In this study, corrosion response of the 2060-T8E30 alloy was analyzed in the following environments: EXCO (an immersion test in accordance with ASTM G34), modified EXCO, and modified ASTM acetic acid salt intermittent spray (MASTMAASIS), which were used to predict the alloy system's exfoliation corrosion (EFC) susceptibility. Here, results show that exposure to the first two environments failed to accurately predict EFC during seacoast exposure while the latter was successful. The final attacks in the 2060 alloy varied from solution to solution. In EXCO solution with a pH of 0.25 and chloride content of 4 M, a severe attack of EFC occurred in the 2060 alloy. In modified EXCO solution with a pH of 3.4 and chloride content of 1.1 M, intergranular corrosion was present. Pitting corrosion appeared on the 2060 alloy in MASTMAASIS environment with pH of 3 and chloride content of 1 M. Additionally, electrochemical impedance spectroscopy signature, video capture microscopy images, and scanning electron microscope images revealed the formation of a new interface on the 2060 alloy resulting from grain lifting in the EXCO solution. Here, it is shown that there is a synergistic action between the corrosion product wedging and the pressure of hydrogen gas during the grain lifting.

In this study, we investigated exfoliation corrosion (EFC) of the alloy 2060 T3E80 in an aggressive solution, namely EXCO. The pH value increased from 0.25 at the initial time point to 3.45 after 96 h of testing. In Stage 1 (0.25 < pH < 3.00), the pH value increased significantly and the aluminum surface experienced anodic dissolution. During this step, a massive hydrogen evolution reaction was observed, and pits formed due to the galvanic effect between the anodic and cathodic sites. The electrochemical impedance spectroscopy (EIS) test and equivalent analogs suggest that the inductive loop at low Nyquist frequencies and phase angle plots correspond to the adsorbed intermediates in the reduction reaction. In Stage 2 (3.00 < pH < 3.45), the pH value slowly increased. The continuous pits along the grain boundaries produced intergranular corrosion (IGC) during this stage. A small amount of delamination occurred at this stage. In Stage 3, pH remains constant and IGC extensively propagate, allowing the accumulation of white gelatinous corrosion products under the grains. These, along with hydrogen bubbles underneath the grain, exerted pressure to the grain that was attacked by IGC, resulting in a lifting of the grain. The grain lifting at this stage was abundant. It is clear from the cyclic potentiodynamic polarization and scanning electron microscopy images that the pits formed on this alloy surface in the EXCO solution can be attributed to fall-out of particles instead of passive film breakdown. (C) 2015 Elsevier B.V. All rights reserved.

Material degradation due to corrosion and corrosion fatigue has been recognized to significantly affect the airworthiness of civilian and military aircraft, especially for the current fleet of airplanes that have served beyond their initial design life. The ability to predict the corrosion damage development in aircraft components and structures, therefore, is of great importance in managing timely maintenance for the aging aircraft vehicles and in assisting the design of new ones. The assessment of aircraft corrosion and its influence on fatigue life relies on appropriate quantitative models that can evaluate the initiation of the corrosion as well as the accumulation during the period of operation. Beyond the aircraft regime, corrosion has also affected the maintenance, safety and reliability of other systems such as nuclear power systems, steam and gas turbines, marine structures and so on. In the work presented in this paper, we reviewed and studied several physics based pitting corrosion models that have been reported in the literature. The classic work of particle induced pitting corrosion by Wei and Harlow is reviewed in detail. Two types of modeling, a power law based simplified model and a microstructure based model, are compared for 2024-T3 alloy. Data from literatures are used as model inputs. The paper ends with conclusions and recommendations for future work.

Aluminum Alloys are widely used for aircraft application in both commercial and military fields. The increasing development in performance and efficiency has led to Al-Cu-Li Alloys, as these are capable of being lighter while achieving similar or superior levels of mechanical properties of their commonly used counterparts. The need to evaluate new alloys such as Al 2060 to their susceptibility to corrosion, especially Exfoliation Corrosion (EFC), becomes evident as this corrosion mechanism is known to be one of the main reasons of failure of aluminum alloys. The aim of this paper is to use electrochemical and surface analysis techniques to evaluate the susceptibility to EFC in Al 2060 in the Exfoliation Corrosion (EXCO) solution proposed in ASTM G34, propose a mechanism and describe the different stages of the damage evolution of the material. Electrochemical techniques include Open Circuit Potential (OCP), Cyclic Potentiodynamic Polarization (CPP), Galvanostatic Polarization (GS), Electrochemical Impedance Spectroscopy (EIS) and Electrochemical Noise (EN).

