Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTIO~
In the past~ iron nickel alloys have been developed
having extremely low thermal expansion coefficients which enable
them to be used over wide temperature ranges without losing
strength and without any substantial change in elasticity.
Examples of such alloys are given in UOS. Patent Nos. 3,157,495
and 4,006,011 and ~ypically contain controlled amounts of cobalt,
columbium and titanium. They are used in such applica~ions as
rocket engine parts and the like which must have superior
resistance to thermal fatigue. A difficulty with alloys of this
type, however~ is theîr notch sensitivity and severe micro
shrinkage upon cooling from the molten state. As a resultg they
have not been used in the cast orm.
The present invention resides in the disc-overy that
critical amounts o boron can be added to nickel-iron b~se alloys
o~ the type described above to eliminate notch sensitivi~y and
dele~erious microshrinlcage in castingsO At the same time, the
alloy retains its low thermal expansion charaeteristics.
Specifically, it has been found that the addi~ion of about .06%
,
- . - . , :
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SM-1093
to .25% boron to certain types of iron-nickel base alloys ~7ill
promote the formation of a eutectic boride during solidification;
and it is the presence of this eutec~ic boride which improves
castability of the alloy. Alloys of this type are characterized
by a wide liquidus to solidus range and can be cast or hot-worked
and used in wrought form, provided that a suitable heat treat-
ment for the wrought form is provided.
The above and other objects and features of the
invention ~ill become apparent from the following detailed
description taken in connection with the accompanying drawings
which form a part of this specification/ and in which:
Figures lA and lB are photomicrographs at magnifica-
tions of 50X and 400X, respectively, showing the formation of a
eutectic boride in the alloy of the invention; and
Figs. 2A and 2B are differential thermal analysis
plots showing the effect of variation in boron in the alloy of
the invention upon the solidus temperature.
The alloy of the invention has ~he ollowing broad
and preferred ranges of composi~ion:
T~BLE I
Broad Preferred
Nickel at least 16% 30-50%
Gobal~ at least 10% 10-20%
Columbium 0-5% 2-4C/o
25 - Tantalum 0-3% 0-1%
Titanium 0-2~5% 1-2%
Aluminum 0-2% .25-1%
Boron at least .06% .06-.30%
Carbon 0-.1% .015-.045%
Iron Bal. Bal.
SM-1093
The carbon should be kept as low as possible in
order that it will not produce carbide clusters in the boride
eutectic about to be described. Additionally, the alloy can
contain up to 0.1% zirconi~ which is desirable to impede the
S formation of Ni3Cb at the grain boundaries of the alloy. Up
to 0.1% oE rare earths can be added which act as scavengers
to prevent delet~rious sulfide formations and the formation
of acicular phases; while up to 1% hafni~m can be added which
acts as a carbide former and widens the liquidus to solidus
temperature range. As is knownj small amounts of tan~alum
are often associated with columbium vbtained from commercial
sources. Normally, these small amounts of tantalum occur in
amounts up to about 3% of the total content of columbium plus
tantalum. As used in the following claims, therefore, the term
"columbium" means pure columbium (if it is available) vr
columbium plus certain amounts of tantalum. A certain amount of
the columbium content, however, can be replaced by pure tantalum
in the ratio of two parts tantalum to one part columbium~
As will be see~ from the following deseription, alloys
in the foregolng range of composition have low thermal expansion
characteristics and are free o notch sensitivity, making them
especially available for use as an alloy used in castings
intended for use over a wide ramperature range.
Properties of the new and improved alloy of the
invention are established by the following Table II which
shows the analysis of five different heats having varying
amounts of boron additions:
S~-lOg3
- T~BLE Il
Analysis (Aim)
Heat
No. C B Ni Co Cb Ti Al Fe
_
D]-939 0.03 0.005 38.2 15.3 3.0 1.7 0.8 Bal.
Dl-940 0.03 0.050 38.2 15.3 3.0 1.7 0.8 ~al.
Dl-979 0.02 0.100 38.2 15.3 3.0 1.7 0.8 Bal.
