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The mechanism of fatigue-induced interface damage in Cu/V nanoscale metallic multilayers was systematically investigated using the state-of-the-art in situ transmission electron microscopy. Upon cyclic compression, Cu/V nanoscale metallic multilayers show an interface-controlled fatigue deformation, and the interface morphology of Cu/V nanopillars changes significantly after 200 cyclic loadings. Cyclic deformation induced nanoscale intrusions/extrusions on the bimetal interface, while no sign of persistent slip bands or crack was observed. Formation of the nano-intrusions/extrusions can be attributed to the reciprocating defect motion and severe defect-interface interactions during the fatigue testing. This study provides a unique mechanistic understanding on the fatigue damage in metallic multilayers at nanoscale and sheds light on tailoring the damage tolerance of heterostructures via interface engineering.

Plastic deformation and fracture behavior of Cu/V nanoscale metallic multilayers (NMMs) were systematically investigated through in situ transmission electron microscopy testing. Two different types of interfaces: sharp interfaces and interfacial transition zones (ITZs), formed in the as-fabricated Cu/V NMMs. Upon deformation, sharp interfaces and ITZs exert distinct influences on the fracture behavior of Cu/V NMMs. Sharp interfaces can impede the crack propagation by deflecting the crack propagation and blunting the crack tip, while the ITZs can induce microcracks and facilitate the fracture process. These findings shed light on the important roles of interface structures in the deformation of a broad class of NMMs.

The competition between dislocation slip and deformation twinning in body-centered cubic (BCC) nanocrystals can be strongly influenced by the deformation conditions. In this study, we investigate the deformation of [112]-oriented BCC tungsten nanowires under different loading modes. It shows that dislocation plasticity is a predominant deformation mode under uniaxial tension, while deformation twinning prevails when non-uniaxial stress is applied. The interfaces of bending-induced twinning are composed of numerous stepwise {112} twin boundaries. These findings shed light on the deformation mechanism of BCC nanocrystals under complex loading conditions.

We have investigated limited angle transmission tomography to estimate speed of sound (SOS) distributions for breast cancer detection. That requires both accurate delineations of major tissues, in this case by segmentation of prior B-mode images, and calibration of the relative positions of the opposed transducers. Experimental sensitivity evaluation of the reconstructions with respect to segmentation and calibration errors is difficult with our current system. Therefore, parametric studies of SOS errors in our bent-ray reconstructions were simulated. They included mis-segmentation of an object of interest or a nearby object, and miscalibration of relative transducer positions in 3D. Close correspondence of reconstruction accuracy was verified in the simplest case, a cylindrical object in homogeneous background with induced segmentation and calibration inaccuracies. Simulated mis-segmentation in object size and lateral location produced maximum SOS errors of 6.3% within 10 mm diameter change and 9.1% within 5 mm shift, respectively. Modest errors in assumed transducer separation produced the maximum SOS error from miscalibrations (57.3% within 5 mm shift), still, correction of this type of error can easily be achieved in the clinic. This study should aid in designing adequate transducer mounts and calibration procedures, and in specification of B-mode image quality and segmentation algorithms for limited angle transmission tomography relying on ray tracing algorithms.

Selective laser melting technique was employed to fabricate a step-wise compositionally graded Fe–Al alloy coupon using the in-situ alloying of blended powders. Microstructural characterization of the fabricated coupons shows a columnar to equiaxed grain structure transition with increasing Al content along the building direction, along which the laser scan speed is increased. In the low Al content regions, the absence of constitutional undercooling (CUC) allows columnar grain growth against the macroscopic thermal gradients, whereas dominance of CUC in high Al regions leads to fully equiaxed and refined microstructures. Nanoindentation studies show a linear increase in hardness and reduction in elastic modulus for Al content up to ~20 at.%; no significant anisotropy in these properties could be seen.

Long periodic stacking ordered phases (LPSOs), consisting of various configurations of stacking faults, play an important role in developing ultrastrong Mg alloys with moderate ductility. However, their formation mechanisms are far from clear as no apparent defects are introduced during their formation as it is commonly believed that stacking faults are induced by defects. Here, we present the atomic and electronic basis for lattice-distortion-mediated formation of stacking faults, i.e., localized face-centred-cubic (FCC) structures, within a Mg-Zn-Y alloy with a hexagonal close-packed (HCP) structure. The atomic motion trajectories from ab-initio molecular dynamic simulations show that the Mg atoms occupying the nearest neighbour positions of Zn and Y solute atoms undergo a local HCP-to-FCC transition. It is revealed that a local lattice distortion caused by the solute atoms enables the Mg atoms to move and rearrange into a local FCC configuration, which is validated by high resolution scanning transmission microscopy and in-situ synchrotron X-ray diffraction. Our simulations provide profound insight into the formation mechanism of stacking faults in HCP Mg and their physical nature of phase transformations; this is not only critically important because conventional defects, such as dislocations and vacancies, are important to deformation are rare for Mg and its alloys, but also because they serve as a potential new approach to the design of advanced Mg alloys when defects are actually phase transformations.

