U.S. patent application number 13/982764 was filed with the patent office on 2014-10-02 for cu-ni-zn-mn alloy.
This patent application is currently assigned to BAOSHIDA SWISSMETAL AG. The applicant listed for this patent is Jean-Pierre Tardent, Florian Dalla Torre. Invention is credited to Jean-Pierre Tardent, Florian Dalla Torre.
Application Number | 20140294665 13/982764 |
Document ID | / |
Family ID | 45875923 |
Filed Date | 2014-10-02 |
United States Patent
Application |
20140294665 |
Kind Code |
A1 |
Torre; Florian Dalla ; et
al. |
October 2, 2014 |
Cu-Ni-Zn-Mn Alloy
Abstract
Precipitation hardened alloy on the basis of copper, zinc,
nickel and manganese exhibiting a high strength and ductility with
values similar to those of stainless steels in combination with
excellent machinability. The inventive alloy family is
characterized by fine fibre-like or globular precipitates that
emerge during intermediate temperature annealing treatments, which
in case of the unleaded variations significantly improves the
machinability. The alloy of invention is particularly suited for
free machining applications such as the production of pen tips and
reservoirs for writing implements of reduced tip dimensions, where
conventional Cu--Ni--Zn--Mn alloys fail due to lack of strength and
where the corrosion resistance in gel-based inks is insufficient
without restriction to other fields of application.
Inventors: |
Torre; Florian Dalla;
(Zurich, CH) ; Tardent; Jean-Pierre; (Les Genevez,
CH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Torre; Florian Dalla
Tardent; Jean-Pierre |
Zurich
Les Genevez |
|
CH
CH |
|
|
Assignee: |
BAOSHIDA SWISSMETAL AG
Reconvillier
CH
|
Family ID: |
45875923 |
Appl. No.: |
13/982764 |
Filed: |
February 3, 2012 |
PCT Filed: |
February 3, 2012 |
PCT NO: |
PCT/EP2012/051890 |
371 Date: |
November 19, 2013 |
Current U.S.
Class: |
420/582 |
Current CPC
Class: |
C22C 30/06 20130101;
C22C 30/02 20130101; C22F 1/08 20130101; C22C 9/04 20130101 |
Class at
Publication: |
420/582 |
International
Class: |
C22C 9/04 20060101
C22C009/04 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 4, 2011 |
CH |
00211/11 |
Claims
1-23. (canceled)
24. A precipitation hardenable copper alloy comprising, in
percentage of weight, between 43.5 and 48 wt. % Cu, between 36 and
40 wt. % Zn, between 9 and 12 wt. % Ni, between 5 and 7 wt. % Mn,
2.0 wt. % or less of Pb, 1.0 wt. % or less of Al, 2.0 wt. % or less
of Sn, between 0.05 and 0.5 wt. % of Fe, 0.2 wt. % or less of
Si.
25. The copper alloy according to claim 24, further comprising 0.15
wt. % or less of As.
26. The copper alloy according to claim 24, having tensile strength
values above 800 MPa and elongations above 5% when subjected to a
low temperature heat treatment comprised between about 300.degree.
C. and about 450.degree. C.
27. The copper alloy according to claim 26, wherein having a beta
phase precipitated in a fine needle-like structure when subjected
to the low temperature heat treatment.
28. The copper alloy according to claim 24, wherein the presence of
Al and Sn results in a high volume fraction of beta during a hot
deformation, and which can be reduced during intermediate
temperature annealing for allowing good cold formability, and
NiSn-rich precipitates and or Ni--Al rich precipitates when
submitted to a low temperature heat treatment comprised between
about 300.degree. C. and about 450.degree. C.
29. The copper alloy according to claim 24, having hardness values
comprised between 190 and 320 HV, tensile strength comprised
between 550 and 700 MPa, and elongation greater than 25% when the
alloy is subjected to a high temperature heat treatment comprised
between 500 and 700.degree. C.
30. The copper alloy according to claim 24, having tensile strength
greater than 800 MPa and tensile elongation greater than 5% when
the alloy is subjected to a low temperature heat treatment
comprised between 300 and 450.degree. C.
31. The copper alloy according to claim 24, having a microstructure
containing fine-grained needle-like or globular-like precipitates
of similar composition or different composition than the matrix and
with grain size below 5 micron when the alloy is subjected to a low
temperature heat treatment comprised between 300 and 450.degree.
C.
32. A precipitation hardenable copper alloy comprising between 45
and 48 wt. % Cu, between 36 and 40 wt. % Zn, between 9 and 14 wt. %
Ni, between 4 and 7 wt. % Mn, between 0.05 and 0.5 wt. % Fe, 1.5
wt. % or less of Ca, 1.0 wt. % or less of Si, 1.0 wt. % or less of
Al, 0.15 wt. % or less of As, and 0.1 wt. % or less of Pb.
33. The copper alloy according to claim 32, wherein Ca forms
precipitates with Cu and/or Zn in a pure alpha or a duplex
alpha/beta structure.
34. Copper alloy product, comprising a copper alloy comprising, in
percentage of weight, between 43.5 and 48 wt. % Cu, between 36 and
40 wt. % Zn, between 9 and 12 wt. % Ni, between 5 and 7 wt. % Mn,
2.0 wt. % or less of Pb, 1.0 wt. % or less of Al, 2.0 wt. % or less
of Sn, between 0.05 and 0.5 wt. % of Fe, 0.2 wt. % or less of
Si.
35. The copper alloy product according to claim 34, comprising
wires, rods, strips, and rectangular shapes and profiles.
36. The copper alloy product according to claim 35, being obtained
via casting, hot extrusion and successive cold drawing and heat
treatment steps.
37. The copper alloy product according to claim 35, wherein wires
have a final diameter smaller than 2.5 mm.
38. The copper product according to claim 34, comprising a writing
implement.
39. The copper product according to claim 38, wherein said writing
implement comprises a pen tip, a tip socket and/or reservoir for
pen tips to be filled with either oil-based, gel-based inks or
other liquids
Description
FIELD
[0001] The present invention generally relates to wrought
Cu--Ni--Zn (nickelsilver) alloys, more particularly to
Cu--Ni--Zn--Mn alloys mainly for the use in areas where machining
operations are substantial.
