Date of Completion

11-27-2018

Embargo Period

11-26-2019

Keywords

In situ TEM, In situ Heating, MEMS, Specimen Preparation, Temperature Calibration, Phase Transformations, Gas Atomized Powders

Major Advisor

Dr. Mark Aindow

Associate Advisor

Dr. C. Barry Carter

Associate Advisor

Dr. Seok-Woo Lee

Field of Study

Materials Science and Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

In situ heating holders offer the possibility of studying thermally activated processes by performing real time, high temperature experiments inside the transmission electron microscope. The poor thermal stability of traditional furnace-type heating holders limits their use to a narrow range of materials and processes. Modern micro electro mechanical system (MEMS) based heating holders have significantly improved the ability to perform such experiments and have led to a revival in the field of in situ TEM. The excellent thermal stability of the MEMS devices allows us to carry out controlled heating and cooling experiments on both particulate and bulk samples at high spatial resolution. Despite these advantages, there are several practical challenges to the use of MEMS-based heating holders. In this dissertation, two important issues that impede the reliable interpretation of data from MEMS-based in situ heating experiments are addressed: measurement of specimen temperature, and preparation/transfer of site-selective specimens from bulk samples. It is shown that the specimen temperature can be obtained from the size-dependent sublimation behavior of monodisperse polyvinyl pyrrolidone capped Ag-nanocubes using the Kelvin equation. This approach gives the temperature of the microheater membrane to an accuracy of ±5 ⁰C, and a systematic evaluation of the different potential sources of error is presented. Next, a protocol is described for using a dual-beam focused ion beam - scanning electron microscope (FIB-SEM) to perform site selective specimen preparation and transfer onto a MEMS microheater. The critical features of this protocol are the specimen geometry and a custom FIB-SEM sample stage that minimizes ion beam exposure during the procedure. This approach is then used to prepare cross-sectional specimens from gas-atomized powder particles of three Al-alloys; solid solution strengthened Al-Mg, precipitation hardenable Al-Mg-Si, and an Al-Mn-Cr-Co-Zr alloy, which contains icosahedral quasicrystalline dispersoids. In situ scanning transmission electron microscopy heating experiments on these samples revealed a wide variety of thermally activated processes such as: solute redistribution to eliminate micro-segregation; dissolution, coarsening, transformation and decomposition of secondary phases; and precipitation within the aluminum matrix.

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