TIAL INTERMETALLIC COMPOUND SINGLE CRYSTAL MATERIAL AND PREPARATION METHOD THEREFOR

Abstract

A TiAl intermetallic compound single crystal material and a preparation method therefor are disclosed. The alloy composition of the material comprises Ti.sub.aAl.sub.bNb.sub.c(C, Si).sub.d, wherein 43≦b≦49, 2≦c≦10, a+b+c=100, and 0≦d≦1 (at. %).

Claims

1. A TiAl intermetallic compound single crystal material, comprising, based on atomic percent, an alloy composition of formula: Ti.sub.aAl.sub.bNb.sub.c(C, Si).sub.d, wherein 43≦b≦49, 2≦c≦10, a+b+c=100, and 0≦d≦1.

2. The TiAl intermetallic compound single crystal material according to claim 1, wherein 42≦a≦55, 4349, 2≦c≦9, and d=0.

3. The TiAl intermetallic compound single crystal material according to claim 1, wherein 44≦a≦51, 43≦b≦47, 6≦c≦9, and d=0.

4. The TiAl intermetallic compound single crystal material according to claim 1, wherein 43≦b≦47, 6≦c≦10, a+b+c=100, and 0.1≦d≦1.

5. The TiAl intermetallic compound single crystal material according to claim 1, prepared with a method comprising the steps of: (1) mixing pure raw materials with a purity of 99.9% or higher of each substance, in proportions according to the formula of the alloy composition, and melting the materials into master alloy ingots in a cold crucible electromagnetic induction levitation melting furnace at a vacuum level of 10.sup.−3 Pa or lower, followed by a gravity casting process or a suction casting process, to obtain master alloy rods; (2) cutting the master alloy rods into upper rods and lower rods which are used as raw material rods and seed crystal rods respectively in an optical floating zone directional solidification furnace; controlling the distance between the upper raw material rods and the lower seed crystal rods to 1-5 mm; arranging the raw material rods and the seed crystal rods coaxially and to be perpendicular to the horizontal plane, feeding an inert gas for protection during directional solidification, rotating the upper and lower rods in opposite directions at a relative rotational speed of 10-40 rpm, starting heating to melt the opposite ends of the upper and lower rods, adjusting the positions of the upper and lower rods to allow the opposite ends to gradually approach and be joined to each other, adjusting the power of the equipment and maintaining the temperature for 5-10 min, and then adjusting the growth rate to 2.5-30 mm/h when the surface of the floating zone becomes smooth and the melting is even, so as to start the directional solidification; and after the solidification is completed, reducing the power slowly, and slowly separating the solidified specimens from the remaining feed rod specimens; and (3) subjecting the prepared TiAl alloy single crystal rod to vacuum heat treatment by furnace cooling or air cooling at 1250-1350° C. for 12-24 hrs and then at 900° C. for 30 min.

6. The TiAl intermetallic compound single crystal material according to claim 5, wherein in Step (1), a water cooled copper crucible is employed for the electromagnetic induction levitation melting, and the master alloy is melted no less than 3 times.

7. The TiAl intermetallic compound single crystal material according to claim 5, wherein in Step (1), the master alloy rods have a size of Φ(4-8) mm×120 mm; differential pressure suction casting is employed in the suction casting process, in which the pressure difference is incubated at 3 MPa; and when the gravity casting process is used, the pressure of the protective gas is two thirds of the standard atmospheric pressure.

8. The TiAl intermetallic compound single crystal material according to claim 5, wherein in Step (2), the inert gas is argon or nitrogen, and the inert gas is fed at a flow rate of 3-5 L/min during the directional solidification.

9. The TiAl intermetallic compound single crystal material according to claim 5, wherein in Step (1), the raw materials of Al, Ti, C or Si have a purity of 99.999% or higher, and the neat metal raw material of Nb has a purity of 99.9% or higher.

10. The TiAl intermetallic compound single crystal material according to claim 5, wherein in Step (1), the lower seed crystal rods have a length of 20-30 mm, and the upper raw material rods have a length of less than 190 mm.

11. The TiAl intermetallic compound single crystal material according to claim 1, prepared with a method comprising the steps of: Step 1: mixing pure raw materials with a purity of 99.9% or higher of each substance, in proportions according to the formula of the alloy composition, and melting the materials into master alloy ingots in a cold crucible electromagnetic induction levitation melting furnace at a vacuum level of 10.sup.−3 Pa or lower, to homogenize the alloy components after 3-4 times of melting, followed by suction casting to obtain rods for directional solidification; Step 2: subjecting the TiAl alloy rod specimens to directional solidification in a high-purity yttrium oxide coated corundum crucible, evacuating to 5×10.sup.−3 Pa, and then feeding high-purity argon protective gas to the system; and Step 3: adjusting the power of an induction power source to heat the specimens, and maintaining the temperature at 1450-1650 K for 15-30 min, to start the directional solidification, in which the withdrawal rate of directional solidification is controlled to 5-20 μm/s; and after continuous growth to a specimen length of 50 mm, subjecting the specimens after directional solidification to rapid quench, with the solid-liquid interface being retained.

