EP1308529A1

EP1308529A1 - Titanium aluminum intermetallic compound based alloy and method of fabricating a product from the alloy

Info

Publication number: EP1308529A1
Application number: EP02024552A
Authority: EP
Inventors: Toshimitsu Nagasaki Res. & Dev. Center Tetsui
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 2001-11-05
Filing date: 2002-11-04
Publication date: 2003-05-07
Anticipated expiration: 2022-11-04
Also published as: US20030111141A1; DE60202731T2; DE60202731D1; JP2003147475A; JP4107830B2; EP1308529B1

Abstract

A titanium aluminum intermetallic compound based alloy superior in creep strength and low cycle fatigue strength made by casting. The alloy has lamellar structure. A volume ratio of non-lamellar structure of the alloy is equal to or less than 3 volume percent. Diameters of lamellar grains included in the alloy are equal to or less than 200 µm. Lamellar spacing of the lamella structure included in the alloy is equal to or less than 2 µm.

Description

Background of the Invention

1. Field of the Invention

The present invention is related to a titanium aluminum intermetallic compound based alloy (hereinafter, referred to as a TiAl based alloy) and a method of fabricating a product from the alloy.

2. Description of the Related Art

TiAl based alloys exhibit superior strength at high temperatures. Therefore, TiAl based alloys are expected to use for rotating components, such as turbine wheels of turbochargers and turbine blades of gas turbines.
Japanese Laid Open Patent Application (Jp-A-Heisei 9-143599) discloses a TiAl based alloy. The alloy consists of, in atomic percent, 45-48% aluminum, 0.5-2.0% nickel, 1.0-3.0% niobium, 0.2-1.0% tungsten, and 1.0-2.0% manganese, and a remainder including titanium plus impurities. The alloy has superior internal friction properties because of the appropriate nickel concentration.
Fabrication of products from TiAl based alloys is often achieved by casting. An issue of casting is that many defects, such as voids, are included in the cast products. Defects are desirably removed to increase mechanical properties of the cast products.
It is widely known that hot isostatic pressing (HIP) effectively removes defects from the cast product. HIP is a process in which cast products are exposed to a high temperature at an elevated pressure.
HIP, however, enlarges the lamellar spacing of lamellar structure of the TiAl based alloys. The enlargement of the lamellar spacing is undesirable, because the narrow lamellar spacing of the lamellar structure is an origin of superior mechanical properties of the TiAl based alloy. The increase in the lamella spacing degrades the creep strength of the TiAl based alloy.
In addition, the low cycle fatigue strength of TiAl based alloys is inferior to conventional metallic material because of its poor ductility. Especially in case of TiAl alloys made by casting, the low cycle fatigue strength is very small due to its large grain size. The poor low cycle fatigue strength is an issue, because gas turbines components are often exposed to severe thermal stress that causes low cycle fatigue.
Therefore, a need exists to provide a TiAl based alloy made by casting that exhibits superior properties in creep strength and low cycle fatigue strength.

Summary of the Invention

An object of the present invention is to provide a TiAl based alloy made by casting that is superior in creep strength.
Another object of the present invention is to provide a TiAl based alloy made by casting that is superior in low cycle fatigue strength.
Still another object of the present invention is to provide an TiAl based alloy made by casting that is superior in both of creep strength and low cycle fatigue strength.
In an aspect of the present invention, a TiAl based alloy has properties as follows. The TiAl based alloy has lamellar structure, and a volume ratio of non-lamellar structure of the alloy is equal to or less than 3 volume percent. Diameters of lamellar grains included in the alloy are equal to or less than 200 µm. Lamellar spacing of lamella structure included in the alloy is equal to or less than 2 µm.
The alloy preferably includes boron, a concentration of which is advantageously in the range of 0.2 to 1.2 atomic percent.
In another aspect of the present invention, an TiAl based alloy includes, in atomic percent, 43-48% aluminum, 2.0-5.0% niobium, 0.2-1.2% tungsten, 0.1-1.0% nickel, 0.2-1.2% boron, and a remainder including titanium plus impurities.
The TiAl based alloy preferably includes manganese, a concentration of which is advantageously in the range of 0.2 to 1.2 atomic percent.
The TiAl based alloy preferably includes chromium, a concentration of which is advantageously in the range of 0.2 to 1.2 atomic percent.
The TiAl based alloy preferably includes silicon, a concentration of which is advantageously in the range of 0.1 to 1.0 atomic percent.
The TiAl based alloy preferably includes carbon, a concentration of which is advantageously in the range of 0.1 to 0.5 atomic percent.
In still another aspect of the present invention, a method of fabricating a product from a TiAl based alloy is composed of
casting a TiAl intermetallic compound based alloy into a product;
executing hot isostatic pressing on the product;
heat-treating the product in a non-oxidizing environment after the hot isostatic pressing;
rapid cooling the product after the heat-treatment.
The product is preferably heated at 1320° to 1370° C. during the heat-treatment.
The product is preferably cooled to 1000 °C. at a cooling rate of 30° to 100° C./min. during the rapid cooling.
The fabricating method preferably includes:
second heat-treatment of the product after the rapid cooling; and
slowly cooling the product after the second heat-treatment.
The product is preferably heated at 900° to 1050° C. during the second heat-treatment.
The product is cooled at a cooling rate equal to or less than 10° C./min. during the slow cooling.
The fabricating method is advantageous when the TiAl based alloy includes boron, a concentration of which is preferably in the in the range of 0.2 to 1.2 atomic percent.

