Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Patent
1994-10-25
1996-07-30
Sheehan, John
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
148407, 148421, 420418, C22C 1400, C22F 118
Patent
active
055407927
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a component according to the introductory part of claim 1. Further the invention relates to a process for producing such components based on intermetallic phases of the system titanium-aluminum with an aluminum content between 42 at. % and 53 at. %.
Presently there is an increasing interest in intermetallic phases as a potentially suitable construction material for components subjected to high stress at high process temperatures. Particularly the intermetallic phases based on titanium-aluminide can be put to a variety of uses because of their good strength at high temperatures combined with low density, e.g. in such cases when the mechanical component stress is partially-related to the occurrence of centrifugal forces. As an example turbine blades can be mentioned in this context.
Of importance in this connection are first of all titanium aluminides with an aluminum content ranging between 42-53 at. %, particularly within the range of 45-50 at. %, in view of their good mechanical properties. The phase diagram of the system titanium aluminum shows in this range of aluminum concentration the intermetallic phases Ti.sub.3 Al and TiAl. However these materials have a poor resistance against oxidation, respectively corrosion, manifesting itself negatively in components produced on this base at operational temperatures between 700.degree. C. and 900.degree. C. The cause of this drawback resides in the fact that the mentioned titanium aluminides at these temperatures do not form a protective, stable oxide layer based on Al.sub.2 O.sub.3, in spite of their high aluminum content. Instead, especially after longer periods of oxidation, layers based on TiO.sub.2 are in fact formed, which have a high oxidation rate. This leads to a quick loss of component wall thickness, thereby damaging the component made of such a material.
From the materials technology in the field of high-temperature materials, e.g. such as those based on NiCrAl, oxidation-inhibiting protective coatings are known, e.g. of the type Ni(Co)CrAlY. However such protective coatings when applied to titanium aluminide could have a negative influence on the material properties of this material, particularly due to interdiffusion processes which can drastically reduce the mechanical properties of the material, particularly its resistance against mechanical loading. Furthermore such protective coatings always have flaws due to conditions of manufacturing and/or operation, such as pores or cracks, which can lead to strong local corrosion of the material --here titanium aluminide--covered by this protective layer.
Finally it is known to improve material surfaces through the so-called aluminizing process, wherein the aluminum content of such a surface is enriched. At first this leads to improved oxidation characteristics, but thereby disadvantageously the intermetallic phase TiAl3 is formed which has a strong tendency to crack. As a result the component subjected to this surface treatment is prone to cracking, respectively brittleness.
It is therefore the object of the invention to create a component of the above-mentioned kind wherein the good mechanical characteristics of the titanium aluminide are defined and the requirements of oxidation and corrosion resistance at process temperatures up to 900.degree. C. can be insured. Furthermore it is the object of the invention to create a process for producing a component of the above-mentioned kind, wherein a reproducible production of such components is made possible without the aforementioned disadvantages.
It has been found that the oxidation resistance of titanium aluminides with aluminum contents between 42 and 53 at. % aluminum depends not only on the exact composition of the material, respectively the alloy, but rather on the microstructure. When exposed in the above-mentioned temperature range of up to 900.degree. C., particularly of 700.degree.-900.degree. C., a titanium aluminide with given composition can form a slow growing Al.sub.2 O.sub.3 layer as well as a rapidly growing TiO.su
REFERENCES:
Binary Alloy Phase Diagrams, Second Edition, vol. I, pp. 225-226, 1990.
Gil Alexander
Quadakkers Willem J.
Dubno Herbert
Forschungszentrum Julich GmbH
Sheehan John
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