12. The TiAl intermetallic compound single crystal material according to claim 11, wherein in Step 1, the rods for directional solidification have a size of φ (4-6 mm)×100 mm; and in Step 2, the high-purity yttrium oxide coated corundum crucible has a size of φ (7-9 mm)×100 mm, and the high-purity argon protective gas is fed such that the pressure is 0.04-0.06 MPa.

13. A method for preparing a TiAl intermetallic compound single crystal material according to claim 1, comprising the steps of: (1) mixing pure raw materials with a purity of 99.9% or higher of each substance, in proportions according to the formula of the alloy composition, and melting the materials into master alloy ingots in a cold crucible electromagnetic induction levitation melting furnace at a vacuum level of 10.sup.−3 Pa or lower, followed by a gravity casting process or a suction casting process, to obtain master alloy rods; (2) cutting the master alloy rods into upper rods and lower rods which are used as raw material rods and seed crystal rods respectively in an optical floating zone directional solidification furnace; controlling the distance between the upper raw material rods and the lower seed crystal rods to 1-5 mm; arranging the raw material rods and the seed crystal rods coaxially and to be perpendicular to the horizontal plane, feeding an inert gas for protection during directional solidification, rotating the upper and lower rods in opposite directions at a relative rotational speed of 10-40 rpm, starting heating to melt the opposite ends of the upper and lower rods, adjusting the positions of the upper and lower rods to allow the opposite ends to gradually approach and be joined to each other, adjusting the power of the equipment and maintaining the temperature for 5-10 min, and then adjusting the growth rate to 2.5-30 mm/h when the surface of the floating zone becomes smooth and the melting is even, so as to start the directional solidification; and after the solidification is completed, reducing the power slowly, and slowly separating the solidified specimens from the remaining feed rod specimens; and (3) subjecting the prepared TiAl alloy single crystal rod to vacuum heat treatment by furnace cooling or air cooling at 1250-1350° C. for 12-24 hrs and then at 900° C. for 30 min.

14. The method according to claim 13, wherein in Step (1), a water cooled copper crucible is employed for electromagnetic induction levitation melting, and the alloy is melted no less than 3 times.

15. The method according to claim 13, wherein in Step (1), the master alloy rods have a size of φ (4-8) mm×120 mm; differential pressure suction casting is employed in the suction casting process, in which the pressure difference is incubated at 3 MPa; and when the gravity casting process is used, the pressure of the protective gas is two thirds of the standard atmospheric pressure.

16. The method according to claim 13, wherein in Step (1), the raw materials of Al, Ti, C or Si have a purity of 99.999% or higher, and the neat metal raw material of Nb has a purity of 99.9% or higher.

17. The method according to claim 13, wherein in Step (1), the lower seed crystal rods have a length of 20-30 mm, and the upper raw material rods have a length of less than 190 mm.

18. The method according to claim 13, wherein in Step (2), the inert gas is argon or nitrogen, and the inert gas is fed at a flow rate of 3-5 L/min during the directional solidification.

19. A method for preparing a TiAl intermetallic compound single crystal material according to claim 1, comprising the steps of: Step 1: mixing pure raw materials with a purity of 99.9% or higher of each substance, in proportions according to the formula of the alloy composition, and melting the materials into master alloy ingots in a cold crucible electromagnetic induction levitation melting furnace at a vacuum level of 10.sup.−3 Pa or lower, to homogenize the alloy components after 3-4 times of melting, followed by suction casting to obtain rods for directional solidification; Step 2: subjecting the TiAl alloy rod specimens to directional solidification in a high-purity yttrium oxide coated corundum crucible, evacuating to 5×10.sup.−3 Pa, and then feeding high-purity argon protective gas to the system; and Step 3: adjusting the power of an induction power source to heat the specimens, and maintaining the temperature at 1450-1650 K for 15-30 min, to start the directional solidification, in which the withdrawal rate of directional solidification is controlled to 5-20 μm/s; and after continuous growth to a specimen length of 50 mm, subjecting the specimens after directional solidification to rapid quench, with the solid-liquid interface being retained.

20. The method according to claim 19, wherein in Step 1, the rods for directional solidification have a size of φ (4-6 mm)×100 mm; and in Step 2, the high-purity yttrium oxide coated corundum crucible has a size of φ (7-9 mm)×100 mm, and the high-purity argon protective gas is fed such that the pressure is 0.04-0.06 MPa.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] FIG. 1 is a binary phase diagram of a Ti—Al alloy in the prior art.

[0041] FIG. 2 shows microstructures at a maximum longitudinal section (a) and the lamellar orientation (b) of the specimens of the present invention after directional solidification.

[0042] FIG. 3 shows microstructures at a longitudinal section in a competition phase of directional solidification of the specimens of the present invention.

[0043] FIG. 4 shows microstructures at a maximum longitudinal section (a) and the lamellar orientation (b) of the specimens of the present invention after directional solidification.

