Date of Award

3-2023

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Department of Engineering Physics

First Advisor

Larry W. Burggraf, PhD

Abstract

Consolidation of pure molybdenum through laser powder bed fusion and other additive manufacturing techniques is complicated by a high melting temperature, thermal conductivity, and ductile-to-brittle transition temperatures. Nano-sized silicon carbide particles (0.1 wt%) were homogeneously mixed with molybdenum powder and the printing characteristics, chemical composition, microstructure, and mechanical properties were compared to pure molybdenum for scan speeds of 100, 200, 400, and 800 mm/s. The addition of silicon carbide improved the consolidation and mechanical properties and the oxygen content was reduced. Two mechanisms of oxygen reduction were identified as responsible for the improvements: oxidation of free carbon and the creation of secondary-phase nanoparticles. Positron annihilation lifetime spectroscopy (PALS) and Doppler-broadening of annihilation radiation were used to monitor the microstructural evolution and compared with electron back scatter diffraction analysis. The grain size and misorientation results do not correlate with positron lifetimes indicating the positrons sample sub-grain regions. PALS identified the presence of dislocations and micro-voids not revealed through electron microscopy techniques and correlated with the findings of secondary-phase nanoparticles in the Mo-0.1SiC specimens. This dissertation produced the first proof-of-concept for the possibility of in-situ positron-based, defect monitoring and diagnostic tool.

AFIT Designator

AFIT-ENP-DS-23-M-084

Comments

A 12-month embargo was observed.

Approved for public release. PA case number on file.

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