Eliminating prior particle boundary delineation

The process prevents prior powder particle boundary delineation by providing one or more surfactant elements which prevent nucleation of carbides at the particle surfaces with the result that carbide precipitation occurs within the powder particles rather than predominantly at the particle surfaces. In one embodiment, a small but effective amount of one or more surfactants is added to prealloyed powder before the powder is enclosed and densified at elevated temperature. In another embodiment, the surfactant is added to the melt of the prealloyed powder prior to atomization. The surfactant should be capable of forming a vapor under the conditions of hot densification, should be a strong oxide and/or sulfide former, must be a weak carbide former, should form oxides and/or sulfides which will not nucleate carbides of other elements, and which, if present in the article made from the powder, will not objectionably affect the desired properties. Useful surfactants include magnesium, barium, calcium, cerium, lanthanum, lithium, neodymium, praseodymium, yttrium, and misch metal.

BACKGROUND OF THE INVENTION 
This invention relates to powder metallurgy and, more particularly, to a 
method for substantially eliminating the precipitation of carbides at 
prior powder particle boundaries in articles made by powder metallurgy. 
The undesired formation of carbides at prior particle boundaries occurs 
during powder metallurgy processes after the powdered alloy has been 
heated above the carbide solvus temperature. When the microstructure of 
the cooled alloy is examined, the prior particle boundaries can be seen 
distinctly delineated. And upon further examination, it has been 
determined that the materials which delineate boundaries are mainly 
reprecipitated carbides. Here and throughout this application it is 
intended by the term "carbides" to include compounds of carbon with one or 
more metals with or without one or more nonmetals such as oxygen, sulfur 
and nitrogen. 
It is not completely understood why such carbides form at the prior 
particle boundaries. When the alloy is heated above the carbide solvus 
temperature, the carbides within the powder particles go into solid 
solution. When the temperature is then lowered below the carbide solvus 
temperature, carbides reprecipitate. While at elevated temperature, the 
powder particles are ideally desired to fuse together to form a solid, 
unitary article without the particles themselves melting. When the 
carbides precipitate along what had been the particle boundaries rather 
than remaining more homogeneously distributed within the particles, the 
resulting article does not have the desired properties, as evidenced most 
notably by low ductility in directions perpendicular to the delineated 
prior boundaries. Furthermore, it has been found that alloys which are 
vulnerable to sulfidation attack, which is particularly undesirable in 
products such as components of engines which burn fuels containing sulfur 
are improved by the present process. 
One method for solving the problem of boundary delineation is proposed in 
Allen U.S. Pat. No. 3,890,816, June 24, 1975, relating to the elimination 
of carbide segregation to prior particle boundaries in nickel-base alloys 
by means of adding a strong MC-type carbide former selected from the group 
consisting of columbium, tantalum, hafnium and zirconium to the alloy 
melt. The added carbide formers seem to prevent boundary delineation by 
strongly binding the carbon within the particles, thus preventing carbide 
precipitation at the boundaries. However, this requires adding a 
significant amount of such carbide formers, which substantially alters the 
composition of the treated alloys and which in turn may significantly 
affect the alloys' properties or may result in different properties, so 
that the modified composition may not receive the same acceptance in 
industry as the unmodified alloys. In the case of superalloys for aircraft 
engines this is a particularly serious problem, since under existing 
regulations concerning qualification testing, such modified alloys cannot 
be used in many of their intended applications without extensive and 
costly testing to determine the properties of articles made from the new 
alloy. 
FNT The same solution was subsequently published in Metals Engineering 
Quarterly, Nov. 1974, pages 47-49. 
SUMMARY OF THE INVENTION 
It is, therefore, a principal object of this invention to provide a process 
for minimizing or substantially eliminating carbide precipation at prior 
powder particle boundaries (boundary delineation) in alloy articles made 
by powder metallurgy techniques without significantly changing the alloy 
composition. 
It is a further object to provide articles made in accordance with the 
process of this invention. 
