The phase transformation and its effects on properties of a Cu−0.12 wt% Zr alloy

• The phase transformation of Cu−0.12 wt% Zr alloy aged at 450 °C is studied in detail.
• The crystallography and morphology of Cu5Zr phase are investigated. Meanwhile, the space structure model of Cu5Zr phase is established.
• The strengthening mechanism of Cu−0.12 wt% Zr alloy aged at 450 °C for 8 h is discussed.

The phase transformation of a Cu−0.12 wt% Zr alloy during the aging process at 450 °C was investigated by Transmission electron microscopy (TEM) and high-resolution transmission microscopy (HRTEM) in the study. The strengthening mechanism was studied as well. The precipitation sequence of Cu−0.12 wt% Zr aged at 450 °C is supersaturated solid solution→Zr-rich atomic clusters→semi-coherent Cu5Zr phase. This transformation can be achieved by the accumulation of Zr-atom layers. The Zr-rich atomic clusters are composed of disk-shaped monolayers of Zr atoms. The Cu5Zr phase precipitate out in the nucleation of Zr-rich atomic clusters. The disc-like Cu5Zr phases with face-centered cubic structure are found in the matrix, whose habit plane is parallel to {111}Cu plane. The orientation relationship with [112]Cu∥[011]Cu5Zr,(111¯)Cu∥(111¯)Cu5Zr exists between the Cu and Cu5Zr phases. The space structure model of Cu5Zr phase can be established. With aging time, the hardness increases continuously first, and decreases slightly after 8 h, indicating overaging. The strengthening effect of Cu5Zr phase precipitated in aging at 450 °C for 8 h is calculated to be 5.8 MPa according to the Orowan strengthing, which is quite consistent with the experimental data.