KR100209305B1 - Manufacturing method of gamma titanium aluminaid alloy - Google Patents

Abstract

In the present invention, the titanium aluminide alloy for high strength heat resistant structure has excellent mechanical properties at room temperature as well as at high temperature, so as to facilitate general machining at room temperature in atomic%, Al: 40-43%, Cr: 1-4%, Nb: 4% or less, B: 0.3-2.5%, and the balance consists of Ti and inevitable impurities, or Al: 43-47%, Cr: 1-4%, Nb: 4% or less, B: 0.1 -0.5%, and the remainder are high strength, high ductility gamma-based titanium aluminide alloys and atomic weight% consisting of Ti and inevitable impurities, Al: 40-43%, Cr: 1-4%, Nb: 4 % Or less, B: 0.3-2.5%, and the balance consists of Ti and inevitable impurities, or Al: 43-47%, Cr: 1-4%, Nb: 4% or less, B: 0.1-0.5% And the remainder of the cast of alloy consisting of Ti and unavoidable impurities and the workpiece of the cast for 1-72 hours at 900-1400 ° C. Having the fine crystal grains of 10-100㎛ performing provides a method for producing high strength, high heat-resistant structural systems gamma titanium aluminide alloy ductility.

Description

High-strength, high-ductility, gamma-based titanium aluminide alloy for heat-resistant structure and manufacturing method

The present invention is a high strength, mostly used for casting parts such as gas turbine engines, automobile engines, turbine blades of high pressure and low pressure compressors, exhaust engine valves and turbocharger rotors that are exposed for a long time in a high temperature and harsh oxidation atmosphere, The present invention relates to a high ductility gamma-based titanium aluminide alloy and a method of manufacturing the same.

Adding about 49 to 60 atomic percent aluminum to titanium forms a TiAl intermetallic compound having a regular structure called gamma (γ). This gamma phase is a high-temperature structural material that can replace nickel base superalloy due to its light weight, high oxidation resistance (~ 1000 ℃), strength holding ability (~ 700 ℃), and creep resistance. It is getting a lot of attention. However, gamma TiAl intermetallic compounds exhibit very low ductility at low temperatures (20 to 600 ° C.), which makes machining difficult and general low strength.

One way to overcome this drawback is to use a two-phase structure in which the Ti ₃ Al (alpha 2) phase coexists with a gamma TiAl matrix. This two-phase structure appears in Ti- (39-49 atomic%) Al composition. In general, in the two-phase region, 47-48 atomic% Al alloy having a high fraction of gamma phase is known to be excellent not only in strength but also in elongation. If the aluminum content is increased to 50 atomic% or more based on this composition, only the gamma phase is present, and the oxidation resistance is slightly increased, but mechanical properties such as elongation, strength and fracture resistance are poor. Therefore, in the conventional alloy, efforts have been mainly made to improve or reinforce the mechanical properties of the material in the composition range of 47 to 48 atomic% Al.

Conventional techniques related to this are well described in US Pat. Nos. 4,294,615, 4,842,819, 4,842,820, 4,879,092 and the like, and mainly attempted to improve mechanical properties by adding third and fourth elements. The aluminum composition range of these conventional alloys is an alloy having a gamma phase predominantly at approximately 46 to 50 atomic%.

U.S. Patent 4,294,615 intends to improve ductility by adding vanadium (V) to gamma TiAl base, and U.S. Patent 4,842,820 adds boron (B) to increase strength and ductility. US Pat. Nos. 4,842,819 and 4,879,092 attempt to improve ductility by adding chromium (Cr), respectively, and improve ductility and oxidation resistance by simultaneously adding chromium and niobium.

However, in order to improve the ductility, they are particularly hot processed such as hot forging, quench solidification, and hot extrusion, and it is not easy to predict whether the characteristics of a simple cast body will be improved from these results. In addition, the mechanical properties of the reported workpiece are also difficult to determine the absolute tensile properties at room temperature because high temperature measurement or bending test is used.

