*4.2. The Formation and Control of Coarse Particles in Cu-Cr-Zr System Alloys*

The maximum solubility of the chromium and zirconium in equilibrium is 0.7 wt.% and 0.15 wt.% [40], respectively, and the solubility of chromium and zirconium in copper is less than 0.4 wt.% Cr and 0.15 wt.% Zr at 960 ◦C [41]. Since the composition of Cu-Cr-Zr-Ni-Si alloy is close to or above the solubility limit at the solution temperature, the coarse particles are almost formed during the solidification process. As can be seen from the results of this work, high-temperature solution treatment (960 ◦C for 1 h) failed to eliminate these coarse particles. The result that the dissolution of second particles was very limited by prolonging the high-temperature solution treatment can be confirmed by Appello [41]. Huang [27] suggested that the Cr-rich phase first solidified molten melt, and Zr-rich phase solidified at the interface of the Cr-rich phase and melt. In other words, the Cr-rich phase became as the solidified core of the Zr-rich phase. Spaic et al. [42] revealed that the Cr-rich phase was the eutectic reaction product formed in the solidification process, which explained the resistance against solution treatment.

It has been reported [43,44] that the second phase particles ranging from 1 nm to 100 nm have a significant strengthening effect on the mechanical properties of the material, so the coarse particles do not contribute to the mechanical strength of the Cu-Cr-Zr-Ni-Si alloy. Holzwarth [45] studied the mechanical properties of the Cu-Cr-Zr alloy under different heat treatment conditions and revealed that the differences in the density and spacing of the coarse particles had little effect on the mechanical properties. Furthermore, due to the difference in the deformation ability of coarse particles and the matrix, it is easy to generate stress concentration at the interface of coarse particles and matrix leading to the occurrence of fatigue and fractures in the hot-working or cold-working process. In addition, the formed coarse particles deplete the amount of solute elements dissolved in the matrix, resulting in the reduction in the precipitation strengthening effect. Therefore, controlling the amount and size of the coarse particles is essential for the precipitation-strengthened Cu-Cr-Zr system alloys. Effective measures can be taken as follows:

