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|United States Patent Application
VECCHIO; KENNETH S.
;   et al.
December 8, 2011
FABRICATION OF STRUCTURAL ARMOR
Fabrication techniques for and examples of metallic composite materials
with high toughness, high strength, and lightweight for various
structural, armor, and structural-armor applications. For example,
various advanced materials based on metallic-intermetallic laminate (MIL)
composite materials are described, including materials with passive
damping features and built-in sensors.
VECCHIO; KENNETH S.; (San Diego, CA)
; ROHATGI; AASHISH; (Alexandria, VA)
; KOSMATKA; JOHN; (Encinitas, CA)
THE REGENTS OF THE UNIVERSITY OF CALIFORNIA
August 8, 2011|
|Current U.S. Class:
|Class at Publication:
||B32B 15/01 20060101 B32B015/01|
1. An article of manufacture, comprising: a metal substrate; and a stack
of alternating metal and intermetallic layers metallurgically bonded to
one another and to a surface of the metal substrate, wherein each metal
layer comprises a first metal and each intermetallic layer comprises an
alloy of the first metal and a second metal, wherein thickness values of
the layers in the stack are spatially graded.
2. The article as in claim 1, wherein the first metal comprises titanium.
3. The article as in claim 1, wherein the second metal comprises
4. The article as in claim 1, wherein the first metal comprises a
5. The article as in claim 1, wherein the first metal comprises titanium,
the second metal comprises aluminum, and each intermetallic layer is
6. An article of manufacture, comprising: a substrate comprising a first
metal; a stack of alternating metal and intermetallic layers
metallurgically bonded to one another and to a surface of the substrate,
wherein each metal layer comprises the first metal and each intermetallic
layer comprises a compound of the first metal and a second metal; and a
plurality of metal wires penetrating through the stack and each having a
portion embedded in the substrate, each metal wire metallurgically bonded
to the stack and substrate.
7. The article as in claim 6, wherein the first metal comprises titanium,
and the second metal comprises aluminum.
8. An article of manufacture, comprising: a stack of alternating metal
and intermetallic layers metallurgically bonded to one another, wherein
each metal layer comprises the first metal and each intermetallic layer
comprises a compound of the first metal and a second metal; and at least
one sensor embedded in the stack operable to measure a parameter
indicative of a condition of the stack.
9. The article as in claim 8, wherein the sensor comprises a vibration
10. The article as in claim 9, wherein the vibration sensor comprises a
11. The article as in claim 10, wherein the piezoelectric material
comprises a lithium niobate crystal.
12. The article as in claim 9, wherein the sensor comprises a temperature
13. The article as in claim 8, further comprising a control mechanism
engaged to the stack to control the stack in response to a signal from
14. An article of manufacture, comprising: a stack of alternating metal
and intermetallic layers metallurgically bonded to one another, wherein
each metal layer comprises a first metal and each intermetallic layer
comprises an alloy of the first metal and a second metal; and a closed
metal box enclosing the stack, wherein each inner wall of the closed
metal box is metallurgically bonded to the stack.
15. The article as in claim 14, wherein the first metal comprises
16. The article as in claim 15, wherein the second metal comprises
17. The article as in claim 14, wherein the first metal comprises a
18. The article as in claim 15, wherein the first metal comprises
titanium and the second metal comprises aluminum.
19. The article as in claim 14, wherein the first metal comprises nickel.
20. The article as in claim 14, wherein the first metal comprises a
21. The article as in claim 14, wherein the first metal comprises
22. The article as in claim 14, wherein the first metal comprises a
23. The article as in claim 14, wherein the first metal comprises iron.
24. The article as in claim 14, wherein the first metal comprises an iron
25. The article as in claim 14, wherein the first metal comprises
26. The article as in claim 14, wherein the first metal comprises a
27. The article as in claim 14, wherein the first metal comprises an
aluminide-forming metal or alloy.
28. The article as in claim 15, wherein the second metal comprises an
29. The article as in claim 15, wherein the second metal comprises an
aluminum metal matrix composite.
30. The article as in claim 15, wherein the second metal comprises an
aluminum-infiltrate ceramic composite.
31. The article as in claim 14, wherein the first metal comprises a metal
or alloy that forms intermetallic compounds with magnesium, including
aluminum and its alloys.
32. The article as in claim 15, wherein the second metal comprises
33. The article as in claim 15, wherein the second metal comprises a
34. The article as in claim 15, wherein the second metal comprises a
magnesium metal matrix composite.
35. The article as in claim 15, wherein the second metal comprises a
magnesium-infiltrate ceramic composite.
36. The article as in claim 14, wherein the first metal comprises a
37. The article as in claim 14, wherein the first metal comprises
titanium, the second metal comprises aluminum, and each intermetallic
layer is titanium trialuminide.
38. The article as in claim 14, wherein each layer in the stack is
39. The article as in claim 14, wherein at least part of the layers in
the stack comprise corrugations.
