First-principles calculations on structural energetics of Cu-Ti binary system intermetallic compounds in Ag-Cu-Ti and Cu-Ni-Ti active filler metals

Structural, mechanical and thermodynamic properties, as well as the electronic structures of Cu-Ti binary system intermetallic compounds in Ag-Cu-Ti and Cu-Ni-Ti active filler metals were calculated systematically using a first-principles density functional theory (DFT). The calculated formation enthalpy index that all the Cu-Ti intermetallic compounds are thermodynamic stable from degradation to pure metals and the relationship between Cu content (x) and formation enthalpy (y) for tetragonal structure meets the function y=0.572+(-1.005/(0.048*sqrt(3.142/2)))*exp(-0.5*((x-47.167)/13.533)<^>2). The mechanical properties, including bulk modulus B, shear modulus G, Young's modulus E and Poisson's ratio v, and elastic anisotropy were derived from the elastic data C-ij. For the tetragonal Cu-Ti intermetallic compounds, the shear modulus G and Young's modulus E are negatively related to the formation enthalpy, while for the orthorhombic Cu-Ti intermetallic compounds, G and E are positively related to the formation enthalpy. Moreover, the elastic anisotropy increases in the following order: Cu4Ti < CuTi3 < Cu4Ti3 < Cu2Ti < CuTi < CuTi2 < Cu3Ti2. The thermodynamic properties were estimated from the electronic structures and elastic constants simultaneously, and the results found that Cu4Ti possess the best thermal conductivity and heat capacity among all the Cu-Ti intermetallic compounds, while CuTi3 shows the worst ones. Finally, the relationship between electronic structures and physical properties was discussed, and get the inference that for the Cu-Ti intermetallic compounds, the mechanical properties are positively related to the strength of the covalent bond, while the thermophysical properties are influenced by the ionic character and covalent character simultaneously and the ionic character shows the dominant role, therefore, CuTi and Cu4Ti3 show the strongest mechanical properties due to the strongest covalent character, while Cu4Ti shows the strongest thermal conductivity and heat capacity due to the strongest ionic character.