DESCRIPTION OF RELATED ART
[0002] With regard to the current market situation the trend goes
from ball point pens typically filled with oil-based inks of
relatively high viscosity towards roller-ball pens with inks of
lower viscosity. These new lower viscosity inks are mainly
water-based gel-inks. Compared to oil-based inks, gel-inks have the
advantage of allowing a greater variety of bright colors and can
have glitter effects, as they usually contain pigments that sink
into the paper. Driven by stylistic arguments and reducing the ink
consumption the trend in writing instruments goes towards finer
pens, which can be more easily be realized with low viscosity inks,
in particular with roller-ball pens. Reducing pen tips to
dimensions smaller than 1.6 mm diameters cause stringent
consequences with respect to the strength of the tip material. In
order that a tip can bear the same load with finer tip dimensions,
higher strength values of the alloy must be assured. Therefore, so
far only stainless steel have been used as tip material for the
finest tips, while Cu-based alloys are regarded as not being
suitable due to their inferior strength. Another common mistrust of
Cu--Ni--Zn alloys compared to stainless steels is the resistance
against corrosion in water-based gel inks. The invented alloys
presented here aim to represent an alternative to stainless steel
alloys used in pen tips, which show mechanical properties (strength
and ductility) as good those of stainless steels and corrosion
properties, which are suitable for pen tip applications, where
gel-based inks are used.
[0003] Introduction
The alloy family Cu--Zn--Ni originally imported from China in
17.sup.th century has later in the 18.sup.th century been
recognized almost parallel in France (1819), Germany (1823) and
England (1832) as a copper-nickel-zinc alloy and given the names
"Maillechort"--the later after their Lionese inventors Maille and
Chorier, "Neusilber" and "Nickelsilver". In recent times
nickelsilver is known for its good combination of properties and
the silvery color has promoted the alloy to be successfully used in
various applications. Today most commercially available Cu--Zn--Ni
alloys contain between 10-25% Ni, which due to its complete
solubility in Cu increases not only the strength of the alloy (by
solid solution strengthening, see below) but also elastic modulus
and the corrosion resistance. On the other hand, Cu--Ni--Zn alloys
of grey color bear significant disadvantages, which are related to
the effect of `fire cracking` [H. W. Schlapfer, W. Form Metal
Science 13 (1979); H. W. Schlapfer, W. Form Metall, 32, 135 (1978)]
that is related to the high internal stresses in the pure
mono-phased alpha alloys containing lead. The term fire cracking
describes a kind of liquid metal embrittlement, which occurs in
certain leaded alpha phase alloys, when cold deformed and annealed,
whereby an explosive intergranular fracture occurs during or after
the annealing process.
[0004] To circumvent this difficulty successive alloy development
progress led to the partial replacement of Ni with Mn, which
allowed to maintain the grey color, meanwhile changing the alloy
from a pure alpha alloy to a duplex like alpha/beta structure,
which is not prone to fire cracking as internal stresses are
released at the phase boundaries. Mn has a more limited solubility
in Cu than Ni, but can be alloyed up to approximately 15 wt. % in
Cu--Zn alloys resulting similarly to Ni in a grey color appearance
of the alloy (e.g. see U.S. Pat. No. 5,997,663).
[0005] Nowadays generations of Cu--Ni--Zn--Mn alloys often contain
about 10-25 wt. % Ni, and 3-7 wt. % Mn. The field of applications
ranges from writing instruments, to eye glass frames, keys,
applications in watch industry, fittings, fine tooling applications
and several other areas, where free-machining operations are
frequent or inevitable resulting in large quantities of waste
material in form of chips (up to 50%). Commonly lead in quantities
of 1.0 to 3.0 wt. % are alloyed to alloys, where free-machining
operations are required, which significantly improving their
machinability.
[0006] Lead-Free Alloys
Pressured by new legislations demanding for environmentally
friendly and nontoxic element additions the demand for lead-free
products in particular in applications of free-machining is
constantly increasing. As a consequence new solutions have to be
found in order to secure the recycling path of Cu-based alloys
containing lead substituting elements
[0007] The most prominent current alternatives to Pb as a chip
breaker in free-machining Cu-alloys are: Bismuth, Silicon and
Tellurium. Bismuth has similar properties and behaviour with
Cu-alloys as lead, i.e. low melting point (Pb: 327.degree. C., Bi:
271.degree. C.), miscible in the liquid and immiscible in the
solid, high density (Pb: 11.3 g/cm.sup.3, Bi: 9.78 g/cm.sup.3), a
lubrication effect during machining and so represents an excellent
chip breaker as is Pb. However, due to the incompatibility of
Bismuth with certain Cu-based alloys (high internal stress causing
stress corrosion cracking) a replacement of Pb with Bi in
die-castings and wrought products is not recommended. Alloys
containing bismuth are also more difficult to recycle, because
recycling is done unmixed and so far fully developed recycling does
only exist for lead containing copper alloys [Adaptation to
Scientific and Technical Progress of Annex II Directive 2000/53/EC;
J. Lohse, S. Zangl, R. Gro.beta., C. O. Gensch, O. Deubzer.
Oko-Institut e.V. (2008)]. Bismuth is industrially rated as less
toxic than lead and other neighboring heavy metals, however
injection of large doses can cause kidney damage. Furthermore, it
is considered that its environmental impact is small, due in part
to the low solubility of its compounds
[http://en.wikipedia.org/wiki/Bismuth; Fowler, B. A. "Bismuth" in
Friberg, L. Handbook on the Toxicology of Metals (2nd ed.).
Elsevier Science Publishers. (1986), 117]. Nonetheless bismuth has
found its way into brass products as chip breaker mainly in Asia.
Several patents describe the effect of Bi as a chip breaker in
free-machining wrought copper alloys [U.S. Pat. No. 5,167,726;
EP1790742].
[0008] Alternatively, silicon has been suggested, as an element
addition to favor chip breaking in brasses, but is due to the
less-favorable chip form, the absence of a self-lubricating effect
causing higher wear damage on tools and the associated difficulty
to recycle such chips neither an easy choice for a free-machining
Cu-based alloy. Furthermore the risk of Fe--Si precipitates during
casting of brasses containing low Fe concentrations further reduces
the machinability. Silicon containing free-machining brasses, which
show high strength levels than Si-free leaded versions of
free-machining brasses are nowadays available and are covered in
large parts by the patent family [EP1038981; EP1452613]. Apart from
its effect on machinability Silicon has the strongest influence in
the Cu--Zn diagram to shift the phase boundary between alpha and
alpa+beta towards the beta rich side (Guillet Zn equivalent of 10;
see: [L. Guillet and A. Portevin, Revue de Metallurgie Memoirs
XVII, Paris, (1920), 561]) and has a positive influence on the
strength, wear resistance and corrosion resistance.
[0009] Other known alternative Pb replacements in copper alloys are
based on additions of Tellurium, Calcium and Graphite acting in
form of intermetallics or particles as chip breakers
[WO2008/093974; WO9113183]. Copper Tellurium alloys (C14500)
contain 0.4-0.7 wt. % Te with minor additions of P and Ag and the
rest being Cu. They form CuTe-intermetallics with a satisfactory
chip breaking effect. Unfortunately, the alloy is not an easy to
manufacture alloy due to the high sensitivity of forming oxides
causing embrittlement. In addition, in brasses, Te forms brittle
ZnTe intermetallics as well results in unfavorable properties.