The present invention stems from our discovery that undesirable carbides, 
which we believe to be of the MC type, form at the powder particle 
boundaries because sites on the surfaces of the particles act to nucleate 
carbides, and that the addition of small amounts of certain elements which 
we term "surfactants" effectively eliminates boundary delineation. We 
believe that this is at least partially because the surfactant elements 
act to change the free energy relationships at the surfaces of the powder 
particles, possibly by reacting preferentially with sulfides and oxides 
which otherwise would be free to act as carbide nucleators, thus, in 
effect, deactivating nucleating sites on the particle surfaces. 
One or more of the elements magnesium, barium, calcium, cerium, lanthanum, 
lithium, neodymium, praseodymium, yttrium, and misch metal are preferably 
used as surfactants in carrying out the method of this invention. Other 
rare earth elements may also be used but their high cost is a drawback. 
Potassium can also be used as a surfactant to reduce boundary delineation, 
but appears to be less effective than the preferred surfactants. 
In carrying out the method of the present invention, a small but effective 
amount of at least one of the surfactants as defined herein is provided so 
that when an alloy powder is heated above and then cooled below the 
carbide solvus temperature incidental to densification of the powder to 
form the desired article, boundary delineation is substantially 
eliminated. The amount of surfactant required to prevent prior boundary 
delineation is readily determined and is small enough that the composition 
of the alloy is not changed significantly. The manufacture of articles 
using powder metallurgy techniques in all other respects may proceed as 
desired.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The process of the present invention is advantageously used in the 
production of a wide variety of articles prepared from prealloyed powder 
having widely different compositions. As is well known, the composition of 
the alloy from which the prealloyed powder is prepared is selected so as 
to provide the properties desired in the finished article. As will be 
pointed out more fully hereinbelow, the surfactant, as defined herein, can 
be included in the composition at the time the alloy is melted using 
well-known conventional melting techniques. However, particularly in the 
case of the more volatile surfactants, best results are attained when, as 
is preferred, the surfactant is added to the prealloyed powder prior to 
densification. The alloy powder can be prepared in any manner desired 
which is compatible with the properties desired in the finished article as 
is well known. For example, following preparation of the molten alloy, it 
can be atomized by any desired technique, either a gaseous or liquid 
atomizing fluid can be used. The particle size distribution is not 
critical but the particle size or distribution of particle sizes is 
preferred which favors close packing ratios to facilitate obtaining 100% 
or close to 100% of theoretical density. 
In some instances, it may be desirable to remove excessively large 
particles, but in general, good results can be attained with a wide range 
of particle sizes. In accordance with good powder metallurgy techniques, 
the powder to be used should be well mixed and should not contain areas 
which are predominantly of only one size or of a narrower range of 
particle sizes than the remainder. 
Surfactant in elemental form or mixed or combined with other tolerable 
elements or compounds is added to the alloy powder in any convenient way. 
When relatively small batches of alloy powder are to be densified, the 
required amount of surfactant may be conveniently handled and added in the 
form of one or more relatively large lumps. Surfactant in lump form is 
believed to work well because the surfactant is believed to be present in 
vapor form at the elevated temperature to which the alloy powder is heated 
for densification. Best results have been attained when the surfactant has 
been added as a powder. In the case of the larger masses of powder, as 
when large shapes weighing about 50 pounds (9.1 kg) or more are to be 
formed, surfactant powder screened to about -100 mesh (U.S.S.) should give 
best results but powder as fine as -325 mesh has given good results. When 
surfactant powder is used, it is thoroughly mixed with the alloy powder to 
facilitate the presence of the surfactant in its active form throughout 
the powder mass when the latter is heated. 