[0044] FIG. 5 shows microstructures at a longitudinal section in a competition phase of directional solidification of the specimens of the present invention.

[0045] FIG. 6 shows microstructures at a maximum longitudinal section (a) and the lamellar orientation (b) of the specimens of the present invention after directional solidification.

[0046] FIG. 7 shows a solid-liquid interface retained after quench of the specimens of the present invention upon directional solidification.

[0047] Note: the microstructures in FIGS. 2-7 are grown from right to left.

[0048] FIG. 8 is a flow chart of a method for preparing a high-strength and high-plasticity TiAl alloy material.

[0049] FIG. 9 shows microstructures of a TiAl alloy single crystal (a) and the lamellar orientation (b) after directional solidification.

[0050] FIG. 10 shows microstructures of segregation in a TiAl alloy single crystal before and after different heat treatment processes (a. before heat treatment, and b. after heat treatment).

[0051] FIG. 11 is an XRD pattern of a TiAl alloy single crystal before and after different heat treatment processes.

[0052] FIG. 12 shows a tensile mechanical property profile at room temperature of a TiAl alloy.

[0053] FIG. 13 shows (a) the morphology of a solid-liquid interface upon solidification of a TiAl—Nb single crystal and (b) the principle of necking and crystal selection.

[0054] FIG. 14 shows an optical morphology of a TiAl—Nb alloy after directional solidification.

[0055] FIG. 15 shows SEM images of segregation in a TiAl—Nb single crystal before heat treatment (a) and after heat treatment (b).

[0056] FIG. 16 shows interlamellar distances of a TiAl—Nb single crystal before heat treatment (a) and after heat treatment (b).

[0057] FIG. 17 shows a displacement-strength curve of TiAl—Nb stretched at 900° C.

DETAILED DESCRIPTION

[0058] The disclosure of the present invention will be further described below with reference to accompanying drawings and examples. However, the scope of the present invention is not limited to the following examples

[0059] Referring to accompanying drawings, a TiAl intermetallic compound single crystal with absolutely controllable lamellar orientation is prepared by Bridgman directional solidification method. A specific embodiment is as follows.

[0060] (1) A Ti—Al—Nb ternary alloy with a primary phase being wholly β phase is used. Based on the phase diagram of a multi-element alloy and the principle of phase selection, the primarily precipitated phase is allowed to be absolutely β phase by adjusting the proportional relation between atomic components, as shown in FIG. 1. Specifically, the content of Nb is increased, and the relative proportion of Al is reduced to form a wide β phase region.

[0061] (2) According to the alloy composition obtained in 1), a master alloy is prepared by melting with a high-purity metal component configuration in cold crucible electromagnetic levitation melting equipment under a high-purity Ar gas atmosphere. The master alloy is melted several times to obtain a homogeneous master alloy ingot which is suction casted into master alloy rods.

[0062] (3) The TiAl alloy rods are placed in a corundum crucible with high-purity sintered yttrium oxide coating on the inner wall and having a dimension of Φ (5-8 mm)×100 mm, and transferred to a Bridgman directional solidification furnace, and 0.04-0.06 MPa high-purity argon protective gas is filled when the furnace is evacuated to 5×10.sup.−3 Pa.

[0063] (4) The specimens are heated by adjusting the power of an induction power source, and incubated at 1450-1650 K for 15-30 min, to start the directional solidification, in which the growth rate upon directional solidification is controlled to 5-20 μm/s.

[0064] (5) After continuous growth to a specimen length of 50 mm at a rate, the specimens after directional solidification are subjected to rapid quench, with the solid-liquid interface being retained.

[0065] The present invention is further described with reference to specific Examples 1-7 and FIGS. 1-7.

Example 1

[0066] In the experiment, the alloy composition was Ti.sub.47Al.sub.45Nb.sub.8 (atomic percent, at %), in which the purity of the metal components was 99.999%. A master alloy was prepared by melting in a cold crucible electromagnetic levitation melting equipment under a high-purity Ar atmosphere at a vacuum level of 5×10.sup.−3 Pa. A homogenous master alloy ingot is obtained after 4 times of melting, which is suction casted into master alloy rods of Φ4×100 mm. The TiAl alloy rod specimens were placed in a corundum crucible with high-purity yttrium oxide coating on the inner wall for directional solidification. The system was evacuated to 5×10.sup.−3 Pa and then 0.05 MPa high-purity argon protective gas was fed. The specimens were heated by adjusting the power of an induction power source, and incubated at 1550 K for 25 min, to start the directional solidification, in which the growth rate upon directional solidification was controlled to 5 μm/s. When the withdrawn length reached a specimen length of 50 mm, the specimens were subjected to rapid quench, with the solid-liquid interface being retained. The microstructure at a maximum longitudinal section of the cylindrical specimen was characterized, and the primarily precipitated phase, the grain size and the lamellar orientation at this withdrawal rate were observed and analyzed. As shown in FIGS. 2(a) and 2(b), a TiAl alloy single crystal with a lamellar orientation parallel to the growth direction is obtained. When the growth rate is as low as 5 m/s, the enriched solute can be sufficiently diffused and the growth proceeds steadily. The grains have a sufficient time to grow, so that the resulting grains are relatively large until a single crystal is grown.

