TiAl-based intermetallic compound with excellent high temperature strength

A TiAl-based intermetallic compound has a metallographic structure which includes a region A having fine .beta.-phases dispersed in a .gamma.-phase. The volume fraction Vf of the .beta.-phases in the region A is set equal to or more than 0.1% (Vf.gtoreq.0.1%). Thus, the .beta.-phases can exhibit a pinning effect to prevent a transgranular pseudo cleavage fracture in the .gamma.-phase, thereby providing an enhanced high-temperature strength of the TiAl-based intermetallic compound.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a TiAl-based intermetallic compound having 
an excellent high-temperature strength, and processes for producing the 
same. 
2. Description of the Prior Art 
A TiAl-based intermetallic compound is expected as a lightweight heat 
resistant material, and those having various structures have been 
conventionally proposed (for example, see U.S. Pat. No. 4,879,092 and 
Japanese Patent Application Laid-open Nos. 25534/90 and 193852/91). 
However, even now conventional TiAl-based intermetallic compounds are not 
put into practical use as a heat-resistant material, because the strength 
thereof is insufficient for high temperatures. That is temperatures 
exceeding about 750.degree. C. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a 
TiAl-based intermetallic compound of the type described above, which has a 
high-temperature strength enhanced by improving the metallographic 
structure thereof, and a process for producing the same. 
To achieve the above object, according to the present invention, there is 
provided a TiAl-based intermetallic compound with an excellent 
high-temperature strength, wherein the compound has a metallographic 
structure which comprises a region having fine .beta.-phases dispersed in 
a .gamma.-phase, the volume fraction Vf of the .beta.-phases in the region 
being equal to or more than 0.1% (Vf.gtoreq.0.1%). 
If the metallographic structure of the TiAl-based intermetallic compound is 
configured in the above manner, it is possible to enhance the 
high-temperature strength of the TiAl-based intermetallic compound. This 
is attributable to the fact that the fine .beta.-phases dispersed in the 
.gamma.-phase exhibit a pinning effect, thereby preventing a transgranular 
pseudo cleavage fracture in the .gamma.-phase. However, if the volume 
fraction Vf of the .beta.-phases is less than 0.1%, a sufficient pinning 
effect cannot be provided. If the .beta.-phases are present between the 
adjacent regions, i.e., in the grain boundaries, a high-temperature 
strength enhancing effect is not provided. 
In addition, according to the present invention, there is provided a 
process for producing a TiAl-based intermetallic compound with an 
excellent high-temperature strength, having a metallographic structure 
which comprises; a first region consisting of either a region having fine 
.beta.-phases dispersed in a .gamma.-phase, or a region consisting of 
.alpha..sub.2 -phases and fine .beta.-phases dispersed in a .gamma.-phase; 
and a second region having a .gamma.-phase which does not include 
.beta.-phase, the volume fraction Vf of .beta.-phases in the first region 
being equal to or more than 0.1% (Vf.gtoreq.0.1%); the process comprising: 
a first step of subjecting a TiAl-based intermetallic compound blank 
having a metallographic structure including a .gamma.-phase and at least 
one of .alpha..sub.2 - and .beta.-phases to a solution treatment at a 
treatment temperature set in a range which permits .alpha.- and 
.gamma.-phases to be present, thereby providing an intermediate product 
having a metallographic structure including .gamma.-phases and 
supersaturated .alpha..sub.2 -phases, and a second step of subjecting the 
intermediate product to an artificial aging treatment at a temperature set 
in a range which permits .alpha..sub.2 - and .gamma.-phases to be present. 
In the above producing process, if the TiAl-based intermetallic compound 
blank is subjected to the solution treatment employing the treatment 
temperature and a quenching, it is possible to prevent a coalescence of 
.alpha..sub.2 - and .gamma.-phases in the intermediate product. If the 
intermediate product is subjected to the artificial aging treatment at the 
above-described temperature, the .gamma.-phase is precipitated in the 
.alpha..sub.2 -phase, and the fine .beta.-phases are precipitated in a 
dispersed fashion in the .gamma.-phase. Further, depending upon the 
treatment temperature in the solution treatment, the .alpha..sub.2 -phases 
may be dispersed together with the .beta.-phases in the .gamma.-phase. 
The above and other objects, features and advantages of the invention will 
become apparent from the following description of preferred embodiments, 
taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, one example of a metallographic structure of a 
TiAl-based intermetallic compound is illustrated in a schematic diagram. 