Brief Description of the Drawings

Fig. 1 is a perspective view of turbine wheels fabricated;
Fig. 2 is a table showing alloys and processes utilized for Examples 1 to 7, and evaluation result;
Fig. 3 is a table showing composition of Alloys 1 to 5;
Fig. 4 shows an optical micrograph of a cross section of Example 1;
Fig. 5 shows an optical micrograph of a cross section of Example 2;
Fig. 6 shows a back scattered electron micrograph of a cross section of Example 2 in the vicinity of a grain boundary;
Fig. 7 shows a back scattered electron micrograph of a cross section of Example 3 before the creep rupture test; and
Fig. 8 shows an optical micrograph of a cross section of Example 3 after the failure of the creep test.

Description of the Preferred Embodiments

In an embodiment of the present invention, a TiAl based alloy has properties as follows. The TiAl based alloy has a lamellar structure, and a volume ratio of non-lamellar structure thereof is equal to or less than 3 volume percent. Diameters of lamellar grains are equal to or less than 200 µm, and lamellar spacing is equal to or less than 2 µm. The small volume ratio of the non-lamellar structure, and the small lamellar spacing improves the creep strength of the TiAl based alloy, while the small diameters of the lamellar grains improves the low cycle fatigue strength.
The TiAl based alloy is composed of, in atomic percent, 43-48% aluminum (Al), 2.0-5.0% niobium (Nb), 0.2-1.2% tungsten (W), 0.1-1.0 nickel (Ni), 0.2-1.2% manganese (Mn), and 0.2-1.2% boron (B), and a remainder including titanium plus impurities.
The aluminum concentration of the TiAl based alloy is advantageously in the range of 43 to 48 atomic percent. An aluminum concentration less than 43 atomic percent degrades toughness of the alloy. On the other hand, aluminum concentration more than 48 atomic percent decreases the ratio of α₂ phase in the lamellar structure, and thus degrades the strength at high temperature.
The doping of niobium improves anti-oxidation resistance. The niobium concentration of the TiAl based alloy is advantageously in the range of 2.0 to 5.0 atomic percent. Niobium concentration less than 2.0 atomic percent is not effective to improve the anti-oxidation resistance. On the other hand, niobium concentration more than 5.0 atomic percent is not preferable because of increases in the specific gravity and the cost of the alloy.
The doping of tungsten improves the strength of the TiAl based alloy at high temperature. The tungsten concentration is advantageously in the range from 0.2 to 1.2 atomic percent. Tungsten concentration less than 0.2 atomic percent is not effective to improvement of the mechanical strength at high temperature. On the other hand, tungsten concentration more than 1.2 atomic percent is not preferable because of increases in the specific gravity and the cost of the alloy.
The doping of nickel increases the internal friction of the TiAl based alloy. The nickel concentration of the alloy is advantageously in the range of 0.1 to 1.0 atomic percent. Nickel concentration less than 0.1 atomic percent is not effective to increase in the internal friction. On the other hand, nickel concentration more than 1.0 atomic percent decreases the ductility at room temperature because of the production of harmful phases, such as Laves phases, in the alloy.
The doping of manganese improves low cycle fatigue strength. The manganese concentration of the TiAl based alloy is advantageously in the range of 0.2 to 1.2 atomic percent. Manganese concentration less than 0.2 atomic percent is not effective to the improvement of low cycle fatigue strength. On the other hand, manganese concentration more than 1.2 atomic percent degrades anti-oxidation resistance.
The doping of boron decreases the size of lamellar grains of the TiAl based alloy. The decrease in the grain size improves the low cycle fatigue strength of the alloy. The boron concentration of the alloy is advantageously in the range of 0.2 to 1.2 atomic percent. Boron concentration less than 0.2 atomic percent is not effective to the decrease in the grain size. On the other hand, Boron concentration more than 1.2 atomic percent degrades the brittleness of the alloy because of the precipitation of boride.
The doping of chromium is preferable to further improve the low cycle fatigue strength. The chromium concentration of the TiAl based alloy is advantageously in the range of 0.2 to 1.2 atomic percent. Chromium concentration less than 0.2 atomic percent is not effective to the improvement of the low cycle fatigue strength. On the other hand, chromium concentration more than 1.2 atomic percent degrades the creep strength because of the formation of β phase in the alloy.
