Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Metal and nonmetal in final product
Reexamination Certificate
2000-03-27
2001-10-16
Mai, Ngoclan (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Metal and nonmetal in final product
C419S019000, C419S026000, C419S029000
Reexamination Certificate
active
06303075
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a high temperature alloy material having excellent oxidation resistance and a method of producing the same.
In particular, the present invention relates to a high temperature alloy material to which excellent oxidation resistance is imparted by allowing a Nb—Ti—Al type alloy material to incorporate Mg, and a method of producing the same.
DESCRIPTION OF THE RELATED ART
In the past, Ni-base superalloys have been generally employed as materials for a gas turbine requiring particularly high temperature strength and excellent oxidation resistance. As the gas turbine provides higher energy efficiency at a higher temperature, various efforts have been made to improve the heat resistance of the Ni-base superalloys, and recently, a unidirectionally solidified material or a single crystal superalloy came to a practical stage. It is, however, well known that the improvement in the heat resistance of the Ni-base superalloy has been already approaching its limit as the melting point of Ni is not so high. Therefore, intermetallic compounds and ceramics having inherently high heat resistance, or metals having high melting points such as Nb have been studied as potential materials which may surmount the limit of the Ni-base superalloys.
The former two materials are still far away from the commercial stage since they are inherently brittle and the resulting low reliability is not yet overcome. In recent years, fiber-reinforced composites have been examined, however they are still under investigation and there are serious difficulties not only in their physical properties, but also in their cost effectiveness, and productivity etc.
On the other hand, high melting point metals such as Nb, W, Mo, and Ta have very high melting points in comparison with Ni. In particular, Nb is a high melting point metal with a melting point of not less than 2400° C., showing ductility at a room temperature, it is free from intrinsic problems related to mechanical reliability, different from intermetallic compounds or ceramics. Though they have some problems related to the mechanical characteristics such as strength at an elevated temperature, they can be handled by the conventionally known techniques employed in development of alloys, and it is likely that they are overcome. A serious problem is that Nb has fatally inferior resistance to oxidation at an elevated temperature. When this problem is solved, Nb has high possibility as a practical material which is more advantageous than the Ni-base alloy, however, it is still under investigation.
To be used for a gas turbine, it should have sufficiently high oxidation resistance at a temperature not less than 1000° C. As the Nb-base alloy has inherently poor oxidation resistance, it is very difficult to improve the oxidation resistance of the alloy itself to a practical level without degrading the toughness of the material at a normal temperature. Therefore, in actuality, it is contemplated that such material is coated with another material having high oxidation resistance such as MoSi
2
to provide oxidation resistance, and in effect, it is known that such coating can provide excellent oxidation resistance.
But the coating materials must be selected from intermetallic compounds and ceramics in order to give sufficient oxidation resistance, therefore the coated film is not free from the possibility of cracking, peeling, or fatigue failure and the like, and it is difficult to provide a practical and highly reliable product by itself.
That means, oxidation resistance must be imparted to a Nb-base alloy mainly by means of coating, however it is required to have a measure to assure enough oxidation resistance even when the coating is broken. Therefore, it is necessary to impart self-repair properties to the oxidation resistant coating film. Such self-repair properties are brought by limited oxidation of the Nb-base alloy material, therefore the Nb-base material, as the base material, needs to provide a dense oxide film on the surface in a high temperature oxidative atmosphere and the resulting oxide film shall have a sufficiently low oxygen diffusion rate.
In the past, various investigations have been made to improve the oxidation resistance of a Nb-base alloy (see for example “The Oxidation Behavior and Protection of Niobium” Journal of Metals, August, p.17, 1990). Usually, a metal oxide is generated on the surface of the metal when it is oxidized at an elevated temperature, however when the oxide is produced in the form of a dense film, the oxidation rate is controlled by the diffusion of the metal element or oxygen permeating through the oxide film. Accordingly, when an oxide film having sufficiently low such diffusion rate is formed, the metal shows low oxidation rate and good oxidation resistance. Oxidation of metal Nb results in an oxide thereof, Nb
2
O
5
, which has relatively high oxygen diffusion rate at an elevated temperature, therefore, it shows high oxidation rate at a temperature over 500° C. and it cannot be used as a high temperature material as it is.
Therefore, it has been examined that an element which is more susceptible to oxidation than Nb is added to form an alloy, and to generate a dense oxide film of the added element. In particular, Al and Si can provide excellent oxidation resistant films, since their oxides Al
2
O
3
and SiO
2
have very low oxygen diffusion rates even at an elevated temperature. In order to produce Al
2
O
3
and SiO
2
in an oxidative atmosphere, Al and Si must be contained in the alloy in a sufficiently high concentration, however, Al or Si cannot be added to Nb in a very large amount. That is when they are added in a large amount, they form intermetallic compounds with Nb, and the intermetallic compounds are brittle as described before, therefore they decrease the toughness to a great extent particularly at a low temperature, though they show good oxidation resistance.
That means, a basic condition for a practical alloy material which has reliable mechanical characteristics, is that it shall contain a ductile Nb-base metal phase (body-centered cubic system phase) at least in an amount of not less than 10% by volume, preferably not less than 40% by volume. However, in Nb metal phase, Al, for example, can only be incorporated in an amount of up to 10% (by atomic ratio) at 1200° C., and this amount of Al is absolutely insufficient for producing a dense Al
2
O
3
film in a high temperature oxidative atmosphere. Therefore, under the basic condition that the Nb-base metal phase shall be contained in the structure, a dense oxide film such as Al
2
O
3
and SiO
2
can not been generated by high temperature oxidation, or they have not been generated at least until now, and it seems very unlikely that it can be done in future as well.
Based on these basic matters, various examinations and improvements have been made, and currently a Nb-base alloy having similar degree of oxidation resistance as that of Ni-base superalloy has been made, which is based on Nb—Ti—Al ternary alloy. (For example, see “The Development of Nb-Based Advanced Intermetallic Alloys for Structural Applications”, Journal of Metals, January, p.33-38, 1996). The relatively high oxidation resistance of these alloys are mainly attributed to the generation of an oxide film containing TiO
2
as its main component, however, since the oxygen diffusion rate through TiO
2
is rather high at a high temperature, the resulting oxidation resistance is insufficient.
SUMMARY OF THE INVENTION
The present invention aims to further improve the oxidation resistance of such Nb-base alloys. More specifically, the present invention is to provide a high temperature Nb-base alloy material which contains a ductile Nb-base metal phase to secure the toughness of the alloy at a normal temperature, and yet it can provide a dense oxide film in a high temperature oxidative atmosphere, thereby it can maintain excellent oxidation resistance even when the coating is broken, i.e. it has a self repairing function and a method of producing the same.
Hirai Hisatoshi
Kitahara Akira
Shobu Kazuhisa
Tabaru Tatsuo
Ueno Hidetoshi
Agency of Industrial Science and Technology
Mai Ngoclan
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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