Background of the invention
[0001] By a previous application the inventors disclosed and claimed a set of alloys having
a boron additive which made possible the achievement of a novel combination of strength
and ductility in certain compositions. That application, EP-A-0 110 268, was assigned
to the same assignee as the subject application and is incorporated herein by reference.
[0002] It is pointed out in the prior application that in many systems composed of two or
more metallic elements there may appear, under certain combinations of composition
and treatment conditions, phases other than the primary solid solutions. Such other
phases are commonly known as intermediate phases. Many intermediate phases are referred
to by means of the Greek symbol such as y or y'. Also, they are referred to by formula
as, for example Cu
3AI, CuZn and Mg
ZPb. The compositions of the intermediate phases which have simple approximate stoichiometric
ratios of the elements may exist over a range of temperatures as well as compositions.
[0003] Occasionally, as in the case of Mg
zPb, which occurs in the Mg-Pb system, a true stoichiometric compound, which compound
is completely ordered, is found to occur. Where each of the elements of the compound
is a metallic element, the intermediate compound itself is commonly called an intermetallic
compound.
[0004] The intermediate phases and intermetallic compounds often exhibit properties entirely
different from those of the component metals comprising the system. They also frequently
have complex crystallographic structures. The lower order of crystal symmetry and
fewer planes of dense atomic population of these complex crystallographic structures
may be associated with certain differences in properties, e.g. greater hardness, lower
ductility, lower electrical conductivity of the intermediate phases as compared to
the properties of the primary solid solutions.
[0005] Although several intermediate intermetallic compounds with otherwise desirable properties,
e.g. hardness, strength, stability and resistance to oxidation and corrosion at elevated
temperatures, have been identified, their characteristic lack of ductility has posed
formidable barriers to their use as structural materials. In fact, some of these materials
are so friable that they have been prepared as solids in order that they may be broken
up into powdered form for use in powder metallurgical processes for fabrication of
articles.
[0006] A recent article appearing in the Japanese literature disclosed that the addition
of trace amounts (0.05 to 0.1 wt.%) of boron to Ni
3AI polycrystalline material was successful in improving the ductility of the otherwise
brittle and non-ductile intermetallic compound. See in this regard Journal of the
Japan Institute of Metals, Vol. 43, page 358, published in 1979 by the authors Aoki
and lzumi. Although the room temperature tensile strain to fracture of the Ni
3AI was improved by the boron addition to about 35%, as compared to about 3% for the
Ni
3AI without boron, the room temperature yield strength remained at about 30 ksi. The
Japanese article did not refer at all, however, to rapid solidification of the boron
containing compositions which they studied.
[0007] By the method of the prior application EP-A-0110268 the addition of 0.01 to 2.5 at.%
boron demonstrated further improvements where the alloy preparation included the step
of rapid solidification. In particular, as it is brought out in this prior application,
preferred properties are found in rapidly solidified compositions containing between
0.5 and 2.0% boron and an optimum combination of yield stress and strain to fracture
is found in rapidly solidified compositions containing approximately 1.0% boron or
less.
[0008] Surprisingly, it has now been found that further property improvements are possible
in the alloy system of the gamma prime Ni
3AI intermediate phase where not only boron is present in the composition as a ternary
element but in addition a metal is present as a quaternary ingredient of such compositions
as a substituent metal.
Brief statement of the invention
[0009] It is, accordingly, one object of the present invention to provide an improved alloy
for operation at higher temperatures.
[0010] Another object is to provide an alloy of nickel and aluminum capable of operating
at elevated temperatures for sustained periods of time.
[0011] Another object is to provide a nickel aluminum alloy having an Ll
2 type crystal structure but having significant ductility and strength.
[0012] Another object is to provide an alloy of aluminum and nickel in which cobalt is substituted
for a portion of the nickel and which has a unique combination of physical properties.
i
[0013] Other objects and advantages of the present invention will be in part apparent and
in part pointed out in the description which follows.
[0014] The invention is defined in the appended claims.
[0015] In one of its broader aspects, objects of the invention can be achieved by providing
a rapidly solidified alloy composition having an L1
2 crystal structure and having a composition
(Ni0.75-xCoxAl0.25)yB100-y,
where x is from 0.025 to 0.15, and y is from 97.5 to 99.9.
