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Alloys like brasses (CuZn), tin bronzes (CuSN), and German silver (CuNiZn), for which the required hardness is achieved by cold working , are defined as naturally hard alloys. Included in this group are also the silver bronzes (CuAg) with 2 – 6 wt% of Ag.
====<!--5.1.4.1-->Copper-Zinc Alloys Test (Brasses)====
<sup>1)</sup> t: Strip thickness max. 0.5 mm
The main disadvantages of these alloys are with increasing zinc content , the also increasing tendency towards tension crack corrosion and the poorer stress relaxation properties , compared to other copper alloys.
One of the special brass alloys used as a contact carrier material is CuSn23Al3Co. This material exhibits significantly higher mechanical strength
than the standard brass alloys. Even so , this material is a naturally hardening alloy, a suitable heat treatment allows to further increase its strength.
<xr id="fig:Phase_diagram_of_copper_zinc_for_the_range_of_0_60_wt_zinc"/><!--Fig. 5.5:--> Strain hardening of Ag 99.95 by cold working
====<!--5.1.4.2-->Copper-Tin Alloys (Tin Bronze)====
Because of their good elastic spring properties and formability , the copper-tin alloys CuSn6 and CuSn8 are standard materials for spring contact elements in electrome-chanical electromechanical components , such as connectors, switches, and relays <xr id="tab:Physical Properties of Copper-Tin Alloys"/><!--(Tab. 5.9)--> and <xr id="tab:Mechanical Properties of Copper-Tin Alloys"/><!--(Tab.5.10)-->. Besides these , other alloys such as CuSn4 and , CuSn5 and the multi-metal tin bronze CuSn3Zn9 have significant usage – mainly in North America. <!--5.10-->
<xr id="fig:Phase diagram of the Cu-Sn system for the range of 0 – 30 wt% Sn"/> shows the copper rich side of the phase diagram for the CuSn system. The mechanical property values achieved by cold forming are superior to these of the brass alloys <xr id="fig:Mechanical properties of tin bronze depending on the tin content (cold working 0 and 50%)"/><!--(Fig. 5.11)-->. They increase significantly with increasing Sn content. The work hardening and softening behavior are shown for the example of CuSn8 in <xr id="fig:Strain hardening of CuSn8 by cold working"/><!--Figures 5.12--> and <xr id="fig:Softening of CuSn8 after 3 hrs annealing after 50% cold working"/><!--Fig. 5.13-->. The stress relaxation properties for CuSn alloys are good for up to 100°C, deteriorate however quickly for temperatures above 150°C.
====<!--5.1.4.3-->Copper-Nickel-Zinc Alloys (German Silver)====
Despite its lower electrical conductivity, the good spring properties, high corrosion resistance, and the good workability make copper-nickel-zinc alloys a frequently used spring contact carrier in switches and relays. As illustrated in the phase diagram , the most commonly used materials are in the α -range which means that they are single-phase alloys <xr id="fig:Copper rich region of the ternary copper-nickel-zinc phase diagram with indication of the more commonly available german silver materials"/><!--(Fig. 5.14)-->. The formability and strength properties of german silver are comparable to those of the copper-tin alloys. The work hardening and softening behavior is illustrated on the example of CuNi12Zn24 in <xr id="fig:Strain hardening of CuNi12Zn24 by cold working"/><!--Figures 5.15--> and <xr id="fig:Softening of CuNi12Zn24 after 3 hrs annealing after 50% cold working"/><!--5.16-->.
The relaxation behavior of Cu-Ni-Zn alloys is superior to the one for the tin bronzes. Additional advantages are the very good weldability, brazing
properties, and the high corrosion resistance of these copper-nickel-zinc alloys.
====<!--5.1.4.4-->Copper-Silver-(Cadmium) Alloys (Silver Bronze)====
Besides the low-allowed CuAg0.1 , other copper materials with higher silver contents (2-6 wt%) are also used as contacts carrier materials. Some of them contain additionally additional 1.5 wt% Cd. The phase diagram <xr id="fig:Phase diagram of copper-silver for the range of 0 – 40 wt% silver"/><!--(Fig. 5.17)--> shows , that in principle the CuAg alloys can be precipitation hardened, but the possible increase in mechanical strength is rather small.
Copper-silver alloys have good spring properties and compared to other spring materials have a high electrical conductivity <xr id="tab:tab5.13"/> <!--(Tab. 5.13)--> and <xr id="tab:tab5.14"/><!--(Tab. 5.14)-->. The mechanical strength values in the strongly worked condition are comparable to those of the copper-tin alloys. Work hardening and softening behavior are shown for the example of CuAg2 [[#figures5|(Figs. 13 – 15)]]<!--(Figs. 5.18 – 5.20)-->. For the relaxation behavior , the silver bronzes are superior to German silver and tin bronze.
Because of their good spring properties combined with high electrical conductivity , silver bronzes are suitable for the use contact springs in relaysunder higher current loads. Taking advantage of their high temperature stability , they are also used as current carrying contacts in high voltage switchgear and as electrode material for resistance welding.