Graphite containing Cu-alloys are expensive due to high production
cost via spray casting technology. Little or no information is
available on Ca-containing Cu-alloys [WO2008/093974], in particular
with respect to Cu--Ni--Zn or Cu--Ni--Zn--Mn alloys.
SUMMARY
[0010] It is part of the aim of this invention to introduce new
microstructural design solutions for the alloy, which allows
having, even in the absence of lead as a chip breaker, good
machinability performances in free-machining operations. This can
be solved on the one hand by adjusting the microstructure related
to its partitioning of the alpha/beta phases and/or by additions of
minor alloying elements forming precipitates with one of the major
alloying elements. The minor alloying elements foreseen for this
task are Fe, Al, Ca, Sn, P, and Si. Although it is known, that
duplex structures, or precipitates favor chip breaking compared to
a monophase structure, our multi-path approach is new with regard
to the field of application as well as the alloy family of
Cu--Ni--Zn--Mn alloys. First mentioned approach in the invention
relating to the fine needle like precipitates of beta or beta'
phase in alpha mother grains is conceptually new and can be applied
not only to this alloy family, but basically all Cu--Zn alloys,
where part of the microstructure is in a metastable condition with
respect to the phase transformation. The second approach of using
precipitation of supersaturated solutions is a well-known process
to increase the strength, but here it fulfills for this specific
family of alloys and specific application two tasks: hardening and
chip breaking and thus can be considered as novel. Lastly adding Ca
as chip breaker has so far not found notation in combination with
the fields of applications and the alloy family considered
mentioned here.
[0011] Conventionally, there are four different hardening
mechanisms known in single phased metals: Precipitation hardening,
cold deformation hardening, solid solution strengthening, and grain
size strengthening (Hall-Petch strengthing). Industrially mainly
the first two mechanisms are of importance. Precipitation hardening
is typically used in low-alloyed Cu-alloys where high electrical
conductivity paired with moderate strength is requested. Spinoidal
decomposition can be regarded as a special variation of
precipitation hardening out of a supersaturated solid solution and
finds application in Cu-alloys mainly in alloys containing
substantial amounts of Sn or Ti. Cold deformation hardening is
typically used for increasing the strength in rods, profile and
wire products independent of the type of alloys. Solution hardening
can be regarded as a side-effect when adding additional elements
for improving different properties of the alloys, but is as such
not of great relevance. Finally grain size hardening is
industrially and technically difficult to control and its hardening
contribution becomes evident only at grain sizes smaller than about
10 micrometers, sizes difficult to achieve in industrial
production.
[0012] Similar to duplex steels, brasses or nickel-silver alloys
having a certain range of Zn content exhibit a duplex alpha
(face-centered cubic, fcc) and beta (body-centered-cubic, bcc)
structure, which apart from representing a fifth mode of increasing
strength, is also beneficially influencing the machinability, grain
size stability and hot workability. Current commercially available
leaded Cu--Ni--Zn--Mn alloys range in the Ni content from 5 to 25
wt. %, a Mn 0-7 wt. %, Zn 25-40 wt. % and rest Cu and impurities
typically <1 wt. %. According to Guillet's rule [L. Guillet and
A. Portevin, Revue de Metallurgie Memoirs XVII, Paris, (1920), 561]
Mn shows with a factor of 0.5 only a slight influence towards the
beta rich side in the phase diagram, while Ni exhibits a factor of
-1.2 keeping the phase diagram on the alpha-rich side, and thus
almost in balance for a Mn content of 6 wt. % and Ni content of 12
wt. %. Thus, as a first approximation the complicated 4 component
system Cu--Zn--Ni--Mn can in this case be treated as the Cu--Zn
binary phase diagram. However, as shown below for more precise
estimates on a multicomponent phase diagram, more advanced
thermodynamic software tools are required. With increasing Ni and
Mn content the strength increases. Typical tensile strength values
for cold drawn materials are 700-800 MPa, while in fewer cases
values up to 900 MPa can be found for strongly cold drawn wires,
however that typically goes at the expense of ductility, so that
tensile elongations are limited to -1%.
[0013] In this application we aim to combine these mechanisms in a
novel family of Cu--Ni--Zn--Mn alloys in such a way, that high
strength and sufficient ductility can be achieved. Hereto the Zn,
Al, Ca, Mn, Si, Ni, Sn, Fe content is adjusted to have a
sufficiently high beta content at elevated temperatures, which can
be later reduced by thermo-mechanical heat treatments for
increasing the cold deformability, followed by a precipitation
hardening process, where on the one hand fine precipitates of beta
or beta' (tetragonal distorted bcc structure) are nucleating in
alpha-mother grains, while on the other hand intermetallic
precipitates are forming. This yields a strong increase in tensile
strength higher than usually reached in Cu--Ni--Zn--Mn alloys. In
these classical compositions a trade of is required between cold
deformability, which increases the strength and the ductility which
remains. Here however, the strengthing is only in part resulting
from cold deformation (increase in dislocation density and point
defects), but from the precipitation strengthening. Thus in final
processing steps only moderate deformation has to be applied,
reaching much higher strength values with still good plasticity.
The following detailed description of the invention addresses the
above mentioned points in more detail.
[0014] Corrosion Properties
Dezincification is understood as the dissolution of Zn in Cu--Zn
alloys and can be regarded as the most severe corrosion effect in
Cu-alloys. More precisely Zn dissolves by a di-vacancy diffusion
process leaving a "hole" in the crystal lattice of the surface
layers [J. Y. Zou, D. H. Wang, W. C. Qiu, Electrochmica Acta, 43,
(1997), 1733-1737]. Thus, Cu-alloys free of Zn show superior
corrosion resistance than brasses. In analogy, alpha brasses are
more corrosion and dezincification resistant than the Zn-rich
beta-brasses. Cu--Ni--Zn alloys show in comparison to brasses
similar corrosion resistance as alpha brasses, but have due to the
higher nickel content a better tarnish resistance and resistance to
stress corrosion cracking. Little information is available on the
corrosion properties and the influence of minor alloying elements
in Cu--Ni--Zn alloys, but can be extrapolated from the effects
known to brasses. There different alloying elements have been
reported to improve corrosion resistance and retard dezincification
in brasses as summarized in Ref. [D. D. Davies, "A note on the
dezincification of brass and the inhibiting effect of elemental
additions", Copper Development Association Inc., 260 Madison
Avenue, New York, N.Y. 10016, (1993), 7013-0009]. Minor additions
of arsenic, phosphorous or antimony are known to show improved
corrosion resistance in all-alpha brasses. Duplex brasses where the
beta phases are completely enclosed by alpha grains do exhibit also
a beneficial effect on the resistance to dezincification.
