Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE LNVENTION
Field of Invention - The invention relates to the field
of metallic articles having surface properties and
microstructures which differ from the properties and
microstructures of the underlying substrate.
Description of the Prior Art - Although most prior art
dealing with composite microstructure materials is not
predicated on a surface melting technique, there is some
prior art which shows articles with remelted surface layers.
U.S, Patent 3,388,618 shows a steel cutting tool
having a surface portion with a refined structure. The
structure is relatively deep, about 1/4 inch.
U.S. Patent 3,838,288 speaks generally of a two step
surface melting process, without giving details, except to
say that the formation of pores is minimized.
U.S. Patent 3,505,126 discloses a method for refining
the grain size of a metal plate by surface melting to half
the thickness of the sheet~ reversing the sheet and remelting
the other half. Some of the example language indicates that
thick plate (1/2") is contemplated.
U.S. Patent 3,773,565 speaks of a surface refinement
technique for steel bearing races in which the surface is
melted to a depth of about lmm and allowed to resolidify.
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A description of surface refinement in aluminum alloys,
by surface melting, is given in APP1. Phys. Letters 21 (1972)
23-5.
SUMMARY OF THE INVENTION
The present invention relates to articles having surface
layers with microstructures which differ significantly from
the microstructure of the underlying substrate. The surface
layer has a refined microstructure which results from surface
melting followed by extremely rapid solidification. To
obtain the desired microstructures,cooling rates of 104F/sec
and up are necessary and this requires that the surface layer
be restricted to very thin layers, less than about 50 mils
in thickness.
If the surface layer is of a suitable deep eutectic, and
the cooling rates during solidification are great enough,
crystallization during solidification may be eliminated and
the surface layer may be amorphous.
If the surface layer is based on an alloy of transition
metals and metalloids, unique precipitate morphologies may
be produced.
Other features and advantages will be apparent from the
specification and claims and from the accompanying drawings
which illustrate an embodiment of the invention.
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In accordance with an embodiment of the invention,
a metallic article having a composite microstructure comprises:
aO a crystalline substrate, bo a resolidified surface layer
: having an ultra fine microstructure with at least one of the
surface layer grain dimensions being less than about 1,000 A
with the total thickness of the surface layer being from about
.1 mil to about 50 mils, c. an epitaxial layer separating the
substrate and the surface layer, with the thickness of the
substrate being at least four times the thickness of the surface
layer.
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DESCRIPTION OF THE DRAWINGS
,
Fig. 1 shows a portion of the nickel-boron phase
diagram, and illustrates a typical deep eutectic system.
Fig. 2 shows a transverse optical micrograph of a
skin melted Pd-Cu-Si article.
Fig. 3 is an optical micrograph of Vickers micro-
hardness indentations in the article of Fig. 3.
Fig. 4 shows an electron micrograph of a fracture
surface in a Pd-Cu-Si article.
Fig. 5 shows a microcrystalline surface layer in a
Ni-Co-Cr-Mo B alloy.
Fig. 6 shows an optical micrograph of overlapping
skin melted layers.
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DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention concerns metallic articles having
surfaces layers with metallurgical structures and properties
which differ from those of the underlying substrate. The
articles have a surface layer with a much finer structure,
different chemical and mechanical properties, and, optionally
a different chemical composition than the underlying substrate.
This invention involves eutectic systems which are systems
having specific compositions with melting points that are less
than the melting points of the alloy constituents. Eutectics
may form between two or more elements or compounds. Fig. 1
shows a portion of the nickel-boron phase diagram showing a
eutectic at 18.4 atomic percent boron with the eutectic trough
extending from 0 to 25 percent boron. For the purposes of
this application deep eutectics are those in which the eutectic
melting point is substantially less than the melting point of
the major e~tectic constituent. In the Ni-B system shown in
Fig. 1, the major component of the eutectic is Ni with a
melting point of 1453C while the eutectic temperature is
1080C.
Briefly, the articlesare produced as follows: a metallic
intermediate article is provided having a shape which closely
corresponds to the desired final configuration. The surface
layer (at least) of the intermediate article is of substantially
eutectic composition over some fraction of the total surface
layer area.
