JP3054697B2 - Method for producing Ti-Al alloy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention mainly relates to TiAl and
The present invention relates to a method for producing a Ti—Al-based alloy comprising an intermetallic compound of TiAl ₃ . This Ti-Al alloy is used not only for jet engines and terrestrial turbine compressors and turbine blades and vanes, but also for automobile engine exhaust valves and pistons, rockets, supersonic aircraft and spacecraft engines and heat-resistant structural materials, It is expected to be used as a new lightweight heat-resistant material, such as heat-resistant tubes for boilers and heat-resistant structural materials.

[0002]

2. Description of the Related Art For example, a well-known Ni-based superalloy used as a superalloy for blades and vanes of a high-pressure turbine generally uses a casting material obtained by a unidirectional solidification technique. That is, in the unidirectional solidification technique, as shown in FIG. 1, a molten metal 3 is charged into a crucible 2 heated by a high-frequency coil 1, and the lower end of the crucible 2 is forcibly cooled by cooling water 4. In this method, the molten metal 3 is solidified in one direction. Reference numeral 5 denotes a driving screw for moving the high-frequency coil 1 in accordance with solidification of the molten metal 3.

What is important here is that the Ni alloy phase and the Ni ₃ Al phase, which is an intermetallic compound, constituting the individual columnar crystal grains are eutectic solidified and crystallized directly from the molten metal. Are both cubic structures, and grow with the <001> direction oriented in the solidification direction. This <001>
Since the direction is a direction with high creep resistance, the unidirectionally solidified material of the Ni-based superalloy necessarily has excellent creep strength.

As described above, Ni-based superalloys obtained by the directional solidification technique have particularly excellent creep strength. Therefore, in recent years, application of this directional solidification technique to Ti-Al alloys has been studied. Have been.

[0005] That is, when a Ti-Al-based alloy is produced by a usual melt-casting solidification method, the obtained slab has a structure in which crystal grains having a lamellar structure composed of a γ phase and an α ₂ phase are randomly oriented. Since the mechanical properties of the lamella grains are extremely strong in anisotropy, if the lamella grains are randomly oriented, the ductility is particularly poor at room temperature.

On the other hand, if the interface of the lamellar structure is aligned in the direction in which tensile stress is applied, it is expected that at least room temperature ductility and yield strength in the temperature range from room temperature to about 900 ° C. can be compatible. Therefore, if a Ti-Al-based alloy is manufactured by the unidirectional solidification technique and the interface of the lamellar structure can be aligned in the solidification direction, for example, as in the ingot shown in FIG. Thus, the provision of a Ti-Al-based alloy having the following characteristics can be realized.

[0007]

SUMMARY OF THE INVENTION However, Ti-Al
When the unidirectional solidification technique is simply applied to the base alloy, the lamellar grains become extremely coarse and the lamellar interface of the individual lamellar grains tends to be oriented perpendicular to the solidification direction, as in the ingot shown in FIG. However, the tissue shown in FIG. 2 cannot be obtained. Note that the structure shown in FIG. 3 is extremely fragile when stress acts in the solidification direction of the ingot.

Here, in the case of a Ti—Al alloy, during the solidification process, 1 or 2 depending on the alloy components and solidification conditions, until the final (γ + α ₂ ) two-phase lamellar structure is formed. Reaction of β → β + α → α and α
→ solid state reaction of α + γ is involved. The β phase is Ti
Is a solid solution phase containing α as a main component, and α phase is an irregular phase of α ₂ phase.

Therefore, in the directional solidification process, it is necessary to control the above two reactions on the basis of a phase diagram to obtain the directional solidified structure shown in FIG. It has been difficult to achieve this by simply imitating the unidirectional solidification technology in the above. That is, Ti-Al
There is no established technology for directional solidification of alloys.

Accordingly, an object of the present invention is to provide a Ti-Al-based alloy having both excellent room-temperature ductility and high-temperature strength characteristics by establishing a unidirectional solidification technique for a Ti-Al-based alloy.

[0011]

Means for Solving the Problems The inventors of the present invention have intensively studied a method for suppressing extreme coarsening of lamella grains and realizing an orientation in which the interface of the lamella structure of each lamella grain is aligned with the solidification direction. As a result, it was found that a desired structure could be obtained by regulating the component composition and the directional solidification conditions, and for the first time, it was possible to provide a directional solidified material of a Ti-Al-based alloy.