Al-containing alpha brasses are well known to show improved
corrosion resistance (Admiralty or Naval brasses) and even
dezincification in duplex brasses was reported to be retarded when
adding up to 2 wt. % Al. The influence of tin on the
dezincification and corrosion of brasses is more ambiguous as it
has a positive effect in beta but a negative effect in alpha
grains. However in combination with Al additions an amount of up to
1 wt. % Sn has been reported to improve corrosion and
dezincification resistance. Silicon exhibits a positive effect when
added below the level of precipitation of Si-rich precipitates in
alpha grains of brasses, which lies at around 0.5 wt. %. Above this
level of Silicon corrosion and dezincification increases as it does
for iron additions. Finally the influence of lead shows positive
effects in alpha brasses, but only if Pb-compounds are forming a
passivation layer [S. Kumar, T. S. N. Sankara Narayanan, A.
Manimaran, M. Suresh Kumar, Mater. Chem. & Phys. 106, (2007),
134-141], while it shows reducing performance in duplex
brasses.
[0015] The present invention aims also for applications where
corrosion properties can be of crucial importance, in particular in
solutions where crevice conditions are present. This is for
instance the case in ball pen tips where the gap between the ball
and the surrounding pen socket is of the order of few micrometers
distance and the ink is not constantly stirred (during storage of
the pen tip). In water-based gel-inks this may locally lower the pH
of the ink and cause local corrosion attack. The right choice of
elements and the appropriate microstructure to reduce corrosion is
thus detrimental to the lifetime of a pen tip.
[0016] More generally the invention also aims to increase the
dezincification and the corrosion resistance in mild and medium
active solutions to levels which are common to stainless steels,
with the goal to replace them in applications, where a combination
of high strength, good corrosion resistance and improved
machinability are the main parameters for the materials choice.
[0017] The invention relates to age hardenable high-strength
Cu--Zn--Ni--Mn-based alloys with superior mechanical properties and
excellent machinability suitable for applications, where intensive
free-machining operations are required as for example for the
production of pen tips and reservoirs for writing implants of
reduced tip dimensions. However, the range application goes beyond
the production of writing instruments and does in general extent to
all applications where heavy free machining operations are
required. The composition of the invented alloy is given as
follows:
TABLE-US-00001 Cu: 42-48 wt. % Zn: 34-40 wt. % Ni: 9-14 wt. % Mn:
4-7 wt. % Pb: 0-2.0 wt. % Al: 0-1 wt. % Sn: 0-2 wt. % Fe: 0-0.5 wt.
% Si: 0-1.0 wt. % Ca: 0-1.5 wt. % As: 0-0.15 wt. % P: 0-0.3 wt.
%
[0018] The invention of the alloy aims to satisfy the current needs
for lead-free machinable Cu--Ni--Zn--Mn alloys suitable
free-machining operations as required for example in writing
applications. In addition, the invented alloys exhibit an
attractive combination of high strength with sufficient ductility
required for subsequent operations or safety margins. While the
flow stress reaches values comparable to those of typical stainless
steels used for pen tip and other free-machining applications,
sufficient cold-formability is often still required in order to
perform further bending operations or other cold-deformation steps,
such as the insertion of the pen ball onto the tip socket. However,
in contrary to stainless steels the machinability of this alloy
family is superior due to the precipitation hardened phases.
Additions of arsenic as well as minor additions of P, Si, Al and Sn
demonstrate beneficial effects on the corrosion resistance.
[0019] The copper alloy disclosed herein exhibits machinability
performance (easier chip handling, less tool consumption) superior
to that of stainless steel used in pen tip and also other
applications allowing for a higher production rate of parts per
hour. When subjected to a special low-temperature heat treatment,
the alloy has a unique microstructure, which even in absence of
lead, is leading to a good machinability performance superior to
that of typical stainless steels used in pen tips. The alloy that
is an ecologically friendly, lead-free free-machining
Cu--Ni--Zn--Mn alloy free of harmful elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The invention will be better understood with the aid of the
description of an embodiment given by way of example and
illustrated by the figures, in which:
[0021] FIG. 1 shows an optical microscopy images of samples heat
treated at 350.degree. C. (FIG. 1a) and 450.degree. C. (FIG. 1b) of
alloy No 1;
[0022] FIG. 2 shows an optical image of the longer screw-like chips
of alloy No 1 produced with the Citizen long turning machine;
[0023] FIG. 3 shows an optical microscopy images of the as-cast
structure (FIG. 3a) and the cold deformed annealed (450.degree. C.)
(FIG. 3b) of alloy No 3;
[0024] FIG. 4 shows pseudo-binary phase diagram (FIG. 4a) and phase
fraction diagram for a specific composition (FIG. 4b) of alloy No
3.
[0025] FIG. 5 represents a screw-type and curly type chips shown
for two types of alloys of the alloy No 3;
[0026] FIG. 6 shows machining tests with Mikron Multistar made at
100 Hz on alloy No 3 with composition A annealed at 450.degree. C.
(FIGS. 6a and 6b); and alloy No: 1 (FIGS. 6c and 6d), chip length
of the leaded alloy No: 1 being smaller than that in alloy No
3;
[0027] FIG. 7 shows as-extruded microstructure (FIG. 7a) and after
2 cycles of cold deformation and annealing at 650.degree. C.) (FIG.
7b) of alloy No 5; heat treated alloy at 540.degree. C. followed by
350.degree. C. (FIG. 7c) and 400.degree. C. (FIG. 7d) low
temperature heat treatment of alloy No 5; and
[0028] FIG. 8 shows optical microscopy image of a sample annealed
at 540.degree. C. followed by a second annealing process at
400.degree. C. (FIG. 8a); secondary electron microscopy image of
alloy with NiSn preciptates in beta phase matrix and at boundary to
alpha grains (FIG. 8b) both of alloy No: 6.
DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS OF THE INVENTION
[0029] The present invention generally relates to wrought
Cu--Ni--Zn (nickel-silver) alloys, more particularly to
Cu--Ni--Zn--Mn alloys mainly for the use in areas where machining
operations are substantial. The present invention relates also to
leaded, leadless or lead-free free-machining Cu--Ni--Zn--Mn alloys
particularly suited for applications in areas where free machining
operations are heavily involved, such as writing instruments, eye
glass frames, medical tools, electrical connectors, locking
systems, fine tooling, fasteners and bearing for automotive
industry, without restriction to other fields of application. In
addition, the present invention aims to replace wrought steel
products in various applications where high strength and sufficient
ductility combined with excellent free-machinability are required
with or without the presence of lead.