This metallographic structure is comprised of an infinite number of 
regions A each having fine .beta.-phases (.beta.-phases having B2 ordered 
structure) dispersed in a .gamma.-phase (a TiAl phase). In addition to the 
.beta.-phases, .alpha..sub.2 -Phases may be dispersed in the .gamma.-phase 
in some cases. 
With such a configuration, the fine .beta.-phases dispersed in the 
.gamma.-phase exhibit a pinning effect, and a transgranular pseudo 
cleavage fracture in the .gamma.-phase is prevented, thereby enhancing a 
high-temperature strength of a TiAl-based intermetallic compound. The 
volume fraction Vf of the .beta.-phases in each of the regions A is set 
equal to or more than 0.1% (Vf.gtoreq.0.1%) in order to provide such 
effect. It should be noted that the .alpha..sub.2 -phases dispersed in the 
.gamma.-phase do not contribute to an enhancement in high-temperature 
strength of the TiAl-based intermetallic compound. 
FIG. 2 is a schematic diagram showing another example of a metallographic 
structure of a TiAl-based intermetallic compound. This metallographic 
structure is comprised of an infinite number of first regions A each 
having fine .beta.-phases dispersed in a .gamma.-phase, and an infinite 
number of regions B each having a .gamma.-phase with no .beta.-phase 
included therein. In the first region A, .alpha..sub.2 -phases, in 
addition to the .beta.-phases, may also be dispersed in the .gamma.-phase 
in some cases. 
Even with such a configuration, an effect similar to the above-described 
effect is provided because of the presence of the first regions A. In 
order to provide such effect, the volume fraction Vf of the .beta.-phases 
in each of the regions A is set equal to or more than 0.1% 
(Vf.gtoreq.0.1%), and the volume fraction Vf of the first regions A in the 
metallographic structure is set equal to or more than 1% (Vf.gtoreq.1%). 
It should be noted that the .gamma.-phase including no .alpha..sub.2 - and 
.beta.-phases and thus, the second region B does not contribute to the 
enhancement in high-temperature strength of the metallographic structure. 
A difference between the metallographic structures of the above-described 
types is attributable to conditions for producing the TiAl-based 
intermetallic compounds. For example, in producing the TiAl-based 
intermetallic compound having the metallographic structure shown in FIG. 
2, there is employed a procedure, which comprises a first step of 
subjecting a TiAl-based intermetallic compound blank having a 
metallographic structure including a .gamma.-phase and at least one of 
.alpha..sub.2 - and .beta.-phases to a solution treatment at a treatment 
temperature which is set in a range permitting the .alpha..sub.2 and 
.gamma.-phases to be present, thereby providing an intermediate product 
having a metallographic structure including the .gamma.-phase and 
supersaturated .alpha..sub.2 -phases; and a second step of subjecting the 
intermediate product to an artificial aging treatment at a treatment 
temperature which is set in a range permitting the .alpha..sub.2 - and 
.gamma.-phases to be present. The TiAl-based intermetallic compound blank 
contains aluminum in a content represented by 36 atomic % 
.ltoreq.Al.ltoreq.52 atomic % and titanium in a content represented by 48 
atomic % .ltoreq.Ti.ltoreq.64 atomic % as well as at least one .beta.-area 
enlarging element E as a third element, which is selected from the group 
consisting of Mo, Nb, Ta, V, Co, Cr, Cu, Fe, Mn, Ni, Pb, Si and W. The 
content of the .beta.-area enlarging element E is set equal to or more 
than 0.5 atomic %. If the contents of aluminum, titanium and the 
.beta.-area enlarging element E depart from the above-described ranges, 
respectively, it is not possible to produce a TiAl-based intermetallic 
compound blank having a metallographic structure of the type described 
above. 
As shown in FIG. 3 the treatment temperature in the solution treatment is 
set at a range equal to or more than an eutectoid line E.sub.L which 
permits a reaction, .alpha.-phase+.gamma.-phase.fwdarw..alpha..sub.2 
-phase+.gamma.-phase, to occur, but is set equal to or less than 
.alpha.-transus line T.sub.L which permits a reaction, 
.alpha.-phase.fwdarw..alpha.-phase+.gamma.-phase, to occur, in a Ti-Al 
based phase diagram. This is for the purpose of preventing the coalescence 
of the .alpha..sub.2 - and .gamma.-phases in the intermediate product. 