Patents relating to pure casts include US Pat. Nos. 4,661,316, 4,849,168, 5,080,860 and the like.

U.S. Patent 4,661,316 is to improve ductility by adding Mn, and 4,849,168 is to improve castability by adding nickel and silicon while adding trace amount (0.3 wt% or less) of B.

However, it is not clear how the room temperature mechanical properties have been improved by performing bending tests or high temperature tensile tests.

On the other hand, U.S. Patent 5,080,860 seeks to improve castability and to obtain a cast structure having a uniform equiaxed structure by adding 0.5 to 2 atomic% B in a gamma base alloy containing trace amounts of Cr and Nb. At this time, the obtained room temperature yield strength was 503-565 MPa and the elongation was 0.1-1.4%. This resulted in a much higher strength than the yield strength (351 to 386 MPa) and the elongation (1.3 to 2,1%) of Ti-48 atomic% Al binary alloy, and the elongation was slightly lowered.

The inventors of the present invention have made continuous studies to solve the above-mentioned problems of the prior art, and the fact that when the boron is added in a small amount and the aluminum content is relatively small, the grain size of the cast rapidly decreases from 600 to 900 µm to 10 to 40 µm. As a result, a new finding was found that the yield strength of the cast body increased to 530-800 MPa and the elongation also showed a high elongation of 0.7-1.6%.

This is due to the complex action of boron and aluminum, and the mode of high temperature solid state transformation (beta phase → alpha phase) generated during solidification at the proper boron and aluminum content changes rapidly. This is because a fine Widmanstatten tissue of is obtained. In order to obtain an extremely fine grain structure for this unique basket weave tissue formation, it is essential to properly select the Al content and the B content.

1 is a chart comparing the tensile properties of the alloy according to the prior art (alloy not containing B and the alloy containing B) and the alloy having a high Al content.

(A) Alloy according to the prior art without containing B (Comparative Example 9, 1290 ° C)

(B) Alloy according to the prior art containing B (comparative example 14, 1290 degreeC)

(C) Alloy according to the present invention with high Al content (Example 2, 1290 ° C.)

2 is a chart comparing the tensile properties of the alloy according to the prior art (including B) and the alloy according to the present invention (high strength alloy and high ductility alloy) having a low Al content.

(A) Alloy according to the prior art containing B (comparative example 14, 1290 degreeC)

(B) High-strength alloy according to the present invention (Example 8)

(C) High Ductility Alloy According to the Present Invention (Example 11)

3 is an optical micrograph comparing the grain structure of the cast structure for each of the following alloys.

(a) Alloy according to the prior art without containing B (Comparative Example 9)

(b) Alloy according to the prior art containing B (Comparative Example 14): Grains can be distinguished in different lamellar directions (fine black lines are boride compounds)

(c) Alloy according to the present invention (Example 7)

Figure 4 is an optical micrograph showing the change of the heat treatment casting structure (basket weave structure) according to the B content change in the alloy according to the present invention.

(a) The alloy according to the present invention (Ti-42.8Al-2Cr-4Nb-1.0B) having a low B content was maintained at 1290 ° C. for 24 hours and then air-cooled and maintained at 900 ° C. for 4 hours.

(b) After maintaining the alloy according to the present invention (Ti-42.8Al-2Cr-4Nb-2.0B) with a large amount of B for 24 hours at 1290 ° C and air-cooled and maintained at 900 ° C for 4 hours.

The present invention is atomic percent, Al: 40-43%, Cr: 1-4%, Nb: 4% or less, B: 0.3-2.5%, and the remainder is made of Ti and inevitable impurities, or Al: 43-47%, Cr: 1-4%, Nb: 4% or less, B: 0.1-0.5%, and the remainder are high-strength, high-ductility gamma-based titanium aluminide alloys made of Ti and inevitable impurities. to provide.

