Metal treatment – Stock – Titanium – zirconium – or hafnium base
Reexamination Certificate
2001-02-22
2003-12-30
King, Roy (Department: 1742)
Metal treatment
Stock
Titanium, zirconium, or hafnium base
C420S418000, C420S420000
Reexamination Certificate
active
06669791
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to TiAl based alloys, a production process therefor, and a rotor blade using the same.
2. Description of Related Art
Recently, as materials used for a turbine blade of a turbocharger, a turbine engine or the like, and materials used for future aircraft, TiAl based alloys, being lightweight (specific gravity of about 4) and having excellent heat-resistance, are attracting much attention. In particular, in the case of a large blade, as the constituent material of the blade become lighter, the centrifugal stress becomes smaller, thus enabling improvement in the maximum attainable rpm, an increase in blade area, and a decrease in applied stress on the disk portion.
This TiAl based alloy is an alloy composed mainly of TiAl and Ti
3
Al, which is an intermetallic compound having excellent high temperature strength. As described above, it has excellent heat resistance, but has a problem in that ductility at room temperature is poor. Therefore, various measures have been heretofore taken, such as control of the microsstructure or ternary addition. For example, in Japanese Unexamined Patent Application, First Publication No. Hei 6-49565, there is disclosed a technique in which Cr or V is added as the ternary addition, in order to improve the ductility of the TiAl based alloy at normal temperature. Furthermore, a laminated structure (lamellar structure) region obtained by alternately laminating the TiAl phase and the Ti
3
Al phase is formed in a matrix structure in order to improve the strength. Moreover, Kim (Young-Won Kim. Intermetallics. (6) 1998 pp. 623-628) has reported that in a TiAl alloy having a lamellar grain with a mean grain diameter of from 30 to 3000 &mgr;m, as the mean grain diameter of the lamellar grains increases, the ductility and tensile stress at a room temperature decrease.
The case of the above described technique however, is not sufficient in view of improvement in ductility at a normal temperature. In particular, with a blade used for an engine for industrial use or the like, foreign matter such as sludge may collide with the blade at the time of operation, or at the time of production of the blade, the blade may be broken due to impact at the time of fixing the blade to the outer periphery of the disk with a hammer. Hence, it becomes necessary to improve the impact property of the TiAl based alloy. With the above technique however, it is difficult to improve the impact property.
Furthermore, in many cases the TiAl based alloy has been heretofore produced by casting. However the casting structure is generally large, and there is a tendency for the impact property of a material to decrease further. In the case of casting, production of small parts such as vehicle parts is relatively easy. However production of large parts has been difficult due to problems with flowability of the molten metal in the mold. On the other hand, isothermal forging is also commonly used as a forging method of the TiAl based alloy. Here, in order to develop a lamellar structure, it is necessary to pass through a zone in which the &agr;-phase exists. With the isothermal forging, however, there is a problem in that since processing at a high temperature of 1150° C. or higher is not possible due to problems of the apparatus, the lamellar structure necessary for improvement of the mechanical property is not developed in the forged material. In addition, production of large parts is also difficult.
BRIEF SUMMARY OF THE INVENTION
The present inventors consider that it is essential to form the above described lamellar grains in the matrix, sin order to improve the strength of the TiAl based alloy. Based on this assumption, the present inventors have changed the mean grain diameter of the lamellar grains to various sizes, and have found that the ductility at room temperature, in particular, the impact property can be greatly improved for a predetermined mean grain diameter, thereby concluding the present invention.
Moreover, the present invention is in conceiving as a method for reducing the mean grain diameter of the lamellar structure, one wherein a TiAl based alloy material is held in an equilibrium temperature range of an &agr; phase or in an equilibrium temperature range of an (&agr;+&bgr;) phase, and then the material is subjected to high-speed plastic working in the cooling process thereafter. The invention is also in finding a specific method for this method.
That is to say, it is an object of the present invention to solve the above described problems in the TiAl based alloy and to provide a TiAl based alloy excellent in workability, and with excellent strength as well as an improvement in ductility at room temperature, in particular, an improvement in the impact properties at room temperature, and a production process therefor.
It is an another object of the present invention to provide a blade comprising a TiAl based alloy having improved impact properties.
To achieve the above objects, the TiAl based alloy of the present invention is characterized by having a fine structure in which lamellar grains having a mean grain diameter of from 1 to 50 &mgr;m are closely arranged, with an &agr;
2
phase and a &ggr; phase being laminated therein alternately. More specifically, the TiAl based alloy of the present invention is characterized by two kinds of fine structures, one being a structure form (hereinafter, referred to as “structure 1”) in which lamellar grains having a mean grain diameter of from 1 to 50 &mgr;m are closely arranged, with the &agr;
2
phase and the &ggr; phase being laminated therein alternately, and the other being a structure form (hereinafter, referred to as “structure 2”) in which a matrix composed mainly of a &bgr; phase fills the gaps between the lamellar grains in the form of net work, and the ratio of this matrix is not smaller than 10% and not larger than 40%.
By having such a microstructure, the strength is improved by means of the lamellar grains themselves formed in the metal structure, and since the lamellar grains having a small grain diameter distribute closely and finely, ductility at room temperature, in particular, impact resistance is improved. As other properties, with the structure
1
, since the high temperature strength increases, it can be used as a turbine blade of a gas turbine or the like. Moreover, with the structure
2
, high temperature deformability is improved due to the effect of the &bgr; phase between the lamellar grains, making plastic working easy. However, the creep strength slightly decreases. Therefore, it can be used as a turbine blade of a steam turbine or the like having a low upper limit for the operating temperature.
In order to achieve the above objects, one of the TiAl based alloys of the present invention may be a TiAl based alloy having a composition comprising 40 to 48 atomic % of Al, 5 to 10 atomic % of one or more kinds selected from Cr and V, with the remainder being Ti and inevitable impurities. The TiAl based alloy having this composition has an equilibrium range of &agr; phase or (&agr;+&bgr;) phase, which is wide at a high temperature. Moreover, according to a production method of the present invention described below, this TiAl based alloy is easily subjected to high-speed plastic working, and becomes a fine microstructure in which lamellar grains are closely arranged. As a result, a TiAl based alloy having excellent ductility at a room temperature, and in particular, excellent impact properties can be obtained. The structure
1
can be obtained by holding the TiAl based alloy in the &agr; region, and the structure
2
can be obtained by holding the TiAl based alloy in the (&agr;+&bgr;) region.
Another TiAl based alloy of the present invention may be a TiAl based alloy having a composition comprising 38 to 48 atomic % of Al, 4 to 10 atomic % of Mn, with the remainder being Ti and inevitable impurities. Also in the TiAl based alloy having this composition, the high-temperature equilibrium range of the &agr; phase or (&a
Shindo Kentaro
Takeyama Masao
Tetsui Toshimitsu
Combs-Morillo Janelle
King Roy
Mitsubishi Heavy Industries Ltd.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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