Construction of metallurgical interface with high strength between immiscible Cu and Nb by direct bonding method

Cu/Nb joints are promising to be used in resistance to radiation damage because of their outstanding properties. Due to the immiscibility in the Cu-Nb system, metallurgical interface is difficult to construct directly between Cu and Nb. In this paper, a direct bonding method was used to construct the metallurgical interface directly between Cu and Nb rods and prepare Cu/Nb joints. The key point of the method is to anneal Cu-Nb assemblies at the temperature between 92% and 98% of the melting point of Cu (T_mCu = 1356 K) for 3 h under a pressure of 106 MPa. The maximum tensile strength and bending strength of the obtained Cu/Nb joints are 222 MPa and 47 MPa, respectively. The Micro-test results show that a diffusion layer with a thickness of about 36 nm forms between Cu and Nb, indicating that a metallurgical interface is successfully constructed between Cu and Nb. The high temperature structure of Cu at the temperature close to the T_mCu provides diffusion paths probably. In addition, a thermodynamic model was established for the direct bonding of Cu and Nb. Through the thermodynamic calculations and differential scanning calorimetry (DSC) tests, the storage energies in the Cu and Nb rods are proved to serve as the thermodynamic driving force for the diffusion between Cu and Nb. - Highlights: • Direct bonding method is a feasible way to construct the metallurgical interface directly. • The key point of the direct bonding method is the annealing temperature (between 0.92 TmCu-0.98 TmCu). • A thermodynamic model for the direct bonding of the immiscible metals was established. • Storage energies of Cu and Nb rods serve as the driving forces for the diffusion and alloying of the Cu-Nb system.