Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1091476 K-0474-a-KCC-C.I.P.
Copper base alloys upon prolonged exposure to
air at room temperature develop non-uniform and unattract-
ive o~ide ~ilms commonly referred to as t~rnish. To pre-
ven~ such tarnishing, a variety o clear coatings have
been developed that are either very costly or are subject
to damage in use and resultant local tarnishing and cor-
rosion under the coating. Thus, while the majority o
copper base alloys possess the required formability for
fashioning into use~ul articles, their lack of tarnish
resistance limits their use in areas where these articles
are also in~ended to serve a decorative function.
The search for a copper base alloy having a
stainless property, that is, one which does not require
a protective coating, has been active for many years. In-
deed, the International Copper Research Association (INCRA)
has developed an alloy of reasonable tarnish resistance
- that is known as INCRA C-57. This alloy consists of cop-
per with S percent by weight tin and 7 percent by weight
alu~ninum. However, this alloy is difficult to fabricate
using conventional brass mill techniques. More specifical-
ly, this alloy does not hot roll very well and its cold
rollability is also quite limited. In addition, the form-
ability of the finished alloy is relatively low, severely
limiting the application and utility of the alloy iTI the
manufacture of formed articles. Thus, in order to gain
the benefits of tarnish resistance in a co~per base alloy,
q~
--2--
, _ _ . .. .. _ . ~ _ .
0 9 1~ 7 6
one has had to resort to compositions which possess
inherently limited workability and formability. Pri-
marily, for this reason such alloys have not gained
widespread commercial acceptance.
The alloy of the present invention offers the ability
to be fabricated into a variety of useful articles which
can be used indoors without any protective coating in the
same manner that stainless steel is used. This unique
ability provides the designer the option of selecting the
, 10 warm color of a copper alloy with the additional advantage -~
of not having to utilize a protective coating to maintain
this appearance.
According to the invention, there is provided a copper
alloy consisting essentially of the following constituents
in the following amounts in percent by weight: Aluminum -
7.0-8.5, Nickel - 1.5-2.5 and Copper - 89.0-91.5 and
having a film resistance of at least 95 kilohms and a
microstructure containing less than 2 volume percent
of intermetallic ~ and ~ phases.
In addition to having the foregoing composition,
alloys of the present invention must be capable of
spontaneously forming a thin nearly transparent film at
ambient temperature which has an electical polarization
resistance of at least 95 kilohms when measured in a
neutral salt solution under applied DC potential. This
high film resistance is achieved by processing the alloy
under conditions which result in a structure consisting of
a matrix of copper-aluminum-nickel solid solution, uniform
in composition and containing a very fine dispersion of
NiAl intermetallic compound.
The alloy of this invention is suitable for indoor
-;
1091476
exposure and has been found to have excellent strength and
formability characteristics in addition to a high order of
stain ~esistance. upon outdoor exposure the alloy of this
invention exhibits a high order of corrosion resistance
but does lose its lustre in time and turns into a uniform
dull bronze color.
Accordingly, an advantage of the invention, at least
in preferred forms, is that it can produce a copper base
alloy which is resistant to oxidation and corrosion at
ambient temperatures.
A further advantage of the present invention, at
least in preferred forms, is that it can provide a copper
base alloy which will assume a uniform color that is both
aesthetically desirable and reasonably permanent.
A further advantage of the present invention, at
least in preferred forms, is that it can provide a copper
base alloy which will maintain an aesthetically attrac-
tive surface appearance without the need for protective
coatings.
A further advantage of the present invention, at
least in preferred forms, is that it can provide a tarnish
resistant copper base alloy that assumes aesthetically
pleasant tones and which offers an ability to be readily
fabricated into a variety of useful articles.
A further advantage of the present invention, at least
in preferred forms, is that it can provide a copper base
alloy of high strength while at the same time providing
good formability.
Fig. 1 is a plot of a portion of the copper-
1091~76
alulilinum nickel ternary phase diagram showing the region
encompassed by the alloy of this invention as weli as
sho~ing a low formability zone and~a low film resistance
zone in which alloys outside of the range encompassed by
this invention will lie. The numerical values for the
points shown on the phase diagram indica~e the film resist-
ance in kilohms;
Fig. 2 is a graph showing tensile properties
versus anneal temperatures for an alloy of the present
invention;
Fig. 3 is a photolithograph of the microstructures
of alloy C of Table I, as cast;
Fig. 4 is a photolithograph of the microstructures
of alloy B of Table I, as cast;
Fig. 5 is a photolithograph of the microstructures
of alloy B of Table I, heat treated; and,
Fig. 6 is a photolithograph of the microstructures
of alloy B of Table I, hot rolled, cold rolled and annealed.