[0067] FIG. 3 shows microstructures in a competition phase of directional solidification at 5 μm/s. Because the phase boundary motilities are different due to the different degrees of misfit on the interfaces of 0°- and 45°-slanted lamellas in the β.fwdarw.α solid/solid phase transition, a critical withdrawal rate of 5 μm/s is present, at which after the α grain nucleation at a lamellar orientation of 0° and 45° occurs, the grains slanted at 45° are finally eliminated since the driving force with which the grains grow at 0° is high. A single crystal having a lamellar orientation that is parallel to the growth direction is thus obtained.

Example 2

[0068] The same alloy composition and method as those in Example 1 were used. The incubation temperature was 1550K and the incubation time was 25 min. The directional solidification was started, during which the growth rate upon directional solidification was controlled to 15 m/s. As shown in FIGS. 4(a) and 4(b), an α phase having a lamellar orientation of 45° is retained at such a withdrawal rate in the β.fwdarw.α solid/solid phase transition, so the final structure is a single crystal having a lamellar orientation of 45°.

[0069] FIG. 5 shows microstructures in a competition phase of directional solidification at 15 μm/s. At such a withdrawal rate, the driving force for grain nucleation at 45° is higher than that at 0° in the solid/solid phase transition, such that the grains slanted at 0° cannot grow, and a TiAl alloy single crystal having a lamellar orientation that is 45° with respect to the growth direction is obtained.

Example 3

[0070] The same alloy composition and method as those in Example 1 were used. The incubation temperature was 1550 K, and the incubation time was 25 min. The directional solidification was started, during which the growth rate upon directional solidification was controlled to 20 μm/s. As shown in FIGS. 6(a) and 6(b), a single crystal having a lamellar orientation that is 45° with respect to the growth direction is obtained.

[0071] FIG. 7 shows a solid-liquid interface retained after rapid quench. The dendrite growth morphology is quaternarily symmetrical, and has an obvious secondary dendrite that is perpendicular to the primary dendrite. It can be inferred that the β phase of cubic system is the primary phase during the directional solidification.

Example 4

[0072] The same method as that in Example 1 was used. The alloy composition was Ti.sub.55Al.sub.43Nb.sub.2, the incubation temperature was 1650 K, the incubation time was 30 min, and the growth rate upon directional solidification was 5 μm/s. A TiAl alloy single crystal having a lamellar orientation that is parallel to the growth direction was obtained.

Example 5

[0073] The same method as that in Example 1 was used. The alloy composition was Ti.sub.48Al.sub.43Nb.sub.9. The incubation temperature was 1450 K, the incubation time was 30 min, and the growth rate upon directional solidification was 10 μm/s. A TiAl alloy single crystal having a lamellar orientation that is 45° with respect to the growth direction was obtained.

Example 6

[0074] The same method as that in Example 1 was used. The alloy composition was Ti.sub.51Al.sub.45Nb.sub.6. The incubation temperature was 1650 K, the incubation time was 15 min, and the growth rate upon directional solidification was 5 μm/s. A TiAl alloy single crystal having a lamellar orientation that is parallel to the growth direction was obtained.

Example 7

[0075] The same method as that in Example 1 was used. The alloy composition was Ti.sub.42Al.sub.49Nb.sub.9. The incubation temperature was 1550 K, the incubation time was 25 min, and the growth rate upon directional solidification was 5 μm/s. A TiAl alloy single crystal having a lamellar orientation that is parallel to the growth direction was obtained.

[0076] The present invention is further described in detail in connection with another method for preparing a high-strength and high-plasticity TiAl alloy material with reference to FIG. 8. The specific embodiment is as follows.

[0077] (1) Referring to FIG. 8, the alloy composition based on atomic percent of the high-strength and high-plasticity TiAl alloy material is (44-51)Ti-(43-47)Al-(6-9)Nb. The primarily precipitated phase is allowed to be absolutely β phase by adjusting the proportional relation between atomic components.

[0078] (2) A master alloy button ingot of TiAl alloy is prepared by electromagnetic induction levitation melting in a water cooled copper crucible, and then mater alloy rods are obtained after a suction casting process.

[0079] (3) The master alloy rods are cut into raw material rods and seed crystal rods which are subjected to directional solidification in an optical floating zone. High-purity argon is introduced and used as a protective gas. The relative rotational speed of the upper and lower sections, the heating power, and the growth rate are adjusted, to control the lamellar orientation of the TiAl alloy and achieve the growth of a single crystal.

[0080] (4) The prepared TiAl alloy single crystal rods are subjected to vacuum heat treatment, by heating for a period of time in the α single-phase region, incubating, and then annealing, to completely eliminate the brittle B2 phase and the residual stress, so as to obtain a high-strength and high-plasticity TiAl alloy material.