Brief description of the figures
[0016] The present invention and the-description which follows will be made clearer by reference
to the accompanying figures in which:
Figure 1 is a plot of the values of the stress of the inventive alloys plotted against
the strain in percent for the base Ni3AI alloy as well as alloys containing substituents for the nickel and aluminum constituents.
Figure 2 is a plot showing the variation in yield strength and ductility for different
cobalt concentration, x, in as-solidified alloys having the composition
Figure 3 is a plot similar to that of Figure 2 but for samples which had been annealed
at 1100°C. Detailed description of the invention
[0017] By a substituent metal is meant a metal which takes the place of and in this way
is substituted for another and different ingredient metal, where the other ingredient
metal is part of a desirable combination of ingredient metals which ingredient metals
form the essential constituent of an alloy system.
[0018] For example, in the case of the superalloy system Ni
3AI or nickel base superalloy, the ingredient or constituent metals are nickel and
aluminum. The metals are present in the stoichiometric atomic ratio of 3 nickel atoms
for each aluminum atom in this system.
[0019] It has been known heretofore that a desirable crystal form and accompanying superior
physical properties can be achieved by forming a single crystal of Ni
3AI. However, polycrystalline Ni
3AI is quite brittle and shatters under stress such as applied in efforts to form the
material into useful objects or to use such an article.
[0020] It was discovered that the inclusion of boron in the rapidly cooled and solidified
alloy system can impart desirable ductility to the rapidly solidified alloy as taught
in application EP-A-0110268 referred to above.
[0021] Now it has been discovered that a certain metal can be beneficially substituted in
part for the constituent metal nickel and hence this substituted metal is designated
and known herein as a substituent metal, i.e. as a nickel substituent in the Ni
3AI structure. Moreover, it has been discovered that valuable and beneficial properties
are imparted to the rapidly solidified compositions which have the stoichiometric
proportions but which have a substituent metal as a quaternary ingredient of such
rapidly solidified alloy system.
[0022] The alloy compositions of the present invention must also contain boron as a tertiary
ingredient as taught herein and as taught in application EP-A-0110268 referred to
above, and must further contain a quaternary component or ingredient as taught in
the subject specification.
[0023] The composition which is formed must have a preselected intermetallic phase having
a crystal structure of the L1
2 type and must have been formed by cooling a melt at a cooling rate of at least about
10
3oC per second to form a solid body the principal phase of which is of the L1
2 type crystal structure in either its ordered or disordered state. The melt composition
from which the structure is formed must have the first constituent and second constituent
present in the melt in an atomic ratio of approximately 3:1.
[0024] As pointed out in the prior application EP-A-0110268, referred to above, compositions
having this combination of ingredients and which are subjected to the rapid solidification
technique have surprisingly high values for both the strain to fracture after yield
and for the 0.2% offset yield stress. For boron levels between 1 and 2% the values
of the strain to fracture generally declines so that a preferred range for the boron
tertiary additive is between 0.5 and 1.5%.
[0025] By the prior teaching of application EP-A-0110268, it was found that the optimum
boron addition was in the range of 1 atomic percent and permitted a yield strength
value at room temperature of about 100 ksi to be achieved for the rapidly solidified
product. The fracture strain of such a product was about 10% at room temperature.
[0026] Surprisingly, it has now been found that the unusual strength properties which are
obtained through the use of the rapid solidification in combination with the boron
additive may be increased to heretofore unprecedented levels with the addition of
a selected quaternary component or ingredient as a substituent to the primary nickel
constituent.
[0027] The quaternary ingredient which may be beneficially included in a composition for
rapid solidification as a substituent to make unprecedented improvements in the aluminide
properties is the element cobalt.
[0028] Further, it has been observed that in the case where an equiaxed structure is formed
with the quaternary composition of nickel, aluminum, boron and cobalt, by rapid solidification,
the properties of the composition are substantially better on the average than in
those cases where the non-equiaxed structure is formed. The equiaxed structure is
believed to result from recrystallization. It is known that recrystallization can
readily occur in a single-phase material.
[0029] The addition of the cobalt as quaternary ingredient and as a substituent for nickel
at about a 5 atomic percent level apparently does not form borides or beta phases
underthe influence of the rapid solidification process.
[0030] Regarding the improved properties achieved, the measurements made following the preparation
of the alloys and the testing of alloys as described herein has yielded some surprising
results. One set of the properties and particularly the tensile strength properties
are indicated in the attached Figure 1 in which the stress in ksi is plotted against
the strain in percent.