Dl-10320.02 0.160 38.2 15.3 3.0 1.7 0.8 Bal.
Dl-12870.02 0.300 38.2 15.3 3.0 1.7 0~ Bal.
Heat No. Dl-939 is a standard prior art alloy
similar to that described in the aforesaid U.S. Patent No.
3,157,495. An alloy of this type is characteri~ed by a ~icro-
structure which shows no eutectic boride phase and contains
large amounts of porosity which leads ~o poor stress rupture
life. All of the heats in the ~oregoing Table II were cast
in investment molds to yield test bars. These bars were
~hen heat-treated and machined to 0.250 inch diameter bars
which were subsequently stress rupture tested. The results
of the stress rupture tests are shown in the following Table III:
TABLE III
,Stress Rupture (1200F!9
Heat Boron*~Life Elong. R~A.
N _ (Wt, ~/O~ ) (%~_
Dl-939 0.0052 0.2 2~0 9.2
Dl~940 0.050 0.7 1.3 6.9
Dl-979 0.094 66.2 3.9 6.7
Dl-1032 0.156 138~5 8.0 9.7
Dl-1032* 0.156 172.2 8.2 11~2
*Combination smooth tensile bar and notched tensile bar.
*J~A~tual Wto % as contrasted with aim of Table Il~
As can be seen from the Eoregoing Table III, the
- standard prior ~rt alloy Dl-939 containing only .0052% boron
has a s~ress rupture lie of only 0.2 hour at 1200F/90 Ksi
wilh a 2% elonga~ion and 9.2% reduction in areaO Furth~r
additions of boron up to 0.05~% (Heat Dl-940) have very little
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~ ~ 66 ~ 2 SIi-1093
effect on the s-ress rupture life which increases Lo only 0.7%
at 1,3% elongation and 6.9C,~ reduction in area. ~lowev2r in
Heat Dl 979 with a boron addition of 0.094~/0, stress rupture
liEe under the same conditions is dramatically ;ncreased to 66.2
hours at 3.9% elongation and 6.7% reduction in area. Boron
additions of 0.156% (Heat Dl-1032) more than double the stress
rupture life to 138.5 hours at 8% elongation and 9.7% reduction
in area. Heat Dl-1032~' is the same as ~hat previously described
except that the test specimen was a combination smooth tensile
bar and notched tensile bar. Here the stress rupture life is
further increased.
As shown in Figs. lA and lB, photomicrographs of the
alloy of Heat Dl-1032 containing 0.16% boron shows large amounts
of eutectic boride and exhib;ts freedom from deleterious micro-
shrinkage. I~ ha~ an average thermal coefficient of expansion
of about 4.7 x 10 in/in/ F at room temperature to 800Fo It
is helieved that the alloy of the invention derives its improved
castability through the formation of eutectic boride during
solidification.
The stress rupture characteristics of Heat Dl-1287
(Table II) containing 0.3~/O boron were not determined; however
photomicrographs of this alloy show the same large amounts of
eutectic borideO It is believed that boron ~ddi~ions materially
above 0.3% w;ll cause the volume of the inner dendritic eutfctic
to become excessive, resulting in large crack paths which could
impair ~he physical properties of the alloy.
The difference in solidification characterist'cs of
this alloy as compared to prior ar~ alloys is shown in the
t:hermal diagrams o~ Figs. 2A and 2B. The upper diagram (Fi~. 2A)
is for a conventional prior ar~ nickel-iron alloy containin~
--5--
S~ g3
0.005% boron (Heat Dl-939); while the d.iagram of ~ is for
Heat Dl-1032 containing 0.16% boron~ Note that the alloy of
the invention contai.ning boron is characteri~ed by a wider
liquidus to solidus ranre.
Although the invention has been shown in connection
with certain speciic embodiments, it Will be readily ap?arent
to those skilled in the art that various c'nanges in form and
arrangement of parts may be made to suit requiremen~s without
departing from the spirit and scope o:~ the invention.
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