[0081] (5) The microstructure of the prepared TiAl alloy is characterized by OM and XRD, and then the mechanical performances are further characterized, to determine the microstructure of the TiAl alloy with the best comprehensive mechanical performances and corresponding preparation parameters thereof.

[0082] The invention will now be further described with reference to specific Examples 8-13 and FIGS. 8-12.

Example 8

[0083] (1) Choice of Raw Materials:

[0084] The alloy composition used in the preparation of master alloy ingots was Ti.sub.47Al.sub.45Nb.sub.8 (atomic percent), in which the purity of the metal component was 99.999% for Ti and Al, and 99.95% for Nb.

[0085] (2) Preparation of Master Alloy Ingots:

[0086] The master alloy ingots were prepared by melting in a water cooled copper crucible electromagnetic induction levitation melting furnace under a high-purity argon atmosphere. The surface of the metal raw materials was mechanically polished to remove the oxide scale on the surface, and the materials were mixed according to the designed proportion of the components. The mixed material was placed in a water cooled copper crucible in a melting furnace in an amount of about 70 g/ingot, and evacuated to 5×10.sup.−3 Pa. High-purity argon (99.999%) having a pressure ranging from 0.8-1 MPa was fed to the furnace. A homogeneous master alloy ingot was obtained after 3-4 times of melting. Then, the master alloy ingot was suction casted into rods of cis 6×120 mm.

[0087] (3) Directional Solidification in Optical Floating Zone:

[0088] The master alloy rods were cut into upper and lower rods, which were used as raw material rods and seed crystal rods respectively in an optical floating zone directional solidification furnace. The lower rods were seed crystal rods of 30 mm in length, and the upper rods were feed rods of less than 100 mm in length. During the directional solidification, the raw material rods and the seed crystal rods were arranged coaxially and to be perpendicular to the horizontal plane. The distance between the upper and lower rods was 5 mm and the interval was at the focusing center of four filaments. High-purity argon protective gas was introduced at a flow rate of 5 L/min. The axial relative rotational speed of the upper and lower rods was adjusted to 30 rpm. Heating was started to melt the opposite ends of the upper and lower rods. The positions of the upper and lower rods were adjusted, to allow the opposite ends to gradually approach and be joined to each other. The power was adjusted to 68% of the total power, and the temperature was maintained for 5 min. When the surface of the floating zone became smooth and the melting was even (that is, no obvious vibration occurred in the floating zone), the growth rate was adjusted to 5 mm/h, to start directional solidification. After growth to 80 mm, the directional solidification was stopped, the power was reduced slowly, and the solidified specimens were slowly separated from the remaining feed rod specimens.

[0089] (4) Vacuum Heat Treatment

[0090] The TiAl alloy single crystal rod after directional solidification were placed in a corundum tube, evacuated to 10.sup.−3 Pa, and then sealed. The tube was transferred to a heat treatment furnace, and subjected to a heat treatment process comprising furnace cooling at 1300° C. for 24 h and then at 900° C. for 30 min.

[0091] (5) Structure and Performance Characterization

[0092] FIG. 9a shows a macroscopic photograph of rod specimens after the directional solidification in an optical floating zone. It can be seen that the specimens is rapidly grown into a single crystal after the short-term competition and elimination in the directional solidification. FIG. 9b shows that the lamellar orientation of the single crystal is parallel to the growth direction. FIGS. 10(a) and 10(b) show the microstructures before and after the heat treatment. In connection with the XRD pattern of FIG. 11, it can be seen that a large amount of B2 phase is distributed in the structure before heat treatment, and the B2 phase is completely eliminated after 24-h heat treatment. FIG. 12 shows a tensile strength-strain curve at room temperature of the high-strength and high-plasticity TiAl alloy prepared. The yield strength is 729 MPa and the plastic strain is 6.9%. Therefore, the alloy has excellent mechanical properties at room temperature.

Example 9

[0093] The same preparation method as that in Example 8 was used. The alloy composition was Ti.sub.44Al.sub.47Nb.sub.9 (atomic percent). The directional solidification process in the optical floating zone included a relative rotational speed of 20 rpm, a heating power of 55% of the total power, and a growth rate of 2.5 mm/h. The vacuum heat treatment process included furnace cooling at 1250° C. for 12 hrs and then at 900° C. for 30 min. The B2 phase was completely eliminated, to obtain a TiAl alloy material having a yield strength of 550 MPa and a plastic strain of 6.0% at room temperature.

Example 10

[0094] The same preparation method as that in Example 8 was used. The alloy composition was Ti.sub.51Al.sub.40Nb.sub.9 (atomic percent). The directional solidification process in the optical floating zone included a relative rotational speed of 25 rpm, a heating power of 70% of the total power, and a growth rate of 10 mm/h. The vacuum heat treatment process included furnace cooling at 1300° C. for 20 hrs and then at 900° C. for 30 min. The B2 phase was completely eliminated, to obtain a TiAl alloy material having a yield strength of 628 MPa and a plastic strain of 6.5% at room temperature.