[0031] It is evident from Figure 1 that the alloy containing the tri-nickel aluminide labelled
Ni
3AI with 1% boron has the lowest stress values and that the two other samples which
were tested had significantly and unexpectedly higher values. The sample with about
5 atomic percent silicon had the highest stress values found and these were of the
order of 185 ksi. However, the same sample had lower strain and failed at a lower
value than the value measured for the tri-nickel aluminide itself. The boron doped
nickel aluminide having the cobalt substituent for nickel was found to have a stress
of approximately 130 ksi but had very significantly greater strain capability than
that of either the nickel aluminum superalloy itself or of the boron doped nickel
aluminide having the silicon substituent for aluminum.
[0032] Further study was made of the rapidly solidified compositions containing the substituent
cobalt as a quaternary additive. The cobalt additive was a substituent for nickel
and the concentration of nickel was decreased as the concentration of cobalt was increased.
The concentration of the cobalt was increased as is illustrated in Figure 2 from x=0.05
to x=0.3 in the expression:
[0033] From the plotted data of Figure 2, it is evident that the yield strength of the quaternary
composition containing cobalt as a substituent for nickel increases significantly
as the additions of cobalt increased from x=0.05 to 0.10 and then to 0.20. In fact,
as is evident from the figure, the yield strength doubles in value at a concentration
of cobalt x=0.20 when compared to the composition free of cobalt, i.e. at x=0. Beyond
x=0.2
0 the value of the yield strength decreases thus demonstrating that there is an effective
maximum in increasing tensile properties which occurs in the range of about x=0.20
cobalt as a nickel substituent and quaternary additive to the composition containing
the boron doped tri-nickel aluminide.
[0034] By contrast, the effect of the addition of the cobalt quaternary ingredient on the
ductility is illustrated from data plotted in Figure 2 to peak at a cobalt concentration
of approximately x=0.05. Additions of the cobalt at the x=0.05, 0.10, 0.20, and 0.30
levels demonstrated that only in the area of the x=0.05 level did the ductility increase
significantly over the boron doped tri-nickel aluminide from which the cobalt was
absent (i.e. 0% Co). The percentage increase from x=0 to x=0.05 Co was a surprising
80% for this relatively narrow range of cobalt addition.
[0035] The ductility of the sample which had the higher concentrations of cobalt declined
to a value at x=0.10 Co which is slightly higher than that at 0% cobalt and to progressively
lower values at x=0.20 and x=0.30 as illustrated in Figure 2.
[0036] With reference now to Figure 3, there is a plot of data obtained after the ribbon
samples which were prepared to contain the x=0.05, 0.10, 0.15, 0.20 and 0.30 cobalt
substituent were annealed at 1100°C for two hours. It may be observed from the data
plotted in Figure 3 that the annealing tended to reduce the overall yield strength
relative to those plotted in Figure 2. The ordinate scale of Figure 3 is about 40%
of that of Figure 2. The annealed ribbons exhibited a peak in strength for the sample
containing the x=20 level of cobalt. This strength maximum occurred at about the same
cobalt concentration in the unannealed ribbon as illustrated in Figure 2.
[0037] It will also be noted from the results plotted in Figure 3 that the ductility value
drops off very rapidly between x=
0.10 and x=0.05 so that it is necessary to have more than the x=0.05 level of cobalt
present as the quaternary additive substituent to avoid the precipitous drop in the
annealed ductility which occurs as the concentration of cobalt is reduced below the
x=0.10 value.
[0038] Based on the results obtained by study of the annealed ribbon as plotted in Figure
3 the optimum substituent concentration of cobalt for nickel in the boron doped tri-nickel
aluminide is approximately x=0.10. At this level, the cobalt quaternary additive resulted
in a composition having a yield strength of 80 ksi and ductility of 14% elongation
measured for the Ni
3Al-based alloy.
[0039] It should be emphasized that the data reported in Figures 2 and 3 are for tests of
ductility and tensile properties which were made at room temperature.
[0040] In the practice of this invention, an intermetallic phase having an L1
2 type crystal structure is important. It is achieved in alloys of this invention as
a result of rapid solidification. It is important that the L1
2 type crystal structure be preserved in the products which are annealed for consolidation
after rapid solidification.