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A portion of the surface area generally corresponding to
that part of the area which is of substantially eutectic
composition, is rapidly melted to a depth on the order of
about .1-50 mils. Melting is performed by heating the surface
of the article with a form of energy which will be substantially
absorbed at the surface. Since melting occurs under
conditions which heat the surface without significantly
heating the substrate, after melting is complete, cooling
of the surface layer by heat flow into the substrate is
extemely rapid, usually at least about 105~C/sec. By varying
the heating parameters, the cooling rate may be controlled.
This high cooling rate results in an ultra fine microstructure,
which in combination with the selected surface chemistry is
responsbile for the novel surface properties of the article.
This invention may conveniently be described in terms of two
major embodiments, the first involves a surface layer which
is at least partially amorphous. The second embodiment
involves a surface layer which is at least partially micro-
crystalline.
The two embodiments are related in that they both comprise
articles which have then resolidified surface layers whose
microstructure is extreme~y fine, with at least one of the
grain dimensions being less than about l,OOOA. In stating
this relationship the amorphous state is regarded as comprising
material whose grain size is on the order of atomic size,
about 5A For convenience these articles are described as
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having a composite microstructure i.e. a fine surface
microstructure and a coarse substrate microstructure.
Embodiment 1 Partially or Wholly Amorphous Surface Layer
Briefly it can be said that amorphous materials lack the
regular~long range order which is characteristic of crystalline
materials and have structures similar to super-cooled liquids
such as glasses.
Amorphous metals, which have heretofore usually been
produced only by complete melting followed by extremely
rapid solidification in shapes of restricted geometry (with
at least one dimension being restricted to less than about
5 mils), have been found to possess properties which often
include high strengths, high hardness, good fatigue resistance,
and resistance to oxidation and corrosion. The rapid
solidification is commonly obtained by cooling the liquid
metal on a cold solid chill,and this is the reason that at least
one dimension must be small. The present invention provides
these advantages of amorphous materials in a surface layer,
on a~d integral with a massive crystalline substrate.
So far as is currently known, compositions which have
been made amorphous have invariably been of approximately
eutectic composition. The closer to exact eutective composition,
the more readily a composition can usually be made amorphous
by rapid solidification. Likewise, the deeper the eutectic
(measured in terms of percent depression of the absolute
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melting temperature of the eutectic point from the absolute
melting temperature of the major phase), the more readily
a eutectic can usually be made amorphous. The process of
the invention is broadly applicable to any deep eutectic.
Classes of eutectics which have been made amorphous by rapid
solidification include:
a. Eutectics between transition metals and metalloids
which usually contain from about 15 to about 30
atomic percent of the metalloid. Examples include
Ni + B, Ni + P, Ni + P + C, Fe + B and Ti-Be (as used
herein the term metalloid includes elements chosen
from the group consisting of C, B, P, Ge, Se, Te,
Ga, As, Sb, Be and Si, and mixtures thereof).
Boron and phosphorous are the preferred metalloids
while iron, cobalt, and nickel are the preferred
transition elements.
b. Eutectics between nontransition metals and metalloids,
for example, Ag + Ge,
c. Eutectics between early transition metals and late
transition metals, elements 21 to 28 are the
transition elements; the low number elements are the
early transition elements typified by Ti while the
high number elements are the late transition metals,
typified by Co. An appropriate eutectic would be
that between Ti and Co.
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d. Eutectics between transition metals and nontransition
metals typified by Cu-Zr,
e. Eutectics between nontransition metals typified by
Au-Sn.
Eutectics of the first group, between transition metals
and metalloids, and eutectics of the third group, between
early and late transition metals are preferred for the purposes
of this invention. Although most of the previously listed
example systems listed are binary, the ternary and higher
0 eutectic compositions exhibiting deep eutectic troughs may
of course be made amorphous, and in fact, indications are
that the more complex eutectics may be made amorphous with
comparatively greater ease than the simple systems. For
this reason, the minimum and preferred minimum eutectic
temperature depressions (from the major constituent melting
point) for various multi component eutectics are listed
in Ta~le I.