[0030] The present invention has among the above mentioned various
fields of applications particular focus on writing instruments,
where the tip material is in direct contact with the ink and the
ball material. Nowadays a number of ball materials, such as various
types of tungsten carbide hard-metal balls with different binders
(Co, Co+Ni+Cr), different types of steels and different types of
ceramic balls are on the market, while the type of inks can be
separated into mainly gel-based and oil-based inks and to a lesser
extent inks based on other liquids. The Cu--Ni--Zn--Mn alloy family
presented here can be combined with all possible combinations of
ball or ink materials.
[0031] The objective of the present invention is to provide a new
high-strength Cu--Ni--Zn--Mn alloy family that thanks to a special
thermo-mechanical treatment and an optimized alloy composition
reaches mechanical properties comparable with those of wrought
stainless steel alloys. The leaded variations exhibit excellent
machinability and are thus promising candidates for all
applications, where high strength, good ductility and excellent
machinability are of utmost importance, i.e. writing instruments,
eye glass frames, keys, applications in watch industry, fittings
and other fine tooling and free-machining applications, without
restricting other fields of application. The lead-free variations
persuade on the one hand by their duplex alpha beta structure, and
on the other hand by the precipitates both result in a significant
improvement of the machinability with respect to untreated Pb-free
Cu--Ni--Zn--Mn alloys. In addition, the lead-free variations do not
contain any user unfriendly amounts of elements, which either may
be harmful for human and/or environment.
[0032] The present invention is realized by providing seven
different Cu--Ni--Zn--Mn alloys on a basis of copper, zinc, nickel,
manganese and other elements. The compositions of the alloys
presented here and in the granted patent family EP1608789B1 are
optimized for special applications, where apart from production
costs the appearance of the alloy is as important as the mechanical
properties, machinability and corrosion properties. Different
dimensions and geometrical forms can be produced from these alloys,
such as wires, strips, rods, tubes and various profiles and square
shapes. In particular wire drawn products such as pen tips for
writing instruments are addressed, which after a hot deformation
process are typically drawn down to the final diameter in
successive cold drawing and heat treatment steps. In this respect
the Mn content of the alloy is limited to the range of 4-7 wt. %.
Higher levels of Mn show a negative effect during cold-forming,
while a lower Mn content increases the risk of fire cracking and
too low beta content during warm extrusion processes. Apart from
the high cost of Ni, a higher Ni content (>14 wt. %) is pushing
the phase diagram towards a purely mono-phase alloy even at
elevated temperatures. A lower Ni content (<9.0 wt. %) bears the
risk that the silvery color turns gradually into a yellowish one
and has to be increased in Cu content in order to maintain the
balance between alpha and beta phases. In cases where steels are
aimed to be substituted a silvery appearance of the Cu--Ni--Zn--Mn
alloys is of great importance. The Zn content is chosen in a range
that allows to vary the microstructure (fraction of beta content)
from 0% to approximately 50%.+-.10%. Zn content >40 wt. % show a
to high amount of beta suitable for cold drawing, while a lower
content than 34 wt. % makes hot extrusion processing difficult. The
content of Pb is kept at a minimum level to assure good to
excellent machinability. The copper alloy is of grey or silver
color/appearance typical for Cu--Ni--Zn--Mn alloys sometimes having
a nuance of a pale yellowish tone.
[0033] For the alloys presented in this invention a thermodynamic
model approach has been applied in order to have a better estimate
on the phase fields and the influence of alloying elements on the
phase fields than it is possible with the Guillet role of thumb
used in brasses [J. .ANG.gren, F. H. Hayes, L. Hoglund, U. R.
Kattner, B. Legendre, R. Schmid-Fetzer: Applications of
Computational Thermodynamics. Z. Metallkunde 93, (2002), 128-142].
This is clearly a more refined approach than common alloy design
approaches and has demonstrated to be a fine tool to evaluate the
stability of each phase as a function of temperature.
[0034] The machinability of most of the alloys described below has
been measured on a Citizen long turning lathe and a Mikron
Multistar turning machine. The following machine parameters have
been used: (see Table 1).
[0035] First Alloy:
The first alloy is based on the granted patent EP1608789
applications and consists of 42-48 wt. % Cu, 34-40 wt. % Zn, 9-14
wt. % Ni, 4-7 wt. % Mn, .ltoreq.0.5 wt. % Fe, .ltoreq.0.03 wt. % P
and .ltoreq.2.0 wt. % Pb.
[0036] Said granted patents mentioned above are based on the idea
that thanks to a special thermal heat treatment an alloy with an
alpha-beta structure stable at elevated temperature and thus
suitable for hot deformation processes can be modified into a pure
alpha alloy when annealed at temperatures between 630-720.degree.
C. resulting in improved cold formability and better corrosion
resistance due to the mono-phased structure. The associated
chemical variations of the major elements are balanced out in order
to guarantee the said microstructural transformation from a duplex
to a mono-phase alpha alloy. According to Guillet's rule of thumb
for the Zn equivalent in brasses, Mn is almost insensitive to the
variation, while Ni is showing an alpha stabilizing effect. Our
thermodynamic calculations in the multi-component system shows that
for minor elements such as Fe a content of 0.5 wt. % increases the
beta phase fraction of the alloy by about -5-10%, without changing
the slope of the curves, while at intermediate temperatures of
about 400.degree. C. Fe provokes a co-existence of the gamma phase
(<5% volume fraction) in an alpha/beta matrix. Phosphorous is
added in order to increase the corrosion resistance.
TABLE-US-00002 TABLE 1 Machining test parameters used for the
alloys included in the present invention. Parameter I II III
Citizen Turning ~4'000 ~8'000 ~10'000 speed [1/min] Lengthwise 0.01
0.02 0.02 depth [mm] Facing 0.01 0.01 0.01 depth [mm] Interpo-
0.001 0.001 0.001 lation [mm] Cutting 25 40 50 speed [m/min] Mikron
Multistar Frequency 85 95 100 [Hz] Turning ~16'000 ~18'000 ~19'000
speed [1/min] Parts per 120 140 120 140 120 140 minute Feed
moderate high Low- moderate Low- low moderate moderate
[0037] The first invention presented here builds-up on the
processing parameters used for the above mentioned granted patents,
i.e. EP1608789, which allow the formation of a mono-phase alpha
Cu--Ni--Zn--Mn alloy. Its primary aim was to develop an alloy
suitable for pen tip applications, where the corrosion resistance
is superior with respect to duplex phased Cu--Ni--Zn--Mn alloys.
This can only be guaranteed in purely mono-phase state not allowing
for microstructural conditions allowing galvanic corrosion leading
to localized microstructural determined crevice conditions.
[0038] Compared to aforementioned alloys developed in the patent
family EP1608789, the alloy presented here is in addition subjected
to heat treatments at lower temperatures of 300-450.degree. C.