CROSS REFERENCE TO RELATED APPLICATIONS
 This application is a divisional of U.S. patent application Ser.
No. 12/615,105, filed on Nov. 9, 2009, which is a divisional of U.S.
patent application Ser. No. 11/629,578, filed on Oct. 30, 2007, which is
a 371 national stage application of and claims the benefit of
International Application No. PCT/US2005/021095, filed on Jun. 15, 2005,
which claims the benefit of U.S. Provisional Patent Application No.
60/580,867, filed on Jun. 17, 2004. The disclosures of the prior
applications are considered part of (and are incorporated by reference
in) the disclosure of this application.
 This application relates to structural armor materials, their
designs, fabrication, and applications.
 Structural armor materials are specially designed to exhibit high
material strengths and resistance to ballistic impacts. Such materials
may be used to protect persons and various objects such as motor
vehicles, aircraft, and buildings, from ballistic and other harmful
impacts. One type of structural armor materials use multi-layer composite
structures of different material layers to form metallic intermetallic
laminate (MIL) composites. MIL composites may be designed to be
relatively light in comparison to various other armor materials. U.S.
Pat. No. 6,357,332, for example, describes examples of MIL composite
armors and associated fabrication processes.
 This application includes, among others, structural designs and
fabrication of metallic materials based on metallic-intermetallic
laminate (MIL) composite materials.
 In one implementation, a metal box is provided to have an opening
and a metal lid plate for closing the opening. A stack of alternating
first metal and second metal layers is placed inside the metal box. A
first metal in the first metal layers and a second metal in the second
metal layers are operable to react under heat and pressure to form an
intermetallic material. The opening is then closed by the metal lid plate
to compress the stack inside the box to contact each inner metal wall of
the box. Pressure and heat are then applied to the closed metal box to
compress the stack and to cause reaction between two adjacent layers in
the stack and reaction between the stack and each inner metal wall of the
box to form metallurgical bonding between adjacent layers in the stack
and between the stack and the metal box.
 In another implementation, a substrate made of a first metal is
provided to include a surface. A metal sheet made of a second metal is
then placed on the substrate in contact with at least a portion of the
surface. Pressure and heat are applied to the substrate and the metal
sheet to compress the metal sheet against the surface to cause reaction
between the metal sheet and the surface and to form an intermetallic
 An article of manufacture is also disclosed as an example. This
article includes a stack of alternating metal and intermetallic layers
metallurgically bonded to one another, and cavities in at least one
intermetallic layer and filled with a filling material. Each metal layer
includes a first metal and each intermetallic layer includes an alloy of
the first metal and a second metal.
 In another example, an article of manufacture is described to
include a metal substrate and a stack of alternating metal and
intermetallic layers metallurgically bonded to one another and to a
surface of the metal substrate. Each metal layer includes a first metal
and each intermetallic layer includes a compound of the first metal and a
second metal. The thickness values of the layers in the stack are
 In yet another example, an article of manufacture may include a
substrate including a first metal and a stack of alternating metal and
intermetallic layers metallurgically bonded to one another and to a
surface of the substrate. Each metal layer includes the first metal and
each intermetallic layer includes a compound of the first metal and a
second metal. The article further includes metal wires penetrating
through the stack and each having a portion embedded in the substrate.
Each metal wire is metallurgically bonded to the stack and substrate.
 In yet another example, this application describes a structure that
includes a stack of alternating metal and intermetallic layers and at
least one sensor embedded in the stack. The layers are metallurgically
bonded to one another. Each metal layer includes the first metal and each
intermetallic layer includes a compound of the first metal and a second
metal. The sensor is operable to measure a parameter indicative of a
condition of the stack.
 Furthermore, this application describes an article of manufacture
that includes a stack of alternating metal and intermetallic layers
metallurgically bonded to one another, and a closed metal box enclosing
the stack. Each metal layer comprises a first metal and each
intermetallic layer comprises an alloy of the first metal and a second
metal. Each inner wall of the closed metal box is metallurgically bonded
to the stack.
 These and other implementations are described in greater detail in
the attached drawings, the detailed description, and the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
 FIGS. 1A, 1B, and 1C illustrate examples of metallic-intermetallic
laminate (MIL) composites.
 FIG. 2 shows an exemplary system for manufacturing a MIL composite
 FIG. 3 is a chart that compares material properties of different
materials including MIL composites materials exhibiting large specific
 FIGS. 4A and 4B illustrate an example of a MIL composite material
having corrugated layers.
 FIGS. 5A and 5B show an example of confined MIL composite material.
 FIGS. 6, 7, 8A, 8B, 8C, 9A, 9B, and 9C show examples of substrates
coated with MIL composite layers.
 FIGS. 10A, 10B, and 10C show an example of a MIL composite material
having passive vibration-damping cavities.
 FIGS. 11A and 11B show examples of MIL composite materials having
 FIGS. 12A and 12B illustrate one implementation of an embedded
vibration sensor in a MIL composite material.