The doping of silicon is preferable to further improve the creep strength of the TiAl based alloy. The silicon concentration of the alloy is advantageously in the range of 0.1 to 1.0 atomic percent. Silicon concentration less than 0.1 atomic percent is not effective to the improvement of the creep strength. Silicon concentration more than 1.0 atomic percent degrades low cycle fatigue strength.
The doping of carbon is also preferable to further improve the creep strength of the alloy. The carbon concentration of the alloy is advantageously in the range of 0.1 to 0.5 atomic percent. Carbon concentration less than 0.1 atomic percent is not effective to the improvement of the creep strength. Carbon concentration more than 0.5 atomic percent degrades low cycle fatigue strength.
A preferred process of fabricating a product from the aforementioned TiAl based alloy is described below.
The fabrication process starts with melting and casting the TiAl based alloy into a cast product. The boron doping into the TiAl based alloy effectively improves the low cycle fatigue strength of the cast product through miniaturizing the size of the lamellar grains of the product.
After the casting, the cast product is then exposed to HIP. The conditions of the HIP are as follows. The cast product is heated and pressed in an environment of substantially pure argon at a temperature of 1200° to 1300° C. for 1 to 3 hours. The applied pressure is equal to or more than 100 MPa. The argon environment effectively prevents the oxidation of the cast product.
The temperature range of 1200° to 1300° C. is advantageous because of the following reasons. HIP at a temperature lower than 1200° C. is not effective to the elimination of defects because of the poor deformablilty of the alloy. On the other hand, HIP at a temperature higher than 1300° C. is not preferable, because the argon environment inevitably includes a small amount of oxygen gas as an impurity and the small amount of oxygen gas may oxidize the cast product, because its partial pressure is increased by highpressure such as 100 MPa.
As mention above, HIP eliminates the defects of the cast product. The creep strength of the cast product, however, is degraded by HIP. HIP increases lamellar spacing of the lamellar structure, and produces non-lamellar structure at grain boundaries. The non-lamellar structure degrades the conformability of the lamellar grains. The increase in the lamellar spacing and the degraded conformability degrade the creep strength of the alloy.
In order to increase the creep strength of the alloy, the cast product is subjected to the following process. Firstly, the cast product is heat-treated in vacuum at a temperature from 1320° to 1370° C. for 20 to 60 minutes, and then rapidly cooled. This process may be referred to as a VHRCP (vacuum heating and rapid cooling process), hereinafter. The rapid cooling of the cast product is achieved by blowing argon gas having room temperature into the furnace used for the heat-treatment. The cast product is preferably cooled at a cooling rate of 30° to 100° C./min. till the metal product is cooled to 1000° C.
The VHRCP restructures the lamellar structure of the alloy, and effectively improves the creep strength. The VHRCP decreases the lamellar spacing and eliminates the non-lamella structure at the grain boundaries. The decrease in the lamellar spacing and the elimination of the non-lamella structure effectively improve the creep strength of the alloy.
The temperature of the heat-treatment in the range of 1320° to 1370° C. is advantageous because of the following reasons. A heat treatment at a temperature lower than 1320° C. does not cause the restructuring of the lamella structure. On the other hand, it is not preferable that the cast product is exposed to an excessively high temperature.
The VHRCP, which is effective to the improvement of the creep strength, produces remaining stress in the cast product. The remaining stress causes dynamic recrystallization, and thereby produces non-lamellar structure at the grain boundaries when an external mechanical stress is exerted on the cast product. The non-lamellar structure degrades the creep strength of the cast product.
In order to prevent dynamic recrystallization, the cast product is annealed at a temperature from 900° to 1050° C. for 3 to 50 hours in air. The annealing effectively removes the remaining stress. The annealing temperature from 900° to 1050° C. is advantageous. An annealing at a temperature lower than 900° C. is not effective to the removal of the remaining stress. An annealing at a temperature higher than 1050° C. destroys the lamellar structure restructured by the VHRCP. The treatment in inert gas atmosphere is not required, because oxidation of the TiAl alloy in the air is small in this temperature range.
As described, the structure and compositions of the TiAl based alloy and the fabricating process effectively improves both the creep strength and the low cycle fatigue strength of the cast product.