Three-dimensional (3-D) ultrasound imaging is a promising modality for many medical applications. Unfortunately, it generates voluminous data in the front end, making it unattractive for high-volume-rate portable medical applications. We apply synthetic aperture sequential beamforming (SASB) to greatly compress the front-end receive data. Baseline 3-D SASB has a low volume rate, because subapertures fire one by one. In this paper, we propose to increase the volume rate of 3-D SASB without degrading imaging quality through: 1) transmitting and receiving simultaneously with four subapertures and 2) using linear chirps as the excitation waveform to reduce interference. We design four linear chirps that operate on two overlapped frequency bands with chirp pairs in each band having opposite chirp rates. Direct implementation of this firing scheme results in grating lobes. Therefore, we design a sparse array that mitigates the grating lobe levels through optimizing the locations of transducer elements in the bin-based random array. Compared with the baseline 3-D SASB, the proposed method increases the volume rate from 8.56 to 34.2 volumes/s without increasing the front-end computation requirement. Field-II-based cyst simulations show that the proposed method achieves imaging quality comparable with baseline 3-D SASB in both shallow and deep regions.

Synthetic aperture sequential beamforming (SASB) is a two-stage process, with fixed transmit and receive beamforming in the first stage, followed by dynamic receive beamforming in the second stage. Compared to 3D synthetic aperture ultrasound (SAU) imaging, 3D SASB has low computational complexity at the front-end, making it suitable for portable devices. Unfortunately, 3D SASB has low volume rate since typically only one subaperture transmits and receives at a time. To increase the volume rate, we propose to transmit and receive multiple subapertures simultaneously. A straight-forward implementation of such a scheme, for even four simultaneous firings, results in an increase in grating lobe and sidelobe levels. To address these issues, we use bin-based random sparse array to reduce the grating lobes caused by the three other transmits, and then optimize the locations of the active receive elements to minimize the sidelobe. Compared to a contemporary 3D SASB method, the proposed multiple-firing sparse array method increases the volume rate from 11 to 45 volumes per second, without increasing computational complexity at the front-end and with only a small degradation in imaging quality.

Volumetric flow rate estimation is an important ultrasound medical imaging modality that is used for diagnosing cardiovascular diseases. Flow rates are obtained by integrating velocity estimates over a cross-sectional plane. Speckle tracking is a promising approach that overcomes the angle dependency of traditional Doppler methods, but suffers from poor lateral resolution. Recent work improves lateral velocity estimation accuracy by reconstructing a synthetic lateral phase (SLP) signal. However, the estimation accuracy of such approaches is compromised by the presence of clutter. Eigen-based clutter filtering has been shown to be effective in removing the clutter signal; but it is computationally expensive, precluding its use at high volume rates. In this paper, we propose low-complexity schemes for both velocity estimation and clutter filtering. We use a two-tiered motion estimation scheme to combine the low complexity sum-of-absolute-difference and SLP methods to achieve subpixel lateral accuracy. We reduce the complexity of eigen-based clutter filtering by processing in subgroups and replacing singular value decomposition with less compute-intensive power iteration and subspace iteration methods. Finally, to improve flow rate estimation accuracy, we use kernel power weighting when integrating the velocity estimates. We evaluate our method for fast- and slow-moving clutter for beam-to-flow angles of 90° and 60° using Field II simulations, demonstrating high estimation accuracy across scenarios. For instance, for a beam-to-flow angle of 90° and fast-moving clutter, our estimation method provides a bias of −8.8% and standard deviation of 3.1% relative to the actual flow rate.

Synthetic aperture sequential beamforming (SASB) is a technique to achieve range-independent resolution in 2D images with lower computational complexity compared to synthetic aperture ultrasound (SAU). It is a two stage process, wherein the first stage performs fixed-focus beamforming followed by dynamic-focus beamforming in the second stage. In this work, we extend SASB to 3D imaging and propose two schemes to reduce its complexity:(1) reducing the number of elements in both transmit and receive and (2) implementing separable beamforming in the second stage. Our Field-II simulations demonstrate that reducing transmit and receive apertures to 32×32 and 16×16 elements, respectively, and using separable beamforming reduces 3D SASB computational complexity by 15× compared to the 64×64 aperture case with almost no loss in image quality. We also describe a hardware architecture for 3D SASB that performs first-stage beamforming in the scan head, reducing the amount of data that must be transferred for offchip processing in the second stage beamformer by up to 256×. We describe an implementation approach for the second stage that performs an optimized in-place update for both steps of separable beamforming and is well suited for GPU.

Plane wave imaging can provide high frame rates and is widely used for fast imaging in medical ultrasound. Due to a lack of transmit focus, its image quality is usually worse than other ultrasonic imaging modalities, such as Synthetic Aperture Ultrasound. In practice, images with high contrast-to-noise ratio (CNR) are highly desirable as they make it possible for the sonographer to detect small cysts with better accuracy. In this work, we propose to use edge detection and smoothing filters to improve the CNR of cyst images while keeping the point spread function unchanged after beamforming. We show that our post-processing technique helps achieve CNR larger than 19 dB for cyst images with 75 firing angles compounded.