Note: Descriptions are shown in the official language in which they were submitted.
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TITANIUM ALUMINIDE BASED ALLOY
Field of the Invention
The invention concerns alloys made through the use of melting and powdered
metallurgical techniques on the basis of titanium aluminides with an alloy
composition of
Ti-z Al-y Nb where 44.5 Atom %< z< 47 Atom %, especially where 44.5 Atom %< z<
45.5 Atom %, and 5 Atom %< y< 10 Atom % with possibly the addition of B and/or
C
at a content between 0.05 Atom % and 0.8 Atom %.
Background of the Invention
Titanium aluminide alloys have properties which make those alloys highly
suitable for use as light weight work materials, especially for high
temperature
applications. For industrial practice those alloys are of special interest
which are based on
an intermetalic phase y-(TiAI) with tetragonal structure and along with the
majority phase
y-(TiAI) also contain minority portions of the intermetalic phase az(Ti3A1)
with
hexagonal structure. These y-titanium aluminide alloys distinguish themselves
by
properties such lightweight (3.85-4.2 g/cm3), high elastic modulus, high
strength and
creep resistance up to700 C, which makes them attractive as work materials for
moving
parts at high operating temperatures. Examples of such uses are for turbine
blades in
aircraft engines and in stationary gas turbines, engine valves and hot gas
ventilators.
In the technically important area of alloys with aluminum content between 45
Atom % and 49 Atom% there appears during the solidification of the melt and
during the
subsequent cooling a series of phase changes. The solidification can take
place either
entirely by way of R-mixed crystal cubic space centered structure (high
temperature
phase) or in two peritectic reactions in which a-mixed crystals with hexagonal
structure
and the y-phase participate.
It is further known that the element niobium (Nb) leads to an increase in
strength,
creep resistance, oxidation resistance and also ductility. With the element
boron which is
practically insoluble in the y-phase a fine graining can be achieved both in
the cast
condition and also after reshaping with subsequent heat treatment in the a-
region. An
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increased portion of the (3-phase in the structure as a result of low aluminum
content and
high concentration of (3-stabilizing elements can lead to a coarse dispersion
of this phase
and to an impairment of the mechanical properties.
The mechanical properties of y-titanium aluminide alloys are, as to their
deformation and break behaviors, but also because of the structural anisotropy
of the
preferred use of laminated structures or duplex-structures, strongly
anisotropic. For a
desired use of structure and texture in the making of components from titanium
aluminides, casting methods, different powdered metal metallurgies and
reshaping
processes as well as combinations of these manufacturing methods are useable.
From the publication of Y-W. Kim and D. M. Dimiduk in "Structural
Intermetallics 1997", editors M. V. Nathanal, R. Darolia, C.T. Liu, P.L.
Martin, D. B.
Miracle, R. Wagner, M. Yamaguchi, TMS, Warrendale PA, 1996, page 531 it is
known
that in the course of different development programs the effect of a large
number of
alloying elements with respect to constitution, structural tuning in different
manufacturing
processes and individual properties have been investigated. The discovered
relationships
are thereby similarly complex as for the case with the other structured
metals, for
example, steels and can only be summarized by rules which are limited and of
very
general form. Therefore certain mixtures can have exceptional combinations of
properties.
A titanium aluminide alloy is known from EP 1 015 605 B 1 which has a
structural
and chemically hornogenous structure. In this case the majority phases y(TiAl)
and a2
(Ti3A1) are separated into a fine dispersion. The disclosed titanium aluminide
alloy with
an aluminum content of 45 Atom% distinguishes itself by exceptionally good
mechanical
properties and high temperature properties.
A general problem of all titanium aluminide alloys is their low ductility. For
a
long time one has not succeeded in improving the pregiven high brittleness and
low
damage tolerance of titanium aluminide alloys arising from the nature of the
intermetallic
phases (compare "Structural Intermetallics 1997", page 531, see above). For
many of the
above mentioned uses indeed plastic fracture elongations of _ 1% are
sufficient. For the
making of turbines and motors however it is necessary that this minimum amount
of
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ductility be guaranteed in industrial manufacturing throughout large batch
numbers.