.
At the outset the alloy of the present invention
is described in both its broadest overall aspects and its
preferred embodiment with a more detailed description
fol~owing
As is shown by the areas within parallelograms
ABCD and EFGH of Fig.~, the alloy consists of three
components: copper, aluminum and nickel, with aluminum
being present in the range of 7.0 - 8.5 weight percent
` 1091476
and preferably 7,7 - 8.3 weight percent; nickel being pre-
~ent in the range of 1.5 - 2,5 wei~ht perccnt and prefer-
ably 1,8 ~ 2,2 weight percent; with the balance being com-
prised essentially of copperl The alloys of the present
invention may include, in addition to the foregoing mater-
ials, conventional impurities typically found in commerciai
copper base alloys. These common impurities may include:
lead, tin, phosphorus, iron, manganese, zinc, and silicon
in an amount up to a collective total of 0.5 wt %.
10~s is ~shown in Fig, 1, the ranges of aluminum,
nickel and copper set forth above have been found to be
critical for the following reasons, Aluminum content
below 7.0 percent re8ult in lower tarnish resistance with
behavior not significantly better than existing aluminum
bronzes such as ca 61400.` Aluminum contents above 8,S
percent result in a drastic reduction of formability and
ductility of the alloy through the appearance of the brit-
tle, complex-structure, intermetallic phases known as beta
and gamma, Likewise, nickel contents below 1,5 percent
result in reduced tarnish resistance and lower 9trength
of the alloy, while nickel contents above 2,5 percent pro-
duce excessive amounts of nickel-aluminum intermetallic
. compounds which not only reduces formàbility but also
through combination with a portion of the aluminum removes
Al from solid solution and reduces tarnish Fesistance.
Certain properties of alloys having compositions outside
-
1091~76
of the claimed range are also shown in Fig, 1. Within the
ore~oing broad ranges of constituen~s (as is shown by
parallelogram ABCD), the desirable~q,ualitie.s of tarnish
resistance, formability, and seren8th are further optimized
by controlling the aluminum content to within the preferred
range of 7,7 - 8,3 weight percent and the nickel content
within the preerred range of 1,8 - 2,2 weight percent as
is ~hown by parallelogram EFGH, In addition to having
constituents within the foregoing ranges depicted by
parallelogram ABCD, the alloy must be capable of forming
a stable oxide fitm having a film resistance of at least
95 kilohoms, 5uch high film resistances are achievable
on the alloys of this invention when they are processed
according to procedures which follow. These procedures
result in the formation of a microstructure that can be
characterized as an alpha phase alloy matrix containing
small amounts of finely dispersed NiAl co~pound. The
elimination of beta and gamma phases from the alloy, in
part, results in the high tarnisll resistance of the alloy
of the invention, No evidence exists in the literature or
prior art that both composition control and structure
- control were required to produce highest tarnish resistance
in abricable copper-aluminum-nickei alloys.
As is stated above, one unique characteristic
-
of-the alloy of the invention is due to the ormation of
a reaction product film of high electronic and ionic re-
.
~os~4q6
resistance. To achieve maximum resistance, it is critical
that the right proportions of metal ions be present in the
film. Too much or too little of any individ~al metal ion in
the film results in lower fil~ resistance and, hence, higher
rates of oxidation. In order to providè for this critical
control of the propor~tions of the metal ions in the film
it is important to closely control both the structure and
composition of the underlying metal alloy whose atoms are
incorporated into this protective oxide film.
By using an electro-chemical technique known as
linear polarization, the electrical resistance of the protective
oxide film can be measured. The subject alloy was found to
attain very high film res~stance along with fabricability
only over the range of compositions claimed herein. Film
resistance was determined by controlled exposure of the subject
alloy to an accelerated corrosion environment and susequent
linear polarization measurements. Alloys lower in aluminum
and/or nickel than the range claimed herein produce lower
f1lm resistance and thus are not significantly better than
existing commercial aluminum bronzes. Alloys higher in
aluminum and/or nicke] have de1eterious amounts of unwanted
phases such as the brittle ~ and ~. These undesirable phases,
when present in significant amounts, (ie 2 volume precent or
more), drastically reduce both fabricability and formability.
m us, it is also critical that the alloys be
.
--8--
1()91476
clear of ~ and~ phases ( ie that they contain less than 2 volume
percent). The achievement of a high tarnish resistant alloy
is thus accomplished by both composition control and structure
control through proper processing conditions.
The invention is further illustrated by the following
examples. At this point, it should be noted that the invention
is not intended to be limited to the procedures set forth
in the examples which follow, but rather these examples are
provided in order to teach one skilled in the art how to
practice the invention and thus, are not intended to limit
the invention in any way.
