Engineering topics
Kacher, Josh
Publications and source records attributed to Kacher, Josh.
Direct Observation of Grain-Boundary-Migration-Assisted Radiation Damage Healing in Ultrafine Grained Gold under Mechanical Stress
Not Available
Influence of native oxide film on corrosion behavior of additively manufactured stainless steel 316L
The influence of the native oxide film on passive film properties and localized corrosion of additively manufactured SS 316L was studied in 1 wt% HCl by XPS characterization, electrochemical polarization curves, and post-test morphology analysis by SEM. Increased Cr oxidation kinetics was observed in the as-polished sample with the native oxide film resulting in formation of an overall more protective and compact film compared to the cathodically-activated sample. Electrochemical analysis showed that corrosive attack varied between dislocation cell boundaries to cell interiors depending on the initial surface state and polarization conditions. In conclusion, a corrosion mechanism is proposed to explain this variation.
High Energy X-ray Photoelectron Spectroscopy of COTS Electronics Interfacial Failure Modes.
Abstract not provided.
Ferrielectricity in the Archetypal Antiferroelectric, PbZrO 3
Antiferroelectric materials, where the transition between antipolar and polar phase is controlled by external electric fields, offer exceptional energy storage capacity with high efficiencies, giant electrocaloric effect, and superb electromechanical response. PbZrO 3 is the first discovered and the archetypal antiferroelectric material. Nonetheless, substantial challenges in processing phase pure PbZrO 3 have limited studies of the undoped composition, hindering understanding of the phase transitions in this material or unraveling the controversial origins of a low-field ferroelectric phase observed in lead zirconate thin films. Here, leveraging highly oriented PbZrO 3 thin films, a room-temperature ferrielectric phase is observed in the absence of external electric fields, with modulations of amplitude and direction of the spontaneous polarization and large anisotropy for critical electric fields required for phase transition. The ferrielectric state observations are qualitatively consistent with theoretical predictions, and correlate with very high dielectric tunability, and ultrahigh strains (up to 1.1%). This work suggests a need for re-evaluation of the fundamental science of antiferroelectricity in this archetypal material.
Dynamics of the gold–silicon eutectic reaction studied at limited length scales using in situ $\mathrm{TEM}$ and $\mathrm{STEM}$
We report the dynamics of the gold–silicon eutectic reaction in limited dimensions were studied using in situ transmission electron microscopy and scanning transmission electron microscopy heating experiments. The phase transformation, viewed in both plan-view and cross-section of the film, occurs through a complex combination of dislocation and grain boundary motion and diffusion of silicon along gold grain boundaries, which results in a dramatic change in the microstructure of the film. The conversion observed in cross-section shows that the eutectic mixture forms at the Au–Si interface and proceeds into the Au film at a discontinuous growth rate. This complex process can lead to a variety of microstructures depending on sample geometry, heating temperature, and the ratio of gold to silicon which was found to have the largest impact on the eutectic microstructure. The eutectic morphology varied from dendrites to hollow rectangular structures to Au–Si eutectic agglomerates with increasing silicon to gold ratio.
Morphology and Electrochemistry Drive Corrosion of Electroless Nickel Immersion Gold Films: A Multi-Technique Analysis.
Abstract not provided.
Discovering the nanoscale origins of localized corrosion in additive manufactured stainless steel 316L by liquid cell transmission electron microscopy
Here, we use liquid cell scanning transmission electron microscopy (STEM) to directly characterize the nanoscale origins of corrosion initiation in Additive manufacturing (AM) 316 L stainless steel. Under applied anodic potentials, we found that the dislocation cellular boundaries were preferentially corroded and that regions of localize corrosion occurred along the cellular boundaries. We directly observed the earliest stages of corrosion by controlling the biasing parameters to decelerate the corrosion processes. The results show that highly localized corrosion occurs via inclusion dissolution along dislocation cell boundaries. More widespread corrosion initiates at the dislocation cell boundaries and spreads throughout the dislocation networks.
Local Epitaxial Templating Effects in Ferroelectric and Antiferroelectric ZrO 2
Not provided.
Characterizing defect structures in AM steel using direct electron detection EBSD
The mechanical properties of additive and traditionally manufactured alloys are largely dependent on the characteristics and distribution of dislocation cell networks that develop during the fabrication process. This work demonstrates the ability to quantitatively characterize these dislocation structures by high angular resolution electron backscatter diffraction analysis using a direct electron detector. The defect structures are characterized in terms of the geometrically necessary dislocation density and the associated Burgers vector and line direction. Furthermore, the results are discussed in terms of potential defect formation mechanisms.
Spall damage mechanisms in laser powder bed fabricated stainless steel 316L
This paper reports on spall damage mechanisms in laser powder bed fusion (LPBF) fabricated stainless steel 316L (SS316L) subjected to uniaxial, strain plate-impact loading and explores failure evolution with increasing impact velocity and peak pressure. Analysis of velocimetry profiles reveals a heterogeneous failure response with several different failure mechanisms activated during impact and resulting spall. The analysis is supported by optical microscopy and electron backscattered diffraction (EBSD) observation of soft recovered impacted samples, which show evidence of localized nano-twinning, twin growth, severe grain rotation, and grain refinement along the spall plane, in addition to pore nucleation, coalescence, and growth. Furthermore, the observed failure mechanisms, including crack formation and propagation along high angle grain boundaries (HAGBs), are not consistently indicative of ductile fracture typical of wrought stainless steel.
Atomistic modeling of surface and grain boundary dislocation nucleation in FCC metals
Dislocation nucleation plays a critical role in the plastic deformation of crystalline materials. However, it is challenging to predict the active mode and associated rate of dislocation nucleation under typical experimental loading conditions through molecular dynamics simulation due to timescale limitations. In this report we use the free-end nudged elastic band method to determine the activation energies and activation volumes of dislocation nucleation in four typical face-centered cubic metals of Au, Al, Cu and Ni. We focus on the representative processes of surface and grain boundary dislocation nucleation. The atomistically determined activation volumes of these dislocation nucleation processes are larger than 10b3 (with b being the Burgers vector length) under typical experimental loading conditions. These results are compared with experimentally measured activation volumes in ultrafine-grained and nanocrystalline metals, thereby providing mechanistic insight into their rate-controlling deformation mechanisms.