[0041] Nickel aluminide is found in the nickel-aluminum binary system and as the gamma prime
phase of conventional gamma/gamma' nickel-base superalloys. Nickel aluminide has high
hardness and is stable and resistant to oxidation and corrosion at elevated temperatures
which makes it attractive as a potential structural material. Although single crystals
of Ni
3AI exhibit good ductility in certain crystallographic orientations, the polycrystalline
form, i.e., the form of primary significance from an engineering standpoint, has low
ductility and fails in a brittle manner integranularly.
[0042] Nickel aluminide, which has a face centered cubic (FCC) crystal structure of the
Cu
3AI type (Ll
2 in the Strukturbericht designation which is the designation used herein and in the
appended claims) with a lattice parameter a
°=3.589 at 75 at.% Ni and melts in the range of from about 1385 to 1395°C, is formed
from aluminum and nickel which have melting points of 660 and 1453°C, respectively.
Although frequently referred to as Ni
3AI, nickel aluminide is an intermetallic phase and not a compound as it exists over
a range of compositions as a function of temperature, e.g., about 72.5 to 77 at.%
Ni (85.1 to 87.8 wt.%) at 600°C.
[0043] In preparing samples pursuant to this invention the selected intermetallic phase
is provided as a melt whose composition corresponds to that of the preselected intermetallic
phase. The melt composition is made to consist essentially of the two constituent
components of the intermetallic phase nickel and aluminum in an atomic ratio of approximately
3:1 and is modified with boron in an amount of from about 0.01 to 2.5 at.%.
[0044] The melt is next rapidly cooled at a rate of at least about 10
3°C/sec. to form a solid body, the principal phase of which is of the L1
2 type crystal structure in either its ordered to disordered state. Thus, although
the rapidly solidified solid body will principally have the same crystal structure
as the preselected intermetallic phase, i.e., the L1
2 type, the presence of other phases, e.g., borides, is possible. Since the cooling
rates are high, it is also possible that the crystal structure of the rapidly solidified
solid will be disordered, i.e., the atoms will be located at random sites on the crystal
lattice instead of at specific periodic positions on the crystal lattice as is the
case with ordered solid solutions.
[0045] There are several methods by which the requisite large cooling rates may be obtained,
e.g., splat cooling. A preferred laboratory method for obtaining the requisite cooling
rates is the chill-block melt spinning process.
[0046] Briefly and typically, in the chill-block melt spinning process molten metal is delivered
from a crucible through a nozzle, usually under the pressure of an inert gas, to form
a free-standing stream of liquid metal or a column of liquid metal in contact with
the nozzle. The stream of liquid metal is then impinged onto or otherwise placed in
contact with a rapidly moving surface of a chill-block, i.e., a cooling substrate,
made of a material such as copper.
[0047] The material to be melted can be delivered to the crucible as separate solids of
the elements required. They can then be melted therein by means such as an induction
coil placed around the crucible. Alternatively, a "master alloy" can first be made,
comminuted, and the comminuted particles placed in the crucible.
[0048] When the liquid melt from the crucible contacts the cold chill-block, it cools rapidly,
from about 10
3°C/sec to 10
7°C/sec., and solidifies in the form of a continuous length of a thin ribbon whose
width is considerably larger than its thickness. A more detailed teaching of the chill-block
melt spinning process may be found, for example, in U.S. Patents 2,825,108, 4,221,257,
and 4,282,921 which are herein incorporated by reference.
[0049] The following examples are provided by way of illustration and not by limitation
to further teach the novel method of the invention and illustrate its many advantageous
attributes:
Example 1
[0050] A heat of a master composition corresponding to about 3 atomic parts nickel to 1
atomic part aluminum was prepared, comminuted, and about 60 grams of the pieces were
delivered into an alumina crucible of a chill-block melt spinning apparatus. The composition
had the formula:
[0051] The crucible terminated in a flat-bottomed exit section having a slot 0.25 inches
(6.35 mm) by 25 thousandth of an inch (0.635 mm)therethrough. A chill block, in the
form of a wheel having faces 10 inches (25.4 cm) in diameter with a (rim) thickness
of 1.5 inches (3.8 cm), made of H-12 tool steel, was oriented vertically so that the
rim surface could be used as the casting (chill) surface when the wheel was rotated
about a horizontal axis passing through the centers of and perpendicular to the wheel
faces. The crucible was placed in a vertically up orientation and brought to within
about 1.2 to 1.6 mils (30-40 µm) of the casting surface with the 0.25 inch (6.35 mm)
length dimension of the slot oriented perpendicular to the direction of rotation of
the wheel.
