Precipitation Hardening Copper Alloys
5.1.6.1 Copper-Beryllium Alloys (Beryllium Bronze)
The cause for precipitation hardening of CuBe materials is the rapidly diminishing solubility of beryllium in copper as temperature decrease. As the phase diagram for CuBe shows, 2.4 wt% of Be are soluble in Cu at 780°C (Fig. 5.28). In this temperature range annealed CuBe alloys are homogeneous(solution annealing). The homogeneous state can be frozen through rapid cooling to room temperature (quenching). Through a subsequent annealing at 325°C the desired precipitation hardening is achieved which results in a significant increase in mechanical strength and electrical conductivity of CuBe (Table 5.17). The final strength and hardness values depend on the annealing temperature and time as well as on the initial degree of cold working (Table 5.18) and (Figs. 5.29 - 5.31).
Fig. 5.28: Phase diagram of copperberyllium with temperature ranges for brazing and annealing treatments
As precipitation hardening alloys CuBe materials, mainly CuBe2 and CuBe1.7 have gained broad usage as current carrying contact springs because of their outstanding mechanical properties. Besides these CuCo2Be and CuNi2Be, which have medium mechanical strength and a relatively high electrical conductivity, are also used as contact carrier materials. After stamping and forming into desired contact configurations these CuBe materials are then precipitation hardened. CuBe alloys are available as semi-finished materials in a variety of cold work conditions. They can also be supplied and used in the already precipitation hardened condition without significant strength losses. In this case the hardening was already performed at the alloy producer.
Since Beryllium is rated as a carcinogen by the European regulation EU-67/548, it has been tried to reach the application properties of the well established CuBe1.7 and CuBe2 alloys with a lower Be content. The development efforts for alternate precipitation hardening materials without toxic and declaration requiring additive materials, for example CuNiCoSi, are aimed at the replacement of CuBe.
Fig. 5.29: Precipitation hardening of CuBe2 at 325°C after different cold working
Fig. 5.30: Precipitation hardening of CuBe2 (soft) at 325°C
Fig. 5.31: Precipitation hardening of CuBe2 (half hard) at different annealing temperatures
Table 5.17: Physical Properties of Selected Copper-Beryllium Alloys (2 Teile!)
Table 5.18: Mechanical Properties of Selected Copper-Beryllium Alloys (2 Teile!)