TABLE I
Number of Min. M.P. (% ) Pref.M.P. ~o/
Components Depression J Depression~ J
2 15 25
3 10 20
4 7 17
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In its simplest form, this embodiment of the present
invention consists of a crystalline substrate with at least
partially amorphous resolidified surface layer having a
thickness of from about .1 to about 50 mils. Since cooling
rates are inversely related to melt thickness, and since
the formation of amorphous materials require high cooling
rates, the surface layer thickness is preferably less than
20 mils and most preferably less than 5 mils. In order to
obtain rapid cooling the substrate must be at least about
four times as thick as the surface layer. Ihe composition
of the substrate and the surface layer are substantially
identical in this simple form. The surface layer and the
substrate will be separated by an epitaxial layer, a layer
which was melted but which solified in oriented crystalline
form with the orientation of the individual crystals in the
epitaxial layer being generally related to the orientation
of the underlying crystals in the substrate. The thickness
of the epitaxial layer will vary from about .001 to about
1 mil.
While the preceding form in which substrate and surface
layer have identical compositions will undoubtedly be useful
in certain applications, this identical composition form may
not offer the precise combination of substrate and surface
layer properties desired. For example in crystalline form the
eutectics of transition metals and metalloids are usually
extremely brittle, although they have significant ductility
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in the amorphous state. Thus it might be desirable to have
an amorphous surface layer on a ductile substrate. For this
reason, the preferred form of this embodiment is one in which
the chemistry of the surface layer differs significantly
from the chemistry of the substrate.
A variety of means, well known to those skilled in the
metallurgical art, may be used to produce a eutectic surface
layer on a substrate of differing composition. Since these
are more method related than product related, they will not
be discussed here.
Once the surface layer is produced of substantially
eutectic composition with a depth of from about .1 to about
50 mils, the surface may be partially melted as previously
described to produce a surface layer which is at least
partially amorphous.
Experimental work has been performed on alloys
containing 90.7% Pd, 4.2% Cu and 5.1% Si. This system behaves
essentially as the Pd-Si system with Cu substituting for Pd.
The melting point of pure Pd is 1552C (1825~K~ while the
melting point of the Pd-5% Si eutectic is 800C (1073K).
Thus the additio~ of 5 weight percent, (about 15.5 atomic
percent) Si to Pd depresses the absolute melting point of
Pd about 41V/o~ A continuous carbon dioxide (infrared) laser
was used to melt a portion of the surface. The laser
operating conditions were: a power output of 3000 watts,
a spot size of .020 inches and a spot travel rate of 50 feet
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per minute. The power density was about 4 x 107 watts
per square inch, and the dwell time of the laser on a
particular spot was about .00003 seconds. The maximum
melt depth was about 7 mils and the average melt depth
was 3-4 mils. Fig. 2 shows a transverse optical micrograph
of a skin melted surface. The featureless region 1 is the
area which has been skin melted. The unmelted remainder
of the sample displays the features typical of the crystalline
eutectic structure. The extremely thin epitaxial layer is
not resolvable optically. Evidence of partial preferential
melting of certain phases can be seen at the interface
between the substrate and surface layer. X-ray analysis of
the skin melted region gave diffuse patterns characteristic
of liquids and glasses rather than the sharp peaks character-
istic of crystalline materials. Transmission electron
microscopy showed a complete lack of structure in the
featureless region (within the limits of the electron
microscope used), indicating that the material in the feature-
less region is in fact amorphous. Previous experimenters
had noted that amorphous Pd-Cu-Si was slightly softer and
significantly more ductile than crystalline Pd-Cu-Si and
these findings were confirmed. Further, the resistance
of the amorphous layer to etchants commonly used to prepare
metallographic samples gives some indication of its
resistance to chemical corrosion. Fig. 3 is a micrograph
of Vickers microhardness identationsin the amorphous region
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of the alloy. The curved slip lines indicate that the
sample has great ductility, and the symmetry and lack
of any straight line pattern again indicates that the sample
is not crystalline. Prior experimenters have observed that
amorphous materials have a unique fracture surface
morphology, which is terms vein like fracture. This
phenomenon is shown in Fig. 4, a replica electron micrograph
of a fracture surface across a skin melted region in
Pd-Cu-Si. The boundaries of the skin melted region 1 are
the free surface 2 of article and the melt depth limit
3. The fracture surface shows the vein like fracture
4 which is an area of interconnected, irregular raised
portion on the fracture surface, while the crystalline
substrate dispJays a more ordinary fracture surface morphology.