: Interaction of high amplitude acoustic waves with materials produces a small increase in the temperature that can be detected and measured using an IR camera. The changes in temperature as a function of time, due to interaction of high amplitude 20 kHz acoustics, with as received and fatigue damaged polycrystalline Ti-6Al-4V samples are compared. The maximum temperature reached by the sample has been found to increase with increasing fatigue cycles. The role of multiple physical mechanisms, responsible for conversion acoustic energy to heat, like the sample geometry (finite dimension), the microstructure (grain size), and dislocation density are examined. The theoretically evaluated temperature changes are observed to be in reasonable agreement with experimental measurements. The significance of the details of microstructure and dislocation properties needed in theoretical evaluation of temperature changes are used to explain the observed differences between experimental measurements and theoretical calculations.

Non-Contact Acousto-Thermal Signature (NCATS) analysis uses conversion of acoustic energy to heat to characterize evolving damage in materials. In the past, the observed temperature changes were interpreted using phenomenological approaches. This paper presents details of the mechanisms and the theoretical models to predict the temperature change due to conversion of acoustic energy to heat. NCATS experimental measurements performed using 20 kHz high amplitude acoustic waves on as received and fatigued polycrystalline Ti-6Al-4V are compared with theoretical calculations based on the mechanisms of transverse thermal currents, inter-crystalline thermal currents, and dislocation density changes. In the as received samples, the transverse thermal currents contribution has been found to be negligible compared with inter-crystalline thermal currents contribution. The experimentally measured maximum temperature change in the as received sample has been found to be 0.5 °C, and the theoretical prediction based on inter-crystalline thermal currents is 0.08 °C. In the fatigue damaged samples, the maximum temperature change increases with increasing damage that can be attributed to the increasing dislocation density. The theoretical prediction of the maximum temperature attained by a sample that is near failure based on dislocation contribution is 2.0 °C, while the experimental measurements have been found to be 0.95 °C. The differences between the theoretical and the experimental measurements are discussed in the context of the uncertainties in several physical parameters used in the theoretical calculations.

This chapter describes the development of crystallographic texture and its effects on mechanical properties in aluminum-lithium alloys. Crystallographic texture evolves during the forming of wrought products from cast ingots of Al-Li alloys and consequently affects the mechanical properties. Practical approaches to control the texture have been developed and have been successfully used in the products obtained from industrial-scale ingots. This texture tailoring has significantly reduced the yield strength anisotropy. In addition, theoretical approaches have been used to model the yield strength anisotropy of aluminum alloys in the presence of complex precipitates.

The invention of AFM has enabled the development of eddy current microscopy techniques with better spatial resolution than that of conventional eddy current imaging systems.In magnetic force microscopy (MFM), a magnetic probe is oscillated above a magnetic surface.The oscillating magnetic probe generates eddy currents.This concept was used in the development of an MFM based eddy current microscopy (Hoffmann et al., 1998).This technique was used to image local variations in electrical conductivity of a sample consisting of TiC precipitates in Al 2 O 3 matrix with nanometer scale resolution.However, since the magnetic field of an MFM tip is small, this technique is not suitable to image small variations in conductivity.The sensitivity of this technique was improved by using large magnetic fields from a tip made ofpermanent magnet (Lantz et al., 2001).This resulted in increased sensitivity but reduced the spatial resolution down to hundreds of nanometers.From the above discussion it is evident that it is difficult to achieve both high resolution and high sensitivity to local variations in electrical conductivity using eddy current microscopy by MFM.To improve the sensitivity, a flexible cantilever capable of detecting small variations in the forces can be employed.However, in MFM techniques, a stiffer cantilever, vibrated at its resonant frequency, is used in order to make the cantilever sensitive only to the long-range magnetic forces.But by using a stiffer cantilever, it is difficult to measure small forces generated due to very small variations in the electrical conductivity.The magnetic tips used in MFM have small magnetic field strength.Therefore, the eddy current density that can be induced in the sample material is limited.

