U.S. patent application number 12/512451 was filed with the patent office on 2010-01-07 for titanium aluminide alloys.
This patent application is currently assigned to GKSS-FORSCHUNGSZENTRUM GEESTHACHT GMBH. Invention is credited to Fritz Appel, Michael Oehring, Jonathan Paul.
Application Number | 20100000635 12/512451 |
Document ID | / |
Family ID | 40527708 |
Filed Date | 2010-01-07 |
United States Patent
Application |
20100000635 |
Kind Code |
A1 |
Appel; Fritz ; et
al. |
January 7, 2010 |
TITANIUM ALUMINIDE ALLOYS
Abstract
Alloys based on titanium aluminides, such as .gamma. (TiAl)
which may be made through the use of casting or powder
metallurgical processes and heat treatments. The alloys contain
titanium, 38 to 46 atom % aluminum, and 5 to 10 atom % niobium, and
they contain composite lamella structures with B19 phase and .beta.
phase there in a volume ratio of the B19 phase to .beta. phase
0.05:1 and 20:1.
Inventors: |
Appel; Fritz; (Geesthacht,
DE) ; Paul; Jonathan; (Hamburg, DE) ; Oehring;
Michael; (Geesthacht, DE) |
Correspondence
Address: |
MICHAUD-DUFFY GROUP LLP
306 INDUSTRIAL PARK ROAD, SUITE 206
MIDDLETOWN
CT
06457
US
|
Assignee: |
GKSS-FORSCHUNGSZENTRUM GEESTHACHT
GMBH
Geesthacht
DE
|
Family ID: |
40527708 |
Appl. No.: |
12/512451 |
Filed: |
July 30, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12331909 |
Dec 10, 2008 |
|
|
|
12512451 |
|
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|
Current U.S.
Class: |
148/538 ;
75/330 |
Current CPC
Class: |
C22C 1/0491 20130101;
C22C 30/00 20130101; C22C 1/04 20130101; C22F 1/183 20130101; C22C
1/02 20130101; C22C 1/0458 20130101; C22C 14/00 20130101 |
Class at
Publication: |
148/538 ;
75/330 |
International
Class: |
C22F 1/18 20060101
C22F001/18; B22F 1/00 20060101 B22F001/00; C22F 1/00 20060101
C22F001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 13, 2007 |
DE |
102007060587.2 |
Claims
1. A method for the production of an alloy, comprising: providing a
composition that comprises titanium, 38 to 46 at % aluminum, and 5
to 10 at % niobium; subjecting the composition to a casting or
powder metallurgical technique to produce an intermediate product;
and subjecting the intermediate product to a heat treatment, the
heat treatment comprising heating the intermediate product at a
temperature above 900.degree. C. for more than sixty minutes, and
cooling the intermediate product at a rate of more than 0.5.degree.
C. per minute.
2. The method of claim 1 wherein the heat treatment comprises
heating the intermediate product at a temperature above
1000.degree. C.
3. The method of claim 1 wherein the heat treatment comprises
heating the intermediate product at a temperature between
1000.degree. C. and 1200.degree. C.
4. The method of claim 1 wherein the heat treatment comprises
heating the intermediate product at said temperature above
900.degree. C. for more than 90 minutes.
5. The method of claim 1 wherein the heat treatment comprises
heating the intermediate product at a temperature above
1000.degree. C. for more than 90 minutes.
6. The method of claim 1, comprising cooling the intermediate
product at a rate of 1.degree. C. per minute to 20.degree. C. per
minute.
7. The method of claim 1, comprising cooling the intermediate
product at a rate of 1.degree. C. per minute to 10.degree. C. per
minute.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. Patent application
Ser. No. 12/331,909 filed Dec. 10, 2008, which claims priority
German patent application DE 10 2007 060 587.2, filed Dec. 13,
2007, the subject matter of these patents is incorporated by
reference herein in their entirety.
FIELD OF THE INVENTION
[0002] The invention relates to alloys based on titanium aluminide,
in particular made through the use of casting or powder
metallurgical processes, preferably based on .gamma. (TiAl).
