Boron-copper-magnesium-tin alloy and method for making same

A high strength, highly electrically conductive copper-based alloy and method for producing the alloy are provided, with the alloy containing boron in the range of 0.0-2.9 at. %, magnesium in a range of about 2.8-7.6 at. %, tin in a range of about 2.1-4.3 at. %, and the balance copper and unavoidable impurities. The method for producing the high-strength, highly conductive alloy includes solution heat treating or annealing the material to dissolve the solute elements into a solid solution including the copper, rapidly quenching the material to freeze the solute elements in solid solution, and aging the material at a temperature in a range of about 400-475.degree. C. to precipitation harden the alloy material.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to copper alloys, and particularly to alloys 
of copper containing boron, magnesium and tin as the alloying elements, 
and to a method for producing these alloys. 
2. Description of Related Art 
Heretofore, copper alloys containing beryllium as the sole or principal 
alloying element, referred to generally herein as copper beryllium alloys, 
have been employed in applications requiring the properties of high 
strength and high electrical conductivity. Beryllium is alloyed with the 
copper principally as a precipitation hardening agent, so as to improve 
the mechanical properties, particularly to increase the tensile strength 
of the copper. 
Beryllium compounds have been shown to cause disease, and beryllium is 
recognized as a carcinogen, therefore, the use of beryllium as an alloying 
agent is being phased out at foundries in the United States. This has 
created a need for other high strength, highly conductive alloys, 
preferably copper-based alloys, for use in applications which have, prior 
to this time, primarily employed copper beryllium alloys. 
Precipitation hardenable copper alloys and processes for producing copper 
alloys having high strength and/or high electrcal conductivity have 
previously been proposed. An example is presented in U.S. Pat. No. 
4,434,016, which is directed to a precipitation hardenable copper alloy 
that includes a substantial quantity of nickel, and further includes 
aluminum, manganese, magnesium, and restricts the amount of silicon to 
very small amounts. The processing of this alloy to produce precipitation 
hardening in the alloy involves a complex series of steps requiring 
mechanical deformation to be performed. 
Other alloys proposed in the prior art include the alloy disclosed in U.S. 
Pat. No. 4,338,130, which, in specifically avoiding the use of beryllium, 
employs not only nickel and silicon, but also requires aluminum and 
chromium to be present as alloying elements. Chromium has further been 
proposed in several other disclosures as being an alloying element in a 
precipitation hardenable copper alloy, or as an alloying element in a 
copper alloy that improves the mechanical properties through mechanisms 
other than precipitation hardening, such as dispersion hardening. Many of 
the prior art hardenable copper alloys depend upon the use of one or more 
steps of mechanical deformation to cold work the material in order to 
increase the mechanical properties sought for the alloy, at the expense of 
decreasing the ductility or formability of the alloys. 
As noted in the '130 patent, the use of magnesium has traditionally been 
avoided, in that magnesium tends reduce electrcal conductivity and rease 
ductility. Magnesium is present in the copper-based alloy of the '016 
patent and its presence indeed is disclosed as being critical, but the 
'016 patent expressly states that magnesium is not to exceed 0.5 wt.%. 
Another beryllium-free copper alloy that has been employed in 
high-strength, high conductivity applications is designated as C81540. 
This alloy is a sand castable chromium-nickel-copper alloy containing 
0.4-0.8 wt.% silicon, 2.0-3.0 wt.% nickel, 0.1-0.6 wt.% chromium, with the 
remaining balance being mainly copper, however, the specification permits 
minor amounts of other elements in the alloy. This alloy achieves its 
strength through the reaction of chromium and silicon, or nickel and 
silicon, or both. 
There continues to exist a need for alloys that have relatively low 
additions of alloying elements, and that can be produced or processed in a 
simple manner, preferably without the need to conduct mechanical 
deformation steps, wherein the finished product has high strength and high 
electrical conductivity. 
It is therefore a principal object of the present invention to provide a 
copper-based alloy composition having high strength and high electrical 
conductivity, while avoiding the use of beryllium as a precipitation 
hardening agent. 
It is a further principal object of the present invention to provide a 
copper-based alloy composition that is precipitation hardenable to provide 
increased hardness and tensile strength, without the need to mechanically 
deform the material in obtaining those properties. 
It is an additional principal object of the present invention to provide a 
precipitation hardenable copper based alloy in which relatively small 
amounts of specific alloying elements are employed. 
It is an additional important object of the present invention to provide a 
copper-based quaternary alloy in which boron, magnesium and tin are 
essentially the only alloying elements. 
It is a further principal object of the present invention to provide a 
process for producing a precipitation hardened copper-based quaternary 
alloy that includes a solution heat treatment followed by rapid quenching 
and then age hardening. 
