Use of a MEMS-Based TEM Heating Stage to Characterize Carbide Precipitation in Alloy 690

Jonas K. Heuer
2018 Microscopy and Microanalysis  
Heating stages have been used for several years to study a number of dynamic metallurgical phenomena in metals and alloys directly in electron microscopes to help visualize processes and determine mechanisms associated with exposure to elevated temperatures. Some applicable phenomena that can be studied with this in situ technique include phase transformations, precipitate formation, grain growth, quenching behavior, and recovery of deformation structures. This study provides an initial
more » ... on of the use of a heating stage based on Micro-Electrical-Mechanical Systems (MEMS) technology. Unlike traditional furnace-based heating stages for Transmission Electron Microscopes (TEMs), MEMS-based heating stages offer higher operating temperatures, provide faster heating/cooling rates, and have reduced thermal drift. An Alloy 690 (A690) specimen was chosen to demonstrate the capabilities of this heating stage. A690 is an austenitic nickel-based alloy commonly used as a steam generator tubing material in pressurized water reactors due to its excellent resistance to intergranular stress corrosion cracking in a broad range of aqueous environments [1-3]. The A690 material used in this examination originated from an inprocess sample of heavily cold-worked material prior to final annealing. Therefore, the material is expected to have a heavily deformed microstructure and contain a matrix that is supersaturated with carbon based on being solution-annealed prior to cold working. This microstructure makes it a good specimen to evaluate the heating stage because it allows a number of temperature-based phenomena to be examined. At lower temperatures (≤ 800°C), these phenomena include recovery of the deformation structure, precipitation of carbides from the supersaturated matrix, and recrystallization and growth of the matrix grains. At higher temperatures (~1100°C), dissolution of the precipitated carbides is expected during solution annealing with potential re-precipitation at sufficiently-slow cooling rates. In situ thermal examinations were performed using a Protochips Fusion heating stage in an FEI Tecnai F20 TEM. This TEM is equipped with a high angle annular dark field detector, a secondary electron detector, and an Oxford Instruments X-Max100-TLE windowless silicon drift detector for energy dispersive X-ray spectroscopy. Specimens were prepared for the heating studies using a two-step process. The first step involved preparing conventional TEM foils by electropolishing 3 mm discs at -32°C and 14-16 V in an electrolyte composed of 20% perchloric acid and 80% methanol, and the second step involved extracting 40×40 µm regions of electron-transparent material around the central perforations in these foils using a Focused Ion Beam (FIB) instrument for mounting onto the heating membranes of MEMS substrates. Each sample was heated from 20°C up to 800-900°C at rates of 1-5°C/sec. Ramping was paused at intervals of 100-300°C where the temperature was held constant for 5-10 minutes so that images could be acquired of the microstructure at each hold temperature. After reaching the ultimate ramping temperature, each sample was cooled from 800-900°C down to 20°C at a rate of 7-8°C/sec, and additional imaging and analysis was subsequently performed at room temperature. The entire heating trial on each sample lasted approximately 45 minutes.
doi:10.1017/s1431927618009959 fatcat:tvbknbhfczbe7bgdfxivorye74