A particular characteristic of amorphous materials
is that they are thermodynamically unstable, and if heated
above a specific temperature (termed the glass transition
temperature) will crystallize. If crystallization occurs
near the glass transition temperature the resultant crystal
structure will be extremely fine with at least some crystal
dimensions on the order of less than about l,OOOAD. Such a
fine grain structure may have advantages for specific
applications since it is more thermodynamically stable than
the amorphous structure and has reasonably good mechanical
properties due to the fineness of its structure (the well
known Hall-Petch relationship predicts improved properties in
fine microstructures).
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This invention also contemplates the articles which result
from the heat treatment of articles with amorphous surface
layers at temperatures above the glass transition temperature.
Such articles have a crystalline substrate, an epitaxial
layer, and a surface layer which is at least partially
microcrystalline (grain size generally less than about l,OOOA).
The fraction of the surface layer which is microcrystalline
will be substantially equal to that fraction of the surface
layer which was amorphous before the heat treatment. This
heat treatment may be applied to any article with an at
least partially amorphous surface layer, regardless of
whether the chemical composition of the surface layer is the
same a~, or different than that of the substrate.
Embodiment 2
The second major embodiment is an article with a
crystalline substrate and microcrystalline surface layer
restricted in chemical composition.
The resolidified surface layer is microcrystalline and
its composition is essentially that of a eutectic between
a material chosen from the group consisting of transition
metals and mixtures thereof and a material chosen from the
group consisting of metalloids, and mixtures thereof.
C, B and P and mixtures thereof are preferred metalloid
elements. The metalloid constituents is prefera~ly present
in an-amount from about 15 to about 30 atomic percent.
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Certain combinations of metalloid elements and transition
metal elements as major alloy constituents are especially
preferred. These include B and P and mixtures thereof in
combination with Ni, Fe and cobalt and mixtures thereof, and
C, B, and P and mixtures thereof in combination with Ni and
Co and mixtures thereof. Of course the preceding are only
guidelines wh;ch suggest the major alloy constituents and
of course other minor alloy constituents, both metal
and metalloid may be present in amounts up to those which
do not seriously affect the formation and presence of the
desired metalloid rich particles. The surface layer
thickness in this embodiment is preferably from about .1
to about 50 mils. The microcrystalline grains in the surface
layer are elongated, with a length to diameter ratio of at
least 5:1, and a diameter of less than about 4,000A and
preferably less than about l,OOOA~. The grains form with
the axis of elongation parallel to the direction of heat flow,
thus the microcrystalline grains in the surface layer are
predominately oriented with their long axis perpendicular
in the surface. Most importantly, at least one of the
phases in the surface layer is supersaturated in the metalloid
constituent. This supersaturation of the metalloid element
contributes to the extremely high hardnesses displayed by
surface layers of this type. Experimental work has been
done on an alloy containing 15~/o CO~ 15% Cr, 5% Mo, 4/O B,
balance Nickel (weight percent composition). The behavior
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of this system is predictable by reference to the Nickel-Boron
phase diagram (Fig. 1), since cobalt, chromium, and molybdenum
all have significant solid solubility in nickel. A eutectic
composition exists at 4 weight percent boron (18.5 atomic
percent) the melting point of pure nickel is 1453C (1726K),
and the eutectic temperature is 1140C (1413K), thus the
addition of 4 weight percent to nickel depresses the absolute
melting point by about 18%. A 3 Kilowatt laser beam with a
.020 inch diameter spot size was traversed across the sample
at a rate of 50 feet per minute to produce the melt zone
shown in Fig. 5, a transmission electron micrograph. The
structure is a lamellar eutectic with a lamellar spacing of
about 380 A. This alloy is a candidate for the production
of amorphous coatings, however, the skin melting experiments
did not produce a cooling rate sufficient to suppress
crystallization and microcrystalline structure resulted.
Fig. 6 is an optical micrograph of a sample of the same
material which was skin melted using overlapping passes and
this figure shows that the skin melting technique can produce
a wide surface layer and in fact the complete surface of an
article might be so treated. The Vickers microhardness of
the matrix in Fig. 4 is about 650 kg/mm2 while the Vickers
hardness of the skin melted surface layer is about 1250 kg/mm2.