In addition, the present invention is the atomic weight%, Al: 40-43%, Cr: 1-4%, Nb: 4% or less, B: 0.3-2.5%, and the rest is made of Ti and inevitable impurities, or A cast of alloy and a workpiece of the cast made of Al: 43-47%, Cr: 1-4%, Nb: 4% or less, B: 0.1-0.5%, and the remainder are Ti and inevitable impurities. Provided is a method for producing a high-strength, high-ductility, gamma-based titanium aluminide alloy having fine grains of 10-100 µm which are heat treated at -1400 ° C. for 1-72 hours.

In addition, the present invention provides a high-strength, high-ductility, gamma-based titanium aluminide alloy having fine grains which are subjected to a hiping treatment at 900-1200 ° C. prior to the heat treatment of the casting of the alloy. Provide a method.

The reason for limitation of chemical composition in the present invention is as follows.

[Aluminum and boron]

If the aluminum (Al) is 47 atomic% or more, the cast structure peculiar to the present invention consisting of extremely fine grains (10-40 μm) due to the formation of the basket weave cannot be obtained, and the content of aluminum is 40 atomic% If less, the fraction of gamma phase in the two-phase tissue of the present invention becomes insufficient.

When the aluminum content is less than 47 atomic% and 43 atomic%, the B content for obtaining a cast structure peculiar to the present invention is 0.1 to 0.5 atomic%. When the aluminum content is 43 atomic% or less, a considerable amount of gamma phase is contained, and in order to obtain a cast structure peculiar to the present invention, 0.3 atomic% or more of B is required. Can be.

Boron (B) is effective in miniaturizing the casting structure and heat treatment structure in addition to the purpose of forming the cast structure unique to the present invention described above, and also has an effect of greatly improving the castability by increasing the fluidity of the casting.

[chrome]

Chromium (Cr) is added for the purpose of changing the equilibrium composition of the gamma phase to improve the ductility of the gamma phase. If the content is less than 1 atomic percent, the effect is almost no, and if more than 4 atomic percent, the ductility is reduced.

[Niobium]

Niobium (Nb) is added for the purpose of improving the strength and oxidation resistance of the TiAl intermetallic compound, but when the content exceeds 4 atomic percent, ductility decreases.

The cast body manufactured by the alloy composition of the present invention can be subjected to a hot (Hot Isostatic Processing (HIPing)) treatment at 900 ~ 1200 ℃ can remove casting defects, such as homogeneous effect and fine processing.

Castings or bottomed castings can be heat treated for 1 to 72 hours in the range 900 ~ 1400 ℃ to obtain a uniform microstructure. In this case, a tissue close to the gamma single phase containing a small amount of the second phase, a gamma phase, an alpha two phase, a two-phase tissue mixed with equiaxed grains, a lamellar tissue composed of a fine gamma / alpha two-phase layered tissue, and the like may be formed. Each tissue is mainly determined by the aluminum content and the heat treatment temperature. Therefore, the mechanical properties of the cast are greatly affected by the heat treatment temperature (and time) as well as the aluminum content.

The cast body of the alloy of the present invention is significantly different from that of the conventional alloy, it is composed of extremely fine grains (10 ~ 40㎛) by the formation of a unique basket weave cast structure, even when the heat treatment under the above conditions the shape is It is only equiaxed, its size is 30 ~ 70㎛, and no grain growth occurs. As a result, the cast or heat-treated castings of the alloy of the present invention have a much higher yield strength than conventional alloys (grain size ˜600 μm) containing no B as well as conventional alloys (grain size ˜200 μm) containing B. The elongation also showed a high value of 1.6% at the yield strength of 570 MPa.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples.