[0012]

That is, the present invention relates to Al: 45.0 to 47.5 atm.
%, Cr, Mo, W, V, Nb, Ta, Mn and Re
One or more selected from the group consisting of 0.5 to 2.0at in total
% And a total of 0.2 to 1.0 at% of one or more selected from B, C, Si and N, and a Ti melt under a temperature gradient of 10 to 80 ° C./cm at the solid-liquid interface. , 50-25
This is a method for producing a Ti-Al-based alloy, characterized by unidirectional solidification at a solidification rate of 0 mm / h.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION In the directional solidification of a Ti-Al-based alloy, when solidification starts with the α phase as a primary crystal, the (0001) plane of the α phase having a dense hexagonal structure is perpendicular to the solidification direction. Due to the orientation, the {111} plane of the γ phase is oriented in the solidification direction in accordance with the crystal orientation relationship between the α phase and the γ phase, resulting in a texture having the orientation shown in FIG. In order to solve this problem, it is important to select a component composition in which the β phase predominates as a primary crystal, and the component composition will be described in detail below.

FIG. 4 shows Ti- (44-56 at%) Al-
About 2at% Mo alloy, Ti-Al-Mo ternary phase diagram and Scheil
The volume fraction of each constituent phase at the time of 100% solidification, calculated by the model shown in FIG. From the figure, it can be seen that when Mo is added at 2 at%, that is, when the third component is added, the ratio of β phase is at least
It can be seen that to secure 50 vol%, the Al concentration needs to be 47.5 at% or less. Similarly, when the case where Mo was not added was investigated, it was found that the Al concentration had to be 44 at% or less in order to make the ratio of the β phase 50 vol% or more. On the other hand, it was also confirmed that when the Al concentration was less than 45.0 at%, good mechanical properties could not be obtained.

Therefore, in order to secure at least 50 vol% of the β phase without sacrificing the mechanical properties, that is, at an Al concentration of 45.0 at% or more, it is necessary to add the third component to the aluminum alloy. It is necessary to regulate the concentration to 47.5at% or less.

Further, the β component is stabilized in the third component, and the tendency of the liquid phase surface near the ternary Ti—Al binary system containing the third element is examined. One or more selected from Cr, Mo, W, V, Nb, Ta, Mn and Re, which have an effect, are used.

The content of one or more of them is 0.5 to 2.0 at% in total. Because 0.
If it is less than 5 at%, the desired effect cannot be obtained, while 2.0 at
%, The B2 phase is likely to appear. The B2 phase is an intermetallic compound phase having an ordered structure based on a body-centered cubic structure, and is generally brittle near room temperature, easily loses strength at high temperatures, and easily deforms. Accordingly, the appearance thereof causes a decrease in ductility at room temperature and a decrease in high-temperature strength.
To avoid these disadvantages, the volume fraction of the B2 phase should be 1.0
It is effective to suppress the content to 1.5 vol%, and for that purpose, it is necessary to limit the additive component to 2.0 at% or less.

From the above description, Ti: 45.0
~ 47.5at% and Cr, Mo, W, V, Nb, Ta, Mn and Re
One or more selected from the group consisting of 0.5 to 2.0at in total
% Added was used as a basic component.

According to Hunt's model, generally, between the solidification rate during unidirectional solidification and the temperature gradient at the solid-liquid interface,
There is a relationship shown in FIG. 5, and depending on the value of the solidification rate and the temperature gradient at the solid-liquid interface, solidification occurs in an equiaxed crystal, a columnar crystal, or a mixed state of both. Here, according to the Ti—Al phase diagram, the curve shown in FIG. 5 corresponding to the α phase is usually located at the upper left in FIG. 5 from the same curve corresponding to the β phase. If there is a substance that becomes a nucleation site preferentially to the phase, as shown in FIG. 6, the same curve corresponding to the α phase can be shifted to the right from the same curve corresponding to the β phase. . As a substance that becomes this nucleation site,
There are borides, carbides, nitrides and silicides of Ti.

If one-way solidification is carried out under conditions suitable for the shaded region in FIG. 6, a solidification process as shown in FIG. 7 is realized. In other words, since the β phase is body-centered cubic, the <001> direction is oriented in the solidification direction, and the α phase between the β phase dendrites is a fine Ti boride, carbide, nitride or silicide nucleus. Crystallizes one after another. The crystallized α-phase interacts with the β-phase dendrite, which inhibits the growth.
It grows while keeping its plane parallel to the solidification direction. On the other hand, the β phase eventually forms a lamellar tissue through a reaction of β → α → (α + γ), and this β → α reaction is affected by the orientation of the surrounding α phase, and the (0001) plane Are oriented parallel to the solidification direction. Furthermore, β phase dendrites
Since coarsening is suppressed by boride, carbide, nitride and silicide of Ti, a desired unidirectionally solidified structure as shown in FIG. 2 is obtained as a result. However, FIG.
The details of the reason why the coagulation process described in (1) is realized are not yet known.