The cooling rate in the solution treatment is set at a value higher than a 
cooling rate in an oil quenching. This is because .gamma.-phases may be 
precipitated in a laminar configuration in an .alpha..sub.2 -phase, if the 
cooling rate is slower than that during an oil quenching. 
The treatment temperature in the artificial aging treatment is set in a 
range equal to or more than 700.degree. C., but equal to or less than the 
above-described eutectoid line E.sub.l. In this range of temperature, fine 
.beta.-phases can be precipitated in a dispersed state in the 
.gamma.-phase. 
The heating time in the solution treatment and the artificial aging 
treatment is set in a range of at least 5 minutes to ensure that these 
treatments are practically effective. 
Particular examples will be described below. 
First, a starting material was prepared by weighing an aluminum shot having 
a purity of 99.99%, a titanium sponge having a purity of 99.8% and a Cr-Nb 
alloy, so that Al was 47 atomic %; Cr was 2 atomic %; Nb was 2 atomic %, 
and the balance was titanium. 
The starting material was melted in a plasma melting furnace to prepare 
about 20 kg of an ingot. Then, the ingot was subjected to a homogenizing 
treatment at 1200.degree. C. for 48 hours for the purpose of homogenizing 
the ingot and removing casting defects. Subsequently, the ingot was 
subjected to a hot isostatic pressing treatment under conditions of 
1200.degree. C., 3 hours and 193 MPa. Further, the resulting material was 
subjected to an upsetting treatment with an upsetting rate of 80% (a high 
rate) at 1200.degree. C. by a vacuum isothermal forging. The upset product 
obtained in this manner was cut into a plurality of TiAl-based 
intermetallic compound blanks. The metallographic structure of these 
TiAl-based intermetallic compound blanks was comprised of an infinite 
number of .gamma.-phases, and .beta.- and .alpha..sub.2 -phases 
precipitated in a grain boundary of the .gamma.-phases. Each of the 
TiAl-based intermetallic compound blanks was heated for 2 hours at 
1200.degree.-1300.degree. C. and was then subjected to a solution 
treatment in which a water-hardening was conducted, thereby providing an 
intermediate product. Each of the intermediate products has a 
metallographic structure having .beta.-phases and supersaturated 
.alpha..sub.2 -phases. No .beta.-phase was precipitated in the 
.gamma.-phase. 
Then, individual intermediate products were subjected to an artificial 
aging treatment in which they were heated for 1 to 12 hours at 
900.degree.-1200.degree. C., thereby providing TiAl-based intermetallic 
compounds according to examples of the present invention and comparative 
examples. 
Table 1 shows conditions in the solution treatment and conditions in the 
artificial aging treatment for the examples (1) to (3) and the comparative 
examples (1) and (2). The comparative example (2) is TiAl-based 
intermetallic compound blank. 
TABLE 1 
______________________________________ 
Artificial Aging 
Solution Treatment 
Treatment 
Temperature 
Time Temperature 
time 
(.degree.C.) 
(hour) (.degree.C.) 
(hour) 
______________________________________ 
Example (1) 
1300 2 900 12 
Example (2) 
1200 2 900 8 
Example (3) 
1300 2 900 1 
Comparative 
1300 2 1200 3 
example (1) 
Comparative 
-- -- -- -- 
example (2) 
______________________________________ 
FIG. 3 shows a diagram showing states of the TiAl-based intermetallic 
compound in the example (1) or the like and thus the TiAl-based 
intermetallic compound having Cr and Nb contents set at 2 atomic %. In the 
examples (1) to (3), the treatment temperature in the solution treatment 
is set in a range equal to or more than the eutectoid line E.sub.L, but 
equal to or less than the .alpha.-transus line T.sub.L. And the treatment 
temperature in the artificial aging treatment is set in a range equal to 
or more than 700.degree. C., but equal to or less than the eutectoid line 
E.sub.L. In the case of the comparative example (1), the treatment 
temperature in the solution treatment is set in the above-described range, 
but the treatment temperature in the artificial aging treatment exceeds 
the eutectoid line E.sub.L which is the upper limit value of the 
above-described range. 