Example 11

[0095] The same preparation method as that in Example 8 was used. The alloy composition was Ti.sub.48Al.sub.43Nb.sub.9 (atomic percent). The directional solidification process in the optical floating zone included a relative rotational speed of 20 rpm, a heating power of 68% of the total power, and a growth rate of 15 mm/h. The vacuum heat treatment process included furnace cooling at 1350° C. for 24 hrs and then at 900° C. for 30 min. The B2 phase was completely eliminated, to obtain a TiAl alloy material having a yield strength of 660 MPa and a plastic strain of 6.2% at room temperature.

Example 12

[0096] The same preparation method as that in Example 8 was used. The alloy composition was Ti.sub.48Al.sub.43Nb.sub.9 (atomic percent). The directional solidification process in the optical floating zone included a relative rotational speed of 20 rpm, a heating power of 70% of the total power, and a growth rate of 15 mm/h. The vacuum heat treatment process included furnace cooling at 1350° C. for 12 hrs and then at 900° C. for 30 mi. The B2 phase was completely eliminated, to obtain a TiAl alloy material having a yield strength of 593 MPa and a plastic strain of 6.8% at room temperature.

Example 13

[0097] The same preparation method as that in Example 8 was used. The alloy composition was Ti.sub.48Al.sub.46Nb.sub.6 (atomic percent). The directional solidification process in the optical floating zone included a relative rotational speed of 30 rpm, a heating power of 60%, and a growth rate of 20 mm/h. The vacuum heat treatment process included furnace cooling at 1250° C. for 12 hrs and then at 900° C. for 30 min. The B2 phase was failed to be removed completely. As shown in the XRD pattern in FIG. 10b, a small amount of B2 phase was remained after 12-h heat treatment. A TiAl alloy material having a yield strength of 656 MPa and a plastic strain of 3.0% at room temperature was obtained.

Example 14

[0098] The same preparation method as that in Example 8 was used. The alloy composition was Ti.sub.44Al.sub.45Nb.sub.8 (atomic percent). The directional solidification process in the optical floating zone included a relative rotational speed of 25 rpm, a heating power of 55%, and a growth rate of 30 mm/h. A TiAl alloy single crystal with a lamellar orientation that is 45° with respect to the growth direction was obtained. The vacuum heat treatment process included furnace cooling at 1250° C. for 12 hrs and then at 900° C. for 30 min/. The B2 phase was completely eliminated, to obtain a TiAl alloy material having a yield strength of 430 MPa and a plastic strain of 7.8% at room temperature.

Example 15

[0099] (1) The alloy composition, based on atomic percent, was Ti-45Al-8Nb-0.3C-0.2Si, with the balance being Ti. The starting raw materials included Al, Ti, C and Si with a high purity of 99.999% and Nb with a high purity of 99.95%. The materials were repeatedly melted 4 times in a cold crucible electromagnetic induction levitation melting furnace at a vacuum level of 5×10-3 MPa, to obtain a TiAl—Nb master alloy ingot.

[0100] (2) A round rod-like alloy of Φ 6 mm was obtained after a differential suction casting process at a pressure difference of 3 MPa.

[0101] (3) An optical floating zone method of directional solidification was used. The master alloy rods obtained after suction casting were cut into upper feed rods of 150 mm in length and lower seed crystal rods of 20 mm in length. The feed rods were amenable to necking and crystal selection treatment. During the directional solidification, the feed rods and the seed crystal rods were arranged coaxially and to be perpendicular to the horizontal plane. The distance between the feed rods and the seed crystal rods was 1-3 mm, and the opposite ends were located at the focusing center of four filaments. A protective gas was introduced at a flow rate of 4 L/min, and the seed crystal rods and the feed rods were adjusted to rotate at 30 r/min in opposite directions. The heating power was ramped to 68% of the total power in 10 min, to melt the alloy, and the temperature was maintained for 5 min. Then, directional solidification occurred at a growth rate of 15 mm/h. Due to the heating feature in the optical floating zone, the solid-liquid interface was a convex interface shown in FIG. 13(a). As shown in FIG. 13(b), the principle was that the grains in the middle portion grew along the growth direction, and the grains at the two sides grew incline to both sides. Therefore, after the feed rods were subjected to the necking and crystal selection treatment, the grains grown in the middle portion eliminated the grains grown on both sides, and grew rapidly into a single crystal. The process from the competitive growth to the final stable growth is shown in FIG. 14. The power was slowly reduced after the directional solidification, and the solidified specimens were slowly separated from the remaining feed rod specimens.

[0102] (4) The prepared TiAl—Nb single crystal was subjected to vacuum heat treatment for eliminating the segregation. The morphology of segregation before heat treatment was as shown in FIG. 15(a). The segregated phase was eliminated by heating for 24 hrs at 1250° C. in the α single-phase region. After 30-min homogenization at 900° C. and air cooling, the final single crystal was obtained. FIG. 15(b) shows that the segregation is completely eliminated by heat treatment. FIG. 16 shows the variation in interlamella distance before and after heat treatment. Due to the high cooling rate of air cooling, there is no room for coarsening of the lamella.