 Various metallic-intermetallic laminate (MIL) composites are known
for their material strengths, especially at high temperatures. Typically,
a MIL composite may be fabricated from pressing a stack of interleaving
metal layers made of a first metal with a high toughness and being
ductile, and a second metal suitable for reacting with the first metal to
form an intermetallic compound at a high temperature. The resultant MIL
layer exhibits a ceramic-like material properties such as a high material
strength and a high material stiffness. The final laminated composite
material essentially retains the ceramic-like strength and stiffness of
the MIL layers and the toughness and some of the ductility of the first
metal. The processing under the high temperature may be controlled to
cause the second metal to completely react with the first metal so that
the final structure is essentially a fused or laminated stack of MIL
layers interleaved with remaining first metal layers. Such MIL composite
materials may be used as armor layers to resist ballistic impacts and to
protect persons, animals, and various objects. Some examples of such MIL
composite materials are described in U.S. Pat. No. 6,357,332, the entire
disclosure of which is incorporated by reference as part of the
specification of this application.
 For example, MIL materials may be made from titanium and aluminum
to form a Al.sub.3Ti/Ti intermetallic compound with excellent material
strength, stiffness, and toughness. Notably, this material can have a
high corrosion resistance and are light in weight. The titanium metal
with a high toughness may be replaced by other metal materials with a
high toughness. Examples include a titanium alloy, nickel, a nickel
alloy, vanadium, a vanadium alloy, iron, an iron alloy, tantalum, a
tantalum alloy, and any combination of two or more materials selected
from titanium, nickel, vanadium, iron, tantalum and their alloys, or any
other metal that forms aluminides. The aluminum may be replaced by an
alloy of aluminum, an aluminum metal-matrix composite, an
aluminum-infiltrated ceramic composite, or all together replaced by
magnesium, magnesium alloys, a magnesium metal-matrix composite, or a
magnesium-infiltrated ceramic composite. In the situation of the second
metal being magnesium-based, the first metals can now include
aluminum-based metals and composites. When the second metal is
magnesium-based, the intermetallic phase will be a compound of magnesium
and the first metal. The example of the MIL composites being based on
titanium and aluminum will be used throughout this patent, but it is
understood that these metals can be replaced by any of the first and
second metals described above.
 FIG. 1 illustrates an exemplary initial stack of alternating metal
layers 110 and 120 for a MIL composite material prior to the pressing
fabrication under a high temperature. The metal layers are 110 are made
of a first metal of a high toughness such as titanium. The metal layers
120 are made of a second metal such as Al or its alloy. After the proper
pressing, the first and second metals react with each other and sacrifice
the part of each first metal layer 110 and the substantially the entire
second metal layer 120 to produce a composite layer 130 sandwiched
between two remainder layers 110A of the first metal layers 110. FIG. 1B
illustrates the structure of the MIL composite material after the
 FIG. 1C further shows an example of a Al.sub.3Ti/Ti MIL composite
material fabricated by hot pressing through a load-temperature cycle in
the air, without utilizing any kind of inert or protective atmosphere.
The resultant intermetallic, Al.sub.3Ti, possesses a high strength, a
high Young's modulus (e.g., 216 GPa), a low density (e.g., 3300 Kg/m3)
and a low ductility. The thickness values of the initial titanium and
aluminum foils are controlled such that the final composite consists of
Al.sub.3Ti as well as un-reacted titanium. The Ti/Al.sub.3Ti MIL
composites show excellent specific mechanical properties (such as
fracture toughness) that can rival those of conventional metals and
ceramics. The high toughness of the MIL composites is primarily
attributed to the alternate layering of brittle and ductile layers. The
ductile phase reinforcement of brittle materials utilizes crack-laminate
interactions to generate a zone of bridging ligaments that restrict crack
opening and growth by generating closure tractions in the crack wake and
utilize the work of plastic deformation in the ductile metal phase to
increase fracture resistance of the composite. Thus, a crack propagating
in the brittle Al.sub.3Ti layers is effectively stopped every time it
hits the ductile titanium layer.
 FIG. 2 illustrates one exemplary hot pressing system for making MIL
composite materials in the air without a gas-buffered environment. This
system allows for fabrication of MIL composites in open air has a number
of advantages. For example, vacuum or inert atmospheres generally require
greater apparatus cost and processing time, and may also limit the
overall size of the samples that can be produced. The open air
fabrication removes these and other limitations.