BACKGROUND OF THE INVENTION
[0003] Titanium aluminide alloys are characterized by a low
density, a high rigidity and good corrosion resistance. In the
fixed state, they have domains with hexagonal (.alpha.), two-phase
structures (.alpha.+.beta.) and cubically body-centered .beta.
phase and/or .gamma. phase.
[0004] For industrial practice, alloys based on an intermetallic
phase .gamma. (TiAl) with a tetragonal structure and containing
minority shares of intermetallic phase .alpha..sub.2(Ti.sub.3Al)
with hexagonal structure in addition to the majority phase .gamma.
(TiAl) are particularly interesting. These .gamma. titanium
aluminide alloys are characterized by properties like low density
(3.85-4.2 g/cm.sup.3), high elastic modulus, high rigidity and
creep resistance up to 700.degree. C., which make them attractive
as lightweight construction materials for high-temperature
applications. Examples of such applications include turbine buckets
in aircraft engines and in stationary gas turbines, and valves for
engines and hot gas ventilators.
[0005] In the technically important area of alloys with aluminum
content between 45 atom percent and 49 atom percent, a series of
phase conversions occur during the solidification from the cast and
during the subsequent cooling. The solidification can either take
place completely via the .beta. mixed crystal with a cubically
body-centered structure (high temperature phase) or in two
peritectic reactions, in which the .alpha. mixed crystal with
hexagonal structure and the .gamma. phase participate. Atom percent
(at %) of an elemental material in an alloy indicates the
proportion of the identified material as 100.times.[the number of
atoms of the identified elemental material)/(total atoms in the
alloy]. This is equivalent to mole percent of the material, or
100.times.[mole fraction X.sub.A], where X.sub.A=n.sub.A/N.sub.tot,
where n.sub.a is the number of moles of elemental material A in the
alloy and N.sub.tot is the total number of moles of atoms in the
alloy.
[0006] Furthermore, it is known that aluminum in .gamma. titanium
aluminide alloys causes an increase in the ductility and the
oxidation resistance. Moreover, element niobium (Nb) leads to an
increase in the rigidity, creep resistance, oxidation resistance,
but also the ductility. With the element boron (B), which is
practically insoluble in the .gamma. phase, a grain refinement can
be achieved in both the as-cast state and after the reshaping with
subsequent heat treatment in the .alpha. area. An increased share
of .beta. phase in the structure as a result of low aluminum
contents and high concentrations of .beta. stabilizing elements can
lead to rough dispersion of this phase and can cause deterioration
of the mechanical properties.
[0007] The mechanical properties of titanium aluminide alloys are
strongly anisotropic due to their deformation and breaking behavior
but also due to the structural anisotropy of the preferably set
lamellar structure or duplex structure. Casting processes,
different powder-metallurgical and reshaping processes and
combinations of these production processes are used for a targeted
setting of structure and texture in the production of components
made of titanium aliminides.
[0008] Moreover, a titanium aluminide alloy, which has a
structurally and chemically homogeneous structure, is known from EP
1 015 650 B1. The majority phases .gamma. (TiAl) and .alpha..sub.2
(Ti.sub.3Al) are hereby distributed in a finely disperse manner.
The disclosed titanium aluminide alloy with an aluminum content of
45 atom percent (at %) is characterized by extraordinarily good
mechanical properties and high temperature properties.
[0009] Titanium aluminides based on .gamma. (TiAl) are
characterized in general by relatively high rigidities, high
elastic modulus, good oxidation and creep resistance with
simultaneously lower density. Based on these properties, TiAl
alloys should be used as high temperature materials. These types of
applications are heavily impaired through the very low plastic
malleability and the low fracture toughness. Rigidity and
malleability, as with many other materials, behave hereby
inversely. The technically interesting high-strength alloys are
thereby often particularly brittle. Comprehensive examinations for
the optimization of the structure were performed in order to
eliminate these disadvantageous properties.
[0010] The previously developed structure types can be roughly
categorized into a) coaxial gamma structures, b) duplex structures
and c) lamellar structures. The currently achieved development
state is represented in detail for example in: Y.-W. Kim, D. M.
Dimiduk, in: Structural Intermetallics 1997, Eds. M. V. Nathal, R.
Darolia, C. T. Liu, P. L. Martin, D. B. Miracle, R. Wagner, M.