It is an additional important object of the present invention to provide a 
process for producing a precipitation hardened copper-based quaternary 
alloy as set forth in the preceding paragraph, and which does not require 
any steps of mechanical deformation or cold working to achieve the desired 
strength-properties. 
SUMMARY OF THE INVENTION 
The above and other objects are achieved in the present invention by 
providing a copper-based quaternary alloy in which boron, magnesium and 
tin are included in the alloy a the three elements alloyed with the 
copper. More specifically, relatively small amounts of boron, magnesium 
and tin are added to copper in order to render the alloy precipitation 
hardenable in a simple process sequence involving solutionizing the alloy, 
rapidly quenching the alloy to freeze the solute elements (boron, 
magnesium and tin) in an unstable solid solution, and then aging the 
material to precipitate stable intermetallic compounds formed of copper, 
boron, magnesium, and tin. 
The alloy composition of the quaternary alloy of the present invention 
includes a range of about 0.0-2.9 at. % boron, about 2.8-7.6 at. % 
magnesium, about 2.1-4.3 at. % tin, and the balance copper and possibly 
trace amounts of unavoidable impurities. A preferred range of compositions 
within the above composition range to obtain optimum electrical 
conductivity includes from about 0.5 at. % boron, about 4.8 at. % 
magnesium, about 3.3 at. % tin, with the balance being copper and 
unavoidable impurities. A preferred range of compositions within the above 
composition range to obtain optimum hardness (strength) includes from 
about 0.0 at. % boron, about 4.4 at. % magnesium, about 3.3 at. % tin, 
with the balance being copper and unavoidable impurities. Thunder where 
optimum strength is the paramount consideration, the alloy would 
essentially be a ternary alloy of copper, magnesium and tin. 
The process for producing a high strength, high conductivity copper-based 
quaternary alloy having alloying additions of boron, magnesium and tin 
includes heating an alloy having a composition within the prescribed range 
to a temperature above about 680.degree. C., and preferably to a 
temperature in the range of 680-700.degree. C., to dissolve at least the 
majority of the boron, magnesium and tin in the copper, and then rapidly 
quenching the alloy from that temperature, as by ice water bath, to freeze 
these solute elements in an unstable solid solution with copper. The 
solutionizing heat treatment is generally carried out for 1-3 hours at 
temperature. The process further includes aging the thus-quenched alloy at 
a temperature in a range of about 350.degree. C. to about 500.degree. C. 
for a predetermined period of time, which will result in significant 
precipitation hardening, whereby intermetallic compounds of boron, copper, 
magnesium and tin will precipitate out of solid solution to harden and 
strengthen the alloy.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 presents electrical conductivity and corresponding hardness data for 
fourteen existing copper-based alloys that contain beryllium as a 
precipitation hardening agent. The data points were established using data 
published in STANDARDS HANDBOOK, Cast Copper Alloys and Copper Alloy 
Products, Part 7--Alloy Data, Revised 1978, Copper Development 
Association, Inc., New York, N.Y. The units of hardness used in FIG. 1 are 
HV (Hardness Vickers Scale), and the units of electrical conductivity are 
%IACS (International Association of Conductivity Standards). 
In the course of identifying precipitation-hardening copper alloys for the 
purpose of replacing such copper beryllium high-strength, high electrical 
conductivity alloys, the B--Cu--Mg--Sn alloys of the present invention 
were developed. As used herein, the term "high strength" generally refers 
to copper alloys having hardnesses comparable to or exceeding values in 
FIG. 1 with corresponding electrical conductivity, as demonstrated by the 
alloys of the present invention. Also, for the purposes of the disclosure 
of the present invention, the term "high electrical conductivity" is used 
to refer to a copper alloys having electrical conductivity comparable to 
or exceeding values in FIG. 1 with corresponding hardness, as also 
demonstrated by the alloys of the present invention. 
It is to be noted that the disclosure herein correlates hardness values 
with strength properties, with higher hardness values corresponding to 
higher strength alloys. The relationship between hardness and strength for 
beryllium copper alloys has been well established, and it will be readily 
apparent to those of skill in the art that a similar relationship between 
hardness and strength will exist for the beryllium-free copper alloys of 
the present invention. Thus, as used herein, the expression "hardness 
(strength)" is intended to indicate a direct measure of hardness, which 
thus provides an indirect measure or indication of the strength of the 
alloy. 
It was determined by the present inventors that the solute elements 
employed in the alloy of the present invention, namely boron, magnesium 
and tin, would have reducing solubility in copper with decreasing 
temperatures, and that stable intermetallic compounds such as CuMgSn and 
Cu.sub.4 MgSn can form in a copper-magnesium-tin ternary alloy. The 
presence of boron in the B--Cu--Mg--Sn quaternary alloy aids in the 
reduction of total solute in copper, thus improving the electrical 
conductivity of the alloy. 