[Comparative Examples 1 to 7]

Seven alloy ingots were prepared with different concentrations of Ti and Al. Each ingot was made under argon atmosphere using a vacuum arc furnace. Each ingot was air cooled after heat treatment at 1200 ° C. for 24 hours for homogenization and microstructure control. Post-heat treatment was performed at 900 ° C. for 4 hours to stabilize the air-cooled tissue. This post heat treatment was applied to all the comparative examples and examples. Table 1 shows the tensile test and results (Φ3 × 18mm rod-shaped specimens). Alloys containing up to 46 atomic percent aluminum show brittle fracture with almost no elongation. This result seems to be attributed to the increase in the volume fraction of the alpha 2 phase. Alloys containing more than 50 atomic percent aluminum show very low elongation as well as strength. On the other hand, alloys containing 47 to 49 atomic percent aluminum show the best strength and elongation. However, their strength is about 260 to 359 MPa, which is not enough, and the microstructure is often uneven.

[Comparative Examples 8 to 10]

TiAl intermetallic compounds of three different compositions were prepared in the same manner as in Comparative Examples 1 to 7. Here, by adjusting the heat treatment temperature (1200 to 1400 ℃; 24 hours) to change the microstructure to derive the optimum mechanical properties. Table 2 shows a comparison of a Ti-47Al binary alloy showing excellent strength and ductility with a quaternary alloy prepared by adding chromium and niobium (US Pat. No. 4,879,092). Chromium tends to improve the tensile properties (strength and elongation) of the cast heat treated at around 1300 ° C. However, these alloys also do not have sufficient strength, and the casting structure is composed of coarse columnar tablets and has a disadvantage in that castability is not excellent.

[Examples 1 to 4 and Comparative Examples 11 to 14]

This Example and Comparative Example shows the mechanical properties of the gamma base intermetallic compound prepared by the same method as Comparative Examples 8 to 10 containing Cr and Nb, an aluminum content of 43% or more and a trace amount of B . Yield strength of the alloy as a result of adding a small amount of B (e.g. 0.4 atomic%) to a conventional alloy containing no B (Comparative Examples 9 and 10), unlike conventional B-containing alloys (US Pat. No. 5,080,860) The rate of increase was very high and the elongation was maintained at over 1%. When the Al content decreases below 46 atomic percent, the yield strength increases rapidly. However, in this case, when the heat treatment temperature is lowered, as shown in Example 2, the yield strength can be obtained at a high elongation of about 1.1 to 1.3% while maintaining the high strength of 570 to 570 MPa. This yield strength is a result of a significantly increased yield strength compared to the conventional Cr-Nb alloy (Comparative Example 9: Ti-47Al-2Cr-4Nb), as shown in FIG. 1, and the conventional Cr-Nb-B alloy ( Compared with Comparative Example 14), the yield strength was significantly increased. On the other hand, the elongation tends to decrease relatively, but also maintains more than 1.0%, indicating sufficient elongation for machining.

Thus, an alloy containing a small amount of B and an Al content of 47 atomic percent or less shows a very high yield strength and a considerable elongation, because there is a remarkable grain size difference in comparison with an alloy containing no B. That is, in the case of an alloy containing a small amount of B as described in the following Example (5-14), a fine basket weave-shaped casting structure develops and the grain size rapidly decreases.

When the Al content increased to more than 47 atomic%, both the strength and the elongation tended to decrease despite the addition of a small amount of B (Comparative Example 11). This is because the basket weave casting structure is not formed in this Al composition and the volume fraction of the gamma phase is significantly increased.

[Examples 5 to 14]

This Example is prepared by the same method as the Comparative Examples and Examples described above, and the composition is similar, but the measurement results of the mechanical properties of alloys with Al content of 43 atomic% or less and different B content. As the Al content decreases below 43 atomic percent, the strength of the alloy tends to increase steadily, resulting in very high yield strengths up to the 800 MPa level (Example 8). On the other hand, the elongation decreased somewhat, but it was nearly 1%. As shown in FIG. 2, this yield strength level corresponds to a 70-80% increase in strength compared to a conventional high strength Cr-Nb-B alloy (Comparative Example 14).