Yamaguchi, TMS, Warrendale Pa., 1996, pg. 531, and M. Yamaguchi, H.
Inui, K. Ito, Acta mater. 48 (2000), pg. 307.
[0011] The structures made of titanium aluminides were previously
mainly refined by boron additives, which leads to the formation
titanium borides (see T. T. Cheng in: Gamma Titanium Aluminides
1999, Eds. Y.-W. Kim, D. M. Dimiduk, M. H. Loretto, TMS, Warrendale
Pa., 1999, pg. 389; and
[0012] Y.-W. Kim, D. M. Dimiduk, in: Structural Intermetallics
2001, Eds. K. J. Hemker, D. M. Dimiduk, H. Clemens, R. Darolia, H.
Inui, J. M. Larsen, V. K. Sikka, M. Thomas, J. D. Whittenberger,
TMS, Warrendale Pa., 2001, pg. 625.)
[0013] For further refining and consolidation of the structure, the
alloys are usually subjected to several high temperature reshapings
through extruding or forging. Also refer to the following
publications: [0014] Gamma Titanium Aluminides, Eds. Y.-W. Kim, R.
Wagner, M. Yamaguchi, TMS, Warrendale Pa., 1995; [0015] Structural
Intermetallics 1997, Eds. M. V. Nathal, R. Darolia, C. T. Liu, P.
L. Martin, D. B. Miracle, R. Wagner, M. Yamaguchi, TMS, Warrendale
Pa., 1997; [0016] Gamma Titanium Aluminides 1999, Eds. Y-W. Kim, D.
M. Dimiduk, M. H. Loretto, TMS, Warrendale Pa., 1999; and [0017]
Structural Intermetallics 2001, Eds. K. J. Hemker, D. M. Dimiduk,
H. Clemens, R. Darolia, H. Inui, J. M. Larsen, V. K. Sikka, M.
Thomas, J. D. Whittenberger, TMS, Warrendale Pa., 2001.
SUMMARY
[0018] The present invention resides in one aspect in an alloy
which contains titanium, 38 to 46 atom percent (at %) aluminum, and
5 to 10 atom percent niobium, and has composite lamella that
contain a B19 phase and a .beta. phase in a volume ratio of
B19:.beta. of 0.05:1 to 20:1.
[0019] The present invention resides in another aspect in a method
for the production of an alloy. The method includes providing a
composition that contains titanium, 38 to 46 at % aluminum, and 5
to 10 at % niobium and subjecting the composition to a casting or
powder metallurgical technique to produce an intermediate product.
The intermediate product is subjected to a heat treatment. The heat
treatment includes heating the intermediate product at a
temperature above 900.degree. C. for more than sixty minutes, and
cooling the intermediate product at a rate of more than 0.5.degree.
C. per minute.
[0020] The present invention resides in another aspect in an alloy
made by the method described herein.
[0021] A component may be made from the alloys described
herein.
BRIEF DESCRIPTION OF THE FIGURES
[0022] FIG. 1A is an electron photomicrograph of an alloy according
to one embodiment of the present invention.
[0023] FIG. 1B is an electron photomicrograph providing a detailed
view of selected lamella structures T of FIG. 1A.
[0024] FIG. 1C is an electron photomicrograph of an alloy according
to another embodiment of the present invention.
[0025] FIG. 2A is an electron photomicrograph providing a more
detailed view of a lamella structure T of FIG. 1A.
[0026] FIG. 2B is an electron photomicrograph providing a still
more detailed view of a lamella structure T of FIG. 1A.
[0027] FIG. 2C is a diffractogram derived from FIG. 2B.
[0028] FIG. 3 is an electron photomicrograph of a crack in the
alloy of FIG. 1A.
[0029] FIG. 4 is a graph of a plot of force on the vertical axis
vs. deflection on the horizontal axis, for a toughness test of an
alloy as described herein.
DETAILED DESCRIPTION OF THE INVENTION
[0030] In one embodiment, the present invention provides a titanium
aluminide alloy with a fine structure morphology, for example, a
morphology in the nanometer range. In another embodiment, the
present invention provides a component made from a homogeneous
alloy.
