Lightweight Heat-Resistant Material for Generator Gas Turbine

Abstract

The present invention overcomes brittleness of various beryllium alloys including beryllium intermetallic compounds and thereby provides a lightweight material for gas turbines. It's achieved by employing an alloy consisting essentially of an alloy of any one or more intermetallic compounds selected from the group of Be12M, but Be13M if M is Zr, Be10M, and Be17M, where M is any one or more metallic elements selected from the group of Ti, V, Mo, W, Zr, Nb and Ta.

Description

TECHNICAL FIELD

The present invention relates to a material for gas turbines used for driving power generators or such, and more specifically to a lightweight heat-resistant material which enables raise in operation temperature of gas turbines as having high heat-resistance, and further accomplishes high energy conversion efficiency as being lightweight.

BACKGROUND ART

A gas turbine is known as a machine to extract driving force from hot gas obtained by combustion of fuel for driving power generators, jet engines and such. It is prominently effective in improvement of energy conversion efficiency to introduce the hot gas in higher temperature state into the gas turbine, namely, to raise operation temperature of the gas turbine. Further, to make rotary members such as rotor blades lighter in weight is also effective in improvement of the energy conversion efficiency.

What are representative among materials in practical use for gas turbines are a group of nickel-base superalloys provided in the name of INCONEL. While the nickel-base superalloys have relatively favorable heat-resistance and oxidation-resistance, practical operation temperature of the gas turbines thereof cannot exceed 1000 degrees C. Moreover, as the alloys are based on nickel, its relative densities are relatively great such as 8 g/cm³ or more. Japanese Patent Application Laid-open No. H07-207391 discloses related nickel-base superalloys for gas turbines.

Application of titanium alloys is under search so as to accomplish higher efficiency than the nickel-base superalloys. Titanium alloys have an advantage of relatively small relative density such as about 4.5 g/cm³, but also have problems in erosion-resistance and such. International Patent Application Publication WO97/10066 discloses a method for improving erosion-resistance of turbine blades made of titanium alloys.

On the other hand, beryllium alloys including beryllium intermetallic compounds are 4.5 g/cm³ or less in relative density and hence prominently attractive in view of weight saving. Japanese Patent Application Laid-open No. 2004-093269 discloses that, by making beryllium alloys including beryllium intermetallic compounds into micro-spheres of approximate 1 mm in diameter, they come to be applicable as a neutron multiplier for atomic fusion reactors. Meanwhile, Japanese Patent Application Laid-open No. 2004-093270 discloses that, by carrying out HIP forming with respect to powder of beryllium alloys including beryllium intermetallic compounds, they come to be applicable as plasma facing materials for atomic fusion reactors. However, it is known that control of impurities in matters including beryllium is difficult and various characteristics considerably change depending on the amount of impurities. Moreover, as most of the materials including beryllium are brittle at the room temperature, methods for using the materials as structural materials are hardly ever known. Further, because of the brittleness, forming and machining, such as cutting, per se are hard to be accomplished aside from the aforementioned HIP forming. Therefore, industrial applicability of the beryllium alloys including beryllium intermetallic compounds has not been well known in the past.

DISCLOSURE OF INVENTION

The present invention is intended for overcoming brittleness of various beryllium alloys including beryllium intermetallic compounds and thereby providing a lightweight material for gas turbines, which has high heat-resistance and high-temperature corrosion resistance.

The present inventors had intently studied relations among phases and metallographic structures contained in beryllium alloys including beryllium intermetallic compounds and brittleness, heat-resistance and high-temperature corrosion resistance. As a result, the inventors had discovered a beryllium intermetallic compound phase of Be₁₂M (when M is any one of Ti, V, Mo, W, Nb and Ta) or Be₁₃Zr, and Be₁₇M₂ (M is any metallic element of Ti, V, Mo, W, Zr, Nb and Ta) is extremely brittle when existing as a single phase, whereas a composite phase in which two or more of Be₁₂M or Be₁₃Zr, Be₁₀M, and Be₁₇M are mixed as cast has ductility and is further given high heat-resistance and high-temperature corrosion resistance, and thereby reached the present invention of a lightweight material for generator gas turbines.

A material for gas turbines in accordance with a first aspect of the present invention, consists essentially of an alloy of any one or more intermetallic compounds selected from the group of Be₁₂M, but Be₁₃M if M is Zr, Be₁₀M, and Be₁₇M, where M is any one or more metallic elements selected from the group of Ti, V, Mo, W, Zr, Nb and Ta.