While the mechanism by which prior boundary delineation is prevented is not 
fully understood, the data we have collected indicates that nucleation of 
the carbides which hitherto have delineated the prior powder particle 
boundaries is prevented where a critical minimum amount of the surfactant 
is present at the powder particle surfaces. The best explanation of the 
phenomenon now known to us is that the surfactant acts to deactivate sites 
which we believe are related to such sulfur and oxygen as may be present 
and which otherwise would serve to nucleate the unwanted carbides. Thus, 
in accordance with the present invention, by surfactant it is intended to 
include elements which would vaporize and thus are capable of being 
present as a vapor under the conditions which obtain during or incident to 
hot densification, which are strong oxide and/or sulfide formers, which 
are weak carbide formers, which do not form oxides or sulfides which in 
turn would nucleate carbides of other elements present in the alloy 
composition, and which when present in the end product will not 
detrimentally affect the desired properties thereof. It is also desirable 
that the surfactant element not be dangerous in use or when present in the 
end product, and its use should not be too costly. While the surfactant 
element or elements can be incorporated in the prealloyed powder when the 
alloy is melted and atomized, best results are attained when the 
surfactant in the form of a fine powder is mixed with the prealloyed 
powder before it is sealed in an enclosure for densification. 
In use, magnesium has given best results, and therefore, is most preferred. 
Because magnesium and the elements useful as surfactants in accordance 
with the present invention are highly reactive, it is necessary to take 
into account the amount of impurities such as oxygen as well as other 
elements that may be present which would, in reacting with the surfactant, 
in effect, consume so much of it that the remaining amount will not be 
effective to prevent boundary delineation. Thus, in specifying the minimum 
amount of magnesium and other surfactant elements to be used in accordance 
with the present invention it is not intended to include that amount 
thereof which is otherwise consumed. When, because of the melting practice 
followed in making the alloy or the atomizing process used in making 
powder therefrom, substantial amounts of such elements as oxygen or other 
reactive impurities are introduced then it is preferred to reduce 
substantially the amount present. For example, in the case of oxygen which 
may be introduced when the atomizing fluid used is water, the powder can 
be substantially deoxidized by heating in hydrogen or cracked ammonia. 
When hot densifying prealloyed powder atomized by means of an inert fluid 
such as argon from an alloy melted under conditions substantially free of 
oxygen such as are provided by vacuum induction melting techniques, an 
addition of 0.01 weight percent (w/o) magnesium would be effective under 
conditions of high purity. A minimum of about 0.013 w/o magnesium would be 
effective to prevent boundary delineation under less stringent conditions 
of high purity. Preferably, about 0.03 w/o and better yet about 0.05 w/o 
magnesium is used to ensure complete elimination of boundary delineation 
when the usual amount of surfactant-consuming impurities are present. 
While larger amounts of the surfactant may be used, once an amount 
sufficient to prevent boundary delineation entirely is added any excess 
would be tolerable so long as it had no undesired effect. 
Other elements which have been found useful as surfactants in accordance 
with the present invention are barium, calcium, cerium, lanthanum, 
lithium, neodymium, praseodymium, yttrium and the commercially available 
mixture of rare earth elements known as misch metal which primarily is 
made up of cerium and lanthanum. On the other hand, zirconium which has 
many of the properties desired in a surfactant but is a strong carbide 
former did not work to prevent boundary delineation. The amount of each of 
the foregoing surfactants effective to prevent all boundary delineation is 
readily determined by taking an amount which is the stoichiometric 
equivalent of the effective amount of magnesium. Though less precise, the 
calculation is facilitated by selecting an amount which is in the same 
proportion to 0.05 w/o magnesium as the atomic weight of the element bears 
to that of magnesium. For example, the amount of barium to be used to have 
an effect close to that of 0.05 w/o magnesium is readily determined by 
multiplying the ratio of the atomic weights of barium to that of magnesium 
by the weight percent of magnesium. Thus, about 0.28 w/o barium can be 
used instead of about 0.05 w/o magnesium. However, transport rates, vapor 
pressure and other properties under the conditions prevailing during hot 
densification can in practice require an adjustment of the precise amount 
required to give the best results. Potassium can also be used as the 
surfactant to reduce boundary delineation, but appears to be less 
effective than the foregoing surfactants. 
Whether finely divided, in lumps or other form, the surfactant and the 
prealloyed powder are placed in an enclosure which is preferably, but not 
neccessarily, evacuated before being sealed. This is conveniently carried 
out by placing the prealloyed powder and the surfactant in a readily 
deformable container made of material compatible with the powder and 
which, upon completion of densification, can be readily removed. 
