MS Thesis Presentation by Carrell E. Weeks
Monday, April 11, 2005
(Dr. W. Steven Johnson, Chair)
"Evaluation of a Gamma Titanium Aluminide for Hypersonic Structural Applications"
Titanium matrix composites have been extensively evaluated for their potential to replace conventional superalloys in high temperature structural applications, with significant weight-savings while maintaining comparable mechanical properties. The purpose of this investigation is the evaluation of a gamma titanium aluminide alloy with nominal composition Ti-46.5Al-4(Cr,Nb,Ta,B)at.% as a matrix material for use in intermediate temperature applications (400-800°C) in future aerospace transportation systems, as very light-weight structures are needed for cost and weight reduction goals.
Mechanical characterization testing was performed over the potential usable temperature range (21-800 ° C). Thermal expansion behavior was evaluated, as thermal mismatch of the constituents is an expected problem in composites employing this matrix material. Monotonic testing was conducted on rolled sheet material samples to obtain material properties. The alloy exhibited good strength and stiffness retention at elevated temperatures, as well as improved toughness. Monotonic testing was also conducted on specimens exposed to elevated temperatures to determine the degradation effects of high temperature exposure and oxidation. The exposure did not significantly degrade the alloy properties at elevated temperatures; however, room temperature ductility decreased.
Analytical modeling using AGLPLY software was conducted to predict the residual stress state after composite consolidation as well as the potential mechanical behavior of  4 laminates with a ?-MET matrix. Silicon carbide (Ultra-SCS) and alumina (Nextel 610) fibers were selected as potential reinforcing materials for the analysis. High residual stresses were predicted due to the thermal mismatch in the materials. Laminates with Nextel 610 fibers were found to offer the better potential for a composite in this comparison as they provide a better thermal match. Coupons of SCS-6/?-MET were manufactured with different volume fractions (10% and 20%). Both manufacturing attempts resulted in transverse cracking in the matrix from the residual thermal stress.