Preferably, the material for the gas turbines contains a composite phase including any two or more selected from the group of Be₁₂M, but Be₁₃M if M is Zr, Be₁₀M, and Be₁₇M.

More preferably, in the material for the gas turbines, an average content x of M satisfies 7.7<x<10.5 (at %), but 7.1<x<10.5 (at %) if M is Zr.

A material for gas turbines in accordance with a second aspect of the present invention is produced by mixing, smelting, and casting one or more metals M selected from the group of Ti, V, Zr, Nb, Ta, Mo, W and Y, with Be and unavoidable impurities as a balance in a range of an average content x of M satisfying 7.7<x<10.5 (at %), but 7.1<x<10.5 (at %) if M is Zr.

Preferably, the material for the gas turbines is further accomplished of a homogenizing treatment after casting and cooled in a furnace.

Preferably, the material for the gas turbines is formed by casting and an average grain size of the alloy is 50 μm or less.

More preferably, the material for the gas turbines is in that casting is accomplished by a unidirectional solidification.

More preferably, the material for the gas turbines is formed by powder metallurgy and an average grain size of the material is 50 μm or less.

In accordance with a third aspect of the present invention, a gas turbine rotor blade is composed of any of the aforementioned materials for the gas turbines.

High heat-resistance and high-temperature corrosion resistance are given to beryllium alloys including beryllium intermetallic compounds, and thereby a lightweight material for generator gas turbines is provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing temperature dependency of specific strength of an alloy B and nickel-base superalloys in practical use.

FIG. 2 shows stress-strain curves of the alloy B at the room temperature, 1000, 1100, and 1200 degrees C., and stress-strain curves of an alloy C at 1200 degrees C.

FIG. 3 is a drawing showing temperature dependency of fracture stress of the respective alloys.

FIG. 4 shows a Larson-Miller plot of creep strength (minimum creep speed) of the alloy B.

BEST MODE FOR CARRYING OUT THE INVENTION

Beryllium (Be) is a metal which can form intermetallic compounds with various metals. As metals forming beryllium intermetallic compounds, Ti, V, Mo, W, Zr, Nb and Ta can be exemplified. Because these metallic elements are equivalent in chemical behavior in relationships with beryllium, embodiments will be described hereinafter with taking Ti as an example.

Be and Ti form intermetallic compounds having various compositions. At a Be-rich side, important intermetallic compounds related to the embodiment of the present invention are three compounds of Be₁₂Ti, Be₁₀Ti and Be₁₇Ti₂. If casting is accomplished when a Be:Ti ratio is within 12:1 through 17:2, namely when a mixing ratio is within 7.7 through 10.5 at % in average content of Ti, a composite phase of any two or more of Be₁₂Ti, Be₁₀Ti and Be₁₇Ti₂ is obtained. On the other hand, at 7.7 at % in average content of Ti, a single phase of Be₁₃Ti is obtained, and, if richer in Be than those, a Be phase will be crystallized to give Be—Be₁₃Ti alloys. Further, if casting is accomplished when a mixing ratio in average content of Ti is 10.5 at %, a single phase of Be₁₇Ti₂ is obtained, and, if richer in Ti than those, the other intermetallic compounds will be crystallized.

A material for gas turbines in accordance with the present invention is composed of a composite phase of any two or more of Be₁₂Ti, Be₁₀Ti and Be₁₇Ti₂ obtained in mixing ratios of Be:Ti between 12:1 and 17:2, namely mixing ratios in average content of Ti from 7.7 through 10.5 at %. Casting is preferably accomplished by a unidirectional solidification. Instead of casting, powder metallurgy may be applied.

Further, after casing or powder metallurgy, a homogenizing treatment is preferably accomplished. The homogenizing treatment is to cancel segregation in a material by heating the material to a predetermined temperature below and near a melting point thereof, keeping the state for a predetermined period of time, and then carrying out furnace cool. Regarding the heating temperature and the keeping period of time, 1100 degrees C. and 12 hours are exemplified. Since the effect of the homogenizing treatment cannot be sufficiently obtained if the heating temperature is too low, the heating temperature is preferably 1000 degrees C. or higher, more preferably 1100 degrees C. or higher and lower than the melting point. Since the effect of the homogenizing treatment cannot be sufficiently obtained if the keeping period of time is too short, the keeping period of time is preferably 2 hours or longer, more preferably 6 hours or longer, and further preferably 12 hours or longer. Since economic efficiency is deteriorated if the keeping period of time is extremely long, the keeping period of time is preferably 48 hours or less. Meanwhile, a required period of time changes depending on the heating temperature, and a higher temperature leads to a shorter period of time.