Therefore, in the present invention, since the presence of a boride, carbide, nitride or silicide of Ti is essential, one or two or more selected from B, C, Si and N are included in the above-mentioned basic components. More than one seed needs to be added. The addition amount of any one or two or more of them is 0.2 to
1.0at%. If the addition amount is less than 0.2 at%, the above effect cannot be expected, while if it exceeds 1.0 at%, coarse precipitates composed of these additional elements and Ti are formed, which impairs room-temperature ductility. It is.

Here, the conditions suitable for the shaded region in FIG. 6 will be specifically described. The temperature gradient at the solid-liquid interface is as follows.
10-80 ° C / cm, and the solidification rate ranges from 50-250 mm / h. Incidentally, this temperature gradient can be realized in a normal unidirectional solidification furnace, which is made of calcia, yttria, and the like, and is heated by a high-frequency heat source or a resistance heating heat source and cooled at the bottom of the crucible by a chill plate. Therefore, the unidirectional solidification material according to the present invention has an advantage that a special unidirectional solidification furnace is not required.

[0024]

EXAMPLE A molten Ti having a component composition shown in Table 1 was heated in a unidirectional solidification furnace shown in FIG. 8 at a temperature gradient of the solid-liquid interface of 10 to 80 ° C./cm.
Under the solidification at various solidification rates shown in Table 1,
When manufacturing ingots of 20 mm and length 120 mm,
The tissues shown in Table 1 were obtained. Further, Table 2 shows the results of investigation on mechanical properties of several examples of the ingots thus obtained.

In FIG. 8, reference numeral 6 denotes a water-cooled chill plate, 7 denotes a graphite heater, 8 denotes a CaO crucible, and 9 denotes a pouring port made of Mo.

Here, the structure was polished on the longitudinal section and several transverse sections of the ingot, respectively, and examined with an optical microscope and a scanning electron microscope.

The mechanical properties were evaluated by using a 5 × 2 × 1 (mm) test piece taken from an ingot and evaluating the room temperature yield stress and tensile ductility, and further, 2.5 × 2.5 × 5 (mm).
A compression test was performed in a temperature range of 25 to 1000 ° C. using the compression test piece of No. 1 to evaluate high-temperature strength.

[0028]

[Table 1]

[0029]

[Table 2]

Here, a Ti melt containing the composition No. 15 in Table 1, that is, Al: 46.5 at%, Mo: 1.5 at% and B: 0.7 at%, is unidirectionally solidified at a solidification rate of 100 mm / h. FIG. 9 shows the lamella tissue obtained. The lamella interface of the lamella grains shown in the figure is almost parallel to the observation surface, so that the lamella spacing is large and looks perpendicular to the solidification direction, but is actually oriented along the solidification direction.

[0031]

According to the present invention, a unidirectional solidification technique for a Ti-Al alloy is established for the first time. Therefore, one of the Ti-Al alloys, which was expected to have both excellent room temperature ductility and high temperature strength properties, was expected. The directionally solidified material can be provided as real.

[Brief description of the drawings]

FIG. 1 is a diagram showing a unidirectional solidification furnace of a Ni-based superalloy.

FIG. 2 is a diagram showing the structure of a Ti—Al-based alloy unidirectionally solidified material that is the subject of the present invention.

FIG. 3 is a view showing a structure of a Ti—Al alloy obtained by a conventional directional solidification technique.

FIG. 4 is a diagram showing the relationship between the Al content and the volume fractions of α, β, and γ phases.

FIG. 5 is a diagram showing a relationship between a solidification rate and a temperature gradient near a solid-liquid interface.

FIG. 6 is a diagram showing the relationship between the solidification rate and the temperature gradient near the solid-liquid interface in the present invention.

FIG. 7 is a diagram showing a coagulation process in the present invention.

FIG. 8 is a view showing a unidirectional solidification furnace used in the present invention.

FIG. 9 is a photograph taken by a microscope showing a structure of a unidirectionally solidified material.

[Explanation of symbols]

 6 Water-cooled chill plate 7 Graphite heater 8 CaO crucible 9 Filler

──────────────────────────────────────────────────続 き Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI C30B 29/52 C30B 29/52 (56) References JP-A-5-230568 (JP, A) JP-A 8-283890 (JP) , A) STRUCTURAL INTERMAL ETALLICS 1997, p. 287-294 (1997) Metal, Vol. 1990, No. 7, p.p. 34-40 Materials, Vol. 47, No. 5, pp. 540-541 (1998) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 14/00 C22C 1/00 C22C 1/02

Claims (1)

(57) [Claims] (1) Al: contains 45.0 to 47.5 at%, and further contains Cr,
One or more selected from Mo, W, V, Nb, Ta, Mn and Re are 0.5 to 2.0 at% in total, and B, C, Si
And one or more selected from N in total of 0.2
-1.0 at%, containing Ti melt unidirectionally solidified at a solidification rate of 50-250 mm / h under a temperature gradient of solid-liquid interface of 10-80 ° C / cm. Manufacturing method of Al-based alloy.