(also called "low temperature heat treatment" below) allowing for a
fine precipitation of beta and/or beta' precipitates. This
precipitates are showing a needle like morphology and are oriented
along the primary crystallographic axis of the fcc mother grains.
FIGS. 1a and 1b shows micrographs with the low temperature heat
treated alloys having fine precipitates of beta' and beta,
respectively. Note that the phase boundary between beta and beta'
(its tetragonal distorted variation) lies between 400 and
450.degree. C. More particularly, FIGS. 1a and 1b shows samples
heat treated at 350.degree. C. (a) and 450.degree. C. (b) of alloy
No 1.
[0039] It must be mentioned that the concept of low temperature
heat treatments is commonly applied to Cu-alloys that are age
hardenable, i.e. where a supersaturated solid solution is present
with minor additions of elements. Here on the other hand not a
chemical driving force for precipitation is used, but the energy
difference between the beta and beta' phase. This is often applied
to steels, where a martensitic transformation causes an increase in
the strength of the alloy. In this invention, this concept has been
adopted, whereby the transformation cannot be induced by plastic
deformation.
[0040] In order to determine the precise temperature range of heat
treatments a special thermodynamic software tool has been applied,
which allows calculating the phase stability fields in a
multi-component system as a function of temperature and chemical
composition [J. .ANG.gren, F. H. Hayes, L. Hoglund, U. R. Kattner,
B. Legendre, R. Schmid-Fetzer: Applications of Computational
Thermodynamics. Z. Metallkunde 93, (2002), 128-142].
[0041] Said alloy results in improved hardness and tensile strength
of 850-950 MPa with remaining elongation levels of 2-10% compared
to the same alloy not subjected to the low temperature heat
treatment (see Table 2). Even higher strength and ductility might
be reachable by further optimization of thermo-mechanical treatment
as it was done for unleaded alloys (see further down).
[0042] The machinability of said alloy exhibits thanks to the
higher strength, the uniform partition of lead particles and the
fine beta precipitates an excellent machinability (>.about.90%
with respect to CuZn39Pb3=100%), which makes it an interesting
candidate for replacing stainless steels in pen tip applications.
Most often chips were very short (<1 mm length) in particular
when machining with the Mikron Multistar (at all conditions set in
Table 1). But also favorable screw-shape chips.
[0043] FIG. 2 shows an optical image of the longer screw-like chips
of alloy No 1 produced with the Citizen long turning machine.
[0044] Second Alloy
The second alloy of the present invention has a very similar
chemical composition as the first mentioned alloy, however
including arsenic, i.e. of 42-48 wt. % Cu, 34-40 wt. % Zn, 9-14 wt.
% Ni, 4-7 wt. % Mn, .ltoreq.0.5 wt. % Fe, .ltoreq.0.03 wt. % P,
.ltoreq.2.0 wt. % Pb and 0.01-0.15 wt. % As.
[0045] The second invention presented here builds-up on the
processing parameters used for the above mentioned granted patents,
i.e. EP1608789, which allow the formation of a mono-phase alpha
Cu--Ni--Zn--Mn alloy.
[0046] Apart from Arsenic, the same influences on the variations of
chemistry are present in this invented alloy as in the first alloy
presented above.
[0047] As mentioned in the background to the invention, As is used
in brasses as a corrosion inhibitor, which due its fast diffusion
in alpha brasses migrates to the di-vacancies and inhibits further
corrosion of the surface layer [J. Y. Zou, D. H. Wang, W. C. Qiu,
Electrochmica Acta, 43, (1997), 1733-1737]. In the Cu--Ni--Zn--Mn
alloy presented here, the presence of As also improves the
corrosion resistance, which shows in aqueous solutions with <1
wt. % NaCl and in water-based inks an increased corrosion potential
and a lower corrosion rate as compared to the alloy without the
addition of As. This in turn has also a positive effect on the ink,
as fewer ions are taken up by the ink, which might lower their
performance.
TABLE-US-00003 TABLE 2 Vickers hardness tests on samples annealed
at 350 and 400.degree. C. for 1, 5, 10 and 24 hours compared to
normal annealing temperatures for recrystallisation. Vickers
hardness [Hv5] Annealing temperature [.degree. C.] 6 h 1 h 5 h 10 h
24 h Initial (650) 240 239 350 249 265 272 265 350 253 260 268 265
400 244 253 239 232 400 241 254 239 234 Tensile tests Ultimate
Yield tensile Total strength strength elongation [MPa] [%]
650.degree. C., 2 h; O 2.3 mm 177 453 47 450.degree. C., 6 h ; O
2.3 mm 537 730 14 350.degree. C., 6 h ; O 2.3 mm 738 815 7.3
450.degree. C., 6 h ; O 1.6 mm 663-681 858-877 7-9 450.degree. C.,
6 h + 350.degree. C., 6 h; O 1.6 734 941 4 mm
[0048] The low level As additions does not exhibit any difference
in the microstructural appearance of the alloy and it exhibits the
same mechanical properties and machinability performance as the
version without As (First alloy).
[0049] Third Alloy
The third alloy of the present invention is unleaded and contains
the following chemical composition: 45-48 wt. % Cu, 37-40 wt. % Zn,
9-14 wt. % Ni, 4-7 wt. % Mn, .ltoreq.0.5 wt. % Fe, .ltoreq.0.03 wt.
% P, .ltoreq.0.15 wt. % As and .ltoreq.0.1 wt. % Pb.
[0050] One aim of the present alloy invention was to increase the
beta content of the microstructure to a level, which shows good
machinability suitable for turning operations. This is realized by
an increased Zn content as compared to the alloy composition of the
first and second alloy of the present invention. FIG. 3a shows the
as-extruded microstructure of the duplex phased alloy.
[0051] A second goal of the invention of this alloy was to increase
the mechanical properties of the alloy by low temperature heat
treatment steps during wire cold deformation. FIG. 3b shows the
microstructure of such a cold deformed and annealed microstructure,
where a heat treatment of 450.degree. C. has been applied.
[0052] Zn content below 37.5% reduces the amount of beta during hot
extrusion (.about.800.degree. C.) to a volume fraction close to
zero percent, while with a content of Zn>39% the beta phase
fraction reaches about 30% at this temperature. However at lower
temperature annealing its content increases to almost 50% and thus
reduces the ability to strongly cold deform the material.
Increasing the Mn content and reducing the Ni content at the same
Cu:Zn ratio increases stability of the beta phase at high
temperatures suitable for hot extrusion, which can be reversed at
intermediate annealing temperatures (600.degree. C.) More
particularly, an optical microscopy images of the as-cast structure
is shown in FIG. 3a and the cold deformed annealed (450.degree. C.)
is shown if FIG. 3b for alloy No 3.