 In one implementation of the fabrication process by reacting under
heat and pressure, the interleaved first and second metal layers are
pressed under pressure. The operating temperature is then raised to a
temperature less than a melting point of the one or more second metals
and metal alloys but sufficiently high so that, at pressure, the solid
state diffusion occurs between the interleaved layers, physically
engaging and locking the layers in place. The temperature of the
pressured, diffused, locked interleaved layers is raised until all the
one or more second metals are reacted with the one or more first metals
to form an intermetallic compound. In this process, the temperature is
raised at a sufficiently slow pace and under sufficient continuing
pressure so that, despite the fact that the reacting proceeds with
increasing difficulty and an ultimate high temperature reached is greater
than a melting point of the one or more second metals, the one or more
second metals remain initially locked in place and ultimately become
reacted without squirting in liquid state from between the first metal
foils. Next, the material is cooled to the room temperature to form the
final structure which includes layers of one or more first metals and
metal alloys that are interspersed with regions of a hard intermetallic
compound. Notably, each step transpires in the open air with the presence
of atmospheric gases. The second metal layers become substantially or
completely reacted with the first metal layers nonetheless that the
temperature of liquefaction of the at least one second metals and metal
alloys from which the second metal layers are made is exceeded during the
 In the example of the Al.sub.3Ti/Ti composite materials, the
diffusion and reaction between titanium and aluminum to form the
intermetallic phase Al.sub.3Ti exhibit different behaviors at
temperatures significantly below and above the melting point of aluminum
(660.degree. C.). At temperatures below the melting point of aluminum,
e.g., from 400.degree. C. to 642.degree. C., Al is the major diffusing
species. The diffusion of Al is not affected significantly by an
interfacial oxide layer, but an interfacial oxide layer reduces the
nucleation rate of Al.sub.3Ti. Growth of the Al.sub.3Ti intermetallic
tends to occur exclusively on the Ti-rich side with a small fraction of
Al inclusions, and linear kinetics are observed until the breakdown of
the oxide layer, after which parabolic intermetallic growth rates are
observed. When oxide films are present between metals, linear kinetics
are seen in the early stages of diffusion and later become parabolic. The
reaction layer formed is composed of Al.sub.3Ti particles in an aluminum
matrix, and solid solutions are generally absent.
 As an example, foils of commercial purity 1100 aluminum and
Ti-3Al-2.5V foils may be stacked in alternating layers and are processed
in the composite synthesis apparatus in FIG. 2. The foil dimensions may
be selected to completely consume the aluminum in forming the
intermetallic with alternating layers of partially un-reacted Ti metal.
Foils may be cleaned in a bath of 2 pct HF in water, rinsed in water, and
then rinsed in methanol and rapidly dried in order to remove oxide layers
and surface contaminants before processing.
 Next, the cleaned foil stack is placed between two cartridge-heated
nickel alloy platens and placed on the crosshead of a screw-driven load
frame. The synthesis apparatus may be surrounded by ceramic fiber blanket
material to reduce heat loss. After foil loading, the pressure is
increased to about 3.8 MPa by load control at room temperature to ensure
good contact between foils. The temperature is initially raised to
600-650.degree. C. for 2 to 3 hours, while maintaining the pressure, to
allow diffusion bonding of the layers and, thus, minimize internal
oxidation between the layers. The temperature is then slowly raised
through the melting temperature of the second metal and the pressure is
reduced to about 2.3 MPa (to eliminate expulsion of liquid phases). The
temperature is further raised above the melting point of the second
metal, where the pressure drops to about 1.5 MPa as a result of the
reaction as liquid phases form over a 2 to 3 hour period; the pressure
will then increase as a result of intermetallic solidification to about
3.5 MPa when the bulk of the reaction is complete. The temperature is
then increased slowly to 670.degree. C. or above to ensure the reaction
has reached the corners of the sample. The sample is then air-cooled
while maintaining the pressure at about 3.8 MPa.
 The Ti-Al.sub.3Ti MIL composites may have the specific stiffness
(modulus/density) nearly twice that of steel, the specific toughness and
specific strength similar or better than many metallic alloys, and
specific hardness close to many ceramic materials. FIG. 3 shows a
comparison of different armor materials. The x-axis is the specific
modulus of a material on a log scale and the y-axis is the specific
compressive strength on a log scale. Good armor materials should be
located in the upper right-hand corner of the plot. The location of the
MIL composites (red ellipse) is shown to the right (higher specific
modulus) of the typical structural metals such as steels, Ti alloys,
Ni-superalloys, Al-alloys, and Ti-based intermetallics. The only metallic
materials of higher specific stiffness shown are the beryllium alloys.
Several ceramic materials are also shown to have higher specific
stiffness than the MIL composites, including SiC, B.sub.4C,
Al.sub.2O.sub.3, and diamonds. Clearly, the MIL composites possess has
superior material properties for structural applications such as
applications demanding high specific stiffness combined with high
 The above examples of MIL composite materials use planar layers.
Alternatively, the composite layers may be nonplanar and include certain
contours or geometries to enhance the structural performance. For
example, the metal layers used in a MIL composite material may have
corrugations or corrugated features. Such metal layers are stacked and
then subjected to heat and pressure to form the MIL structure. The
corrugations in different metal layers may be identical to one another
and may be different. The corrugations in different metal layers may be
spatially shifted relative to one another. Prior to the processing under
heat and pressure, the stacked layers may have air pockets or voids
between different layers due to the presence of the corrugations. Upon
processing, all layers are fused together into a solid composite
 FIG. 4A illustrates fabrication of identically corrugated plates
411 and 412 using a pair of corrugated pressing plates 410 and 420. In
this example, planar plates of two different metal materials are
interleaved to form a stack. This stack of planar plates is then pressed
between the pressing plates 410 and 420 to become corrugated plates with
identical corrugated patterns under heat and pressure to form a
corrugated panel with a MIL structure. Multiple corrugated panels may be
used to form a final structure.
