Microstructure formed by eutectic reaction in a binary Cu–12.3Zr alloy

A Cu–Zr alloy with a Zr concentration of 12.3 mass% was used to examine experimentally the morphology and crystallography of the microstructure formed by the eutectic reaction for the Cu phase and the most Cu-rich intermetallic compound in the binary Cu–Zr system. A typical lamellar microstructure with interlamellar spacings around 0.4 μm was developed during solidification after induction melting. According to the observations by transmission electron microscopy, the lamellar microstructure consists of the Cu and Cu9Zr2 phases. Here, the space group and Bravais lattice of the Cu9Zr2 phase are considered as F4̄3m and face-centered cubic, respectively. In the lamellar microstructure, the parallel orientation relationship with [100]Cu//[100]Cu9Zr2 and [010]Cu//[010]Cu9Zr2 exists between the Cu and Cu9Zr2 phases, and the Cu/Cu9Zr2 interface is almost parallel to the {100} planes of these phases. During annealing up to 60 h after solidification, the lamellar microstructure changes into a granular microstructure at 1123 and 1223 K due to discontinuous coarsening and spheroidization, whereas it is rather stable at 1023 K. For the specimen annealed at 1223 K for 60 h, the concentration of Zr in the Cu9Zr2 phase was determined to be 24.3±0.2 mass% by electron probe microanalysis. This value is close to the Zr concentration of 24.18 mass% for stoichiometric Cu9Zr2.