EXAMPLES

Seven turbine wheels, which have a shape shown in Fig. 1, were fabricated by casting. The turbine wheels were then subjected to the aforementioned HIP, VHRCP, and/or annealing. HIP is executed for 3 hours under the conditions of a temperature of 1250° C. and a pressure of 150 MPa. The fabricated seven turbine wheels are respectively referred to as Examples 1 to 7.
Fig. 2 shows alloys and fabrication process of Examples 1 to 7. Examples 1 to 7 are made from one of Alloys 1 to 5 that have different compositions as shown in Fig. 3. Alloy 1 has a comparative composition, which includes none of boron, chromium, silicon and carbon. Alloys 2 to 5 are embodiments according to the present invention. Alloy 2 is standard composition. Alloy 3 includes chromium, which improves the low cycle fatigue strength. Alloys 4 and 5 respectively include silicon and carbon, which improve creep strength.
As shown in Fig. 2, Example 1 was fabricated from Alloy 1, and subjected to HIP, VHRCP, and annealing for removing remaining stress.
Examples 2 to 4 were fabricated from Alloy 2, while subjected to different processes. Example 2 was exposed to HIP, not exposed to VHRCP and annealing. Example 3 was exposed to HIP and VHRCP, not exposed to annealing. Example 4 was exposed to HIP, VHRCP, and annealing.
Examples 5, 6, and 7 were respectively fabricated from Alloy 3, 4, and 5. All of Examples 5, 6, and 7 were exposed to HIP, VHRCP and annealing.
It should be noted that Example 2 was not subjected to VHRCP and annealing, and Example 3 was not subjected to the annealing. Examples 2 and 3 were utilized for showing the effects of the VHRCP and annealing.
Test pieces for a low cycle fatigue test and a creep rupture test were machined from the center portions of Examples 1 to 7, respectively. The test pieces for the low cycle fatigue test were cylindrical rods measuring 3 mm in diameter and 8 mm in length. The test pieces were subjected to the low cycle fatigue test under the conditions of 0.7% strain and a temperature of 750° C. to determine cycles to low cycle fatigue failure. The creep test pieces were cylindrical bars measuring 6 mm in diameter and 30 mm in length. The creep test pieces are subjected to stress of 100 MPa at 900° C. to determine creep rupture lives.
The results of these tests are shown in Fig. 2. Example 1, which does not include boron and was subjected to HIP, VHRCP, and annealing, exhibits superior creep strength. The superior creep strength arises from the VHRCP and annealing. Example 1, however, is poorest in low cycle fatigue strength. Fig. 4 shows an optical micrograph of a cross section of Example 1. The grains of Example 1, which is not doped with boron, have a diameter larger than 200 µm. The large grain size causes the poor low cycle fatigue strength of Example 1.
As shown in Fig. 2, Example 2, which includes boron, exhibits superior low cycle fatigue strength. Fig. 5 shows an optical micrograph of a cross section of Example 2. The grain size of Example 2 (that is, size of lamellar grains) is smaller than 200 µm, that is, much smaller than that of Example 1. The small grain size of Example 2 effectively improves the low cycle fatigue strength.
Example 2, however, exhibits poor creep strength. Fig. 6 shows a back scattered electron micrograph of a cross section of Example 2 in the vicinity of a grain boundary. The lamellar spacing of Example 2 is larger than 2 µm. In addition, Example 2 includes non-lamellar structure. The non-lamellar structure has a volume percent more than 3%. The large lamellar spacing and the non-lamellar structure of Example 2 causes the poor creep strength.
As shown in Fig. 2, Example 3, which includes boron and were subjected to HIP and VHRCP, exhibits superior low cycle fatigue strength. As discussed above, this results from the small grain size caused by the boron doping.
In addition, Example 3 is superior to Example 2 in creep strength. Fig. 7 shows a back scattered electron micrograph of a cross section of Example 3 before the creep rupture test. The lamellar spacing is small and less than 2 µm. In addition, non-lamellar structure is not observed in the vicinity of grain boundary. The backscattered electron micrograph proves that Example 3 has a desirable structure for improving the creep strength.
Example 3, however, suffers dynamic recrystallization during the creep test. Fig. 8 shows an optical micrograph of a cross section of Example 3 after the failure of the creep test. A large number of small grains formed by the recrystallization are observed on the boundaries of the lamellar grains. Creep voids are found where the recrystallized grains exist.
The recrystallization is considered as being caused by remaining stress. The rapid cooling during the VHRCP produces large remaining stress at the grain boundaries, and the remaining stress causes dynamic recrystallization during the creep test. Removal of the remaining stress is considered as being effective to improvement of creep strength.
Example 4, which includes boron and is exposed to HIP, VHRCP, and annealing, exhibits superior low cycle fatigue strength and creep strength. The superior low cycle fatigue is achieved by the doping of boron, while the improvement of the creep strength is achieved by the VHRCP and annealing. The annealing removes remaining stress, and thus suppresses the dynamic recrystallization.
Example 5, which was fabricated from Alloy 3, exhibits superior low cycle fatigue strength, while it is inferior to Example 4 in creep strength. This results from the doping of chromium that improves ductility while deteriorating strength. In this invented alloy, the doping of chromium is effective for the application requesting superior low cycle fatigue strength.
Example 6, which is fabricated from Alloy 4, exhibits superior creep strength, while it is inferior to Example 4 in low cycle fatigue strength. This results from the doping of silicon that improves mechanical strength while deteriorating ductility. In this invented alloy, the doping of silicon is effective for the application requesting superior creep strength.
Example 7, which is fabricated from Alloy 5, also exhibits superior creep strength, while it is inferior to Example 4 in low cycle fatigue strength. This results from the doping of carbon that improves strength while deteriorating ductility. In this invented alloy, the doping of carbon is effective for the application requesting superior creep strength.
Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been changed in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