[0053] As described in the aforementioned invention of the first
alloy the same low temperature heat treatment has been applied.
According to the thermodynamic calculations shown in FIGS. 4a and
4b the face-centered cubic (fcc) structure (alpha) is solidifying
first followed by a body-centered-cubic phase (beta). At about
420.degree. C. the beta phase is partially transforming into a beta
prime phase (b'), which is in accord with the microstructural
observations of low temperature heat treatments (see FIG. 1 and
FIG. 3b). The phase MnNi phase displayed in FIG. 4 could not be
manifested in the microstructure, due to too low reaction kinetics.
The same is the case for low volume fraction phases
thermodynamically stable at low temperatures, but due to low
reaction kinetics not appearing. More particularly, FIGS. 4a and 4b
show pseudo-binary phase diagram (a) and phase fraction diagram for
a specific composition (b) of alloy No 3.
[0054] Said microstructure has been achieved with a Zn content of
38 and 39 wt. %. Lower Zn content lowers the amount of beta phase
significantly, while Zn larger than 40 wt. % are showing a too low
density of alpha grains.
[0055] Mechanical strength of this alloy reaches values between
850-1050 MPa and tensile elongations of 2-20%. Such high strength
values combined with good tensile elongations have not been
reported so far to the knowledge of the inventors. One main key
ingredient in achieving an optimum combination between strength and
ductility is to perform two cycles of a low temperature heat
treatment after significant cold deformation. This cyclic heat
treatment allows for a maximum driving force to precipitate fine
beta needles, at the expense of decreasing the dislocation density,
which allows for further cold deformation. Meanwhile
recrystallization and grain growth of alpha grains is kept to a
minimum so that softening effects are avoided.
TABLE-US-00004 TABLE 3 Tensile test data for alloy N.degree.: 3
Tensile tests Ultimate Yield tensile Total strength strength
elongation Condition [MPa] [%] Composition A; 450.degree. C., 6 h;
O 640 815 19 2.3 mm Composition A; 450.degree. C., 6 h; + 809 904
18.9 350.degree. C., 2 h; O 2.3 mm Composition A; 450.degree. C., 6
h; O 687-702 891-898 12 1.6 mm Composition A; 450.degree. C., 6 h +
724-809 848-904 8-19 350.degree. C., 2 h ; O 1.6 mm Composition B;
350.degree. C., 6 h; O 815-835 1020-1040 1 1.6 mm Composition B;
450.degree. C., 6 h + 895-929 1000-1016 2-4 350.degree. C., 2 h; O
1.6 mm
[0056] FIG. 5 shows screw-type and curly type chips shown for two
types of alloys of the alloy No 3.
[0057] Due to the uniform dispersion of softer and harder phases in
the microstructure a good machinability (>70% with respect to
CuZn39Pb3=100%) is reached. Chip length is significantly longer
than in the leaded alloys, however not affecting significantly the
machining performance. Note that the surface quality is
significantly better compared to the surface of the leaded alloy
No: 1 (see FIG. 6).
[0058] FIGS. 6a to 6d represent machining tests with Mikron
Multistar made at 100 Hz on alloy No 3 with composition A annealed
at 450.degree. C. (FIGS. 6a and 6b); and alloy No: 1 (FIGS. 6c and
6d). Chip length of the leaded alloy No: 1 is smaller than that in
alloy No 3.
[0059] Forth Alloy
The forth alloy of the present invention is also unleaded and
contains the following chemical composition: 45-48 wt. % Cu, 36-40
wt. % Zn, 9-14 wt. % Ni, 4-7 wt. % Mn, .ltoreq.0.5 wt. % Fe,
.ltoreq.1.5 wt. % Ca, .ltoreq.1.0 wt. % Si, .ltoreq.1.0 wt. % Al,
.ltoreq.0.03 wt. % P, <0.15 wt. % As and <0.1 wt. % Pb.
[0060] The main focus of this alloy was to introduce Ca into the
material for it to act as a chip breaker when forming precipitates
with Cu, Si, Al and Fe. In absence of Fe, Al and Si additions of Ca
forms precipitates with Cu as has been demonstrated in the patent
application WO2008/093974. Additions of at least one of the other
alloying elements Si, Al or Fe further improve the machinability of
this alloy.
[0061] The main difficulty with this type of alloy is the avoidance
of oxidation of Ca as it strongly reacts with oxygen. This can be
avoided by pre-alloying of Ca with Zn in inert atmosphere.
Subsequent alloying with a pre-alloy of Cu--Mn incl. the above
mentioned amounts of Fe, Si, Al.
[0062] Fifth Alloy
The fifth alloy of the present invention can be unleaded and has
the following chemical composition: 43.5-48 wt. % Cu, 36-40 wt. %
Zn, 9-12 wt. % Ni, 5-7 wt. % Mn, .ltoreq.1.0 wt. % Al, .ltoreq.0.5
wt. % Sn, .ltoreq.0.5 wt. % Fe, .ltoreq.0.03 wt. % P, .ltoreq.0.15
wt. % As and .ltoreq.2.0 wt. % Pb.
[0063] The main focus of this alloy was to generate a variation of
the aforementioned unleaded Cu--Ni--Zn--Mn alloy (No: 3) that is on
the one hand age hardenable, i.e. forms secondary precipitates from
a supersaturated solid solution matrix and on the other hand is
suitable for hot and cold deformation, i.e. allows to be
transformed from a duplex rich in beta structure into a duplex
structure poor in the beta phase fraction. This was realized by
including additions of Fe, Al and Sn.
[0064] Technically and economically speaking high beta phase
fraction in the alloy during extrusion is beneficial as it allows
for lowering the extrusion force and temperature. Subsequent cold
drawing steps require however a high volume fraction of alpha
grains, which if the chemistry is optimized can be achieved with
dedicated heat treatment steps. This metallurgic ally difficult
task has been fulfilled satisfactorily by the addition of Al and
Sn.
[0065] The as-extruded microstructure shows a very fine
recrystallized two-phased structure, with grain sizes well below 20
mm (FIG. 7a). Al acts in this respect as effective grain growth
inhibitor. Subsequent heat treatments above 600.degree. C. exhibit
some grain growth. Low temperature heat treatments exhibit a peak
hardening at 350.degree. C. with Vickers hardness values of >250
HV (see Table 4 and FIG. 7).
[0066] FIGS. 7a to 7d show as-extruded microstructure (FIG. 7a) and
after 2 cycles of cold deformation and annealing at 650.degree. C.)
(FIG. 7b) of alloy No 5. Heat treated alloy at 540.degree. C.
followed by 350.degree. C. (FIG. 7c) and 400.degree. C. (FIG. 7d)
low temperature heat treatment of alloy No 5.
