Phase Stabilities of Stable β– and Metastable β’– Cu₄Ti Intermetallic Compounds in The Cu–Ti–Ni System：a Density Functional Theory Study

Abstract

Cu–Ti alloy has two types of the precipitates such as meta–stable β’–Cu₄Ti and thermodynamically stable β–Cu₄Ti. In general, the precipitation hardening alloys can be strengthened more by additional cold working after aging. However, these precipitates dissolve in Cu matrix during cold working, lowering the conductivity considerably. To find a way to increase the stability of the precipitates during cold working, we have studied the stability of the second phase using the density functional theory (DFT) calculations. First, DFT simulation showed that the cohesive energy of β–Cu₄Ti is lower than that of β’–Cu₄Ti by –5kJ/mol, while β’–Cu₄Ti has lower interfacial energy due to the small lattice mismatch between Cu and β’–Cu4Ti. Second, we evaluated the change of cohesive and interfacial energies for two precipitates with Ni to confirm the effect of Ni on the stability of them in Cu–Ti alloy. As a result, Ni can lower the cohesive energies of these precipitates although there is no effect on the interfacial energies of them. Finally, Cu–4 at.％ Ti alloys with 0, 0.5 and 1 at.% Ni were cast by vacuum induction melting in Ar atmosphere. Experiments showed that the Ni addition to alloy encourages the discontinuous precipitation. After severe deformation, the precipitates in the alloy with Ni has more retained in matrix than alloy without Ni.