Nickel-aluminum-boron powders prepared by a rapid solidification process

Nickel-aluminum alloys containing boron in powder form are disclosed. These alloys are subjected to melt-spinning to form a brittle filament consisting in large measure of a metastable solid solution phase. This is then pulverized to powder configuration. Such powders exhibit excellent sprayability to form a dense, homogeneous, hard coating on a metallic substrate. The alloys, also exhibit excellent resistance to high temperature oxidation.

BACKGROUND OF THE INVENTION 
This invention relates to nickel-aluminum-boron powders and more 
particularly to such powders having excellent capability to produce a 
dense structurally strong coating with excellent resistance to wear, 
oxidation, and corrosion. 
DESCRIPTION OF THE PRIOR ART 
Metal powders are well-known in the art and widely used. They can be 
deposited as a coating on a base metal by various spray techniques, e.g. 
flame spray and plasma spray. A dense, well-bonded coating of suitable 
chemical composition, structures, and properties deposited onto a 
relatively inexpensive base metal is useful to economically extend the 
service life of a product made of the base metal where such a product is 
subjected to corrosive and oxidizing media in service conditions. 
Spray metal powders are also used to produce dense, hard, high structural 
strength coatings for resistance against various kinds of wear e.g. 
abrasive, sliding, fretting, etc. 
The spray coatings are also suited for dimensional restoration of worn 
parts. 
The present invention relates to boron-containing nickel-aluminum alloys 
produced as powders by a rapid solidification process wherein such powders 
are characterized by (a) a high degree of compositional uniformity, (b) 
excellent sprayability i.e. ability to form a dense, hard coating with 
high interfacial bond strength between the coating and the substrate, (c) 
high hardness, and (d) excellent resistance to various hot corrosive 
media. 
Several techniques are well-known in the art to economically fabricate 
rapidly solidified alloys. One well-known example is melt-spin chill 
casting whereby metal is spread as a thin layer on a conductive metallic 
substrate moving at a high speed to form a rapidly solidified ribbon. 
Many alloy compositions are well-known in the art based on transition 
metals containing large amounts of metalloid elements e.g. boron, carbon, 
phosphorous, or silicon wherein such alloys when subjected to rapid 
solidification processing by the method of melt-spinning form ribbons 
which possess high ductility, strength, and hardness (ref. to U.S. Pat. 
No. 3,856,513 and U.S. Pat. No. 3,986,867). 
SUMMARY OF THE INVENTION 
This invention features nickel-aluminum-boron alloys prepared by a rapid 
solidification processing method in powder form characterized by 
homogeneous, ultrafine crystalline structure, excellent sprayability to 
form a dense, well-bonded coating on a metallic substrate and having high 
hardness and excellent resistance to various corrosive and oxidizing 
media. 
Alloy compositions are categorized as given below: 
EQU Ni.sub.40-58 Al.sub.30-40 B.sub.12-20 
wherein the subscripts are in atom percent; Ni, Al, and B are nickel, 
aluminum, and boron respectively; wherein the total content of Ni, Al, and 
B must be equal to 100. 
Using the method melt-spin-chill casting, the alloys of the above formulae 
are subjected to rapid solidification processing (i.e. processing in which 
the liquid alloy is subjected to cooling rates on the order of 10.sup.5 to 
10.sup.7 .degree. C./second) whereby they form brittle ribbons. The 
rapidly solidified ribbons of such alloys having high hardness values, 
800-1100 Kg/mm.sup.2, are readily pulverized by the standard technique of 
hammer milling, etc. into powders under 80 mesh and preferably under 170 
mesh suitable for application as spray coating. 
DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the present invention nickel-aluminum alloys are further 
alloyed with 12-20 atoms percent of boron. The alloys may also contain 
limited amounts of the elements which are found in commercial nickel-base 
alloys without changing the essential behavior of the alloys. Typical 
examples include Ni.sub.40 Al.sub.40 B.sub.20, Ni.sub.45 Al.sub.40 
B.sub.15, and Ni.sub.48 Al.sub.35 B.sub.17. 
The alloys of the present invention upon rapid solidification processing 
from the melt by melt-spin-chill casting at cooling rates on the order of 
10.sup.5 to 10.sup.7 .degree. C./second, form brittle ribbons consisting 
of a high degree of compositional uniformity. The brittle ribbons are 
readily pulverized into powders using standard comminution techniques e.g. 
a rotating hammer mill. Powders typically have an average particle size of 
less than 80 mesh (U.S. Standard), preferably less than 170 mesh (U.S. 
Standard) comprising platelets having an average thickness of less than 
0.1 mm and each platelet being characterized by an irregular shape 
resulting from fracture of the solidified material. The powders prepared 
by fracture of hard, brittle melt-spun ribbons are characterized by a 
smooth edge and surface. The hard, non-deformable powders, because of 
their shape characteristics exhibit excellent free-flowing 
characteristics. The smooth surfaces and edges of the particles also 
prevent agglomeration or interlocking of the particles. The above 
characteristics of the powders of the present invention enable their 
smooth and uniform flow aided by a carrier gas from a powder feed unit to 
the plasma flame through a nozzle. Uniform flow of powders through the 
plasma flame is essential for consistent sprayability of "spray" powders. 
During the process of spraying these crystalline powders, a fairly even 
distribution of particles throughout the cross-section of the spray stream 
is achieved so that sprayed material becomes evenly distributed on the 
substrate being coated and that there are no gaps between passes of the 
spray stream. 
The present rapidly solidified powders of nickel-aluminum-boron alloys 
exhibited capability to form excellent, dense and homogeneous coatings by 
the plasma flame spraying technique. Such coatings have high 
macro-hardness between 80-83 on R.sub.n 15 scale in as-sprayed condition. 
For example, a coating made by plasma spraying of rapidly solidified 
nickel-aluminum-boron powders on a mild steel plate had a microhardness of 
83 on the R.sub.n 15 scale. Such high hardness of the coating will make 
them suitable for various applications involving sliding wear, fretting 
wear, hard particle erosion, and the like. These powders form a very 
dense, extremely well-bonded coating. The coatings of these powders have 
high strength. Typically, the tensile strength of the coating averaged 
around 8000 psi. Many commercial coatings made by plasma spraying mixtures 
of tungsten carbide and cobalt powders intended for wear resistant 
applications typically have macrohardness in the 74-84 range of the 
R.sub.n 15 scale (see, Handbook on Plasma Spray Materials by Bay State 
Abrasives, Dresser Industries, Westboro, Mass.). However, wear-resistant 
applications of tungsten carbide spray coatings are limited to low 
operating temperatures, such as under 1000.degree. F., (540.degree. C.). 
For many applicatons, e.g. in aerospace industry at high operating 
temperature, such as above 1000.degree. F., tungsten carbide coatings are 
unsuitable because of their poor high temperature oxidation resistance. 
Coatings made of commercial titanium-carbide and chromium carbide powders 
mixed with nickel powders have the capability of operating at higher 
temperatures, reaching 1500.degree. F. (815.degree. C.); however, these 
coatings have lower hardness values, e.g. 70-82 on the R.sub.n 15 scale. 
The boron content of the present alloys range between 12 to 20 atom 
percent. With the boron content over 20 percent, the alloys produce 
brittle coatings. At metalloid contents below 12 percent, the alloys are 
difficult to form as rapidly solidified ribbons by the method of melt 
deposition on a rotating chill substrate i.e. melt-spinning. This is due 
to the inability of the melts with low metalloid content to form a stable 
molten pool on the quench surface, as necessary to form rapidly solidified 
ribbons by the melt-spinning procedure. Furthermore, at low contents of 
metalloid, the alloys produced sprayed coatings of less desirable hardness 
and strength. Superior mechanical properties, excellent high temperature 
oxidation and corrosion resistance, high hardness and excellent 
sprayability of the present rapidly solidified nickel-aluminum-boron alloy 
powders will make them suitable for many applications as sprayed coatings 
requiring good oxidation, corrosion, and wear resistance. 
The as-sprayed coating made with the present nickel-aluminum-boron powders 
of exhibited high hardness and excellent resistance to oxidation and 
structural degradation upon exposure to high temperatures, reaching 
1500.degree. to 1700.degree. F. (815.degree.-930.degree. C.) Such coatings 
will be useful in wear resistant applications at high operating 
temperatures which may exceed 1500.degree. F.