 FIG. 4B shows one exemplary structure formed from four identical
corrugated panels that are parallel in their corrugated directions and
are spatially shifted to misalign the corrugated patterns. The space or
volume 430 between two adjacent panels may be filled with a suitable
metallic material, such as a metal foam, air, fire retardant, penetration
resistant fibers, or any number of materials. Alternatively, successive
corrugated panels of the composite laminate material may also be aligned
with their corrugations in an orthogonal orientation.
 A corrugated panel is load-bearing in one of its two planar axis,
and that several spaced-parallel corrugated panels may suitably bear high
loads within, as well as transversely to, their substantial planes.
Accordingly, arrayed composite laminate panels are suitable for good
construction materials, such as for the sides of armored fighting
vehicles, aerospace structures and for buildings.
 The above and other corrugated composite laminate materials benefit
all mechanical and strength advantages associated with corrugation. For
example, a corrugated panel may be capable of better supporting a load
aligned with axis of corrugations in the plane of the material without
buckling or bending. To this extent the utility of the material for
construction, including for load-bearing walls and the sides of armored
vehicles, is enhanced. As another example, the corrugations help to turn
the path of an impacting projectile. To account for the statistically
small probability that the projectile should hit centrally in the trough
of a corrugation, it is possible to back one panel of corrugated armor
with another, offset, panel. If structural strength is desired in two
perpendicular directions in the plane of a composite laminate material
described, then corrugated panels of the material having their
corrugations running in one direction may be alternated with other panels
of the material having their corrugations running at a 90-degree angle.
 The above planar and corrugated MIL composite materials may be used
to construct various advanced materials for structural, armor, and
structural-armor applications. For example, a MIL composite material may
be metallurgically bonded to and confined by a metallic box to form a
confined MIL composite structure. As another example, the MIL composite
layer structure and the associated processing may be used to fabricate a
surface layer over a metallic substrate or plate provide a hard
protection layer that is resistant to wear and corrosion. In addition,
the MIL composite materials may be designed to further harden it by
embedding localized materials that are spatially distributed within a MIL
composite material. A MIL composite material may also be designed to
include, at selected locations, built-in cavities with loose powder and
other suitable materials to create an internal vibration-damping
mechanism. Furthermore, a MIL composite material may also be designed as
an "intelligent" material to include sensors at selected locations to
measure and monitor a parameter of the material at these selected
locations, such as the magnitude of the impact to the material, the
temperature, and other measurable parameters. Examples and
implementations of these and other MIL-based materials are now described
in the following sections.
 FIGS. 5A and 5B illustrate an exemplary confined MIL material in
which a MIL composite material 501 is bonded to and confined within the
metallic walls of a metallic box. Notably, the bonding between the MIL
composite material 501 and the metallic walls of the box or container is
through a reaction between the metallic materials in contact and is
metallurgical in nature. A single fabrication process may be used to
accomplish both the bonding and the confining processes. Such confined
MIL composites may be used to expand the applications and the versatility
of MIL composite materials in structural, armor, and structural-armor
applications. The confined MIL composite materials have excellent
toughness and are effective in stopping and dissipating the energy of
 Referring to FIG. 5B, the MIL composite material 501, with either
planar composite layers or corrugated composite layers, is shown in a
cross sectional view to be in contact with four metallic walls 510, 520,
530, and 540 of the box. The box as shown is rectangular in shape and may
be in other alternative geometries such as a closed cylindrical shape.
 The following describes one exemplary fabrication process for
making such a confined MIL composite material where the MIL composite
material is assumed to be a Ti/Al.sub.3Ti MIL composite, and the metallic
walls of the box are assumed to be made of titanium as an example but may
be any of the various first metals.
 In preparation, the sheets and plates of titanium and aluminum are
cleaned by an appropriate method, e.g., mechanical brushing or
hydrofluoric acid bath. Sheets of Ti and Al are then interleaved to form
a stack. The stacking order is such that aluminum makes the top most and
the bottom most layers of the stack. Alternatively, titanium sheets and
plates may be replaced by any other metal such as nickel, iron, nitinol
 The Ti plates may be welded together to form a box with an opening
on one side. The inner dimensions of the box are designed to be close to
the dimensions of the stack of the Ti and Al sheets. The stack of cleaned
Ti and Al sheets are placed inside the box. The number of sheets is
selected to make the height of the stack slightly greater than the height
of the box. The box is then closed by pressing down a lid Ti plate on top
of the stack. The lid plate is then welded the rest of the box.
