Page 29 - ASME IMECE 2015 Program

WEDNESDAY Special Events

director. For about two years, he also served as program director of the Materials Division Sia Nemat-Nasser Award Presentation
Sensors and Sensing Systems program. For relaxation, he spends his
weekends soaring over the Shenandoah Valley, and he is a certified flight 3:00pm–3:30pm
instructor in gliders (CFI-G) with about 1,800 total flying hours.
Title: Effect of Internal Boundaries and Defects on Mechanical
Behaviors of Crystalline Nanowires

MATERIALS DIVISION AWARD LECTURES Yong Zhu
Sponsored by: Materials Division Department of Mechanical and Aerospace Engineering, North Carolina
2:30pm–7:00pm State University
Ballroom of the Americas B, Hilton of the Americas
Abstract: It is well known that free surfaces play a critical role in the
The following awards and lectures will be presented:
mechanical behavior of crystalline nanowires (NWs). However, the NWs,
especially those synthesized using bottom-up approaches, typically
Materials Division ORR Award Presentation
contain different types of internal boundaries and defects. It is thus of
2:30pm–3:00pm interest to understand whether and how such internal boundaries and
defects would affect the mechanical behaviors of crystalline NWs. In this
Title: The Role of Microstructure in Predicting Fatigue Performance and talk I will discuss two recent studies that revealed important roles of
Variability internal boundaries and defects.

Michael D. Sangid For the penta-twinned Ag NWs, we reported a dislocation-mediated,
School of Aeronautics and Astronautics, Purdue University time-dependent and fully reversible plastic behavior. In-situ tensile testing
in scanning and transmission electron microscopes (SEM/TEM) showed
Abstract: Scatter observed in the fatigue life of engineering alloys can be
that penta-twinned Ag NWs undergo stress relaxation on loading and
attributed to variability of defect arrangements within the material’s
complete plastic strain recovery on unloading, while the same experi-
microstructure, which ultimately limits the life cycle of components. Hence,
ments on single-crystalline Ag NWs do not exhibit such a behavior.
there is great interest in linking the microstructure to fatigue performance,
Molecular dynamics simulations revealed that this behavior originates from
as a means to add value into materials and component designs. With this as
the surface nucleation, propagation and retraction of partial dislocations.
a motivation, a multiscale fatigue model is developed, which obtains
More specifically, vacancies reduce dislocation nucleation barrier,
hot-spots within the microstructure, using rate-dependent crystal plasticity
facilitating stress relaxation, while the TBs and their intrinsic stress field
finite element simulations, based on elastic stress anisotropies and local
promote retraction of partial dislocations, resulting in full strain recovery.
plastic strain accumulations during one-cycle of loading. The quantitative
High-angle annular dark-field scanning TEM image of the cross-sectional
information of the local microstructure around these hotspots and the
sample confirmed the presence of vacancy defects near the TBs.
corresponding slip system activity is leveraged in a physics based life
prediction model, which quantifies the energetics of the active failure In another study, we discovered giant anelasticity in crystalline NWs
mechanisms within the material. Namely, the energy of dislocation governed by stress-gradient-induced migration of point defects. Under
arrangements are modeled within persistent slip bands in terms of their bending, ZnO and Si NWs were found to exhibit giant anelastic relaxation,
stability with respect to the dislocation motion, thus representing the four orders of magnitude greater than the largest ever reported value in
model’s failure criterion for crack initiation. Using Monte Carlo algorithms, bulk materials, with a time scale on the order of minutes. The large
statistically equivalent synthetic microstructures are constructed to predict magnitude of the anelasticity is due to the ultrahigh strain applied to the
the scatter and probability of fatigue failure relative to the observed defect NWs, while the small diffusion distance (e.g., NW diameter), enormous
distributions. Experimental methods are employed to validate these stress gradient and large diffusivity result in the short relaxation (recovery)
models, namely the evolution of spatial strains across the microstructure are time scale. The giant anelasticity led to very high energy dissipation,
measured using in situ experiments via (a) high-energy x-ray diffraction and suggesting that crystalline NWs with point defects could serve as highly
(b) concurrent digital image correlation and electron backscatter diffraction. efficient damping materials.
Biography: Michael D. Sangid received his BS (2002) and MS (2005) in me- Biography: Yong Zhu is an associate professor of mechanical and aerospace
chanical engineering from the University of Illinois at Urbana-Champaign engineering at North Carolina State University. He was a postdoctoral
(UIUC). After his master’s degree, he spent two years working in Indianapo- research associate at the University of Texas at Austin before joining NC
lis, IN, for Rolls-Royce Corporation, specializing in material characterization, State in 2007. He holds joint appointments in the Departments of Materials
fatigue, fracture, and creep of high-temperature aerospace materials before Science and Engineering and Biomedical Engineering at NC State.
resuming his education in 2007. He received his PhD in mechanical
engineering from UIUC in 2010 and continued as a post-doctoral associate. Zhu’s research is focused on mechanics of nanomaterials and their
In the spring of 2012, Sangid started as an assistant professor at Purdue application in stretchable electronics. He has published nearly 50
University in the School of Aeronautics and Astronautics with a courtesy peer-reviewed journal papers and 4 book chapters. Among his notable
appointment in materials engineering, where he continues his work on contributions, Zhu has pioneered in developing microelectromechanical
building computational materials models with experimental validation efforts. systems (MEMS) for in situ electron microscopy mechanical testing of 29
He is a recipient of the TMS Young Leaders Award, the ASME Orr Award, and nanomaterials, which provide electronic measurement of load while
the AFOSR, ONR, and DARPA Young Investigator/Faculty Awards. enabling the simultaneous acquisition of atomic structures. He has made