Doctoral Dissertations

Date of Award

5-2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Tai-Gang Nieh

Committee Members

Yanfei Gao, Hahn Choo, Guoxun Chen

Abstract

The objective of this dissertation is to provide a fundamental understanding of mechanical behavior of body-centered cubic (BCC) high-entropy alloys (HEAs). Incipient plasticity in metallic alloys is of great interest to the scientific community. In my research, I selected a NbTiZrHf-HEA for the initial study using nanoindentation. The critical shear stress was found to approach the theoretical strength, indicating a dislocation nucleation-controlled process. The activation volume was much larger than that for pure metals, suggesting cooperative migration of multiple atoms. The activation energy was also found to be higher than that in a typical face-centered cubic HEA, which is attributable to the nucleation of full dislocations in the former but partial dislocations in the latter.In view of the pronounced effect of interstitials on the strengthening of traditional BCC-metals, oxygen or nitrogen was further added into the BCC-HEA to study their effect on the incipient plasticity. Both interstitial elements increased the critical stress, and nitrogen produced a larger hardening effect than oxygen. The calculated activation volumes were similar in magnitude, implying a similar aforementioned mechanism governing the dislocation nucleation in the studied HEAs. First-principle calculations revealed that the presence of interstitials induced charge transfer between the interstitial and its surrounding metals, resulting in the increased critical stresses for incipient plasticity. I further investigated the incipient yielding under dynamic loading via nanoscratching and found that it occurred at lower stresses compared to the static loading during nanoindentation. Concurrently, tribological behavior of the HEA was studied and compared with its conventional counterparts using nanoscratch tests. The HEA exhibited improved wear resistance and reduced friction coefficient. Wear resistance scaled linearly with hardness, but friction behavior exhibited elastic and plastic regimes. The reduced friction coefficient was discussed in lights of the acting plowing and adhesion mechanisms during nanoscratching.Elastic properties of the HEA were also evaluated via both experiments and first-principle calculations, and their results agreed excellently. Additionally, rule-of-mixtures based on lower-bound prediction could be conveniently used to reasonably estimate elastic moduli of single-phase HEAs.Finally, I also made several suggestions of possible future research topics based on the above results and my experiences.

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