In the above description, Ti can be replaced by any one or more metallic elements of V, Mo, W, Zr, Nb and Ta, and it leads to obtaining similar intermetallic compounds. More specifically, when M is any one or more metallic elements selected from the group of Ti, V, Mo, W, Zr, Nb and Ta, if casting is accomplished when a Be:M ratio is within 12:1 through 17:2, namely when a mixing ratio is within 7.7 through 10.5 at % in average content of M, a composite phase of any two or more of Be₁₂M, Be₁₀M and Be₁₇M₂ is obtained. However, if M is Zr, Be₁₃M (namely, Be₁₃Zr) instead of Be₁₂M is crystallized, and if casting is accomplished when a Be:M ratio is within 13:1 through 17:2, namely when a mixing ratio is within 7.1 through 10.5 at % in average content of M, a composite phase of any two or more of Be₁₃M, Be₁₀M and Be₁₇M₂ is obtained. These materials are equivalent in the present invention.

As a material for gas turbines in practical use, high-temperature strength, high-temperature creep characteristics, and high-temperature corrosion resistance are regarded as important. For giving further detailed descriptions about these characteristics, working examples in accordance with the present invention and comparative examples will be described hereinafter.

WORKING EXAMPLES

(Melting And Casting)

Melting and casting were accomplished from a beryllium raw material and a titanium raw material shown in Table 1. The beryllium raw material was a vacuum fusion ingot of 100-150 mm in magnitude, and the titanium raw material was a rod of 50-70 mm in diameter and 200-300 mm in length. These raw materials were mixed in predetermined mixing ratios of Be to Ti and cast.

TABLE 1
Chemical compositions of the casting materials (unit: wt %)
		beryllium raw	titanium raw
	element	material	material
	Be	99.45	—
	Ti	—	Bal.
	O	0.053	0.1
	Mg	0.014	0.011
	Si	0.016	0.020
	Al	0.020	0.38
	Cr	0.035	—
	Fe	0.18	0.046
	Co	—	—
	Ni	0.030	—
	Cu	0.028	—
	Cl	—	—

A smelting furnace used for melting and casting was a vacuum induction smelting furnace which was provided with an induction heating apparatus of a frequency of 1200 Hz, was capable of evacuating the interior down to 0.5 mmHg, and was provided with a crucible of 20-litter capacity. As the crucible, one made of beryllia (BeO) was applied, and, intervening between an induction heating coil and the crucible, a magnesia mold or a graphite mold with a lining of magnesia was applied.

After carrying out lining on the mold and the crucible, drying was carried out for 24 hours so as to remove humidity thereon entirely.

After this, the mold or the crucible was inserted into the furnace body, subsequently the interior of the furnace was evacuated down to 0.5-1 mmHg, and further argon gas was filled therein up to 0.5 atm. Melting was carried out with an output of 150-180 kW. After the raw materials were completely melted, the melts were further heated up to a temperature higher by 100-150 degrees than the melting point, subsequently the interior of the furnace was exhausted down to 0.5-1 mmHg, and then the heating was completed. The molten metal were subject to casting by tilting the furnace body. Ingots were cooled for 4 hours or more, 18 hours at a maximum, and then extracted. Further, as mentioned later, some samples were subject to the homogenizing treatment after being extracted, in which the samples were re-heated and kept at 1000-1300 degrees C. for 12 hours or more and the furnace cool were carried out.

(Mechanical Property Evaluation)

Four kinds of alloys having average chemical components shown in Table 2 were melted and cast in accordance with the aforementioned melting and casting method. However, regarding the alloys B and D, an arc melting method was applied. The alloy A has a single phase of Be₁₂Ti, the alloys B and C have composite phases of two or more phases of Be₁₂Ti, Be₁₀Ti and Be₁₇Ti₂, and the alloy D has a single phase of Be₁₇Ti₂. Further, the alloy B′ is what is obtained by carrying out the aforementioned homogenizing treatment with respect to the alloy B.

From the obtained alloys, test pieces of about 3×3×6 mm³ were cut out by using a fine cutter, and precisely rectangular solid test pieces were obtained there from by using a rotary grinder. The obtained test pieces were subject to a compression test and a creep test.