Thin-walled metal containers which closely fit the volume of powder to be 
densified therein have given good results. As is well known, the 
temperature range at which hot densification is carried out is determined 
at least in part by the composition of the alloy. It is not desired to 
melt the powder and, therefore, the maximum usable temperature is 
sufficiently below the solidus temperature of the alloy to ensure against 
local melting. The specific manner in which hot densification is carried 
out forms no part of the present invention except that it is believed to 
be necessary to achieve best results that the temperature be high enough 
to vaporize the surfactant during the process and that densification be 
carried out so that about 99% or more of theoretical density be achieved 
at least in those portions of the densified article in which boundary 
delineation is not wanted. 
The following examples are illustrative of the present invention and, 
unless otherwise indicated, were each carried out using conventional 
practices. 
EXAMPLE 1 
About 10 lbs. (4.54 kg) of prealloyed powder of -60/+ 325 mesh (U.S.S.) 
particle size and having the following composition in weight percent were 
mechanically blended with -325 mesh magnesium powder to provide in the 
mixture a magnesium content of about 0.05% by weight. 
______________________________________ 
w/o 
______________________________________ 
Carbon 0.166 
Manganese &lt;.01 
Silicon 0.01 
Sulfur &lt;.001 
Chromium 8.98 
Molybdenum 2.46 
Cobalt 14.36 
Vanadium 0.89 
Titanium 4.81 
Aluminum 5.53 
Boron 0.015 
Iron 0.05 
Zirconium 0.062 
______________________________________ 
The balance was nickel and 29 parts per million (ppm) of oxygen, 10 ppm 
nitrogen and inconsequential impurities. A stainless steel can, made of 
A.I.S.I. Type 304, was filled with the substantially homogeneously blended 
mixture and was then evacuated, as is preferred, to about 
5.times.10.sup..sup.-3 mm Hg and sealed. The sealed container was heated 
long enough for it and its contents to be brought to a temperature of 
about 2150.degree. F (about 1177.degree. C) and then extruded to provide a 
10:1 reduction. The extruded billet was trimmed and then sections were 
prepared from various parts of the billet for microscopic examination. All 
were found to be free of delineated boundaries. A 200.times. micrograph 
prepared from one of the unetched specimens is shown in FIG. 1. Another 
specimen prepared in the same manner then heated at 2275.degree. F 
(1246.degree. C) for 24 hours, air cooled and then etched is shown 
enlarged 100.times. in FIG. 2. 
For purposes of comparison, a specimen was prepared from a billet which had 
been prepared in the same manner as that used in Example 1 except that no 
surfactant was added. The prealloyed powder used had the same composition 
and had been prepared at the same time as the powder used to make the 
billet of Example 1. The specimen was heat treated and etched as described 
in connection with the specimen shown in FIG. 2 and is shown in FIG. 3 
enlarged 100.times.. The prior powder particle boundary delineation 
characteristic of the prior art can be clearly seen. 
Stress rupture specimens, both smooth and combination smooth and notched, 
prepared from the billet of Example 1, which retained 0.037 w/o of the 
0.05 w/o magnesium that had been added to the container prior to 
densification, were tested at 1350.degree. F (732.degree. C) under a load 
of 100,000 psi (7030.7 kg/cm.sup.2). The smooth test specimen had a life 
of 61 hours, a 6% elongation and a 9% reduction in area. The combination 
test specimen had a life of 47 hours, a 5% elongation and an 11% reduction 
in area. The material from which the micrograph shown in FIG. 3 was 
prepared when similarly tested using combination smooth and notched test 
specimens gave an average from two tests of 41 hours stress rupture life, 
4.5% elongation and 10% reduction in area. 