Table I gives approximate Vickers hardness numbers for some
other "hard" materials.
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TABLE II
Material Vickers Hardness (k~/mm2)
Hardened Tool Steel 650-700
Matensite 800
Tungsten Carbide 1600-3000*
Cemented Tungsten Carbide 1300-1400
(with Cobalt)
Quartz 1100
Ni-15Co-15Cr-5Mo-4B 650
substrate
Ni-15Co-15Cr-5Mo-4B 1250
surface layer
* varies with crystallographic orientation
Thus it can be seen that the skin melted microcrystalline
surface layer (supersaturated with B) compares very favorably
with other 'lhard" materials. Equally as interesting as the
hardness is the ductility of these coatings, which is
evident in their resistance to cracking and spalling under
the heavy localized loads which accompany hardness testing.
Fig. 6 shows a transmission electron micrograph of the surface
layer, showing a lamellar structure with a spacing of 380A.
It is believed that in this case, the cooling rate was
sufficient to suppress normal solidification at the equilibrium
freezing point and that this fine structure resulted from the
"solid state~l decomposition and crystallization of the
supercooled liquid at a temperature below the normal
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solidification temperature, but above the glass transition
temperature for the alloy (about 500C).
This invention presents a novel composite type of
article which possesses a unique combination of properties
which heretofore have not been available in a single article.
Those coatings of the present invention which are at least
partially amorphous will ~ave great utility in the prevention
of corrosion and other forms of chemical attack. Such
amorphous coatings will be useful in the chemical industry,
in bearing surfaces which must operate with contaminated
lubricants, and in other similar applications. Many of the
amorphous materials also display high surface hardness
in addition to chemical corrosion resistance and such
coatings will be exceptionally useful for heavy duty
bearing applications. Another use for such coatings is the
protection of articles, such as gas turbine blades and
vanes, from erosion and corrosion. Such erosion, caused by
ingested dust and grit is a major factor which limits
~he effective life of gas turbines. The safe operating
temperature of such coatings will necessaily be less than
the glass transition temperature of the surface composition.
The supersaturated, microcrystalline coatings will have
similar uses, except that the coatings will not be subject to
crystallization and hence will not be temperature limited
except to the extent that grain growth occurs. In addition,
the extreme hardness of certain of these coatings indicates
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that cutting tools might be fabricated with microcrystalline
cutting surface.
Both of the previously described embodiments of
articles with composite microstructures, may be regarded as
being the results of a continuously variable process. Consider
a deep eutectic comprised of a transition metal and a
metalloid, N + 4% B, for example. This material satisfies
both the criteria set forth for embodiments 1 and 2 above.
If this material were melted in a fashion which resulted in
extremely high cooling rates, perhaps about 108~F/sec, an
amorphous structure might be produced. The resultant article
would be that described above in Embodiment 1. If melting
conditions were selected that produced lesser cooling rates,
perhaps 106F/sec, a microstructure consisting of a nickel solid
solution matrix containing discrete, approximately equiaxed
borides might be produced. The borides would form
precipitation in the material below the normal solidification
temperature. If the cooling rate was reduced even more,
to perhaps 104F/sec, a fibrous eutectic microstructure
would result by a normal solidification mechanism. In all
three of these situations, the solidified material would
be supersaturated in boron because the cooling rates employed
are so far removed from e~uilibrium conditions.
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All three microstructures are unusual and are believed
novel when considered as a composite microstructure, in
combination with a bulk crystalline substrate. Even the
third microstructure described, the fibrous eutectic
structure is novel by virtue of its extreme fineness.
It will be appreciated that these three resultant
surface microstructures which fo~ the previously
de~cribed embodiments, are the result of a continuous
process of skin melting. Because solidification rates
vary as a function of position within the melt, surface
layers with mixtures of these three microstructures are
quite likely to form.
Although the invention has been shown and described
with respect to a preferred embodiment thereof, it should
be understood by those skilled in the art that various
changes and omissions in the form and detail thereof may
be made therein without departing from the spirit and the
scope of the invention.
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