Date of Award

Fall 2021

Project Type

Thesis

Program or Major

Materials Science

Degree Name

Master of Science

First Advisor

Marko MK Knzevic

Second Advisor

Todd TG Gross

Third Advisor

James JK Krzanowski

Abstract

This research explores the effects of processing history and compressive strain on a microstructurally flexible high entropy alloy (HEA), Fe42Mn28Co10Cr15Si5 (in at%) – named DP-5Si-HEA. The material was characterized by electron backscatter diffraction (EBSD), scanning electron microscopy (SEM), and neutron diffraction. The as-rolled material had metastable gamma austenite (γ), stable sigma (σ), and stable epsilon martensite (ε) phases at room temperature. Friction stirrer processing (FSP) at different tool speeds changed the microstructure by decreasing the grain size and varying the phase fractions because of the low stacking fault energy of the chosen HEA. The HEA exhibited peak ultimate tensile strength of ~1850 MPa because of refined microstructure, transformation induced plasticity (TRIP) because of γ→ε phase transformation, and transformation-induced Hall-Petch-type barrier effect. Furthermore, an investigation on the effect of reusing powder on compressive, tensile, and creep properties of Inconel 718 (IN 718) fabricated by laser powder bed fusion (LPBF) was carried out. To achieve that, room, and high temperature (6000C, 6500C,7000C, 7500C) tensile tests, room temperature compression tests, and high temperature (5500C, 6500C, 7000C) tensile creep tests for samples prepared by both virgin as well as a reused powder in LPBF were performed. The study showed the reused powder had higher strength at room and high-temperature tensile tests except for 6500C. The virgin powder lasted longer at 550C and 650C creep tests, but the reused powder lasted longer at 700C. It was concluded in this study that the parts made by virgin powder samples should be used for temperatures at creep conditions until 650C for real-life applications. Moreover, finite element analyses were carried out on micro single edge notched bend (MSENB) specimens to obtain fracture toughness of Cu/TiN and Al/TiN nanocomposites within the realm of Linear Elastic Fracture Mechanics. The shape function or geometric factor was first calculated by considering the effects of the geometry, the cono-spherical indenter stress field, and the phase interface shielding/anti-shielding on the crack tip driving force. It was found that adding either Al or Cu nanolayers to TiN nanolayers did not weaken or significantly improve fracture toughness relative to the single TiN thin film. Based on the results, some recommendations were also made for future experiments.

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