Doctoral Dissertations

Date of Award

12-2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

T.G. Nieh

Committee Members

Yanfei Gao, Hahn Choo, Guoxun Chen

Abstract

Recently, concentrated multicomponent alloys, or high-entropy alloys (HEAs) consisting of several principal components in approximately equiatomic proportions, have increasingly attracted research interest because of their unusual intrinsic characteristics, such as severe lattice distortion and sluggish diffusion. These features confer HEAs promising mechanical properties, making HEAs potential for structural applications. However, limited knowledge on intrinsic strength, precipitate stability and tribological behavior of HEAs is available.In Chapters 2 and 3, we systematically analyzed yield strengths of a series of fcc Ni-based and bcc Nb-based equiatomic alloys. By subtracting all possible strengthening contributions, the intrinsic strength (or lattice friction stress) of each alloy was extracted. It was found that lattice friction stress scaled linearly with the lattice distortion in these equiatomic alloys. A simplified model was developed to interpret this result. It was demonstrated that the enhanced strength in HEAs was mainly attributed to the lattice distortion. In Chapter 4, I reported the study of the coarsening of L1₂ precipitates in an fcc (NiCoFeCr)₉₄Ti₂Al₄ HEA. Temporal evolutions of the morphology, size, and volume fraction of these coherent precipitates were examined and evaluated using electron microscopes. Treating the fcc (NiCoFeCr)₉₄Ti₂Al₄ alloy as a pseudo ternary Ni-Ti-Al alloy, I analyzed the coarsening kinetics of the precipitates. One of the major findings was that, owing to the relatively slow atomic diffusion in HEAs, coarsening of L1₂ precipitates in the current HEA was found to be slower than that in the conventional Ni-based alloys. This result demonstrated good thermal stability of the L1₂ precipitates. In Chapter 5, I investigated the tribological behavior of an amorphous Zr₂₀Ti₂₀Cu₂₀Ni₂₀Be₂₀ HEA (a-HEA). Nanoscratch tests were carried out to measure the coefficient of friction (COF) and wear resistance of the amorphous alloy. The morphology of scratched surface and subsurface was further examined using electron microscopes. Due to high hardness and large elastic recovery, the current a-HEA exhibited good wear resistance and low COF, suggesting that a-HEAs are probably good candidate materials for tribological applications.Finally, a conclusion of all my studies was given. In addition, a future perspective based on my research results were presented in Chapter 6.

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