[0031] In one aspect, the present invention provides an alloy based
on titanium aluminides which may optionally be made through the use
of casting or powder metallurgical processes, preferably based on
.gamma. (TiAl), using a composition that contains titanium (Ti), 38
to 42 atom percent (at %) aluminum (Al), and 5 to 10 at % niobium
(Nb), and wherein the composition comprises composite lamella
structures with B19 phase and .beta. phase in each lamella, with a
volume ratio of the B19 phase to the .beta. phase in each lamella
between 0.05:1 to 20:1. In an optional embodiment, the volume ratio
is between 0.1:1 and 10:1.
[0032] It has been shown that in some alloys or intermetallic
connections described herein, composite lamella structures,
including composite lamella structures in the nanometer size, are
created. The lamella structures include modulated lamellas made of
the crystallographically different, and alternatingly formed, B19
phase and .beta. phase. The created composite lamella structures
are largely surrounded by .gamma.-TiAI.
[0033] These types of composite lamella structures can be
established in alloys using known production technologies, i.e.
through casting, reshaping and powder technologies. The alloys are
characterized by an extremely high rigidity and creep resistance
with simultaneously high ductility and fracture toughness.
[0034] Example alloys as described herein can be provided with any
of the following titanium-based compositions (wherein titanium
makes up the balance of the at % of each composition): [0035]
Titanium, 38.5 to 42.5 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at %
Cr; [0036] Titanium, 39 to 43 at % Al, 5 to 10 at % Nb, and 0.5 to
5 at % Zr; [0037] Titanium, 41 to 44.5 at % Al, 5 to 10 at % Nb,
and 0.5 to 5 at % Mo, [0038] Titanium, 41 to 44.5 at % Al, 5 to 10
at % Nb, and 0.5 to 5 at % Fe, [0039] Titanium, 41 to 45 at % Al, 5
to 10 at % Nb, and 0.1 to 1 at % La; [0040] Titanium, 41 to 45 at %
Al, 5 to 10 at % Nb, and 0.1 to 1 at % Sc;, [0041] Titanium, 41 to
45 at % Al, 5 to 10 at % Nb; and 0.1 to 1 at % Y; [0042] Titanium,
42 to 46 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at % Mn; [0043]
Titanium, 41 to 45 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at % Ta;
[0044] Titanium, 41 to 45 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at
% V; and [0045] Titanium, 41 to 46 at % Al, 5 to 10 at % Nb, and
0.5 to 5 at % W.
[0046] Each of the titanium aluminide alloys disclosed above can
optionally include boron (B) and/or carbon (C). For example, any of
the described titanium aluminide alloys may include 0.1 to 1 at %
boron and/or 0.1 to 1 at % carbon. The already fine structure of
the alloy is hereby further refined.
[0047] Within the framework of the invention, the remainders of the
specified alloy compositions are made of titanium and unavoidable
impurities.
[0048] In accordance with the invention, alloys are thus made
available, which are suitable as a lightweight construction
material for high temperature applications, such as turbine buckets
or engine and turbine components.
[0049] According to one aspect, alloys as described herein can be
produced using casting metallurgical or powder metallurgical
processes or techniques, or using these processes in combination
with reshaping techniques.
[0050] In some embodiments, the alloys with composite lamella
structures have a very fine microstructure and a high rigidity and
creep resistance with simultaneously good ductility and fracture
toughness with respect to alloys without the composite lamella
structures.
[0051] It is known that titanium aluminide alloys with aluminum
contents of 38-45 at % and other additives, for example, refractory
elements, contain relatively large volume shares of the .beta.
phase, which can also be present in a controlled form as B2 phase.
The crystallographic lattices of these two phases are mechanically
instable with respect to homogenous shearing processes, which can
lead to lattice conversions. This property is mainly attributed to
the anistropic bond ratio and the symmetry of the cubically
body-centered lattice. The tendency of the .beta. or B2 phase
towards lattice transformation is thus very distinct. Different
orthorhombic phases can be formed through a shear transformation of
the cubically body-centered lattice of the .beta. or B2 phase, to
which phases B19 and B33 belong in particular.