[0067] Cycles of annealing (.about.600-700.degree. C.) and cold
deformation treatments cause an alteration in the microstructure
with an increasing content of the beta volume fraction to
.about.50%, whereby the alpha grains form the matrix surrounded by
beta grains. When successively annealed at lower temperatures
<450.degree. C. fine precipitates in form of needles nucleate
(FIGS. 7c and 7d).
[0068] According to thermodynamic simulations Ni-Aluminides are
formed right after having reached the solidus curve and maintain a
constant level of about 0.02% and thus act as strong grain growth
inhibitors as mentioned before. In addition Al has a strong effect
on the variation of the beta fraction reaching a minimum value at
around 600.degree. C. which towards higher and lower temperatures
is increasing.
[0069] The tensile properties of the alloy show values ranging from
850-900 MPa with elongations of 2-12% (see Table 4).
[0070] Sixth Alloy
The sixth alloy of the present invention is also age hardenable and
has the following chemical composition: 43.5-48 wt. % Cu, 36-40 wt.
% Zn, 9-12 wt. % Ni, 5-7 wt. % Mn, .ltoreq.1.0 wt. % Al,
.ltoreq.2.0 wt. % Sn, .ltoreq.0.5 wt. % Fe, Si.ltoreq.0.2 wt. %,
.ltoreq.0.03 wt. % P, .ltoreq.0.15 wt. % As and .ltoreq.2 wt. %
Pb.
[0071] The main focus of this alloy was to evaluate the influence
Sn in the system, which has been added to provoke precipitation of
NiSn phases.
[0072] A strong increase in the beta fraction with increasing Sn
content has been observed, which allows for very low extrusion
temperatures resulting in a high volume fraction of beta phase.
Laboratory heat treatment and drawing tests have shown that this
volume fraction can be decreased significantly allowing for
subsequent good cold formability.
TABLE-US-00005 TABLE 4 Mechanical testing results of alloy
N.degree.: 5. Vickers hardness measurements for various
combinations of heat treatments 500/4 h 540/4 h 560/4 h 580/4 h
600/4 h 640/4 h 175 171 171 157 157 146 500/4 h 540/4 h 560/4 h
580/4 h 600/4 h 640/4 h 300/8 h 300/8 h 300/8 h 300/8 h 300/8 h
300/8 h 232 232 227 227 241 234 500/4 h 540/4 h 560/4 h 580/4 h
600/4 h 640/4 h 350/8 h 350/8 h 350/8 h 350/8 h 350/8 h 350/8 h 252
244 237 206 221 234 500/4 h 540/4 h 560/4 h 580/4 h 600/4 h 640/4 h
400/8 h 400/8 h 400/8 h 400/8 h 400/8 h 400/8 h 223 206 221 214 225
212 Tensile tests: Cold Ultimate work Annealing Annealing Yield
tensile Total reduction temperature time strength strength
elongation [%] [.degree. C.] [h] [MPa] [MPa] [%] 46 650 6 284 581
32.9 42.3 650 2 435 686 27.3 37.5 500 5 693 871 11.7 6 350 6 692
899 2.4
[0073] Low temperature age hardening tests have shown a maximum
hardening at .about.350.degree. C. The scanning electron microscopy
(SEM) image shown in FIG. 8 shows the material in the overaged
condition heat treated at 400.degree. C., where NiSn precipitates
are visible as white dots in the beta phase and localized to the
phase boundary.
[0074] FIGS. 8a and b show optical microscopy image of a sample
annealed at 540.degree. C. followed by a second annealing process
at 400.degree. C. (FIG. 8a); Secondary electron microscopy image of
alloy with NiSn preciptates in beta phase matrix and at boundary to
alpha grains (FIG. 8b) both of alloy No: 6.
[0075] Vickers hardness measurements revealed a hardness of 230-240
HV for the age hardening at 350.degree. C., while values between
220-230 HV were measured for heat treatments at 300 and 400.degree.
C. comparable with values given in Table 4 for alloy No: 5, but
slightly lower.
[0076] Seventh Alloy
The seventh alloy of the present invention is also an
age-hardenable alloy and has the following chemical composition:
43.5-48 wt. % Cu, 36-40 wt. % Zn, 9-12 wt. % Ni, 5-7 wt. % Mn,
.ltoreq.0.1 wt. % Al, .ltoreq.0.1 wt. % Sn, .ltoreq.0.5 wt. % Fe,
.ltoreq.1.0 wt. % Si, .ltoreq.0.3 wt. % P, .ltoreq.0.15 wt. % As
and .ltoreq.2.0 wt. % Pb.
[0077] Again as the alloy inventions No: 4 and 5, this invention
aims for an age hardenable Cu--Ni--Zn--Mn alloy that apart from
precipitations of alpha in beta or vice versa also contains typical
alloying elements suitable for age hardenability. Here Silicon and
Phosphorus are chosen as candidates.
[0078] Silicon has the strongest effect of all alloying elements on
the alpha beta phase boundary in brasses and thus has to be added
to the alloy with great care. Thermodynamic simulations have shown
that additions of up to .about.0.5 wt. % are still tolerable with
respect to the balance of alpha/beta ratio (3:1, at 800.degree.
C.), while a Si content of 1.0 wt. % reverses the fraction of
alpha/beta completely for a Zn content of 37 wt. %.
[0079] Similar as the Ni-Aluminides precipitates in the previous
mentioned alloy (No: 5) here, Ni5Si2 precipitates are formed right
after temperature has been lowered to below the solidus curve.
However their detection is a non-trival task and was not successful
with the instruments at hands. In low-alloyed copper the
precipitates are nucleating and growing to rounded platelets [D.
Zhao, Q. M. Dong, B. X. Kang, J. L. Huang, Z. H. Jin, Mater. Sci.
Eng. A361, (2003). 93-99].
[0080] Additions of Phosphorous beyond the level used for
de-oxidation is common in copper alloys containing either Fe or Ni.
Such alloys are known for their excellent performance with respect
to the combination of high conductivity paired with high strength.
Typically they form small 20-50 nm sized circular particles of Fe2P
[M. Motohisa, J. Jpn. Copper Brass Res. Assoc. 29. (1990), 224-233;
D. P. Lu, J. Wang, W. J. Zeng, Y. Liu, L. Lu, B. D. Sun, Mater.
Sci. Eng. A421, (2006), 254-259] or hexagonal platelets, of NiP2
having sizes of 50-150 nm [J. S. Byun, J. H. Choi, D. N. Lee,
Scripta Mater. 42, (2000), 637-643].
[0081] The age hardened stage of these alloys show a high
mechanical resistance reaching hardness values beyond 250 HV and
tensile strength above 1000 MPa with tensile elongation of
1-5%.
* * * * *
References