Pits that form on high-strength Al-Cu and Al-Cu-Mg alloys during exposure to neutral chloride-bearing environments, with oxidizing power determined primarily by the presence of dissolved oxygen, are occasionally observed to be decorated with Cu-rich corrosion product deposits. In many instances, scrupulous examination of mature pits reveals decoration with dendritic deposits of metallic Cu. An example of this is given in Fig. 1, which shows metallic Cu deposited at the lip of a pit formed on 2024-T3 (Al-4.4 Cu-1.5 Mg-0.6 Mn) during exposure to a neutral 5% salt-spray environment for 1 week. 1 In the absence of any external source of Cu ions, the observation of highly localized Cu deposits indicates the operation of a very potent redistribution process. The presence of metallic Cu deposits likely requires that Cu ions exist in the solution phase during the corrosion process. While the plating-out of Cu ions onto corroding Al is easily explained, how corrosion of an Al alloy might produce Cu ions is unclear. The problem is that the open-circuit potential (OCP) of a Cu-bearing Al alloy is typically hundreds of millivolts negative of the equilibrium potential for Cu, E Cu , b and oxidation of Cu directly from the alloy is thermodynamically prohibited.Recently, it was discovered that Al 2 CuMg intermetallic particles in high-strength Al-Cu-Mg alloys dissolve by a dealloying process. Electron optical examination revealed that corroding intermetallic particles liberate 10-100 nm diam metallic Cu clusters into a hydrous corrosion product gel as they coarsen and decompose. 2 In parallel activities, X-ray photoelectron spectroscopy and Auger electron spectroscopy composition depth profiles have been used to confirm that the clusters are detached from the underlying alloy substrate and are metallic in nature. 3 Figure 2a is a scanning electron micrograph of a 2024-T3 sample exposed to 0.1 M NaCl solution which shows corrosion in the vicinity of an Al 2 CuMg intermetallic particle. Figure 2b is a backscattered electron image that reveals the presence of metallic Cu clusters in the corrosion product film over the particle.The presence of metallic Cu clusters in a corrosion product layer provides means for explaining the formation of Cu ions during AlCu-Mg alloy corrosion. The overall mechanism proposed for this process is depicted schematically in Fig. 3. According to this mechanism, the Al 2 CuMg particle is anodically polarized by the surrounding ␣-Al matrix phase. 4 The particle dealloys under anodic polarization, leaving behind a porous copper-rich particle remnant. As this fine, porous structure coarsens to reduce its surface energy, small 10-100 nm diam metallic Cu clusters become detached from the particle remnant. The clusters are captured in the hydrous corrosion product gel over the corrosion site and move away due to mechanical transport as the film grows. Clusters may also be carried away from their site of origin by mechanical action due to solution movement. This nonfaradaic detachment process electrically isola...

The integrated nanogenerator (NG) based on vertical nanowire (NW) arrays is one of the dominant designs developed to harvest mechanical energy using piezoelectric nanostructures. Finite element method (FEM) simulations of such a NG are developed using ZnO NWs in compression mode to evaluate its performances in term of piezoelectric potential generated, capacitance, induced mechanical energy, output electrical energy, and effi ciency. This evaluation is essential to correctly understand NG operation. Three main issues are highlighted. The mechanical and electrical structures of the NG as an integrated system are optimized, and strategies for concentrating the mechanical strain fi eld in the NWs and increasing the force sensitivity are developed. In addition, the infl uence of NWs length and diameter on NG performances is investigated. The optimization results in a piezoelectric nano composite material where global performances are improved by mean of long and thin NWs.

Recent work has shown that Al 2 CuMg (S-phase) particles constitute approximately 60% of the second-phase particles in 2024-T3 (Al-4.4Cu-1.5Mg-0.6Mn) with diameters greater than 0.2 m. 1 The Al 2 CuMg particles in question are shown in Fig. 1. Corrosion damage is localized at these particles, resulting in selective particle dissolution or pitting in the matrix adjacent to the particle. These particles also appear to play a significant role in redistribution and enrichment Cu on dissolving Al-Cu-Mg alloy surfaces. However, there is some uncertainty regarding the electrochemical characteristics of this phase and whether it is anodic or cathodic with respect to the surrounding matrix phase. Open-circuit potential (OCP) measurements, 2 and polarization curves 3 conducted on bulk-phase Al 2 CuMg samples indicate that the phase is electrochemically active with respect to the matrix phase. However, observations of pit morphology which show pitting in the matrix adjacent to S-phase particles suggest that the phase is electrochemically noble.We have synthesized the S phase in bulk to attempt to verify earlier reports of its electrochemical characteristics. We have also conducted further experiments to understand the nature of the S-phase particle-␣-Al matrix phase galvanic couple and to understand dissolution phenomena associated with this particle during corrosion of Al-Cu-Mg alloys. In this paper, we report results on synthesis of the phase in bulk form, verification of its structure and composition, and determination of the electrochemical characteristics by OCP measurements and potentiodynamic polarization. These results indicate complexities in the electrochemical characteristics beyond the relative nobility or activity of this compound compared to its microstructural surroundings. ExperimentalBulk-phase synthesis and characterization.-The S phase was synthesized by charging an alumina crucible with 21.25 g 99.999% Al, 20.00 g 99.99% Cu, and 8.75 g 99.9% Mg. The charge was heated to 990ЊC at a rate of 25Њ min Ϫ1 and held for 30 min to insure full melting. The melt was mechanically agitated, then cooled at 7Њ min Ϫ1 to 510ЊC and held for 65 h. The melt was finally cooled to room temperature at 5Њ min Ϫ1 . At this composition, isothermal treatment at 510ЊC causes the melt to segregate into solid S and an Alrich liquid. 4 The long isothermal treatment time was used to maximize the S-phase crystal size. Ingots were sectioned and polished by hand in oil. The finishing step was carried out using a 1.0 m diamond paste. Polished surfaces were degreased by ultrasonic cleaning in ethanol.Bulk intermetallic samples were characterized using standard X-ray powder diffraction and backscattered electron Kikuchi patterns (BEKPs). 5-7 The acquisition of high-quality BEKPs suitable for phase identification was accomplished using a new charge coupled device (CCD) based camera with a single-crystal scintillator designed by Sandia National Laboratories and built in conjunction with Photometrics, Inc., of Tuscon, AZ. This camera was insta...