Since the ductility is sensitively dependent on structure in industrial
manufacturing
processes it is extremely difficult to assuredly obtain a highly homogenous
structural
configuration. For high tensile strength alloys a maximum tolerable defect
size, for
example the maximum grain or lamina colony size, is very small so that for
such alloys a
very high structural homogeneity is desirable. This homogeneity can however,
because of
the unavoidable fluctuation of the alloying mixture from, for example 0.5
Atom % in
aluminum content, only be reached with difficulty.
At the present time of the many possible structural types of y-titanium
aluminide
alloys only lamellar and so called duplex structures are taken into
consideration for high
temperature uses. Upon the cooling from the single phase region the a-mixed
crystals
first appear while plates of the y-phase crystallographically become oriented
and separate
from the a-mixed crystals.
Compared to this, duplex structures consisting of lamina colonies and y-grains
arise when the material has been heated into the second phase area a+'y. Then
upon
cooling the a-grains lying in the second phase area again change into two
phased lamina
colonies. Above all, coarse structural components exist in y-titanium
aluminide alloys
since during the running through of the a-area large a-grains are formed. This
can indeed
happen during the solidification when large stalk crystals of the a-phase are
formed from
the melt. Accordingly as much as possible the single phase area of the a-mixed
crystals
must be avoided during processing. Since in practice however fluctuations in
the
composition and processing temperatures appear and thereby locally vary the
constitution
in work pieces, the formation of large lamina colonies is not to be prevented.
Proceeding from this state of the art the present invention has as its object
the
making available of a titanium aluminide alloy with a fine and homogeneous
structural
morphology, as to which alloy the variations of the alloy composition as well
as
unavoidable temperature fluctuations which appear during manufacturing
processes of
industrial practice have hardly any or no significant effect on the
homogeneity of the
alloy, and especially without having to make any basic changes in the
manufacturing
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processes. Therefore a further object of the invention is to make available a
structural
component consisting of a homogenous alloy.
Summarv of the Invention
This object is solved by means of an alloy based on titanium aluminide made
through the use of melting and powdered metallurgical technologies with an
alloy
composition of Ti-z Al-y Nb where 44.5Atom %< z< 47 Atom %, especially where
44.5
Atom %< z< 45.5Atom %, and 5 Atom %<y < lOAtom %, which is further formed in
that this alloy contains molybdenum (Mo) in the range of between 0.1 Atom % to
3.0
Atom%. The remainder of the alloy is made up of Ti (titanium).
Investigations have shown that the alloying of molybdenum with titanium
aluminide having a niobium portion usually results in an alloy for which the
(3-phase is
not stable over the entire temperature region, and therefore in a customary
process
procedure such as extrusion the remainder of the high temperature (3-phase
dissolves, and
a better structural homogeneity of the alloy is obtained. In this way over the
entire
temperature range relevant to the before mentioned manufacturing process a
portion of
the volume of the 0-phase without grain coarseness is realized. This type of
alloy
according to the invention therefore, because of the fine and very uniform
dispersion of
the (3-phase, has a homogenous structure with high strength values.
Therefore an alloy is presented by the invention which is suitable as a
lightweight
work material for high temperature applications, such as for turbines blades
or engine and
turbine components. The alloy of the invention is made through the use of
casting
metallurgy, melting metallurgy or powdered metal metallurgy methods or by the
use of
these methods in combination with reshaping techniques.
Above all in the case of Ti-(44.5 Atom % to 45.5 Atom %) Al - (5 Atom % to 10
Atom %) Nb the addition of molybdenum at a content of about 1.0 Atom % to 3.0
Atom
% leads to good microstructures with a high structural homogeneity.
Moreover an alloy according to the invention has a composition of Ti-z Al-y Nb-
x B where 44.5 Atom %< z< 47 Atom %, especially where 44.5 Atom %< z< 45.5
Atom %, 5 Atom %< y< 10 Atom % and 0.05 Atom %< x< 0.8 Atom %, or a
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composition of Ti-z Al-y Nb-w C where 44.5 Atom %< z< 47 Atom %, especially
where
44.5 Atom % < z< 45.5 Atom %, 5 Atom % is < y < 10 Atom % and 0.05 Atom %< w<
0.8 Atom %, each of which alloys contains molybdenum (Mo) in the region of
between
0.1 Atom % to 3 Atom %, especially in the region of between 0.5 Atom % to 3
Atom %.