 If desired, a small tube may also be welded to the box such that
the box can be evacuated or back filled with an inert gas. Alternatively,
the process of placing the stack in the box and welding of the box may be
conducted within an evacuated chamber.
 Upon sealing the stack in the box, the entire assembly is heated in
a box furnace through a specific time-temperature routine to allow the
sheets to react to form the MIL composite as well as the metallurgical
bond to the box in the process. The result is a confined MIL composite
that is metallurgically bonded to the box.
 Such a confined MIL composite material block may be used as a
building block for various structures. Multiples of confined MIL
composite material blocks may be jointed together to construct large
structures. Since the external surfaces of the block is a metal (e.g.,
Ti), a suitable technique for joining two metal parts may be used to join
different confined MIL composite material blocks. For example, welding
may be used to joint two blocks together. Large armor panels in various
shapes may be constructed from joined blocks.
 In some applications, a surface of a metallic substrate, plate, or
part may be coated with a hard layer to provide improved surface
hardness, strength, and resistance to wear and corrosion. A MIL composite
structure and the associated fabrication process may be used to form such
a hard layer. The MIL composite layer is designed to have a hardness
greater than the hardness of the substrate to which it is bonded, and the
specific hardness can be tailored by suitable choice of materials. The
hardened surface layer may include layers of, for example,
titanium-trialuminide or titanium metal and titanium-trialuminide or
titanium metal and titanium trialuminide interspersed with another hard
ceramic, such as boron carbide, silicon carbide, tungsten carbide,
aluminum oxide, silicon dioxide, or any number of other hard ceramic
materials or interdispersed with metallic particles of elements such as:
titanium, tungsten, nickel, iron, copper, or any number of other metals
and their alloys.
 In one implementation, a hardened surface layer may be formed on a
metallic substrate such as, but not limited to, titanium and its alloys.
The hardened layer may exist on either, any, or all of the exposed or
unexposed surface of the substrate. This hardened layer may be a layered
structure. As an example, FIG. 6 shows that the layered structure may
include alternating layers of titanium (or a titanium alloy), and layers
of an intermetallic phase that is generally titanium-trialuminide. The
hardness of the intermetallic layer is greater than the hardness of the
substrate, while the hardness of the titanium (or the titanium alloy)
layer can be the same, greater, or less than that of the substrate. The
titanium (or titanium alloy) in the layered structure may be replaced by
other metals such as nickel, various kinds of steels etc., in which case
the intermetallic layer formed will be an aluminide of this other metal
(i.e. an aluminide of nickel, aluminide of iron etc). In another
variation, the titanium trialuminide layer may be interspersed with
regions of other hard phases such as ceramics. The coefficient of thermal
expansion of the various layers in the coating can be carefully matched
to that of the substrate layer through the use of various combinations of
ceramic materials and/or metals in the intermetallic layer(s) to aid in
achieving a good bond with the substrate material.
 Alternatively, the hardened layer may not be a layered structure
and may be a single layer of titanium trialuminide that is
metallurgically formed on the Ti substrate.
 The choice of coating material combination, e.g., the use of
Titanium/Titanium-trialuminide as the coating material, may be used on a
substrate of a material other than titanium. For example, the substrate
may be a nickel alloy and the coating may be a MIL composite layer having
interleaved Titanium and Titanium-trialuminide layers. As another
example, the substrate may be titanium and the coating may be a MIL
composite layer having interleaved nickel and nickel-trialuminide layers.
 In fabrication of a hard surface layer on a substrate, alternate
layers of titanium and aluminum sheets of pre-determined thickness are
first placed on the titanium substrate. As an example, thickness of the
titanium and aluminum sheets may range from about 0.001'' to about 0.1''.
In some implementations, the thickness of all the titanium sheets and
that of all the aluminum sheets may be respectively equal to each other,
although the thickness of the sheets can vary within the stacking
sequence in order to change the resulting hardened layer thickness
sequence. The length and the width of all the sheets may be sized to
cover a selected portion or the entirety of the substrate.
 Next in fabrication, the sheets are pressed under pressure and heat
in the ambient air as described with reference to the system shown in
FIG. 2. The reaction between the metals on the substrate surface creates
a ceramic-like intermetallic layer or a MIL composite layer. This
fabrication in the ambient air does not need a processing chamber with a
vacuum system and can be generally cheaper and easier than other layer
deposition methods such as vapor deposition, sputter coating, case
hardening which often require specialized environments and equipment. In
another aspect, the present processing technique may be used to form a
wide range of thickness of the hard layer. The thickness of the hard
layer in our process is orders of magnitudes greater than that achieved
by the conventional techniques such as vapor deposition, sputter coating,
case hardening etc. The process also may be used to provide the
flexibility of varying the hardness distribution of the surface layer by
utilizing the interspersed material of varying hardness.
 A MIL composite layer as the hard surface on a substrate may be
further configured to include various features to improve the performance
of the hard layer.