The compression test were carried out by using an Instron-type testing machine “Autograph AG-10TE” produced by SHIMADZU CORPORATION under a strain rate of about 1×10⁻⁴ s⁻¹. Test temperatures were the room temperature and high temperatures of 1000, 1100 and 1200 degrees C. The high-temperature tests were carried out by heating the test pieces in vacuum with a “Siliconit” heating element, and a pushing jig made of SiC were used. Stress-strain curves are drawn from obtained load-displacement curves, and thereby fracture stresses and fracture elongations were found. Further, with respect to any test pieces on which 0.2% fluid stresses were observed, they were regarded as yield stresses thereof.

The creep test were carried out by using a super-high-temperature creep testing machine “HCTT-3000” produced by TOSHIN KOGYO CO., LTD. under a loaded stress of 80 MPa and a temperature of 1200 degrees C. in an argon atmosphere. Heating was accomplished by using a tungsten heater enclosing the whole of the testing machine. To a pressure plate, an alumina plate with a BN coating was applied. Thereby, minimum creep speeds (stationary creep speeds) were found.

TABLE 2
Chemical compositions of alloys (units: wt %, however,
values in parentheses are converted to values in at %)
element	A	B	C	D
Be	69.6	67.04	63.8	58.99
Ti	30.0 (7.5)	31.6 (8.1)	35.8 (9.5)	40.0 (11.3)
V	<0.01		<0.01
W	<0.05		<0.05
Mg	<0.001	<0.01	<0.001	0.010
Al	0.084	0.11	0.058	0.18
Si	0.025	0.026	0.025	0.009
Cr	0.007	0.11	0.004	0.11
Fe	0.056	0.39	0.034	0.46
Co	<0.001	<0.01	<0.001	<0.01
Ni	0.003	0.064	0.003	0.067
Cu	0.009	0.017	0.009	0.016
Mn	0.007		0.007
Sc	<0.001		<0.001
BeO	0.22	0.44	0.19	0.12

Results of the compression test are shown in Table 3. The respective tests are accomplished in n=2. With respect to the alloys B and C, 0.2% proof stresses could be found and are hence shown therein altogether. With respect to any conditions or alloys but them, test pieces fractured at early stages without plastic deformation. FIG. 2 shows stress-strain curves of the alloys B and C obtained from the compression test. Further, FIG. 3 shows temperature dependency of fracture stresses of the respective alloys. Further, FIG. 1 shows temperature dependency of static specific strengths of the alloy B and a nickel-based superalloy as a comparative example. Here, the specific strengths were calculated with assumptions of a density to be 2.2 g/cm³ with respect to the alloy B, and a density to be 8.5 g/cm³ with respect to the nickel-base superalloy. The alloy B exceeds the nickel-base superalloy in specific strength at the room temperature, and further prominently exceeds the nickel-base superalloy at high-temperatures of 1000 degrees C. or higher.

TABLE 3
Results of the compression test
					1200° C.
	R.T.	1000° C.	1100° C.		0.2%
	fracture	fracture	fracture	fracture	proof	fracture
	stress	stress	stress	stress	stress	elonga-
alloy	(MPa)	(MPa)	(MPa)	(MPa)	(MPa)	tion (%)
A	595.0	718.9	477.0	148.2
	488.3	473.5	436.6	112.3
B	467.3	790.1	594.3	359.4	357.8	1.2
	460.3	767.5	522.7	265.2	259.8	0.7
B′	481.3	800.3	613.5	367.2	365.8	2.3
heat-	482.1	797.6	608.9	376.5	369.3	2.7
treated
C	361.8	736.1	620.7	97.5	85.5	12.8
	197.7	529.0	471.6	89.2	83.3	2.0
D	319.6	481.0	281.0	155.1
	121.6	70.1	43.3	86.5

Results of the creep test with respect to the alloy B at 1200 degrees C. is shown in Table 4. Moreover, FIG. 3 is a drawing in which the creep test results are made in a Larson-Miller plot, and estimation of characteristics at the other temperatures estimated from the result of 1200 degrees C. is shown by a dotted line therein.

TABLE 4
Results of the creep test with respect to the alloy B
	test		specific
	temperature	stress	strength	minimum creep speed
alloy	(° C.)	(MPa)	(MPa)	1/sec	%/hr
B	1200	80	33.3	3.40E−07	1.22E−01

(Chemical Property Evaluation)

Alloys shown in Table 5 were melted and cast, and an oxidizing evaluation test was accomplished.

Test pieces provided for a compatibility test were what was cut out in a disc shape of Φ8×2 mm from the ingot by means of electric spark wire-cutting and then processed with mirror grinding on both sides thereof.