Alternatively the alloy is made up of Ti-z Al-y Nb-x B-w C where 44.5 Atom %<
z< 47 Atom %, especially where 44.5 Atom %< z< 45.5 Atom %, 5 Atom %< y< 10
Atom %, 0.05 Atom %< x < 0.8 Atom % and 0.05 Atom % < w < 0.8 Atom % and
additionally of molybdenum in the region of between 0.1 Atom % to 3 Atom %.
By means of the given alloying and the corresponding alloying proportions high
strength y-titanimium aluminide alloys with a fine dispersion of the (3-phase
are created
for a wide range of processing temperature.
In the case of the present invention the strived for structural stability and
process
security are thereby achieved in that the appearance of single phase regions
are avoided
over the entire temperature region traversed in the manufacturing processes
and upon use,
by the aimed for inclusion of the cubic space centered (3-phase. Principally
the (3-phase
appears as the high temperature phase for all technical titanium aluminide
alloys at
temperatures _ 1350 C.
From the literature it is known that this phase can be stabilized at low
temperatures by different elements such as Mo, W, Nb, Cr, Mn, and V. The
special
problem with the alloying of these elements exists however in that the (3-
stabilizing
elements have to be very accurately tuned to the Al content. Moreover in the
case of the
addition of these elements undesirable exchange effects appear which lead to
higher
portions of the 0-phase and to a coarse dispersion of this phase. Such a
constitution is
most disadvantageous for the mechanical properties. Further, the properties of
the (3-
phase are dependent on the alloying elements and their composition. Especially
the
constitution must be so chosen so that a precipitation of the brittle w-phase
from the (3-
phase must be substantially avoided. Because of this relationship an alloying
composition is presented whereby for the mechanical properties an optimum
composition
and dispersion of the 0-phase can be realized for a wide region of processing
temperatures. At the same time the best possible strength properties are
achieved.
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According to a preferred form of the invention the alloy likewise contains
boron,
preferably with a boron content in the alloy in the area of from 0.05 Atom %
to 0.8 Atom
%. The addition of boron leads advantageously to the formation of stable
precipitates
which likewise contribute to the mechanical hardening of the alloy and to the
stabilization
of the structure.
The object of the invention is further solved by a construction component made
from an alloy of the invention. To avoid repetition reference is made to the
previous
exposition.
Brief Description of the Drawings
In the following the invention, without limitation to the general thought of
the
invention, is described by way of exemplary embodiments with reference to the
accompanying schematic drawings, to which in regard to publication reference
is made
for all details of the invention not more closely explained in the text. The
drawings are:
Fig.l shows a raster electron microscope picture of a cast block having an
alloying
of Ti-45A1-8Nb-0.2C (Atom %);
Fig. 2a to 2c each shows a picture of the structure in an alloy of Ti-45A1-8Nb-
0.2C (Atom %) taken by a raster electron microscope after different processing
steps;
Fig. 3a and 3b each shows a picture of the structure in an alloy of the
invention of
Ti-45A1-Nb-2 Mo (Atom %) after different processing steps, and
Fig. 4 is a diagram with tension-elongation curves resulting from tests of the
alloy
Ti-45A1-5Nb-2 Mo (Atom %).
Detailed Description
Fig. 1 shows two pictures of a structure in a cast block made of the alloy Ti-
45A1-
8Nb-0.2 C (Atom %). The pictures as well as the further pictures in the
following figures
were taken by means of back scattered electrons in a raster electron
microscope.
The structure (Fig.1) shows lamina colonies of the aZ-phase and the y-phase,
which originate from former y-lamina. The former y-lamina are separated by
stripes of
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bright pictured grains of P-phase or B2-phase. The a-lamina next formed in the
(3- a-
conversion decay upon further cooling into a2-lamina and y-lamina.
In figures 2a to 2c two further pictures of the structure of the alloy Ti-45A1-
8Nb-
0.2C taken in the raster electron microscope and after different processing
steps are
shown. Fig. 2a shows the structure after extrusion at 1230 C. The extrusion
direction
runs horizontally. The structure shows grains of the a2- and P-phase, with the
cubic space
centered P-phase having vanished.
Fig. 2b shows the structure of the alloy after the extrusion at 1230 C and a
further
forging step at 1100 C. This structure shows grains of the az- and y-phase
and a few a2/y
lamina colonies.
In Fig. 2c is shown the structure of the alloy after extrusion at 1230 C and a
subsequent heat processing at 1330 C. This structure exhibits likewise grains
of the az-
and y-phase. The picture shows a fully laminar structure with lamina of the a2-
and y-
phase. The lamina colony size has a value of about 200 m, with colonies also
appearing
which are clearly larger than 200 m.