 For example, the thickness of the titanium and the aluminum layers
may be spatially graded so that the thickness of each titanium layer may
be different from the thickness of other titanium layers in the stack,
and that the thickness of each aluminum layer may be different from the
thickness of other aluminum layers in the stack. This graded layer
structure may be used to fine tune the hardness to or close to a desired
 As another example, the MIL composite layer may be filled with
perforation patterns or holes embedded with suitable hard materials to
further improve the hardness of the layer. FIG. 7 illustrates one example
where perforated aluminum sheets with different kinds of perforation
patterns are used to form the MIL composite material and the perforations
are filled by either a hard ceramic or another metal.
 When the perforations are filled with ceramics, such ceramics may
include boron carbide, tungsten carbide, silicon carbide etc. FIGS. 8A
and 8B show photographs of the top surface view and cross-sectional view,
respectively, of an exemplary wear and corrosion coating made from
perforated aluminum sheets filled with boron-carbide powder. Because each
perforated aluminum sheet is sandwiched between two Ti sheets before
fabrication, cavities are formed in the intermetallic layer after the
fabrication and are filled with the ceramic materials infiltrated with Al
or the intermetallic phase. FIG. 8C shows an enlarged view of the
cross-sectional view of FIG. 8B to illustrate examples of the
 A metal substrate coated with hard layer may also be strengthened
by having vertical hard metal wires embedded in the hard layer and the
substrate. FIGS. 9A, 9B, and 9C illustrate one example of a wire
toughened substrate coated with a MIL composite layer. Before the
processing under head and pressure to form the hard layer, holes may be
formed, e.g., drilled, into the aluminum and titanium sheets for forming
the MIL composite layer and similar matching holes are also formed in the
substrate. Titanium wires of similar diameter as that of the holes are
inserted through the sheets into the substrate. Upon completion of
processing, the Ti wires are metallurgically bonded to the MIL composite
layer and the substrate. FIG. 9A shows a top-surface photograph of a
sample fabricated with Ti wires connecting the substrate plate and the
hardened surface layer. FIG. 9B shows a cross-section photograph of a
sample fabricated with Ti wires connecting the substrate plate and the
hardened surface layer. FIG. 9C shows a micrograph of the cross-section
of one of the Ti wires.
 The wired toughened structure may be fabricated in the following
process in one implementation. The substrate, with alternating layers of
titanium and aluminum sheets placed on its surface, is placed in between
two heater platens, e.g., the system in FIG. 2. These platens are
compressed between the constraints of a load frame with mechanical
fixtures designed to distribute the load uniformly across the substrate
area. The entire assembly is then reacted in a load-temperature cycle.
Upon the completion of the reaction, the assembly is cooled gradually to
minimize separation of the layers due to thermal expansion mismatch
between the various constituents of the assembly. The final structure
includes the substrate, un-reacted titanium layers, and titanium
trialuminide layers. In the case of a decorated composite sample, the
titanium trialuminide layer also includes embedded ceramic within the
 In another aspect, MIL composite layers may be structured to
include vibration-damping cavities filled with loose powder materials to
absorb vibration energy. This damping mechanism is passive and is built
into a MIL composite material structure. Hence, the so fabricated MIL
composite material can inherently damp vibrations. In one implementation,
such damping cavities may be spatially distributed at positions of high
amplitude displacement of vibrations in a MIL composite material. For
example, titanium (metal)-Titanium Trialuminide (intermetallic)
composites may be designed with such cavities to achieve enhanced
vibration damping properties in specific vibration modes while
simultaneously possessing high strength, high toughness, low density and
good corrosion resistance.
 FIGS. 10A, 10B, and 10C illustrate one implementation of the
fabrication process for making a MIL composite material with damping
cavities. First, the vibration modes for a give MIL composite material
are determined. Damping cavities are designed to be located at positions
of high amplitude displacement of these vibration modes. Holes of a
desired size are formed, e.g., by drilling, in titanium and aluminum
sheets at such pre-determined locations. The drilled layers are then
stacked on top of each other to create a pattern of cylindrical cavities.
FIG. 10A illustrates one example of a MIL composite material with marked
hole locations. Two differently sized holes are shown. The drilled layers
may be a 3-layer structure by stacking Al/Ti/Al on top of each other.
Next, a titanium ring of the same outer diameter as the cavity is pushed
into the cavity. The height of the ring is made to be the same as the
height of the cavity. Alternatively, Ti cups may be used to replace the
rings. Furthermore, rings or cups may be eliminated and "bare" hole are
used to receive the powder material.
 FIG. 10B shows that separate stacks of Ti/Al/Ti sheets without any
holes are provided and are reacted under load and at a high temperature
within a load frame to form MIL composite plates of Ti/Al.sub.3Ti/Ti.
