Titanium aluminides based on the g-phase
(TiAl) offer the potential for component weight savings of up
to 50 percent over conventional superalloys in 600 to 850 °C aerospace
applications (ref. 1). Extensive development efforts over the
past 10 years have led to the identification of "engineering"
g-alloys, which offer a balance of
room-temperature mechanical properties and high-temperature strength
retention (ref. 1). The g class of
titanium aluminides also offers oxidation and interstitial (oxygen
and nitrogen) embrittlement resistance superior to that of the
a2 (Ti3Al) and
orthorhombic (Ti2AlNb) classes of titanium aluminides.
However, environmental durability is still a concern, especially
at temperatures above 750 to 800 °C. Recent work at the NASA
Lewis Research Center led to the development of an oxidation-resistant
coating alloy that shows great promise for the protection of g
Aluminizing treatments, conventional MCrAlY (M = Ni or Fe) coatings, and ceramic oxidation-resistant coatings for g-based titanium aluminides have not proven successful because of poor mechanical properties, thermal expansion mismatch, and chemical incompatibility. Promising coating alloys have been identified in the Ti-Al-Cr system (ref. 2). These alloys exhibit excellent oxidation resistance and are generally compatible with the g substrate alloys; however, they are brittle (ref. 2).
A Ti-Al-Cr oxidation-resistant coating alloy recently developed at NASA Lewis offers excellent substrate compatibility and some improvement in mechanical properties, without sacrificing oxidation resistance (refs. 3 and 4). The alloy composition, Ti-51Al-12Cr (in atomic percent), was selected so that the microstructure consists of the g-phase and a minor volume of the oxidation-resistant Ti(Cr,Al)2 Laves phase. By basing the coating alloy on the g-phase, we can optimize the mechanical properties and substrate compatibility. The volume fraction of the Laves phase is kept to a minimum because it is extremely brittle.
The Ti-51Al-12Cr coating alloy was applied to the General Electric
g-alloy, Ti-48Al-2Cr-2Nb (in atomic
percent), by low-pressure plasma spray. Oxidation tests at 800
and 1000 °C in air indicated that the coating alloy successfully
protected the substrate from oxidation (see the figure). Evaluation
of the isothermal fatigue behavior of coated Ti-48Al-2Cr-2Nb at
elevated temperatures in air is in progress.
Table of Contents
Last updated April 30, 1997
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