As in the structure illustrated in Fig. 2a, also in the structures illustrated
in Figs.
2b and 2c the cubic space centered phase does not appear. So the P-phase in
this
temperature range with a heat processing after the extrusion is
thermodynamically not
stable.
In Figures 3a and 3b are illustrated raster electron microscope pictures of
the
structure of an alloy in accordance with the invention. Proceeding from an
alloy of
Ti-45A1-5Nb the alloying agent molybdenum was added at 2Atom%. This starting
alloy
Ti-45A1-5Nb-2Mo is based on a composition as described in European Patent EP 1
015
650 B1.
Figs. 3a and 3b show the structure of this alloy of the invention after an
extrusion
at 1250 C and a subsequent heat treatment at 1030 C (Fig 3a) as well as
observed at
1270 C (Fig 3b).
The structure of Fig. 3a exhibits grains of the a2-phase, the y-phase and the
brightly pictured (3-phase, with the latter being arranged in strips. The
structure in Fig. 3b
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shows lamina colonies of aZ-and y-phases as well as grains of the brightly
pictured ~3-
phase, which again have precipitated from the y-phase.
The structures of Fig. 3a and 3b are fine, very homogenous and show uniform
distribution of the (3-phase. After the heat treatment of 1030 C a globular
structure is
presented, with it having grains of (3-phase in strips parallel to the
extrusion direction,
while the material heat treated at 1270 C exhibits a very homogenous, fully
lamellar
structure with uniformly distributed P-grains (Fig 3b).
The colony size of the alloy Ti-45A1-5Nb-2Mo has a value of between 20 to 30 m
and is therefore at least about 5 times smaller than in the fully laminar
structure of y-
titanium aluminide alloy. Moreover, in the (3-phase the y-phase has been
eliminated so
that the (3-grains are very finely subdivided. Therefore, in summary, a very
fine and
homogenous structure has been achieved.
Tests have shown that this fine and homogenous structure morphology after heat
treatment is present for the entire high temperature range up to 1320 C. The
structures
show clearly that over the entire temperature range relevant for the
manufacturing
processes a sufficient volume of the (3-phase is provided and the grain growth
is
effectively suppressed.
In tension tests carried out on the material which was heat treated at 1030 C,
at
room temperature a stretch limit of 867 MPa, a tensile strength of 816 MPa and
a plastic
elongation at rupture at 1.8% were measured.
Fig. 4 shows measured tension-elongation curves from test of the alloy Ti-45A1-
5Nb-2Mo in tension tests. The test material was extruded at 1250 C and
subsequently
subjected to a heat treatment for two hours at 1030 C and was then subjected
to an oven
cooling. The curves taken at 700 C and 900 C show that the alloy is suitable
for many
high temperature applications. By the alloying of a small amount of molybdenum
a very
uniform microstructure in the alloy is achieved so that this alloy can be well
used as a
high temperature work material.
Moreover in Fig. 4 the results of a tension test at room temperature (25 C) on
the
material of the invention is illustrated, with the tension 6 in MPa being
shown against the
elongation s in %. Thereby an elongation limit increase was found which
otherwise up to
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now has not been observed for y-titanium aluminide alloys. This represents an
indication
of an especially fine and homogenous structure. The elongation limit increase
indicates
that the material can react to local tensions by plastic flow, which is very
beneficial for
ductility and damage resistance.
The homogeneity of the alloy of the invention in the region of relevant
processing
temperatures is not dependent on technically unavoidable fluctuations of the
temperature
or of the composition.
The titanium aluminide alloys of the invention are made through the use of
metallurgical casting or powdered metal techniques. For example, the alloys of
the
invention can be processed by hot forging, hot pressing and hot extrusion and
hot rolling.
The invention offers the advantage that despite the fluctuations of the
alloying
composition appearing with the industrial finishing and unavoidable processing
requirements as previously, a titanium aluminide alloy with very uniform
microstructure
and high strength has been made available.
The titanium aluminide alloy of the invention achieve high strength up to a
temperature in the region of 700 C to 800 C as well as good room temperature
ductility.
Therefore the alloys are suitable for numerous areas of application and can
for example
be used for highly loaded components or as titanium aluminide alloys for
exceptionally
high temperatures.
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