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Materials Science and Engineering Seminar
Dr. Sriram Vijayan
Department of Materials Science & Engineering
The Ohio State University
Materials fabricated using fusion based additive manufacturing (AM) processes or multi pass welding processes will experience spatially varying temporal and thermal transients due to the localized high energy density delivered by the heat source. Similarly, components used in critical engineering applications experience varying thermal transients when "in service," e.g. in next generation nuclear reactor cores, gas turbine engines, re-entry space vehicles and solder joints in micro-electronic packages. These varying thermal transients (a combination of extreme thermal gradients (104 - 106 K/m) and/or rapid thermal cycling (102- 103 K/s)) result in metastable microstructures, which can significantly impact part performance. In order to improve the performance of a part in the "as fabricated" condition or prevent the deterioration of a part "in service," the dynamic processes facilitating the microstructural evolution in the solid state, under such extreme thermal conditions need to be understood.
Currently, this information can only be obtained through post-mortem characterization of the sample obtained from the part, in the electron microscope (EM). Previously, micro electro mechanical systems (MEMS) based in situ transmission electron microscopy (TEM) heating stages have been used for the study of thermally activated phenomena at high spatial resolution, under isothermal conditions. However, to observe these dynamic processes under "far from equilibrium" conditions at high spatial resolution, the development of new MEMS-based in situ TEM heating stages that mimic processing and/or "in service" conditions are needed. My current research is focused at utilizing MEMS-based in situ heating devices to replicate extreme thermal conditions of AM inside the TEM. This was achieved by modifying a commercially available MEMS micro heater device using a focused ion beam scanning electron microscope. This simple modification enabled us to generate extreme thermal gradients across a TEM lamella. To validate these thermal gradients, infra-red thermography and "Ag nanocube sublimation" techniques were used to measure temperature distribution across this modified device and the FIB cut lamella respectively. The ex situ and in situ temperature measurements confirmed that a large thermal gradient (~106 K/m) could be generated across a lamella inside the TEM. Additionally, the MEMS micro heaters allow for rapid heating and cooling rates of ~ 103 K/s, by combining these capabilities, the modified MEMS device is used to mimic processing and/or "in service" like conditions inside the TEM. In this presentation, the results from in situ TEM experiments that mimic metal AM process conditions in "previously melted" layers of electron beam melted
Ti-6Al-4V using the modified MEMS device will be presented, and the dynamic processes responsible for the solid-state transformations in Ti-6Al-4V will be discussed. Furthermore, the challenges associated with MEMS-based in situ heating experiments and other potential applications of this modified MEMS device will be critically analyzed.
Sriram Vijayan is a Research Associate in the Department of Materials Science & Engineering at The Ohio State University. He completed his PhD in Material Science and Engineering from the University of Connecticut (UConn). Prior to graduating from UConn, Sriram obtained his Master’s degree in Metallurgical & Materials Engineering from the Indian Institute of Technology-Madras, Chennai and Bachelor’s degree in Metallurgy and Materials Technology from the Jawaharlal Nehru Technological University, Hyderabad, India. His research at the Center for Electron Microscopy and Analysis (CEMAS) focuses on using micro electro-mechanical systems (MEMS) based in situ TEM tools to understand thermally activated phenomena observed in materials under extreme conditions. He is leading the characterization efforts of the OSU team for a Multi University Research Initiative (MURI) funded by Office of Naval Research, which explores the fundamental aspects of additive manufacturing (AM). His other research interests include understanding microstructural evolution in metallic nuclear fuels under in reactor thermal conditions and developing high throughput characterization workflows for AM builds. Previously he has developed techniques such as; a site-specific specimen preparation approach for MEMS based in situ devices, and a non-contact technique to measure the temperature of a sample inside the TEM. These techniques are critical to perform reliable and reproducible MEMS-based in situ TEM heating experiments. The specimen preparation and transfer protocol developed for MEMS based devices has been widely appreciated by the microscopy community and won him the best poster award at the Microscopy and Microanalysis meeting held in St Louis, MO in 2017. He is an active member of The Minerals, Metals and Materials Society (TMS) and Microscopy Society of America (MSA). He is also actively involved in co-organizing symposia at TMS and MS&T annual meetings.
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