EXAMPLE I
TARNISH RESISTANCE
Alloys of the compositions shown in Table I were
prepared by melting together the constituent metals under
a charcoal cover in a clay-graphite crucible. After thorough
mixing, the heats were cast into steel molds. The resulting
structure varied from essentially single phase as shown in
Fig. 3 for alloy C as-cast to a two-phase alpha plus 10%
beta structure shown in Fig. 4 for alloy B as-cast. Due to
relatively rapid cooling during casting with resultant
segregation of the constituents, non-equlibrium structures
are common in the as-cast condition with phases being present
that are not expected from the equilibrium diagrams as
well as the accurrence of local variations in composition
from point to point in the alloy known as coring. These
_g_
-; ~091476
effects are sufficient to reduce the ~ilm resis~ance sig-
nificantly below the values attainable when the me-tal is
homogenous with respect to distribution of the alloy con-
stituents. Also, coarse grain boundary precipitates of
NiAl compound may cause a low film resistance, This is
illustrated by the data in Table I for the heat treated
condition of.alloy B where ~enerally lower film resistance
is found for structures having coarse precipitates as
shown in Fig. 5 (heat treated condition - heat B). If
the NiAl compound is finely dispersed throughout the
structure as is achieved in alloy B and processing of this
invention a~ shown in Table I for the HR ~ CR ~ Ann con-
di~ion of alloy B and by Fig, 6 (hot rolled, cold rolled,
and annealed - heat B), then tarnish resistance and mechan-
ical properties are enhanced; The film resistance data
in Table I supports the previous statements.
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1091476
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~ 1091~76
It should be noted that even if the alloy contains
the correct constituents in the specified ranges, the objects
of the invention will not be achieved unless the alloy is free
of aand ~ (ie contains less than 2 volume percent) and has
the bulk of the aluminum in solid solution with any excess Al
and Ni in the form of a finely divided NiAl dispersion throughout
the solid solution matrix.
An alloy composition range of 7.7 - 8.3% aluminum,
1.8 - 2.2% nickel with the balance copper appears to be optimum,
although high tarnish resistances are noted throughout the
entire broad range of 7.0 - 8.5% alumunum, 1.5 - 2.5% nickel,
balance essentially copper when the alloy is processed in
accordance with this invention. The alloy is of significantly
improved tarnish resistance and can be utilized indoors without
protective coatings. Oxidation is very slow but the alloy
will eventually mellow to an attractive uniform film rather
than the non-uniform and unattractive tarnish film which forms
on most currently available copper base alloys. The excellent
behavior of the alloy is not affected by normally encountered
levels of common impurities such as iron, zinc, manganese, tin,
lead, silicon or phosphorus. The alloy can be produced
with reasonable ease using conventional brass mill equipment.
Formability of the finished sheet and strip is sufficient
to satisfy the vastmajority of users.
Exploration of the Cu corner of the Cu-Al-Ni
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109:1476
system included alloys o~ from 6 to 10% Al and 1 to 7% Ni.
The best of these alloys, considering both film resist~nce
and fabricability/formability behavior, were those in the
preferred 7.7 - 8.3% Al`and' 1.8 - 2~2~/o Ni range. Alloys
of this composition hot roll and cold roll very well,
have good formability in drawing, stretching and bendin~
modes and form an excellent high resistance film upon
exposure.
EX~MPLE II
TARNISH RESISTANCE
~lloys having the composition listed in Table II
were prepared from electrolytic-tough-pitch copper, nickel
pellets and aluminum pellets. Preparation followed the
ensuing sequence: Rods o~ copper were melted under a
charcoal cover and heated to approxima'tely 2100 F; a
portion or all of the desired aluminum content was add'ed
to the melt and stirred at the same temperature, nickel
in the form of'nickel pellets or a 50-50 copper-nickel '
master alloy was added next and the melt held at a temper-
ature of 2000-23Q0 F until complete solution occurred.
Any remaining aluminum was next added and stirred in The
melt was ctabilized at a temperature of about 2100 F and
cast into a steel mold and allowed to solidify
The resulting ingots were reheated to 1500 F
to 1650 F and hot rolled to at least 75% reduction in
thickness, finishing at about 1000 F. The material was
-13-
., . . . ... . _ ___ .. ... __ . .~ . __ . ... .. .. .