EXAMPLES 1-5 
A number of alloys having compositions within the scope of the invention, 
as given in Table 1, were fabricated as ribbons having the thicknesses of 
.about.0.001-0.003 inches by the melt-spinning-chill casting method using 
a rotating Cu-Be cylinder having a quench surface speed of .about.5000 
ft/minute. The alloys were found to have excellent ribbon fabricability. 
The ribbons were found by Xray diffraction analysis to consist 
predominantly of a single solid solution phase. The as-quenched ribbons 
were found to be quite brittle to bending, being amenable to ready 
comminution to powder. The as-quenched ribbons exhibited high 
microhardness values ranging between 800 to 1100 Kg/mm.sup.2 (see Table 
1). 
TABLE 1 
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Alloy Composition 
Hardness 
Example (atom percent) 
(Kg/mm.sup.2) 
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1 Ni.sub.40 Al.sub.40 B.sub.20 
950 
2 Ni.sub.45 Al.sub.37 B.sub.18 
1020 
3 Ni.sub.50 Al.sub.30 B.sub.20 
1004 
4 Ni.sub.55 Al.sub.32 B.sub.13 
810 
5 Ni.sub.52 Al.sub.36 B.sub.12 
966 
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EXAMPLES 6-7 
Two alloys having compositions Ni.sub.45 Al.sub.37 B.sub.18 and Ni.sub.50 
Al.sub.35 B.sub.15 were prepared into rapidly solidified powders by 
pulverization of brittle, rapidly solidified melt-spun ribbons. The 
powders had a particle size ranging between -170 mesh to +325 mesh. The 
powders were plasma sprayed on the surface of mild steel plate grit 
blasted with alumina. The spray parameters used are given in Table 2 
below. 
TABLE 2 
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Working Gas: Argon 
Working Gas Flow: 40% at 50 psig 
Powder Gas Flow: 15% at 50 psig 
Powder Feed Rate: 6.1 pounds/hour 
Deposition Rate: 4.5 pounds/hour 
Arc Current: 700 Amperes 
Arc Voltage: 30 volts 
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Plasma sprayed coatings produced with the above powders have a dense 
coating with a homogeneous structure. The coatings also exhibited high 
hardness values ranging between 80-83 on the R.sub.n 15 scale. 
EXAMPLE 8 
The tensile strength of a plasma sprayed coating 0.006-0.008 thick having 
the composition Ni.sub.45 Al.sub.37 B.sub.18 (atom percent) was measured 
and indicated as average value of 8200 psi. 
EXAMPLE 9 
A mild steel coupon was coated on all sides by plasma spray deposition 
technique with a 0.020" thick coating of an alloy, Ni.sub.45 Al.sub.37 
B.sub.18. The coated coupon was exposed to 1700.degree. F. for 100 hours. 
The coating exhibited excellent resistance to oxidation and spalling.