Date of Award

2024-08-01

Degree Name

Doctor of Philosophy

Department

Mechanical Engineering

Advisor(s)

Arturo Bronson

Abstract

The reactivity of liquid Hf-Nb-Ti-Ta alloy in B4C/C packed bed to form boride-carbide precipitates for ultra-high temperature ceramics was investigated at 2800 K. The objectives were to predict the thermodynamic phases formed and verify them with microstructural characterization. A pseudo-isopiestic technique was used to significantly lower the oxygen potential as the liquid Hf alloy infused and reacted into a B4C packed bed within a graphite enclosure. The technique had a low temperature region (at 1300 K) to control the oxygen potential with a Y-Al/Y2O3-Al2O3/carbide mixture as a liquid Hf alloy at 2800 K and reacted with B4C/C. In addition, any oxygen within the graphite enclosure reacted to form CO which transported oxygen from the hot region to the low temperature at which the invariant point of oxide/Y-Al melt/carbide fixed the oxygen potential.

The liquid Hf alloys reacted with the B4C to produce a metal matrix composite of Hf-Nb-Ti alloys with and without Ta with precipitates of primarily Hf-Nb diborides and Hf-Nb-Ti monocarbides. Although the liquid Hf has a significant reactivity with oxygen, limited dissolved oxygen developed within the liquid Hf alloy, because the pseudo-isopiestic technique significantly controlled the oxygen potential. The liquid Hf alloy upon cooling segregates into hafnium rich and niobium rich phases of borides and monocarbides along with graphite precipitation. A calculated Hf-Nb/Ti/B4C ternary system depicting diborides and monocarbides in the midsection surrounded by liquid regions at 2800 K explains the reactivity of the Hf liquid alloys with B4C.

Language

en

Provenance

Received from ProQuest

File Size

93 p.

File Format

application/pdf

Rights Holder

Laura Leticia Sandoval

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