Claims

An titanium aluminum intermetallic compound based alloy, wherein a volume ratio of non-lamellar structure is equal to or less than 3 volume percent, and diameters of lamellar grains are equal to or less than 200 µm, and a lamellar spacing of lamellar structure is equal to or less than 2 µm.
The titanium aluminum intermetallic compound based alloy according to claim 1, comprising boron.
The titanium aluminum intermetallic compound based alloy according to claim 2, wherein a boron concentration is in the range of 0.2 to 1.2 atomic percent.
The titanium aluminum intermetallic compound based alloy according to claim 1, comprising (measured in atomic percent):

43-48% aluminum;

2.0-5.0% niobium;

0.2-1.2% tungsten;

0.1-1.0% nickel;

0.2-1.2% boron; and

a remainder including titanium plus impurities.
The titanium aluminum intermetallic compound based alloy according to claim 4, further comprising manganese.
The titanium aluminum intermetallic compound based alloy, according to claim 4, wherein a manganese concentration is in the range of 0.2 to 1.2 atomic percent.
The titanium aluminum intermetallic compound based alloy, according to claim 4, further comprising chromium.
The titanium aluminum intermetallic compound based alloy according to claim 7, wherein a chromium concentration is in the range of 0.2 to 1.2 atomic percent.
The titanium aluminum intermetallic compound based alloy according to claim 4, further comprising silicon.
The titanium aluminum intermetallic compound based alloy according to claim 9, wherein a silicon concentration is in the range of 0.1 to 1.0 atomic percent.
The titanium aluminum intermetallic compound based alloy according to claim 4, further comprising carbon.
The titanium aluminum intermetallic compound based alloy according to claim 11, wherein a carbon concentration is in the range of 0.1 to 0.5 atomic percent.
A method of fabricating a product from titanium aluminum intermetallic compound based alloy comprising:

casting an titanium aluminum intermetallic compound based alloy into a product;

executing hot isostatic pressing on said product;

heat-treatment of said product in a non-oxidizing environment after said hot isostatic pressing;

rapidly cooling said product after said heat-treating.
The method according to claim 13, wherein said product is heated at 1320° to 1370° C. during said heat-treatment.
The method according to claim 14, wherein said product is cooled to 1000 °C. at a cooling rate of 30° to 100° C./min. during said rapid cooling.
The method according to claim 15, further comprising:

second heat-treatment of said product after said rapid cooling; and

slowly cooling said product after said second heat-treatment.
The method according to claim 16, wherein said product is heated at 900° to 1050° C. during said second heat-treatment.
The method according to claim 16, wherein said product is cooled at a cooling rate equal to or less than 10° C./min. during said slowly cooling.
The method according to claim 13, wherein said titanium aluminum intermetallic compound based alloy includes boron.
The method according to claim 19, wherein a boron concentration of said titanium aluminum intermetallic compound based alloy is in the range of 0.2 to 1.2 atomic percent.