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Wednesday, June 27, 2007 - 5:00 PM
HTM1.7

Microstructure and Mechanical Properties of Gamma Titanium Aluminide Tiles for High Temperature Structural Applications

N. M. Wereley, K. Kothari, University of Maryland, College Park, College Park, MD; R. Radhakrishnan, T. S. Sudarshan, Materials Modification, Inc., Fairfax, VA

A rapid consolidation process called plasma pressure compaction (P2C) has been used to produce near-net shape parts of γ-TiAl. The P2C process consists of a plasma activation stage, which removes all oxides and other contaminants on the surface of the powders. The powders are rapidly heated to high temperatures and with synergistic application of pressure, high densities are achieved with consolidation times of 20 minutes or less.  One inch diameter discs of γ-TiAl with 0.25 inch thickness have already been consolidated via P2C to produce four different microstructure types with minimal grain growth.  This was done by consolidating the powders in different phase fields of Ti-Al binary system. The different microstructure morphologies, alloy composition and grain size had a significant impact on strength and stiffness.  With the success of P2C in consolidating one-inch discs, γ-TiAl tiles of dimension 4 inches x 4 inches x 0.6 inches were consolidated via P2C.  -325 mesh (<45 µm) γ-TiAl powders of Ti-48%Al-2%Cr-2%Nb (at%) composition were consolidated in the α2 + γ phase field (~1200 C) and pure α phase field (~1400 C)  to generate duplex and fully lamellar microstructures.  Samples with a duplex microstructure and average grain size of 5 μm exhibited the best set of mechanical properties with flexure strength, ductility, elastic modulus and fracture toughness as high as 1238 MPa, 2.3%, 154.58 GPa and 17.95 MPa m1/2, respectively.   Preliminary high temperature four-point bending test results show good flexural strength retention up to 800 C. Exposure in air at temperatures above 800 C, had an adverse effect on the flexure strength of the consolidated samples.   This is being verified by high temperature tensile tests in air and vacuum at temperatures up to 1000 C.

Summary: A novel powder consolidation process called plasma pressure compaction has been used to consolidate gamma titanium aluminide tiles of dimension 4 inches x 4 inches x 0.6 inches. Duplex and fully lamellar microstructures generated in the tiles have been characterized for their mechanical properties at temperatures up to 1000 C. The results show good room temperature ductility with good strength and modulus retention at high temperatures.