami behavior and the growth rates were approximately constant. The transient-like behavior is attributed to the steep, deformation-induced stored energy gradients surrrounding precipitate particles where the recrystallized grains are nucleated.     

Managing greenhouse gas emission in the indian aluminum industry

Fluorocarbons are pollutants that destroy the ozone layer in the upper atmosphere and allow more ultraviolet radiation to reach the surface of the earth. Over-exposure to such radiation damages plants and greatly increases people’s risk of skin cancer. Aluminum refineries and smelters, which consume large amounts of energy, are committed to continuous improvement in greenhouse gas abatement. Although India is under no international pressure to reduce greenhouse gas emissions, the Indian aluminum industry could undertake such a commitment voluntarily. This analysis shows where immediate improvements are possible, and presents a tentative action plan for the industry. For more information, contact H. Mahadevan, Larsen & Toubro Limited, Mineral Processing Division, Saki Vihar Road, Powai (W), Mumbai, Maharashtra 400072, India; +91-22-8581401; fax +91-22-8581633; e-mail hm@epc.ltindia.com.