Date of Award

8-2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Taigang Nieh

Committee Members

Yanfei Gao, Peter K. Liaw, Dayakar Penumadu

Abstract

A polycrystalline, face-centered cubic high entropy alloy (HEA) FeCoCrMnNi was investigated by nanoindentation and stress relaxation. A literature review on HEAs was presented in Chapter 1 and a description of methodology was followed in Chapter 2.

In Chapter 3, the nature of incipient plasticity was characterized with instrumented indentations over various loading rates and temperatures. The maximum shear stress for initiating the incipient plasticity attained the theoretical strength and was relatively insensitive to grain orientation. However, it strongly depended upon the temperature, indicating a thermally activated nucleation process. The estimated activation energy and volume signify that the nucleation is mediated by point defect.

Chapter 4 deals with the time dependence of the incipient plasticity. By holding the sample at a constant stress lower than that prompting instantaneous pop-in, pop-in is observed after a period of delay during which creep occurred. The creep suggested that the dislocation loop in equilibrium with the holding load expanded to the critical radius by vacancy-controlled dislocation climb due to the chemical potential gradient of the vacancy from beneath the stressed region to the free surface.

Chapter 5 discusses the size dependence of the pop-in. Six tip radii were selected to perform the nanoindentation: 50, 200, 233, 467, 638, 743 and 2013nm [nanometer]. At small tip radius, the pop‑in size is linearly proportional. However, it disappeared with a 50nm tip and approached a constant when the tip radius exceeds 638nm. A model on the basis of the competition between image force and volume dependence on radius was proposed to explain these features.

Chapter 6 turns to mechanistic investigation of the global deformation using stress relaxation from 293 to 1073K [Kelvin]. The alloy retains work hardening capability up to 1073K and the stress reduction decreases with the increasing temperature. The activation volume increases from 12b3 [Burges Vector cubed] at 873K to 62b3 at 1073K and the activation enthalpy from 0.2eV [electronic volt] at 673K and 0.9eV at 1073K. These values plus the Portevin-Le Chatelier effect within the testing temperatures indicate that the impurity may play an important role during high-temperature deformation.

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