Congratulation on Dr. Song’s graduation! Gyuho will start his new postdoc position at Prof. Sophie Wang’s lab at UConn. I wish Gyuho a great success at a new place (too far!).
Author: Lee, Seok-Woo
Jessica passed her dissertation proposal! Congratulations!
Jessica passed her dissertation proposal! Many Congratulations!!!
Title: Influence of grain boundaries on the mechanical properties of ceramic materials with high interface density
Abstract:
The development of lightweight, high strength, cost-effective transparent materials is in high demand for military vehicles, vessels, electronics, and sensor applications. Recent advances in the synthesis of transparent nanocrystalline ceramics have made it a superior choice over conventionally used glass due to their excellent mechanical properties, but their plasticity and fracture mechanisms have not been clearly understood, yet. In our preliminary study, transparent magnesium aluminate spinel with grain sizes ranging from 3.7 to 80 nm has been successfully synthesized using environmentally controlled pressure-assisted sintering. Hardness measured by nanoindentation has revealed a breakdown of the Hall-Petch relationship at a critical grain size of 18.5 nm with a measured hardness of 22.5 GPa. Below the critical grain size, as the grain size decreases, hardness decreases. This result indicates the emergence of a new plasticity mechanism such as grain boundary sliding. Micropillar compression was implemented to determine the yield strength as a function of grain size. Unlike the hardness dependence on grain size, below the critical grain size, yield strength did not decrease but rather remains the same. In addition, fracture occurred with no plasticity, indicating that a grain boundary would serve as a crack source, which is different from its role as a source of plasticity under nanoindentation. Our preliminary results clearly show that grain boundaries affect the mechanical behavior differently under different deformation condition. In order to gain a deeper understanding of the mechanical behavior of nanocrystalline ceramics, therefore, it is critical to understand the role of grain boundaries in plasticity and fracture processes under a given deformation mode.
In this proposal, therefore, we propose to perform experimental and computational studies to understand the effects of grain boundaries on the mechanical properties of nanocrystalline ceramics under various deformation modes. Micromechanical testing will be used to characterize their mechanical response under indentation, compression, bending, and fatigue. Electron microscopy will be performed to examine the evolution of defect structures before and after mechanical tests. Constitutive modeling and atomistic simulation will also be performed to understand the grain-boundary-assisted mechanisms that govern plasticity and fracture processes. The successful completion of this investigation will provide a fundamental understanding of the influence of grain boundaries on the mechanical properties of transparent ceramics and allow for the more rapid development of transparent ceramics with improved mechanical properties.
Jessica receives the GE fellowship! Super-congrats!
Jessica receives the GE fellowship in recognition of her excellent leadership, outreach service, and research capability. Many congratulations!!!
Tyler’s paper was published at Journal of Materials & Design!
Tyler’s paper was published at Journal of Materials and Design! Many Congratulations! This work is the collaboration with multiple groups (Mark Aindow (UConn), Avinash Dongare (UConn), Cyril Williams (US Army Research Laboratory)).
Tyler J. Flanagan, Sriram Vijayan, Sergey Galitskiy, Jacob Davis, Benjamin Bedard, Avinash Dongare, Mark Aindow, Cyril L. Williams, Seok-Woo Lee, “Shock-induced deformation twinning and softening in magnesium single crystals,” 194, 108884, Materials & Design (2020) [PDF] [web]
Abstract
Magnesium is widely regarded as an excellent structural material, primarily because it forms the basis for a range of light-weight high-strength alloys. Recently, high-strain rate deformation of magnesium has received a great deal of attention due to the complicated deformation modes that involve combinations of dislocation slip and deformation twinning. In this study, single crystal magnesium samples were shock-compressed along the c- and a-axis, then released back to ambient conditions. Post-mortem transmission electron microscopy revealed that extension twins developed for both c- and a-axis shock loading. Also, the nanoindentation hardness values for these shocked samples were compared to those for samples compressed under quasi-static conditions; it was found that the hardness decreased with increasing strain rate for both c- and a-axis loading. Molecular dynamics simulations were performed to elucidate the detailed mechanisms of deformation twinning in terms of inertial confinement of sample geometry and different stress relaxation speed between impact and lateral directions. The conversion from work-done to heat was discussed to explain the influence of shock-induced heating on the residual hardness. These results give new insights into the residual mechanical response in shock-compressed materials and may help to develop a more fundamental understanding of shock phenomena in metallic materials.
