Naturally Hard Copper Alloys
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 (Brasses)
Copper-zinc alloys are widely used as contact carrier materials in switching devices for electrical power engineering because of their high electrical conductivity, the higher mechanical strength combined with good formability compared to pure copper (Tables 5.7 and 5.8), and at the same time their reasonable economic costs. Especially suitable are the brasses with up to 37 wt% Zn content which are according to the phase diagram all made up from the " -phase of the CuZn system (Fig. 5.5). it is important to note the strong dependence of the electrical conductivity and mechanical strength on the zinc content (Fig. 5.6).
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.
Fig. 5.5: Phase diagram of copper-zinc for the range of 0-60 wt% zinc
Fig. 5.6: Mechanical properties of brass depending on the copper content (after cold working of 0 and 50%)
Fig. 5.7: Strain hardening of CuZn36 by cold forming
Fig. 5.8: Softening of CuZn36 after 3 hrs annealing after 25% 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 components such as connectors, switches, and relays (Tables 5.9 and 5.10). Besides these other alloys such as CuSn4 and CuSn5