`~` 1091~76
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then coldrolled from ,about,250'lto.120", annealedatabout
` 1300 F for45 ~inutes, cold rolled to.060", annealed atahout
1300F fox45 minutes, andcoldrolled to.030", Tensile
properties were evaluated in the50% coldrolledcondition and
also in theannealedcondition where temperatures of525F and
1250F were used, Samples of the varlou~ alloys were abraded
with silicon carbide abrasive, then exposed to,an acceler-, -
ated ~tmospheric corrosion test comprising alternate wet/dry
cycles with a weak sodium bisulfite solution especially
formulated to produce oxide films of thesamenatureas those
ound on copper alloys after several years o~ outdoor ex-
posure in an urban industrial environment.
The relative ohmic resistances of the oxide film
on the alloy samples were measured by the electrochemical
technique known as linear'polarization. ~e ohmic resis-
tance is determined from a plot of the DC current versus
DC voltage of a sample exposed in an electrolyte in which
the film is stable. DC voltage is controlled by mean9 oE
an instrument known as potentiostat.
Corrosion rate and thus tarnishing rate has
repeatedly been demonstrated to be inversely proportional
to oxide film resi'stance and therefore the criticality of
the composition and structure of the alloy o the present
invention is demonstrated by film resistance,measurements.
- The data in Table II shows that alloy compositions
outside the range specified in this present invention pro-
.
-14-
,, . .... .. _ .. _ .. ~ . _ . __ __ ._ . _ __ ,._~ .... , ..... _. . . .
` 109147~i
` '' . ,
duce ilm resistance significantly lower than alloys within
the specified range
Alloy E in Table II is wlthin the specified range
and shows a relative film resistance of 136 kilohms; alloy
.
F having an Al content below the specified range results
in a film resistance of only 45 kilohms; alloy G with an
Al content within the specified range but with an Ni con-
tent above the specified range results in a film resistance
of only 64 kilohms; alloy H with an Al content still within
the range but at the low end and with an Ni content above
the specified range results in a film resistance of only
52 kilohms; and alloy I with both Al and Ni contents above
the specified range although closer in film resistance,
still only results in a film resistance o 91 kilohms and
in addition, a large loss in formability and fabricability
occurs in alloy I which is due to the presence of exce~sive
amounts of the intermetallic compounds ~ and ~ with limited
ductility. As is stated above, to be considered tarnish
resistant, in the terms of this invention, the alloy should
possess a film resistance of at least 95 kilohms and pre-
ferably, at least 100 kilohms.
,
.
~ ` 1091476
TABLE II
- Film Resistance
- . Alloy Composition, wt ~/O Kilohms
; ` ~'
E 90, 22 Cu ~ 7 . 66 Al -~ 2 ,12 Ni 136
- F 91, 31 Cu ~ 5, 85 Al ~ 2. 84 Ni 45
G 86, 78 Cu + 8.19 Al + 5. 03 Ni 64
H 88.46 Cu + 7,32 Al + 4,22 Ni 52
85. 87 Cu + . 9. 02 Al ~ 5 .11 Ni 91
EXA21PLE I I I .
MECHANICAL AND FORMING PROPERTIES
.
: Alloys of this invention (such as alloy E) are
characteriæed by high strength, good ductility, and good
formability, Figure 2 illustrates the excellent mechanical
property levels of alloy E o~ Table II attainable in the
~ 50% cold rolled condition and after a range o annealing
~' temperatures, The alloys listed in Table III were prepared
.
in the same manner as in Example II and were tested for
mechanical properties and ormability, When Al.andtor Ni
leveLs are at higher levels than specified, (alloys G and
I).ductility and formability decreases significantly, When
Al and/or Ni levels are at lower levels than specified
(alloys F. and J) ductility increases but a significant
loss occurs in strength levels, Properties determined on
commercial 70/30 brass are shown for comparison,
-16- .
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1091~76
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1~ - 1091~1L76
EXAMPLE IV
IMPURITY EFFECTS.
An al~loy was prepared in the same manner as in
Example III comprising 7,97% Al, 2.03% Ni, copper and the
following impurities: .03% Pb, ,03% Sn, ,03% P, .03% Si,
,05% Fe, .05% Mn, and ,10% Zn. The mechanical properties
were as follows, for the 50% cold rolled condition, yield
strength 114,000 psi, tensile strength 141,000 psi, elong-
ation 2,8%; for the 700 C annealed condition, yield
strength 46,000 psi, tensile strength 85,000 psi, elong-
ation 38.5%, The limiting dra~ rati-o was 2.12 and the
. Olsen Bulge Height was .413". Relative oxide film resis-
tance was somewhat decreased to 95 kilohms due to the
presence of the impurity additions, Thus, no penalty in
mechanical properties or formability resulted from the
presence of the relatively large total impurity level as
compared with state-of-the-art commercial practice and
only a moderate penalty in tarnish resistance occurred.
-18-
.... ,.. , .
.. .