[0052] Without wishing to be bound by any one particular theory,
the invention is based on the idea of using lattice transformations
through shear conversion for an additional refining of the
microstructure of the titanium aluminide alloys. This type of
process is not previously known for titanium aluminide alloys in
scientific literature. In the case of the alloys as described
herein, the formation of brittle phases like .omega., .omega.' and
.omega.'', which are extremely disadvantageous for the mechanical
material properties, are also avoided, due to shear
conversions.
[0053] In some embodiments, the structural refining of the alloys
described herein is achieved without the addition of grain-refining
or structure-refining elements or additives such as boron (B) and
the alloys thus contain no borides. Since the borides occurring in
TiAl alloys are brittle, they lead to the brittleness of TiAl
alloys as of a certain content and generally represent potential
crack nuclei in boron-containing alloys.
[0054] In another aspect, some alloys as described herein comprise
composite lamella structures with the B19 phase and .beta. phase in
each lamella, wherein the lamellas are surrounded by the
TiAl-.gamma. phase.
[0055] In various embodiments, the volume ratio of the B19 phase
and .beta. phase each in a lamella is between 0.05:1 and 20:1, for
example, 0.1:1 and 10:1.In some embodiments, the volume ratio of
the B19 phase and .beta. phase in a lamella is between 0.2:1 and
5:1, and the volume ratio may be between 0.25:1 and 4:1. In certain
embodiments, the volume ratio of the B19 phase and .beta. phase in
a lamella is between 1:3 and 3:1. For example, the volume ratio may
be between 0.5:1 and 2:1. Embodiments having a particularly fine
structure in the alloy composition have a ratio, in particular the
volume ratio, of the B19 phase and .beta. phase in a lamella,
between 0.75:1 and 1.25:1, for example, particular between 0.8:1
and 1.2:1. In one instance, the volume ratio may be between 0.9:1
and 1.1:1.
[0056] Moreover, it is possible in a further embodiment of the
alloys according to the invention that lamellas of the composite
lamella structures are surrounded by lamellas of type .gamma.
(TiAl), preferably on both sides of the lamella.
[0057] The alloys are further characterized in that the lamellas of
the composite lamella structures have a volume share of more than
10%, optionally more than 20%, of the total alloy.
[0058] Moreover, the fine lamella-like structure in the composite
structures are retained if the lamellas of the composite lamella
structures TiAl have the phase .alpha..sub.2-Ti.sub.3Al with a
volume share of up to 20%, wherein in particular the (volume) ratio
of the B19 phase and .beta. phase in the lamellas remains unchanged
and constant.
[0059] The alloys according to the invention are suitable as high
temperature lightweight construction material for components that
are exposed to temperatures of up to 800.degree. C.
[0060] An alloy as described herein can be produced using casting
or powder metallurgical techniques. The casting or powder
metallurgical techniques are used to produce an intermediate alloy
product containing the titanium, aluminum, niobium and optional
other components, if any, in the appropriate proportions. The
intermediate alloy product is then subjected to heat treatment
including heating at temperatures above 900.degree. C., preferably
above 1000.degree. C., in particular at temperatures between
1000.degree. C. and 1200.degree. C., for a predetermined period of
time of more than 60 minutes, preferably more than 90 minutes,
yielding a heat-treated intermediate alloy product. The
heat-treated intermediate alloy product is then cooled with a
predetermined cooling rate of more than 0.5.degree. C. per
minute.
[0061] In one embodiment, the heat-treated intermediate alloy
product is cooled with a predetermined cooling rate between
1.degree. C. per minute and 20.degree. C. per minute, preferably up
to 10.degree. C. per minute.
[0062] Light (high temperature) materials or components for use in
thermal engines like combustion engines, gas turbines, and aircraft
engines may be made of an alloy as described herein, e.g., from an
alloy based on an intermetallic bond of type .gamma.-TiAl made
through casting or powder metallurgical processes or techniques and
heat treatment.
[0063] Accordingly, an alloy as described herein can be used for
the production of a component. To avoid repetitions, reference is
made expressly to the above expositions.
[0064] As indicated above, the alloys described herein may be
created through the use of conventional metallurgical casting
methods or through known powder metallurgical techniques, and can
for example be processed through hot forging, hot pressing or hot
extrusion and hot rolling.