In carrying out the following examples, prealloyed powder was used having 
the following composition 
TABLE I 
______________________________________ 
w/o 
______________________________________ 
Carbon 0.17 
Silicon 0.01 
Sulfur 0.002 
Chromium 8.99 
Molybdenum 2.47 
Titanium 4.79 
Aluminum 5.60 
Cobalt 14.14 
Vanadium 0.90 
Zirconium 0.07 
Boron 0.012 
Iron 0.06 
______________________________________ 
The balance was nickel except for incidental impurities which included 23 
ppm oxygen and 10 ppm nitrogen. Each of the following surfactants in 
powder form, except for barium and calcium which were in rod form, were 
measured out to provide an amount equivalent on an atomic weight basis to 
0.05 w/o magnesium in 5 lbs (2.27 kg) of the prealloyed powder. It may be 
noted that surfactants which had been oil packed were washed with 
chloroform and ether, care being taken to minimize exposure to the 
atmosphere. The surfactants used were barium, calcium, cerium, lanthanum, 
lithium, neodymium, praseodymium, yttrium and misch metal, the latter 
being a well known mixture of rare earths made up mostly of cerium and 
lanthanum. The weighed out powder samples of Ce, La, Li, Nd, Pr, Y, and 
misch metal were each blended with 5 lbs of the prealloyed powder under an 
argon atmosphere to minimize oxidation and then each blend was used to 
fill a 3 inch (7.62 cm) by 5 inch (12.7 cm) long Type 304 stainless steel 
can having a wall thickness of 0.25 inch (0.64 cm). The Ba and Ca in rod 
form were placed in the bottom of their respective cans before the cans 
were filled with the 5 lbs of prealloyed powder. The filled cans were 
outgassed and sealed. The sealed cans were heated to 2150.degree. F 
(1177.degree. C) and then reduced 10:1 by extending down to 1.04 inch 
(2.64 cm) round. 
In Table II below, the amount of each of the surfactants added, the amount 
retained in the extruded specimens as well as the nitrogen, oxygen and 
hydrogen contents are indicated in weight percent or parts per million. 
TABLE II 
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Ex. Surfac- Added Retained 
N O H 
No. tant (w/o) (w/o) (w/o) (ppm) (ppm) 
______________________________________ 
2 Ba 0.28 0.04 0.001 49 8 
3 Ca 0.08 0.03 0.001 43 11 
4 Ce 0.29 0.21 0.001 62 5 
5 La 0.29 0.40 0.002 74 4 
6 Li 0.014 &lt;.01 0.001 71 11 
7 Nd 0.30 0.30 0.001 73 4 
8 Pr 0.29 0.25 0.001 82 7 
9 Y 0.18 0.12 0.004 66 6 
10 Misch metal 
0.29 * 0.001 477 6 
______________________________________ 
*0.14 w/o Ce and 0.14 w/o La were found. 
The indicated retained level of lanthanum greater than the amount added is 
believed to have been the result of local segregation. The high oxygen 
content coupled with the normal nitrogen content found in Example 10 is 
believed to indicate the misch metal as added contained rare earth oxides. 
Specimens prepared from each of the extrusions of Examples 2-10 were found 
to demonstrate that in each boundary delineation had been eliminated. 
For further comparison, an identical process was carried out using an 
addition of 0.19 w/o zirconium powder having a particle size of -325 
(U.S.S.). The zirconium bearing product showed substantially the same 
prior powder particle boundary delineation characteristic of the prior 
art. 
EXAMPLES 11 AND 12 
To illustrate another embodiment of the process of the present invention, a 
100 lb (45.36 kg) vacuum induction melted ingot having the composition 
indicated for Example 11 in Table III below (w/o unless otherwise 
indicated) was remelted and 0.15 w/o magnesium in the form of NiMg was 
added to the melt just prior to atomization. The same procedure was 
followed with respect to Example 12 but 0.30 w/o magnesium in the form of 
NiMg was added to the melt. 
TABLE III 
______________________________________ 
Ex. No. 11 
Ex. No. 12 
______________________________________ 
C 0.174 0.190 
Mn &lt;.01 &lt;.01 
Si &lt;.01 0.02 
S &lt;.001 &lt;.001 
Cr 8.61 8.56 
Mo 2.44 2.43 
Co 14.40 14.25 
V 0.89 0.89 
Ti 4.79 4.74 
Al 5.52 5.39 
B 0.01 0.02 
Fe 0.07 0.04 
Zr 0.067 0.068 
O(ppm) Powder 40 42 
Extruded Bar 122 150 
N(ppm) 20 10 
Mg added to the melt 
0.15 0.30 
Retained in Powder 
0.055 0.099 
Retained in Extruded Bar 
0.049 0.074 
______________________________________ 
The balance of the composition in each case was nickel plus inconsequential 
impurities. 