The use of magnesium has traditionally been avoided, or is present in only 
very small amounts, in copper alloys that have been proposed for end uses 
in which good electrical conductivity is desired. This is evidenced in 
U.S. Pat. No. 4,388,130, which, as previously discussed, discloses that 
even small amounts of magnesium will significantly reduce conductivity. 
U.S. Pat. No. 4,434,016 discloses that the use of a very minor amount of 
magnesium, asserted to otherwise be a critical alloying element for the 
alloy disclosed therein, was not seen to reduce the electrical 
conductivity of that alloy. That patent, while recognizing that further 
enhancement of a property referred to as stress relaxation might be 
obtained with further increases in magnesium content, nevertheless 
discloses that the magnesium content should not exceed 0.5% by weight (1.3 
at. %), so as to avoid inferior strength-to-bend properties. 
In the present invention, the alloy contains a significantly greater amount 
of magnesium, preferably in a range of about 2.8-7.6 at. % while still 
achieving high electrical conductivity. Tin is also present in the 
copper-based alloy in a preferred range of about 2.1-4.3 at. %. Boron is 
added to enhance electrical conductivity in the preferred range of about 
0.0-2.9 at. %, and the balance is preferably copper and possibly trace 
amounts of unavoidable impurities. 
Within this overall preferred composition, an especially preferred 
composition of solute elements that enhances electrical conductivity is 
about 0.5 at. % boron, 4.8 at. % magnesium, 3.3 at. % tin, and the balance 
is copper and possibly trace amounts of unavoidable impurities. The 
preferred composition that enhances hardness (strength) is about 0.0 at. % 
boron, 4.4 at. % magnesium, 3.3 at. % tin, and the balance is copper and 
possibly trace amounts of unavoidable impurities. These alloys are 
expected to demonstrate superior strength and/or electrical conductivity 
properties, when produced in accordance with the process of the present 
invention. 
High strength, high conductivity copper alloys will generally attain their 
desired strength properties through precipitation hardening, also referred 
to as age hardening, or by dispersion hardening or cold working, or both. 
The quaternary alloy of the present invention is a precipitation 
hardenable alloy. A preferred process for producing this alloy includes 
the steps of giving the alloy a solution heat treatment, quenching the 
alloy at a sufficiently fast rate to freeze the majority of the solute 
elements, boron, magnesium and tin, in a solid solution with copper, and 
then heating the thus-quenched alloy to a temperature sufficient to 
precipitate out the intermetallic compounds formed with the solute 
elements. 
More specifically, the alloy is preferably subjected to an annealing or 
solutionizing heat treatment at or above 680.degree. C., and preferably in 
the range of about 680.degree. C. to 700.degree. C. The annealing is 
conducted for a length of time sufficient to bring all or the majority of 
the boron, magnesium and tin into solution with the copper. An appropriate 
duration may preferably be three (3) hours. The subsequent quenching of 
the alloy material is preferably a rapid quench, for example, by quenching 
in ice water. The aging or precipitation hardening step is preferably 
conducted at a temperature in the range of about 350 to 500.degree. C. 
The achievement of high electrical conductivity in as-cast parts, followed 
by solutionizing then aging is important to the nonferrous foundry 
industry. FIG. 2 is a graph which plots the electrical conductivity as a 
function of magnesium concentration and aging time. Boron and tin are held 
constant at 0.5 at. % and 3.3 at. % respectively. The balance is copper 
and unavoidable impurities. The highest electrical conductivity is 
achieved when the magnesium composition is 4.8 at. %. A preferred alloy 
for high electrical conductivity is 0.5 at. % boron, 4.8 at. % magnesium, 
and 3.3 at. % tin, balance copper and unavoidable impurities. Other tests 
on alloys of similar composition have demonstrated that the high levels of 
electrical conductivity are attainable with magnesium contents up to about 
5.2 at. %. The rate that electrical conductivity increases as a function 
of aging time at an aging temperature of 400.degree. C. is evident in FIG. 
2. The processing history prior to age hardening includes a solution heat 
treatment or anneal at 690.degree. C. for three hours, followed by a 
quench in ice water. 
FIG. 3 is a graph which plots the hardness (strength) as a function of 
magnesium concentration and aging time for the same alloys in FIG. 2. 
Boron and tin are held constant at 0.5 at. % and 3.3 at. % respectively. 