[0052] The wheel was rotated at 1200 rpm. The melt was heated to between about 1350 and
1450°C. The melt was ejected as a rectangular stream onto the rotating chill surface
under the pressure of argon at about 1.5 psi to produce a long ribbon which measured
from about 40-70 11m in thickness by about 0.25 inches (6.35 mm) in width.
[0053] The ribbon was tested for bend ductility and a value of 1.0 was found. This value
of bend ductility designates that the ribbon can be bent fully to 180° without fracture.
Example 2
[0054] The procedure of Example 1 was repeated using the equipment used as described in
Example 1 to prepare a master heat of the boron doped nominal Ni
3AI composition but one which was modified to the following composition:
[0055] This alloy was designated Alloy 92.
[0056] The melt was cast as also described in Example 1.
[0057] Ribbons were cast from the heat as also described in Example 1. The ribbons were
tested for bend ductility and a value of 0.04 was found for the ribbon thus prepared.
This value of bend ductility was calculated by dividing the ribbon thickness by the
bend radius at which the ribbon fractures.
Examples 3 through 12
[0058] Ten additional heats constituting Alloys 96, 101, 111 through 117 and 125 were prepared
having the compositions as set forth in the Table I below. These heats were prepared
in the manner described with reference to the first example described above and were
tested for bend ductility in the same manner as disclosed in Example 2. The values
for bend ductility which were obtained are listed in Table I.
[0059] It was also found that there is a correlation between the full bend ductility (bend
ductility=1.0) of the samples which were prepared and the formation of an equiaxed
configuration in the crystallographic structure which was formed. The Table indicates
also those samples for which an equiaxed format was found and also those for which
the non-equiaxed format was found.
[0060] As is evident from the results listed in Table I of all the compositions evaluated
in which an element was substituted for nickel, only the substitution of cobalt resulted
in a composition which had the equiaxed structure and full bend ductility.
Examples 13-15.
[0061] Three additional master heats were prepared using the procedure as described in Example
1. The compositions of the three heats of these examples are given in the attached
Table II for the respective examples.
[0062] The samples prepared in this manner were also tested for full bend ductility and
the results are also included in Table II below. Further, the structure was determined
and the Table lists the structure in terms of whether it is equiaxed (E) or non-equiaxed
(N).
[0063] The ribbons from these Examples were tested in tension without any preparation. The
resulting 0.2% offset yield strength (0.2% flow stress) and the ductility (strain
to failure after yield (i.e., total plastic strain s
P) are shown in Figure 2 as a function of the atomic percent concentration of the cobalt
in the composition. Each circle and triangle on Figures 2 and 3 represents an experimentally
determined data point.
[0064] The total plastic strains reported in Figure 2 should be regarded as minimum material
properties since the thin ribbons are largely susceptible to premature failure induced
by surface defects. Thus, the total plastic strain (ductility) would be expected to
be much higher for bulk material in which surface defects will play a much less influential
role. In fact, although not done for the ribbons of Examples 12-15, the apparent ductility
of ribbon-like specimens can generally be increased by mechanical polishing of either
the flat width surfaces or of the edges, or both, to remove surface and near-surface
defects and asperities.
[0065] The improved ductility of the nickel aluminide modified with boron and the quaternary
cobalt additive when processed by the method of the present invention may be tested
by the 180° reverse bend test wherein the ribbons are sharply bent 180° without the
use of mandrels or guides.
[0066] Samples of the ribbons prepared as described in Examples 12-15 above were subjected
to heat treatment at 1100°C for two hours. The tests conducted on the strip prior
to the heat treatment were performed again on samples of the strip which had been
subjected to the heat treatment. The results obtained from these tests are plotted
in Figure 3. Referring now to Figure 3, it is evident that there has been a reduction
in the values on the ordinate scale. The ordinate of Figure 3 is approximately 40%
of the scale shown in Figure 2. The abscissa which shows the concentration of the
cobalt quaternary additive in weight percent is not changed and is the same in Figure
3 as it is in Figure 2. As is evident from Figure 3, an annealed tri-nickel aluminide
having a preferred combination of properties is one having about 10 atom percent cobalt
substituent for the nickel of the aluminide.