The prealloyed powder thus produced was sealed in cans as was described in 
connection with Example 1 and reduced 10:1 by extrusion as previously 
described. Specimens for microscopic examination were prepared and 
examined as described in connection with the previous examples and the 
material was found free of delineated boundaries. 
Stress rupture specimens, both smooth and combination smooth and notched, 
prepared from the extruded bars of Examples 11 and 12 were tested at 
1350.degree. F (732.degree. C) under a load of 100,000 psi (7030.7 
kg/cm.sup.2). The results of the tests are given in Table IV. 
TABLE IV 
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Specimen Life Elong. R.A. 
Ex. No. Type (hrs.) (%) (%) 
______________________________________ 
11 Smooth 39 6 7 
Combo. 40 4 8 
12 Smooth 36 4 4 
Combo. 39 5 6 
______________________________________ 
While the present invention has been illustrated by means of one of the 
so-called superalloys, it is not intended thereby to limit the scope of 
the invention. As was noted hereinabove, the present invention is useful 
in eliminating boundary delineation in a wide range of compositions. It 
can be used in connection with any composition susceptible to boundary 
delineation with a minimum of change to the composition and its 
properties. The process can be used in treating alloys of one or more of 
the transition metals iron, nickel or cobalt, including the precipitation 
hardening superalloys, tool steels, the nickel-iron electronic alloys, and 
stainless steels. All such alloys, as a practical matter, contain at least 
about 0.005% carbon. The process of the present invention is particularly 
useful in preventing boundary delineation in precipitation hardening 
alloys containing about 0.01 to 0.50% carbon, up to about 2.0% manganese, 
up to about 1.0% silicon, up to about 25% chromium, about 20 to 80% 
nickel, up to about 60% iron, up to about 25% cobalt, up to about 12% 
molybdenum, up to about 8% tungsten, about 0.5 to 10% titanium, about 0.2 
to 10% aluminum, up to about 7% niobium, about 0.002 to 0.30% boron, up to 
about 10% tantalum, up to about 0.50% zirconium, up to about 3.0% hafnium, 
up to about 5.0% rhenium and up to about 1.5% vanadium. Illustrative of 
alloys included in the foregoing are the following which contain about 
______________________________________ 
Alloy A Alloy B Alloy C Alloy D 
______________________________________ 
C 0.02-0.06 0.05-0.09 0.04-0.09 
0.15-0.20 
Mn 0.15 Max. 0.02 Max. 0.15 Max. 
0.02 Max. 
Si 0.20 Max. 0.10 Max. 0.20 Max. 
0.02 Max. 
Cr 14-16 11.9-12.9 12-14 8-11 
Co 16-18 18-19 7-9 13-17 
Mo 4.5-5.5 2.8-3.6 3.3-3.7 2-4 
W -- -- 3.3-3.7 -- 
Ti 3.35-3.65 4.15-4.50 2.3-2.7 4.50-5.0 
Al 3.85-4.15 4.80-5.15 3.3-3.7 5.0-6.0 
Nb -- -- 3.3-3.7 -- 
B 0.02-0.03 0.016-0.024 
0.006-0.015 
0.01-0.02 
V -- 0.58-0.98 -- 0.70-1.20 
Zr 0.06 Max. 0.04-0.08 0.03-0.07 
0.03-0.09 
Fe -- 1 Max. -- 1 Max. 
______________________________________ 
and the balance nickel plus incidental impurities. 
The terms and expressions which have been employed are used as terms of 
description and not of limitation, and there is no intention in the use of 
such terms and expressions of excluding any equivalents of the features 
shown and described or portions thereof, but it is recognized that various 
modifications are possible within the scope of the invention claimed.