Date of Award

2022-05-01

Degree Name

Doctor of Philosophy

Department

Metal And Material Engineering

Advisor(s)

Ramana V. Chintalapalle

Abstract

High entropy alloys (HEAs) have emerged as an exciting class of materials with the potential for a wide range of structural, high-temperature applications. HEAs may be defined on a compositional basis as five or more principal elements in equimolar ratios. This definition led to the term “high entropy” due to the high configurational entropy which is obtained with principal elements in these amounts. The combination of elements has contributed to materials that may favor solid-solution phases over intermetallic phases and have the potential to possess highly desired material properties including high yield and creep strength. The inclusion of refractory metals into these alloys further expands their application potential. In fact, several refractory high entropy alloys (RHEAs) have been explored in recent years. The MoNbTaW RHEA has been one of the first and most significant ones in this family. While current interests span various compositions, the attention is focused on CrNbTaVW and CrMoNbTaW RHEAs for their application in extreme environments. The CrMoNbTa in the as-received condition processed through vacuum arc melting was found to be a multiphase microstructure containing three phases. The microstructure was assessed after a 6-hour anneal in the temperature range of 600-9000C for which it was thermally stable. The oxidation resistance was evaluated in the temperature range of 600-14000C for both 12 and 24 hours, obtaining a moderate oxidation resistance. The surface oxidation scale was found to consist of two main oxides of Cr-Nb-Ta and Ta-Nb-W which become more defined at high temperatures. Atomistic simulations on the dislocation behavior for this alloy family were also performed. The stress required to move a dislocation improved in HEA compositions as compared to the pure metals. It was found that Cr-containing compositions obtain higher strength values which were believed to be influenced by the increase in lattice distortion. Long dislocations of pure metals and HEA compositions were evaluated in both a void-free and void-containing lattice. The presence of an obstacle such as a void improved the strengthening of pure metals and HEAs. The combined experimental and theoretical results of the present work provide fundamental insights into these RHEAs to design advanced materials for high-temperature applications.

Language

en

Provenance

Received from ProQuest

File Size

123 p.

File Format

application/pdf

Rights Holder

Rebecca Alexandra Romero

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