Jessica’s paper was published at Journal of Materials & Design!
Jessica’s paper was published at Journal of Materials & Design. Many Congratulations!!!
- Jessica M. Maita, Gyuho Song, Mariel Colby, Seok-Woo Lee, “Atomic arrangement and mechanical properties of amorphous boron,” – Materials & Design, 193, 108856 (2020) [PDF] [web]
Abstract
Amorphous boron can be synthesized by chemical vapor deposition (CVD) onto a tungsten wire substrate, and this core-shell fiber has been widely used in high-performance composites due to its superior mechanical properties. Although the amorphous boron coating makes a significant contribution to the high fracture strength, its mechanical properties have not been studied rigorously due to its thin thickness and strong adhesion to the substrate. Furthermore, the medium-range atomic ordering of CVD amorphous boron has not been clearly understood, and the determination of the closest crystalline structure has been a challenge. In this study, high-resolution transmission electron microscopy (HRTEM), nanoindentation, and in-situ micropillar compression were performed to investigate the atomic arrangement and mechanical properties. Electron diffraction and autocorrelation function analysis revealed α-rhombohedral boron ordering as the closest crystalline structure. Micropillar compression displayed near-ideal yield strength (~13 GPa), but nanoindentation a relatively moderate Young’s modulus (~320 GPa), leading to a modulus of resilience (2.64 × 108 J/m3) unprecedentedly higher than most advanced engineering materials. Our structural and mechanical data will be discussed in terms of lattice point spacing and the influence of surface defects on fracture strength, respectively. Our results will be potentially useful to improve mechanical properties of amorphous boron core-shell fibers and related composites.
Beautiful artwork from DMD: Rotating Tyler’s Au microparticles!
This is the the artwork of UConn Digital Media and Design (DMD) Graduate Assistants Yucheng Hang, 杭雨城, and Maria Raykova. In an art-meets-science partnership, Yucheng and Maria created imaginative designs based on microscopy images provided by UConn Tech Park – Innovation Partnership Building researchers.
Artist’s Statement
This is my second piece of artwork in this series connecting the micro world with the macro world. The geometry of the original image inspired me to animate it and create a feeling of being in the universe. -Yucheng Chang
Original Image (displayed in comments below): solid-state de-wetted gold microparticles on sapphire substrate. Courtesy of Tyler Flanagan, CAMMA, IPB at UConn Tech Park.
The official link of this video is the following.
https://www.linkedin.com/feed/update/urn:li:activity:6664174078837968896/
Kiera attended the virtual Frontiers 2020!
Kiera participated in the virtual Frontiers 2020!
The title of her poster is “Effects of Alloying on Dislocation Nucleation in [001] SrNi2P2“. Kiera used spherical indentation and investigated the dislocation nucleation stress for differently alloyed SrNi2P2. She discovered that a large atomic inclusion (Rh) can induce a residual shear stress that lowers a dislocation nucleation stress. So, alloying has been known to make the motion of dislocation difficult, but it could the nucleation of dislocation easy!
Her online poster presentation is available at this Portfolium link. You must log-in with your UConn ID (or google/Facebook) to see her presentation.
Frontiers in Undergraduate Research exhibitions provide opportunities for UConn’s talented undergraduate researchers to share their work with the university community. Consistent with the University’s steps to prevent the spread of COVID-19, Frontiers 2020 has moved online.
Previously, Amanda Giroux (2018, currently at Electric Boat) and Hetal Patel (2019, currently at UC Berkeley) attended this event from our group. Kiera Burns is the third one (2020)! I am very proud of the excellence of our UConn undergraduate students!
Jessica receives the outstanding leadership award!
UConn MSE department gives the outstanding leadership award to a graduate student annually. This year, Jessica is selected. Many congratulations!!!
This is the second one from our research group! (after Gyuho!).
Jessica’s leadership is highlighted in MSE webpage!
Jessica has made a great contribution to MSE department as well as our local society. Her great leadership is highlighted in MSE webpage. It is a great article!
MSE Graduate Student Reaches Out to Help High Schoolers Transition to College