Table 2 shows textures on the metallographic structure for the examples (1) 
to (3) and the comparative examples (1) and (2) 
TABLE 2 
______________________________________ 
Vf of Vf of phases Vf of phases 
first dispersed in first 
dispersed in grain 
region 
regions A (%) boundary (%) 
A (%) .beta.-phase 
.alpha..sub.2 -phase 
.beta.-phase 
.alpha..sub.2 -phase 
______________________________________ 
Example (1) 
82 5 0 0 0 
Example (2) 
75 2 1 0 0 
Example (3) 
60 0.5 0 0 0 
Comparative 
0 0 0 3 7 
example (1) 
Comparative 
0 0 0 2 5 
example (2) 
______________________________________ 
FIG. 4A is a photomicrograph (2,000 magnifications) showing the 
metallographic structure of the example (1), and FIG. 4B is a schematic 
tracing of an essential portion shown in FIG. 4A. This metallographic 
structure corresponds to that shown in FIG. 2 and hence, has first regions 
A each having .gamma.- and .beta.-phases, and second regions B each having 
a .gamma.-phase with no .beta.-phase included therein. 
FIG. 5A is a photomicrograph (2,000 magnifications) showing the 
metallographic structure of the comparative example (1), and FIG. 5B is a 
schematic tracing of an essential portion shown in FIG. 5A. In this 
metallographic structure, .alpha..sub.2 - and .beta.-phases are 
precipitated at the grain boundary of each .gamma.-phase, but no 
.alpha..sub.2 - and .beta.-phases exist in the .gamma.-phase. 
FIG. 6 is a photomicrograph (500 magnifications) showing the metallographic 
structure of the comparative example (2). In FIG. 6, relatively white and 
small island-like portions are .beta.-phases, more dark colored and 
smaller island-like portions are .alpha..sub.2 -phases, and the other 
portions are .gamma.-phases. The .beta.-phases and .alpha..sub.2 -phases 
are precipitated at the grain boundary of the .gamma.-phases, but no 
.alpha..sub.2 - and .beta.-phases exist in the .gamma.-phase. 
FIG. 7 shows results of a tensile test in a range of from ambient 
temperature to 900.degree. C. for the examples (1) to (3) and the 
comparative examples (1) and (2). In FIG. 7, a line a.sub.1 corresponds to 
the example (1); a line a.sub.2 to the example (2); a line a.sub.3 to the 
example (3); a line b.sub.1 to the comparative example (1), and a line 
b.sub.2 to the comparative example (2). 
It can be seen from FIG. 7 that the examples (1) , (2) and (3) indicated by 
the lines a.sub.1, a.sub.2 and a.sub.3 have an excellent high-temperature 
strength, as compared with the comparative examples (1) and (2) indicated 
by the lines b.sub.1 and b.sub.2. In the examples (1), (2) and (3), the 
high-temperature strength is increased with an increase in volume fraction 
Vf of the .beta.-phases in the first region A. Especially in the case of 
the examples (1) and (2) indicated by the lines a.sub.1 and a.sub.2, the 
high-temperature strength is higher than the ambient-temperature strength 
at about 660.degree. to about 880.degree. C., and the maximum strength is 
shown at 800.degree. C. 
In the TiAl-based intermetallic compound of this type, the volume fraction 
Vf of .beta.-phases is set equal to or more than 0.1% (Vf.gtoreq.0.1%) in 
order to insure a high-temperature strength attributable to the presence 
of the .beta.-phases. 
Table 3 shows the conditions in the solution treatment, the volume fraction 
Vf of the first regions A, the volume fraction of the .beta.-phases in the 
first regions A, and the elongation for examples (4) to (8) and a 
comparative example (3). The artificial aging treatment was carried out at 
900.degree. C. for 12 hours. 
TABLE 3 
______________________________________ 
Vf of .beta.- 
Vf of phases 
Solution Treatment 
first in first Elon- 
Temperature 
Time region region gation 
(.degree.C.) 
(hour) A (%) (%) (%) 
______________________________________ 
Example (4) 
1250 2 39 4.5 1.3 
Example (5) 
1280 2 31 4.0 1.2 
Example (6) 
1300 2 15 2.0 1.0 
Example (7) 
1320 2 5 1.8 0.8 
Example (8) 
1340 2 2 0.2 0.25 
Comparative 
1400 2 0 0 0.2 
example (3) 
______________________________________ 
FIG. 8 is a graph taken from the relationship shown in Table 3, wherein 
spots (4) to (8) and (3) correspond to the examples (4) to (8) and the 
comparative example (3), respectively. 
It is apparent from FIG. 8 that the elongation of the TiAl-based 
intermetallic compound has a point of inflection at about 1% the volume 
fraction Vf of the first regions A. Therefore, in order to insure a 
ductility of a TiAl intermetallic compound, the volume fraction of the 
first regions A is set equal to or more than 1% (Vf.gtoreq.1%).