Date of Award

Summer 8-5-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering

Department

Doctoral Studies

First Advisor

Borkar, Tushar

Second Advisor

Schwam, David

Third Advisor

Fodor, Petru

Subject Headings

Materials Science, Mechanical Engineering

Abstract

High-entropy alloys (HEAs) are a novel family of solid-solution alloys that have gained international interest due to their exceptional characteristics. Because of the need from the transportation and defense sectors, lightweight HEAs have attracted researcher’s curiosity as prospective advanced materials. Low-weight high entropy alloy synthesizes using arc melting with a mass ratio of AlCrFeMnTix(0.1,0.15,0.2). The synthesized HEA is comprised of a mixture of body center cubic (bcc) and ordered bcc (L21) solid solution phases. The synthesized HEAs have heat treated at 650C, 800C, and 1150C for 1hr, 4hr after solutionized at 1150C for 2 hr to understand the effect of temperature evaluation in these HEAs. To investigate the role of titanium in improving the strengthening of Al1.5CrFeMnTix(x=0.1,0.15,0.2) and tailor the mechanical properties via studying strengthening precipitation mechanisms. The results show that the density of the alloy is 5.88 g cm-3, which fulfills the criteria of low-weight HEA; also, from the XRD, the BCC+ L21 phases are available inside the HEA Composition matrix. Ultra-high temperature ceramics (UHTCs) consist of different ceramics like boride, carbide, diboride, and nitride of the transition metals. The UHTCs show prominent characteristics such as higher melting point, higher oxidation resistance, lower thermal conductivity, higher yield strength, and microhardness. High entropy ceramic (HEC) is a multi-component alloy similar to the high entropy alloy (HEA) specifically developed for hypersonic vehicles, nuclear reactors, and high temperature applications. In the current study, mechanical alloying(MA) and spark plasma sintering(SPS) were used to develop equiatomic HEC (Hf0.2Nb0.2Ta0.2Ti0.2Zr0.2)N. The HECs were mechanically alloyed using the high-energy ball mill at 500 RPM for 6 hours. The ball to powder (BPR) ratio of 10:1 was maintained during the milling. This mechanically alloyed powder was sintered using SPS for 1800°C, 1900°C, and 2000°C to investigate the effect of temperature on the densification of these HECs. The consolidated samples were further analyzed using SEM, microhardness, XRD, and wear testing to understand the effect of different sintering temperatures on microstructure, phase transformation, and mechanical and tribological behavior of these high entropy nitride ceramics. Oxidation treatments have been performed at 1000°C-1200°C for 2 hours duration.

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