Date of Award

6-2007

Document Type

Thesis

Degree Name

Master of Science in Aeronautical Engineering

Department

Department of Aeronautics and Astronautics

First Advisor

Shankar Mall, PhD

Abstract

Research on boron-enhanced Ti-6-4 has demonstrated the following improvements to the Ti-6-4 alloy: up to 40% increase in ultimate tensile strength, up to 30% increase in modulus/stiffness, while maintaining greater than 10% ductility at RT conditions. The increased properties are attributed to small additions of boron (-or= wt%), which refine the microstructure and result in a small volume fraction (~6 vol%) of fine TiB whiskers. Previous research indicates potential for substantial improvements in fatigue, fatigue crack growth, and fracture toughness. However, uncertainty regarding these second-tier mechanical properties is currently limiting implementation of this class of titanium alloys. This study of fatigue variability of a powder-metallurgy, boron-enhanced Ti-6-4 alloy identifies the most prevalent damage mechanism and elucidates the impact on fatigue design limits. The alloy was produced via a unique prealloyed powder-metallurgy process. The powder mesh size used was -35, which equates to powder particles with a diameter of 500 µm and smaller. Specimens were ultimately machined from a rolled plate. The mean fatigue behavior compared favorably with available data on conventional Ti-6-4, both wrought and powder-metallurgy product forms. However, inclusions in the material were responsible for a few poor fatigue results, which ultimately govern the fatigue design limits. Variability assessment and fatigue crack growth analyses indicate that if the frequency and size of inclusions can be reduced, this material could become a more viable alternative for select turbine engine and aircraft applications.

AFIT Designator

AFIT-GAE-ENY-07-J24

DTIC Accession Number

ADA470054

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