TABLE 5
	comparative	working examples of the
ele-	examples	present invention
ment	E	F	G	H	I	J	K
Be	77.6	69.8	70.0	65.1	60.8	69.1	62.3
Ti	22.0	29.8	34.4	34.4	38.8	30.5	37.0
V	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01	<0.01
W	<0.05	<0.05	<0.05	<0.05	0.15	<0.05	<0.05
Mg	0.005	0.003	<0.001	<0.001	<0.001	<0.001	<0.001
Al	0.026	0.030	0.077	0.067	0.055	0.10	0.20
Si	0.008	0.023	0.028	0.025	0.024	0.018	0.051
Cr	0.003	0.005	0.006	0.006	0.003	0.026	0.053
Fe	0.043	0.032	0.046	0.037	0.036	0.081	0.22
Co	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
Ni	0.002	0.001	0.003	0.002	0.002	<0.001	0.008
Cu	0.011	0.013	0.016	0.009	0.006	0.012	0.017
Mn	0.004	0.005	0.010	0.009	0.006	0.006	0.013
Sc	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
BeO	—	—	0.36	0.31	0.10	0.13	0.10
U	0.0036	0.0030	—	—	—	—	—

Oxidation reactivity was evaluated by keeping the respective alloys and Inconel 738 as a comparative example in high-temperature dry air for a definite period of time, and measuring amounts of increase in masses per unit surface area. The heating temperatures were 1000 and 1200 degrees C., and the times of heating were evenly set to be 24 hours. Results of measurement of oxidation mass increase are shown in Table 6.

TABLE 6
Oxidation mass increases of the respective
alloys after 24 hours (g/m2)
	alloy	1000° C.	1200° C.
	E	123	—
	F	5.1	—
	G	15.1	159.1
	J	5.7	8.8
	K	3.8	8.7
	Inconel 738	17.7	22.0

In the alloy E of the comparative examples, as being understood from comparison with Inconel 738, the oxidation mass increase cannot be considered to be small enough. The alloys F, G, J, K, as long as comparing values at the temperature of 1000 degrees C., have smaller oxidation mass increases as compared with one of Inconel 738, therefore it should be understood that these alloys have prominently high high-temperature corrosion resistance.

INDUSTRIAL APPLICABILITY

High heat-resistance and high-temperature corrosion resistance are given to beryllium alloys including beryllium intermetallic compounds and thereby a lightweight material for gas turbines is provided.

Claims

1. A material for gas turbines, consisting essentially of: an alloy of any one or more intermetallic compounds selected from the group of Be12M, but Be13M if M is Zr, Be10M, and Be17M2, where M is any one or more metallic elements selected from the group of Ti, V, Mo, W, Zr, Nb and Ta.

2. The material for gas turbines as recited in claim 1, wherein the alloy includes a composite phase including any two or more selected from the group of Be12M, but Be13M if M is Zr, Be10M, and Be17M2.

3. The material for gas turbines as recited in claim 1, wherein an average content x of M satisfies 7.7<x<10.5 (at %), but 7.1<x<10.5 (at %) if M is Zr.

4. The material for gas turbines as recited in claim 2, wherein an average content x of M satisfies 7.7<x<10.5 (at %), but 7.1<x<10.5 (at %) if M is Zr.

5. A material for gas turbines, wherein the alloy is produced by mixing, smelting, and casting one or more metals M selected from the group of Ti, V, Zr, Nb, Ta, Mo, W and Y, with Be and unavoidable impurities as a balance in a range of an average content x of M satisfying 7.7<x<10.5 (at %), but 7.1<x<10.5 (at %) if M is Zr.

6. The material for gas turbines as recited in claim 5, wherein a homogenizing treatment and furnace cooling are accomplished after casting.

7. The material for gas turbines as recited in claim 1, wherein the alloy is produced by casting and an average grain size of the alloy is 50 μm or less.

8. The material for gas turbines as recited in claim 7, wherein the casting is accomplished by a unidirectional solidification.

9. The material for gas turbines as recited in claim 1, wherein the alloy is produced by powder metallurgy and an average grain size of the material is 50 μm or less.

10. The material for gas turbines as recited in claim 5, wherein the alloy is produced by casting and an average grain size of the alloy is 50 μm or less.

11. The material for gas turbines as recited in claim 5, wherein the casting is accomplished by a unidirectional solidification.

12. The material for gas turbines as recited in claim 5, wherein the alloy is produced by powder metallurgy and an average grain size of the material is 50 μm or less.

13. A gas turbine rotor blade comprising the material for gas turbines as recited in claim 1.

14. A gas turbine rotor blade comprising the material for gas turbines as recited in claim 5.