[0103] (5) The single crystal after heat treatment was processed into withdrawn specimens with a nominal size of Φ 3 mm×20 mm. The tensile curve at a tensile rate of 1×10.sup.−3S-1 and a tensile temperature of 900° C. is as shown in FIG. 17, indicating that the yield strength of the TiAl—Nb single crystal at 900° C. is 637 MPa, the elongation is 8.1%, and the ductile-brittle transition temperature is greater than 900° C., which are far higher than a common TiAl alloy.

Example 16

[0104] An alloy having a composition of Ti-45Al-8Nb-0.4C-0.5Si (with the balance being Ti) was prepared by using the same method as that in Example 15. However, a gravity casting process was used to obtain round rod-like specimens of Φ 8 mm. Due to the use of necking and crystal selection treatment, single crystal specimens could be obtained rapidly from the alloy of this diameter. After the same heat treatment for eliminating the segregation, the tensile strength is 618 MPa and the elongation is 9.2%.

Example 17

[0105] The same preparation method as that in Example 15 was used. The alloy composition was Ti-45Al-8Nb-0.4Si-0.6C, with the balance being Ti (based on atomic percent). The same heat treatment process was used. Because the presence of a small amount of C and Si does not change the phase transition temperature greatly, but brings a high temperature strengthening effect, the yield strength of the material at 900° C. becomes 650 MPa, and the plastic strain becomes 7.6%.

Example 18

[0106] The same preparation method as that in Example 15 was used. The alloy composition was Ti-45Al-8Nb-0.5Si, with the balance being Ti. The withdrawal rate was changed to 40 mm/h. Despite the small temperature gradient, a single crystal was still obtained at a rapid growth rate due to the use of necking and crystal selection treatment. After heat treatment, the yield strength at 900° C. is 595 MPa, and the elongation is 8.7%.

Example 19

[0107] The same preparation method as that in Example 15 was used. The alloy composition was Ti-43Al-10Nb-0.3C-0.3Si, with the balance being Ti. Although the element Nb brings a reinforcement effect, the segregation is increased correspondingly. The brittle segregated phase can be eliminated by the heat treatment process. The results of tensile test at 900° C. show that the yield strength is up to 668 MPa, and the elongation is 6%.

Example 20

[0108] The same preparation method as that in Example 15 was used. The alloy composition was Ti-45Al-8Nb-0.4C, with the balance being Ti. The directional solidification process in the optical floating zone was changed and the growth rate was changed to 5 mm/h. A low growth rate was favorable to the formation of a single crystal, as indicated by a shortened distance of an elimination section. After the segregation eliminating vacuum heat treatment, the yield strength of the single crystal alloy material at 900° C. is 602 MPa, and the plastic strain is 7.6%.

Example 21

[0109] The same preparation method as that in Example 15 was used. The alloy composition was Ti-45Al-8Nb, with the balance being Ti. The directional solidification process in the optical floating zone included a relative rotational speed of 20 rpm. Due to the decreased rotational speed, the temperature becomes more uneven, such that the grains in the middle portion grow rapidly into a single crystal. After stretching at 900° C., the yield strength is 620 MPa, and the plastic strain is 7%.

Example 22

[0110] The same preparation method as that in Example 15 was used. The alloy composition was Ti-45Al-8Nb-0.4Si-0.6C, with the balance being Ti. The heating power in the optical floating zone was 65% of the total power. Although a low heating temperature leads to a small temperature gradient that is unfavorable for the formation of a single crystal, the necking and crystal selection enables the formation of a single crystal at such a heating power. After stretching at 900° C., the yield strength is 639 MPa, and elongation is 7.2%.

Example 23: Application and Performance Comparison

[0111] The properties of TiAl single crystal alloys prepared by the optical floating zone method were tested by conventional tensile test at room temperature and high temperature. It is found that the alloys have significantly better properties at room temperature and high temperature than other similar alloys (see table for details).

[0112] The brittleness at room temperature of TiAl intermetallic compounds has always been a major problem limiting their application. Generally, the TiAl alloys have an elongation of 2-3% at room temperature. In contrast, the TiAl alloy obtained in the present invention has an elongation of 6.9% at room temperature, while a high strength is maintained (729 MPa). By means of the high plasticity at room temperature, the inherent difficulty in machining the TiAl alloy at room temperature is solved, and the TiAl alloy has a room temperature elongation of 2 to 3% Puzzle, so that the alloy is easy to be machined into a required shape, and its brittleness at room temperature is improved. The performance comparison with some TiAl alloy single crystals is shown in Table 1.

[0113] Excellent yield strength at high temperature (900° C./637 MPa): The yield strength at 900° C. of the alloy of the present invention is 637 MPa, which is 30-50% higher than that of other TiAl alloys. It is expected that the alloy can be used at a temperature increased from current 650-700° C. to 900° C. (at present, the Ti-48Al-2Cr-2Nb alloy is successfully used by GE in the 6.sup.th and 7.sup.th-stage blades of a low pressure turbine in Boeing 787 aircraft, at a working temperature of 650° C.). The comparison of performances at 900° C. with other TiAl alloys is shown in Table 2.

