Non-Schmid slip behavior in shape memory alloys
 
by Sertan Alkan
 
(University of Illinois at Urbana-Champaign, Department of Mechanical Science and Engineering)
 
DATE : November 17, 2017 (Friday)
TIME : 13:00-14:00
ROOM : VYKM 2
 
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The plastic deformation mechanisms degrading the functional properties of ordered shape memory alloys will be discussed. In particular, tension-compression slip asymmetries and anisotropic glide resistances will be interrogated on both experimental and theoretical grounds for NiTi alloy. The interplay between the atomistic scale dislocation core displacements and the applied stress tensor components will be demonstrated to play a decisive role in the deviations from the critical resolved shear stress rule, also known as non-Schmid effects.  
The theoretical predictions will be compared with the experimental glide resistance measurements on single crystals within the framework of high magnification in-situ Digital Image Correlation (DIC) technique. Physical insights from the electronic structure will be provided to build a comprehensive understanding on the underlying mechanisms for non-Schmid behavior. The theoretical and experimental anisotropic glide resistance levels will bridged to the macro-scale crystal plasticity models by generating generalized yield surfaces which can embrace the dislocation core - applied stress tensor interactions.
 
 
Short Bio: Sertan Alkan received B.S. (2010) and M.S. (2013) diplomas from Department of Mechanical Engineering at Bogazici University. He is currently a PhD. student in Mechanical Science and Engineering Department at University of Illinois at Urbana-Champaign. During his M.S., he worked on modelling mechanical response of edge cracks in shape memory alloys particularly focusing on the martensitic transformation induced toughening. His PhD. studies involve characterization of fatigue crack growth behavior in nanotwinned Ni-Co alloys via Digital Image Correlation (DIC) technique and establishing a multiscale (continuum and atomistic) theoretical model encompassing the interaction of the crack-tip emitted dislocations with the grain and twin boundaries. Currently, his research mainly focuses on characterization of the slip mediated plasticity and twinning in shape memory alloys and high entropy alloys via DIC technique and atomistic scale simulations within the framework of Density Functional Theory and Molecular Dynamics/Statics.
 

Rechargeable Next-Generation Magnesium/Oxygen Batteries
 
 by Gülin Vardar
 
(Massachusetts Institute of Technology, Nuclear Science and Engineering)
 
DATE : November 25, 2016 (Friday)
TIME : 11:00-12:00
ROOM : VYKM 2
 
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Electrochemical energy storage devices that are robust, energy-dense, and cheap will accelerate the commercialization of electric vehicles.  
Magnesium/Oxygen (Mg/O2) batteries are a promising system with the potential for very high energy densities. Furthermore, a rechargeable
Mg/O2 battery could be a cheaper and potentially safer alternative to lithium Li-ion batteries currently in use. The goal of this talk is to explore candidate magnesium electrolytes for use in Mg/O2 batteries,  
and to assess the reaction mechanisms and performance of Mg/O2 cells   
that employ these electrolytes.
 
Short Bio: Gülin Vardar received B.S. diplomas from Boğaziçi University in Mechanical Engineering and Physics in 2010. She received M.S. and PhD. diplomas from the University of Michigan (Ann Arbor) in Materials Science and Engineering. She is currently a postdoctoral research associate in Massachusetts Institute of Technology.
 

Electrothermal Modeling of AlGaN/GaN Heterostructure Field Effect Transistors
 
by Nazlı Dönmezer
 
(Middle East Technical University, Department of Mechanical Engineering)
 
DATE : October 7, 2016 (Friday)
TIME : 14:00-15:00
ROOM : VYKM 2
 
Nitride-based semiconductors and materials have been promising candidates for wide variety of technological applications such as nitride based power electronics, satellite communication, and light    emitting diodes. AlGaN/GaN based Heterostructure Field Effect Transistors (HFETs), that are used in high power and frequency applications have been intensively used due to their high-efficiency    power switching and large current handling capabilities. In these devices the high power densities and localized heating form small, high temperature regions called hotspots. Analysis of the heat removal from hotspots and temperature control of the entire device is necessary for the reliable design of HFET devices. Due to the resolution limits of the current experimental characterization    techniques and the geometry of the device that limits the accurate temperature measurement, thermal simulations are necessary. The aim is to build an accurate yet efficient electro-thermal model for the analysis and improvement of HFETs.
 
 
Short Bio: Dr. Nazli Donmezer is an Assistant Professor in the Mechanical Engineering Department of Middle East Technical University. She received her PhD. from Woodruff School of Mechanical Engineering   at the Georgia Institute of Technology and her M.S. from Middle East   Technical University in 2013 and 2009 respectively. During her PhD  she  worked on the development of a multiscale model  to simulate the   thermal response of devices with nanometer sized hotspots under the   supervision of Dr. Samuel Graham. She was a recipient of the   Schlumberger "Faculty for the Future" (FFTF) scholarship during her   PhD. studies. Dr. Donmezer joined the faculty at METU in Fall 2014.   She is currently leading a research group  where the goal is to   characterize the electro-thermal behavior of  the nitride devices and materials.