The balance is copper and unavoidable impurities. The highest hardness 
(strength) is achieved when the magnesium composition is 4.8 at. %. A 
preferred alloy for high hardness (strength) is 0.5 at. % boron, 4.8 at. % 
magnesium, and 3.3 at. % tin, balance copper and unavoidable impurities. 
The hardening behavior as a function of aging time at an aging temperature 
of 400.degree. C. is evident in FIG. 3. Note that the aging time for the 
highest hardness (strength) is 10 hours whereas in FIG. 2 the highest 
electrical conductivity is achieved after aging 100 hours. The processing 
history prior to age hardening includes a solution heat treatment or 
anneal at 690.degree. C. for three hours, followed by a quench in ice 
water. 
Very high hardness (strength) and good electrical conductivity are 
attainable in the alloys of the present invention. FIG. 4 is a graph of 
hardness (strength) as a function of varying amounts of boron. The age 
hardening in FIG. 4 is conducted at 400.degree. C., for the various noted 
times. 
The magnesium composition in the alloys used in obtaining the data present 
in FIG. 4 is 4.4 at. %, and tin is present at 3.3 at. %, and the balance 
is copper and unavoidable impurities. A preferred alloy when high hardness 
(strength) is desired is 0.0 at. % boron, 4.4 at. %; magnesium and 3.3 at. 
% tin. A hardness of HV 250 is achievable after aging for one hour at 
400.degree. C. with that alloy composition. The processing history prior 
to age hardening includes a solution heat treatment or anneal at 
696.degree. C. for three hours, followed by a quench in ice water. 
FIG. 5 is a graph of the electrical conductivity as a function of varying 
amounts of boron for the same alloys reported in FIG. 4. The composition 
of element magnesium is 4.4 at. % and element tin is 3.3 at. %, the 
balance being copper and unavoidable impurities. These data have the same 
thermal processing history and chemical compositions as those presented in 
FIG. 4. 
It can thus be seen that high electrical conductivity can be attained in 
such alloy compositions, particularly with increased aging time. Also, in 
viewing both FIGS. 4 and 5 together, it can be seen that the elimination 
of boron from the alloy can yield increased hardness (strength) 
properties, while the addition of relatively small amounts of boron will 
increase the electrical conductivity, with some possible sacrifice of 
hardness (strength) in the resulting alloy. It will be readily apparent to 
persons skilled in the art, upon reading this disclosure, that the alloy 
composition of these alloys can be modified within the ranges disclosed in 
order to achieve desired conductivity/strength combinations. 
Selection of the aging temperature can cause significant changes in the 
properties of the alloy. FIGS. 6 and 7 provide aging data at 400.degree. 
C., 425.degree. C., 450.degree. C. and 475.degree. C., for the preferred 
alloy composition: 0.5 at. % boron, 4.8 at. % magnesium, 3.3 at. % tin, 
balance copper with unavoidable impurities. The kinetics of the 
precipitation process are generally unacceptably slow at temperatures 
below 400.degree. C. FIG. 6 shows that, at 400.degree. C., an achievable 
electrical conductivity is 42% IACS after aging for 100 hours. At 
450.degree. C. and 475.degree. C. an achievable electrical conductivity is 
36% IACS after aging for one hour. 
FIG. 7 shows that, at 400.degree. C., an achievable hardness (strength) is 
HV 227 after aging for 10 hours. At 450.degree. C. and 475.degree. C., 
achievable hardnesses (strengths) after aging for one hour are HV 210 and 
HV 202, respectively. 
The process of the present invention thus preferably entails a 
solutionizing heat treatment and rapid quench, and subsequently aging the 
alloy at an aging temperature equal to or in excess of 400.degree. C., for 
example, 450.degree. C., and further entails aging the as-quenched alloy 
for a time preferably not exceeding one-hundred (100) hours, and, even 
more preferably, not exceeding about ten (10) hours. It is believed that 
aging temperatures in excess of 500.degree. C. will not yield hardnesses 
of above 200 HV, and therefore are not likely to be of any substantial 
commercial importance. The solutionizing heat treatment is preferably 
conducted at a temperature in a range of about 680-700.degree. C., for a 
time ranging from 1-3 hours. 
Alloys having compositions within the ranges disclosed herein, and 
processed in accordance with the method described above, have high 
strength and are highly electrically conductive. Accordingly, the alloys 
are promising candidates to be used in applications in which copper 
beryllium alloys have heretofore been used. 
It is to be understood that the foregoing description of the preferred 
embodiments of the present invention is for illustrative purposes only, 
and variations and modifications may become apparent to those of ordinary 
skill in the art upon reading this disclosure and viewing the figures 
forming a part of this disclosure. Such variations and modifications do 
not depart from the spirit and scope of the present invention, and the 
scope of the invention is to be determined by reference to the appended 
claims.