This paper presents the development of a new non-contact acousto-thermal signature (NCATS) nondestructive evaluation technique. The physical basis of the method is the measurement of the efficiency of the material to convert acoustic energy into heat, and a theoretical model has been used to evaluate this. The increase in temperature due to conversion of acoustic energy injected into the material without direct contact was found to depend on the thermal and elastic properties of the material. In addition, it depends on the experimental parameters of the acoustic source power, the distance between sample and acoustic source, and the period of acoustic excitation. Systematic experimental approaches to optimize each of the experimental variables to maximize the observed temperature changes are described. The potential of the NCATS technique to detect microstructural-level changes in materials is demonstrated by evaluating accumulated damage due to plasticity in Ti-6Al-4V and low level thermal damage in polymer matrix composites. The ability of the technique for macroscopic applications in nondestructive evaluation is demonstrated by imaging a crack in an aluminum test sample.

The interaction of low-frequency electromagnetic waves with metallic nanostructure consisting of nanoparticles has been investigated. The existence of helicons in metallic nanostructure is predicted based on the enhancement of effective mass of the electrons in metallic mesostructures in low-frequency electromagnetic field. The enhancement of the effective mass of electrons subjected to low-frequency electromagnetic field results in the suppression of the cyclotron frequency. When the excitation frequency approaches the cyclotron frequency, helicons can be observed, which in typical metals occur at MHz frequencies. With the decrease in the cyclotron frequency in nanostructures, the helicons can be observed at low frequencies. We present an experimental setup to detect and image helicon resonances in metallic nanostructures at low frequencies and room temperature conditions. The approach is based on modifying an existing atomic force microscope to image surface topography and the magnetic field images of helicon waves simultaneously in metallic nanoparticles. The magnetic field images of the helicon waves and different resonance modes are presented for platinum nanoparticles deposited using through thin film ablation. The contrast in the magnetic field images is explained based on the generation of resonance modes due to helicon wave propagation in the nanostructure. The features in the helicon resonance images are examined as a function of the particle size and frequency of the electromagnetic wave.

Plastic deformation introduces changes in a material which include increases in: dislocations, strains, residual stress, and yield stress. However, these changes have a very small impact on the material properties such as elastic modulus, conductivity and ultrasonic wave speed. This is due to the fact that interatomic forces govern these properties, and they are not affected by plastic deformation to any large degree. This is evident from the fact that the changes in electrical resistance and ultrasonic velocity in plastically deformed and virgin samples are very small and can only be determined by highly controlled experiments. Except for X-ray diffraction, there are no direct nondestructive methods for measuring strain and the residual stress. This paper presents an application of the non-contact acousto-thermal signature (NCATS) NDE methodology to detect plastic deformation in flat dog bone Ti-6Al-4V samples. Results of the NCATS measurements on samples subjected to incremental amounts of plastic deformation are presented. The maximum temperature attained by the sample due to acoustic excitation is found to be sensitive to the amount of plastic strain. It is observed that the temperature induced by acoustic excitation increases to a peak followed by a decrease to failure. The maximum temperature peak occurs at plastic strains of 12-14%. It is observed that there is a correlation between the peak in maximum temperature rise and the strain at the experimentally determined ultimate tensile strength. A microstructural based explanation for this will be presented. The results are discussed in reference to utilizing this technique for detection and evaluation of plastic deformation.

Material state characterization is often performed using nondestructive evaluation (NDE) techniques with an objective to either understand or evaluate the microstructural state of the material or indirectly determine the damage that may have occurred in the material during manufacturing or service. Understanding material state is crucial, since material properties define the material's ability to perform its designed function. A number of nondestructive evaluation/nondestructive testing/nondestructive inspection (NDE/NDT/NDI) sensors are available to measure properties of a material. Basic laws of physics dictate the function of the sensor. This article attempts to explain the physics behind eddy current, ultrasonic, and acoustic emission sensors routinely used in NDI of aircraft structures.

A model-assisted approach for the design and execution of probability of detection (POD) studies is proposed. General agreement was achieved between experimental and full-model assisted results for eddy current inspection of cracks at fastener sites located at both the first and second layers. The accuracy of the POD results was found to be dependent upon the NDE model and assumptions in the model-assisted POD evaluation. Insight is presented for improving the quality of future studies.