The influence of mixed hexagonal close-packed and face-centered cubic ordering on precipitation pathways and kinetics in sputtered superalloys

Superalloys, Precipitates, Nanotwins, Aging, Sputter Deposition

Emigh, Megan Genevieve

关键词: Superalloys, Precipitates, Nanotwins, Aging, Sputter Deposition;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/102442/EMIGH-DISSERTATION-2018.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Superalloys have often been used for applications where a metal that can withstand high stresses, retain high strength at high temperatures, and resist oxidization is desirable.Such applications include parts in the hot zone of jet engines, exposed machine components during highly reactive processing techniques such as fluoroelastomer extrusion, and industrial uses such as heat exchanger tubing or gas turbines. Recently, exploration into small-scale devices known as microelectromechanical systems (MEMS) have shown that there is a dearth of known materials for these devices when properties such as electrical conductivity or toughness are required, concomitant with exposure to high temperatures and/or reactive environments. The lack of known suitable materials is a result of the MEMS industry typically relying on well-studied silicon-based materials such as silicon nitride, silicon carbide, or polysilicon. Therefore, superalloys have been recently proposed to complement emerging MEMS applications as such devices continue to be pushed into novel design spaces.This present examination will show that using a standard MEMS fabrication technique, direct current magnetron sputtering (DCMS), on a superalloy can cause precipitation behavior that is inconsistent with bulk behavior. The rich planar defects present in the DCMS Haynes® 242™ used in this investigation caused the material to contain both face-centered cubic (FCC) and, unexpectedly, significant amounts of hexagonal close-packed (HCP) domains. This mixed close-packed crystal structure caused the Ni-Mo-Cr alloy Haynes® 242™ to directly precipitate the stable HCP-based DOa-type Ni3Mo phase during a standard aging heat treatment. This precipitation entirely bypassed the normally expected metastable FCC-based Ni2(Mo,Cr) phase. This finding was corroborated using multiple examination techniques such as x-ray diffraction, transmission electron microscopy (TEM), in situ TEM, and high-resolution scanning transmission electron microscopy. Despite the departure from normal aging behavior, it was determined that when planar defects were eliminated with a solution anneal and a complete FCC solid solution was recovered, the material exhibited normal precipitation behavior upon subsequent heat treatments. Stemming from these results, it is evident that careful scrutiny of the effects of non-standard processing techniques on superalloys must be employed prior to their introduction to MEMS. On the other hand, these findings may portend an increased design space centered on as-yet undiscovered HCP-based ordering in superalloys that are normally exclusively FCC-type. This postulation arises from the close link between the starting microstructure and subsequent precipitation. In addition to careful scrutiny of HCP and FCC-based precipitation in sputtered Haynes® 242™, this thesis will also examine mass transport in sputtered metals via DCMS diffusion couples. Finally, recommendations on appropriate heat treatments for DCMS superalloys will be proposed in order to ensure such materials exhibit desired precipitation behavior. These recommendations focus on careful control of key microstructural defects that are native to the as-deposited DCMS superalloy films.