[0114] Due to the excellent performances at room temperature and high temperature, the TiAl single crystal is expected to have an extended extent of use in the engine blades of Boeing aircrafts and airbuses, to replace the engine blades used at a temperature of 650-900° C., thus bringing a huge benefit in energy saving and emission reduction, and other aspects. In addition, it has important application prospect in the components such as car compressor turbines and exhaust valves, tail skirts of momentum space interceptor engines, nozzles of satellite engines, reversible turbine rotors for aerospace vehicle and so on.

TABLE-US-00001 TABLE 1 Comparison of mechanical properties of PST single crystals of TiAl alloy Lamellar Yield strength Elongation Composition (at. %) orientation (MPa) (%) Ti—45Al—8Nb (Example 8) 0° 729 6.9 Ti—45Al—8Nb (Example 14) 45°  430 7.8 Ti—43Al—3Si [1] 0° 673 0.6 Ti—43Al—3Si 45°  333 2.1 Ti—43Al—3.5Si [1] 45°  393 1.0 Ti—45.5Al—1.5Si [2] 0° 518 3.0 Ti—45Al—2Si [2] 0° 489 1.3 Ti—46Al—1Si [2] 0° 419 3.3 Ti—47Al—2W [3] 0° 350 1.4 Ti—47Al—2W 45°  275 2.0 Ti—43.5Al—3Si—0.5Re [4] 0° 522 1.7 Ti—46.5Al—1.5Mo—0.6B [5] 0° 440 2.9 Ti—46.5Al—1.5Mo—0.7B [5] 0° 500 2.5 Ti—46Al—1.5Mo—0.2C[6] 0° 690 3.0

TABLE-US-00002 TABLE 2 Comparison of mechanical properties at elevated temperature (900° C.) of some TiAl alloys with different microstructures, including fully lamellar (FL), nearly fully lamellar (NFL), near gamma (NG), nearly lamellar (NL), degraded fully lamellar (DFL), refined fully lamellar (RFL) and duplex (DP). Yield strength Elongation Strain rate Composition (at. %) (MPa) (%) (s-1) Ti—45Al—8Nb (Example 15) 637 8.1   1 × 10.sup.−3 Ti—48Al—2Cr (DP) [7] 308 78   1 × 10.sup.−3 Ti—48Al—2Cr (DP) [7] 279 81   1 × 10.sup.−3 Ti—47Al—0.7Si—0.4Nb—0.4Cr [8] (FL) 370 38 — Ti—46.5Al—2Cr—3Nb—0.2W [9] (RFL) 340 19 0.2 × 10.sup.−3 Ti—45Al—10Nb (FL) [10] 562 16 0.5 × 10.sup.−3 Ti—45Al—10Nb (NFL) [10] 460 21 0.5 × 10.sup.−3 Ti—45Al—10Nb (NG) [10] 420 23 0.5 × 10.sup.−3 Ti—45Al—8Nb (FL) [11] 505 — 0.5 × 10.sup.−3 Ti—45Al—8Nb (NL) [11] 490 — 0.5 × 10.sup.−3 Ti—45Al—8Nb (DFL) [11] 435 — 0.5 × 10.sup.−3 [0115] 1. D. R. Johnson, H. Inui, M. Yamaguchi, Acta Mater. 44, 2523-2535 (1996). [0116] 2. D. R. Johnson, Y. Masuda, H. Inui and M. Yamaguchi, Acta Mater. 45, 2523-2533 (1997). [0117] 3. I. S. Jung, H. S. Jang, M. H. Oh, J. H. Lee, D. M. Wee, Mater. Sci. Eng. A 329-331, 13-18 (2002). [0118] 4. T. Yamanaka, D. R. Johnson, H. Inui, M. Yamaguchi, Intermetallics 7, 779-784 (1997). [0119] 5. D. R. Johnson, K. Chihara, H. Inui, M. Yamaguchi, Acta Mater. 46, 6529-6540 (1998). [0120] 6. H. N. Lee, D. R. Johnson, H. Inui, M. H. Oh, D. M. Wee, M. Yamaguchi, Acta Mater. 48, 3221-3233 (2000). [0121] 7. G. L. Chen, W. J. Zhang, Z. C. Liu, S. J. Li, Y. W. Kim, Gamma titanium aluminides, 31-40 (1999). [0122] 8. H. Clemens, I. Rumberg, P. Schretter. Intermetallics, 2(3), 179-184 (1994). [0123] 9. T. Tetsui. Structural Intermetallics, 489-493 (1997). [0124] 10. Y. W. Kim. Mater. Sci. Eng. A. 192-193, 519-533 (1995). [0125] 11. Z. C. Liu, J. P. Lin, S. J. Li. Intermetallics 10(7), 653-659 (2002).