This significant difference in strength and elongation is due to the sharp difference in cast structure as shown in FIG. That is, in the case of the conventional Cr-Nb alloy (Comparative Example 9), the casting structure is composed of coarse columnar crystals, while the conventional high-strength Cr-Nb-B alloy (Comparative Example 14) is equiaxed crystal grain by B addition. This is formed and crystal grains tend to be considerably finer. On the other hand, the present invention (Example 7) shows a significantly finer grain structure than the conventional B alloy because the grain size was drastically reduced due to the development of the fine basket weave peculiar to the present invention. The development of the basketweave tissue is caused by the solid phase transformation of the high temperature beta phase to the alpha phase.

The effect of the B content was remarkably observed in the two-phase tissues of gamma + alpha 2 (annealed at 1200 ° C.), examples 10-13 illustrate this well. When the B content is increased, the elongation is increased to 1.5 atomic percent, reaching a maximum of 1.6 percent, and then decreasing. At this time, the strength decreased slightly, but the yield strength of 580 MPa was maintained. This result shows that the yield strength not only increased significantly but also the elongation increased compared to the conventional high strength B alloy (Comparative Example 14), as shown in FIG. 2, to reach the level of the conventional B-free alloy (Comparative Example 9). It shows one fact.

The reason why the elongation increases as the B content increases is that as shown in FIG. 4, the fine grain structure of the basket weave which is clearly observed when the B content is relatively small is reduced to a fine isometric structure as the B content increases. Because it changes. At this time, since only isotropic crystallization of grains occurs, the size does not change significantly. As a result, the alloy containing a large amount of B shows a fine equiaxed grain structure similar in size to the basket weave structure. If the B content is more than 1.5 atomic%, the elongation is lowered due to the formation of coarse boron compound, which is considered to have the maximum elongation at ~ 1.5 atomic% B.

Example 14 shows an effect of changing the Al content at the optimum B content (~ 1.5 atomic%). As the Al content decreases, the yield strength increases to 630 MPa or more, while the elongation slightly decreases, but the elongation higher than 1.3%. This shows that it is maintained.

The present invention provides a high-strength, high-ductility, gamma-based titanium aluminide alloy having fine grains used for parts of a cast body that are exposed to high temperatures and harsh oxidation atmospheres for a long time.

Claims (6)

High-strength, high-ductility heat-resistant gamma system composed of atomic weight%, Al: 40-43%, Cr: 1-4%, Nb: 4% or less, B: 0.3-2.5%, and the rest are Ti and inevitable impurities Titanium aluminide alloy. Of the cast and alloy of the alloy consisting of atomic weight% of Al: 40-43%, Cr: 1-4%, Nb: 4% or less, B: 0.3-2.5%, and the remainder of Ti and inevitable impurities A method for producing a high-strength, high-ductility, gamma-based titanium aluminide alloy having fine grains of 10-100 µm in which a processed material is heat-treated at 900-1400 ° C. for 1-72 hours. The method for producing a high-strength, high-ductility, gamma-based titanium aluminide alloy having fine grains subjected to a hiping treatment at 900-1200 ° C prior to the heat treatment of the casting of the alloy. GaA system for high strength, high ductility, heat resistant structure consisting of atomic weight%, 43: 47%, Cr: 1-4%, Nb: 4% or less, B: 0.1-0.5%, and the rest is Ti and inevitable impurities Titanium aluminide alloy. Castings and alloys of alloys consisting of atomic weight%, Al: 43-47%, Cr: 1-4%, Nb: 4% or less, B: 0.1-0.5%, and the rest are Ti and inevitable impurities Method for producing a high strength, high ductility gamma-based titanium aluminide alloy having a fine grain of 10-100㎛ to heat-treat the workpiece at 900-1400 ℃ for 1-72 hours. The method of producing a high strength, high ductility, gamma-based titanium aluminide alloy having fine grains, which is subjected to a HIPing treatment at 900-1200 ° C. prior to the heat treatment of the casting of the alloy.