Aluminizing for TiAl turbocharger rotor by reduced pressure metal-organic chemical vapor deposition (RPMOCVD) and the ef
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Haruki Hino Kawasaki Heavy Industries Ltd., 1-1 Kawasaki, Akashi, Hyogo 673, Japan (Received 22 October 1993; accepted 24 March 1994)
A complex-shaped TiAl turbine rotor has been uniformly aluminized by a metal-organic chemical vapor deposition under reduced pressure (RPMOCVD), and a thick TiAl3 layer, which affects the oxidation resistance, can be formed on the surface by subsequent heat treatment. The oxidation resistance has been studied with an oxidation test at 1173 K for 760 ks in static air. The microstructure has been investigated by SEM, EPMA, AES, and XRD. A heat treatment at above 933 K, which is the melting point of Al, is required to enhance the oxidation resistance of TiAl. With increasing the surface roughness of TiAl, the formation of TiAl3 increases, and consequently the oxidation resistance is more improved.
I. INTRODUCTION Titanium aluminide (y-TiAl) is superior to the specific strength (strength/density) and high temperature strength up to around 1000 K. It could be a candidate for materials in an aircraft and/or automobile1 in which less weight and an improved response are required. The titanium aluminide, however, will be inferior to the oxidation resistance at high temperatures when it is compared with a nickel-base superalloy, such as Inconel 713C. The temperature of the exhausted gas from a gasoline turbine reaches up to 1000 K, which is 100-200 K higher than that from a diesel turbine. Any improvement, therefore, for the oxidation resistance of TiAl is required for the use of a turbocharger rotor. There are fewer studies on the oxidation of TiAl that on the mechanical or the metallurgical properties. While studies on improvement of the oxidation resistance of TiAl are important,2"6 those skills have not been established yet. It is known that aluminum can be deposited by the pyrolysis of Al alkyls, notably tri-iso-butyl aluminum. Such coatings have been applied to corrosion protection layers for steel and other metals.7 The studies by Pierson8 and by Malazgirt and Evans9 have demonstrated that high purity, nonporous, and adherent coatings can be obtained by chemical vapor deposition of an Al alkyl at atmospheric pressure. Generally vapor processings have such merits as higher throwing power and better step coverage for complex-shaped substrates, such as the turbocharger rotor. Vandenbulcke10 has shown the advantage of a reduced pressure CVD in the structure 1984
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J. Mater. Res., Vol. 9, No. 8, Aug 1994
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uniformity of deposits. Umakoshi et al.n have reported that the oxidation resistance of TiAl3 has been much better than that of TiAl. Some potential methods and the effectiveness for the oxidation resistance have been presented.12 According to the report, an aluminizing seems to be one of the most hopeful methods for improving the oxidation resistance of TiAl.
II. EXPERIMENTAL A. Specimens and an apparatus The TiAl turbocharger rotors are prepared as follows.4 A raw material that consists of sponge Ti, Al, and some additives is first
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