[en] This work aims to shed light on the underlying mechanism of hydrogen embrittlement, a critical issue for metallic materials in industries that rely on hydrogen storage, transportation, and application. Here we focus on investigating the mechanical response and microstructural evolution of single-phase ferritic model alloys to hydrogen by using a novel in situ backside electrochemically nanoindentation setup, enabling nanoindentation-related mechanical tests to be conducted during hydrogen charging. As the top surface of the specimen examined by mechanical experiment is not contaminated by the corrosive electrolyte after hydrogen charging, post-mortem microstructural characterizations by different techniques are feasible. The correlation between mechanical behavior and chemical composition in response to hydrogen is investigated by quantifying the hydrogen and measuring the hydrogen diffusivity of FeCr alloys.

[en] Ultrasonic guided wave tomography (GWT) provides an attractive solution to map thickness changes from remote locations. It is based on the velocity-to-thickness mapping employing the dispersive characteristics of selected guided modes. This study extends the application of GWT on a liquid-loaded plate. It is a more challenging case than the application on a free plate, due to energy of the guided waves leaking into the liquid. In order to ensure the accuracy of thickness reconstruction, advanced forward models are developed to consider attenuation effects using complex velocities. The reconstruction of the thickness map is based on the frequency-domain full waveform inversion (FWI) method, and its accuracy is discussed using different frequencies and defect dimensions. Validation experiments are carried out on a water-loaded plate with an irregularly shaped defect using S0 guided waves, showing excellent performance of the reconstruction algorithm. (paper)

[en] Highlights: • Nanoindentation is used to investigate the deformation of individual phases in a complex microstructure. • High resolution strain partitioning in multiphase alloys is obtained by digital image correlation and electron microscopy. • A new non-rigid image registration method to study the local strain partitioning is proposed. In multiphase alloys, the mechanical properties are controlled by both the local properties of individual microstructural constituents, as well as the mutual effect of these constituents as an aggregate. To this end, we systematically studied the local mechanical properties and deformation mechanisms of the microstructural constituents in a ZnAl4Cu1Mg0.31 alloy using nanoindentation tests at room temperature (25 °C) and 85 °C. The obtained strain rate sensitivity and activation volume suggest dislocation-dominated deformation in the primary η-Zn phase and grain/phase boundary sliding in the eutectoid structures. Further, the strain partitioning between individual microstructural constituents and their roles on macroscopic deformation at 85 °C was investigated using quasi in-situ digital image correlation (DIC), supplemented with non-rigid image registration. The DIC measurements showed that eutectic and eutectoid colonies carry higher strain than the primary η-Zn phase grains. The presented approaches and results can therefore be used to design new Zn-Al alloys and also other multiphase alloys with improved mechanical properties.

[en] Undetected inclusions in engineering components cause tremendous industrial expenses in maintenance and repairs each year, with additional risks of catastrophic failures. This paper introduces a powerful method for inclusion imaging and reconstruction in irregularly-shaped components, based on a cutting-edge imaging technique—full waveform inversion (FWI). We propose an ultrasonic scanning setup for nondestructive evaluation (NDE) that fits a variety of components with different shapes and sizes. The FWI theoretical expressions are summarized, aiming for creating clear explanations for the NDE and material characterization communities. Systematic analysis of the FWI performance using different setups has been conducted, and a variety of case studies show different aspects of complexity that the FWI technique can address. Multiple inclusions have been successfully reconstructed in gears, exhibiting the potential of applying the proposed technique in overcoming various NDE challenges related to the rapidly growing structural and material complexity nowadays. (paper)

[en] Ultrasonic techniques are able to accurately detect and characterize flaws in homogeneous structures. Elastic reverse time migration (ERTM) is a powerful tool to reconstruct high-resolution images of flaws. To achieve images with better quality, the solution can be obtained by iteratively finding an image generating the modeled data which can best match the measured data in a least-squares sense, i.e. least-squares migration (LSM). Combing ERTM and LSM, conventional elastic least-squares reverse time migration (ELSRTM) methods are based on the assumption of a constant density, which can lead to inaccurate amplitudes and parameter crosstalk artifacts in the reconstructed images. In this paper, an ultrasonic imaging technique based on the ELSRTM which considers density as well as longitudinal-(L-) and shear-wave (S-wave) velocity variations is explored for imaging flaws in heterogeneous structures. The ELSRTM with density variations can simultaneously reconstruct density and L- and S-wave velocity images, which can provide amplitude-preserving images and mitigate crosstalk artifacts. This method is applied to numerical as well as physical laboratory experiments and the results appear promising for flaw identification in heterogeneous structures. (paper)