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Gerstl, Stephan S. A. |
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Three-Dimensional Nanometer Scale Analyses of Precipitate Structures and Local Compositions in TiAl Engineering Alloys |
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2006 |
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231 |
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Titanium aluminide (TiAl) alloys are among the fastest developing class of
materials for use in high temperature structural applications. Their low density
and high strength make them excellent candidates for both engine and airframe
applications. Creep properties of TiAl alloys, however, have been a limiting factor
in applying the material to a larger commercial market.
In this research, nanometer scale compositional and structural analyses of
several TiAl alloys, ranging from model Ti-Al-C ternary alloys to putative
commercial alloys with 10 components are investigated utilizing threedimensional
atom probe (3DAP) and transmission electron microscopies.
Nanometer sized borides, silicides, and carbide precipitates are involved in
strengthening TiAl alloys, however, chemical partitioning measurements reveal
oxygen concentrations up to 14 at. % within the precipitate phases, resulting in the
realization of oxycarbide formation contributing to the precipitation strengthening
of TiAl alloys.
iv
The local compositions of lamellar microstructures and a variety of
precipitates in the TiAl system, including boride, silicide, binary carbides, and
intermetallic carbides are investigated. Chemical partitioning of the microalloying
elements between the a2/g lamellar phases, and the precipitate/g–matrix phases are
determined. Both W and Hf have been shown to exhibit a near interfacial excess
of 0.26 and 0.35 atoms nm-2 respectively within ca. 7 nm of lamellar interfaces in a
complex TiAl alloy. In the case of needle-shaped perovskite Ti3AlC carbide
precipitates, periodic domain boundaries are observed 5.3±0.8 nm apart along their
growth axis parallel to the TiAl[001] crystallographic direction with concomitant
composition variations after 24 hrs. at 800°C. |
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Northwestern University |
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Ph.D. thesis |
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