[0065] Examples of composite lamella structures of the type
described herein are shown in the figures. The example composite
lamella structures are based on an alloy comprised of titanium
(Ti), 42 atom % aluminum (Al) and 8.5 atom % niobium (Nb).
[0066] FIG. 1A shows a picture of a structure alloy, which was
taken with the help of a transmission electron microscope. The
overview picture in FIG. 1 shows that the composite lamella
structures, which are labeled with T in FIG. 1, have a striped
contrast to the structure of the .gamma. phase surrounding the
structures.
[0067] FIG. 1B shows a picture of the alloy structure with a higher
magnification, whereby it can be seen that the modulated composite
lamella structures (reference letter T) are surrounded by the
.gamma. phase respectively are embedded in the .gamma. phase.
[0068] The structures shown in FIG. 1A and 1b were obtained or set
through extrusion.
[0069] FIG. 1C shows a cast structure of the same alloy, i.e., an
alloy containing titanium, 42 at % aluminum, and 8.5 at % niobium,
in which a composite lamella structure (indicated in the Figure by
the reference letter T) is also formed, which is surrounding by the
.gamma. phase.
[0070] FIG. 2A shows a high resolution illustration of the atomic
structure of the composite lamella structures above the .gamma.
phase. The composite lamella structures are made up of the
controlled B19 phase and the uncontrolled .beta. phase, which
border the .gamma. phase (in the lower area). It can be seen from
the picture in FIG. 2A that the composite lamella structures
contain the two crystallographically different phases B19 and
.beta./B2, which are arranged at separation distances of a few
nanometers. The composite lamella structures contain the phases B19
and .beta., which are both considered ductile. The volume ratio of
the B19 phases to the .beta. phases in a composite lamella
structure is 0.8:1 to 1.2:1. Due to the ductile phases B19 and
.beta., the structure is mainly made of easily malleable lamellas,
which are embedded in the previously relatively brittle .gamma.
phase.
[0071] FIG. 2B shows an illustration of a B19 structure with a
magnified representation. The corresponding diffractogram, which
was calculated from the section shown in FIG. 2B and is
characteristic for the B19 structure, is shown in FIG. 2C.
[0072] FIG. 3 shows an electron-photomicrograph of a crack C in the
aforementioned alloy. It can be seen from the figure that the crack
C is diffracted at the modulated composite lamella structures (T)
and that the composite lamella structures form ligaments that can
bridge the edge of the crack. This type of behavior is considerably
different from the crack propagation in the previously known Ti--Al
alloys, in which a cleavage fracture occurs in the microscopic
dimension observed here. In the alloy according to the invention,
crack propagation is prevented due to the formed composite lamella
structures.
[0073] The fracture toughness of structure important for the
technical application was determined with the help of notched
Chevron samples in the bending test at different temperatures. The
recorded register curve of such a test is shown in FIG. 4. The
indentations marked by the arrows can be seen in the curve, which
indicate that crack propagation intermittently occurs during the
loading of the sample, but is stopped again and again. Such a
behavior is typical for alloys that are made up of a brittle phase
(.gamma. phase), in which the relative ductile phases B19 and
.beta. are embedded.
[0074] As mentioned above, the alloys according to the invention
can be made through the technologies known for TiAl alloys, i.e.
via casting metallurgy, reshaping technologies and powder
metallurgy. For example, alloys are melted in an electric arc
furnace and are re-melted multiple times and are then subjected to
a heat treatment. Moreover, the production methods of vacuum arc
casting, induction casting or plasma casting, which are known for
primary cast blocks made of TiAl alloys, can be used for
production. After the solidification of casting primary cast
material, hot-isostatic presses can also be used as the compression
method at temperatures of 900.degree. C. to 1,300.degree. C. or
heat treatments in the temperature range of 700.degree. C. to
1,400.degree. C. or a combination of these treatments, in order to
close pores and to establish the microstructure in the material as
described herein.
[0075] Although the invention has been described with reference to
particular embodiments thereof, it will be understood by one of
ordinary skill in the art, upon a reading and understanding of the
foregoing disclosure, that numerous variations and alterations to
the disclosed embodiments will fall within the scope of this
invention and of the appended claims.
* * * * *