Notably, the external layers of the plates are Ti layers in order to
metallurgically bond with the external Al layers of the drilled stack of
Ai/Ti/Al layers to form an intermetallic layer. Next, the drilled stack
of Al/Ti/Al layers is placed on top of one MIL composite plate made in
FIG. 10B. A suitable granular material is filled into the cavities in
desired volume fraction. Examples of the granular material include but
are not limited to tungsten carbide, solid glass spheres, titanium
diboride, aluminum oxide, and silicon carbide. The holes are then covered
up with another MIL composite plate. As illustrated in FIG. 10C, this
entire assembly is processed under pressure and heat to cause reaction
between the contacted layers of the drilled stack and the two MIL
composite plates. Optionally, Ti sticks or strips may be placed with each
hole to provide support of the layers above and below the hole.
 The above and other MIL composite structures may incorporate
sensors embedded or buried within each structure so that a condition or
behavior of the structure may be measured and monitored. Such sensors
provide material "intelligence" of a given structure. In addition, a
control mechanism may be implemented in such an intelligent material
structure to control a condition or behavior of the structure and the
sensors and the control mechanism may be connected to form a control
feedback loop so that the entire structure may be intelligently
self-controlled. Such materials are smart and multifunctional materials.
Various useful functions may be implemented within the materials such as
damage detection, health monitoring, temperature sensing, actuation etc.
 As an example, a piezoelectric sensor may operate as both a source
and a receiver of ultrasonic pulses, a network of such embedded sensors
may be used to determine the location and extent of internal structural
damage. Such a network could also be used to locate external impacts on
the structures, such as those caused by projectiles. Further,
piezoelectric crystals may be used to dissipate vibration energy as
electrical energy by connecting them in an electrical circuit.
 FIG. 11A illustrates one example of an intelligent MIL composite
material where sensors 1110 are embedded at selected locations. For
example, vibration sensors 1110 are distributed in the top and the bottom
intermetallic layers of the material to measure any impact and the
distribution of the impact. FIG. 11B further shows that each embedded
sensor 1110 may include conductive wires to output an electrical signal
that represents the magnitude of the impact at the location of the
sensor. Various vibration sensors may be used, including piezo-electric
sensors that generate electrical signals in response to compression
applied to the sensors.
 Embedding sensors within metallic materials such as a MIL composite
material, however, may experience a high temperature during fabrication
process that can exceed 500.degree. C. and may reach the range of about
1000.degree. C. A Ti/Al based MIL composite is fabricated by reacting
alternate layers of Ti and Al between 660.degree. C. and 750.degree. C.
with the reaction occurring over typically 6-8 hours. The Al reacts with
Ti to form Al3Ti resulting in a final structure comprising alternate
layers of Ti and Al3Ti. Piezoelectric sensors embedded within the MIL
composites, hence, should sustain the integrity and function at such high
temperatures. Lithium-niobate piezoelectric sensors may be designed to
operate at such temperatures. Other sensors may also be used.
 FIGS. 12A and 12B show one example of a MIL composite material with
an embedded piezoelectric sensor. In this particular example, the basic
materials used in the fabrication of the MIL composite may be 0.020''
thick Ti-6-4 alloy sheets and 0.024'' thick 1100 Al alloy sheets. The
fabrication of the composite with embedded piezoelectric sensors may be
carried in the following five processing steps. First, several Ti-6-4
alloy sheets and 1100 Al alloy sheets are stacked alternately and reacted
under pressure and heat to form two MIL composite plates, each
approximately 0.325'' thick. In the second step, four 0.75'' diameter
holes corresponding to 4 sensors are machined out in another stack (total
height .about.0.150'') of alternately placed Ti-6-4 sheets and 1100 Al
alloy sheets. A close-fitting titanium ring, with a hole machined in its
wall, is placed in each of the four holes in the sheets. Rectangular
slots are machined in the sheets, adjacent to each of the holes and a
steel tube are placed in the slot. One end of the steel tube is inserted
into the hole in the titanium ring, while the other end of the tube
extended outside the stack. Thus, at the end of the second step, an
un-reacted stack of Ti and Al sheets with titanium rings is provided and
steel tubes are placed in their appropriate locations.
 Next in the third step, the assembly of the second step is
compressed in a load frame at about 600.degree. C. for several hours and
is then cooled down. In the fourth step, the piezoelectric sensors are
carefully placed in their respective holes and their lead wires are
passed through an alumina tube placed within the metal tube. The sensors
are then sealed in the cavity using a high-temperature cement paste
(Ceramabond 571, Aremco Products, USA) which is then cured for several
hours at room temperature followed by curing at about 100.degree. C.
Finally, one pre-reacted plate (from step 1) is placed on each side of
the stack containing the sensors (step 4), and then processed in the same
manner as in step 1.
 In one implementation, the piezoelectric sensors may be a
36.degree. Y-cut LiNbO3 crystals from Boston Piezo-optics (Massachusetts,
USA). The Curie temperature for the crystals is about 1200.degree. C. and
is higher than the maximum processing temperature of about 700.degree. C.
in fabricating the MIL composite material. The crystals may be sized to
be 0.5'' in diameter and 0.078'' tall with a co-axial electrode pattern.
 Only a few implementations are described. However, other
implementations, variations and enhancements may be made.
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