Compound Growth due to Isothermal Annealing of Cu-Clad Al Wire

Abstract

To better understand the annealing mechanisms that occur in Cu-clad Al (CA) wire, the solid-state reactive diffusion between Cu and Al was experimentally examined by a metallographical technique. The hard CA (HCA) wire was prepared by drawing to decrease diameter from 10 mm to 1.5 mm, and then annealed at 250°C (523 K) for 3 h (10.8 ks). The annealed HCA wire is merely called the ACA wire. The HCA and ACA wires were isothermally annealed at temperatures of 150–270°C (423–543 K) for various periods of 12–960 h (43.2 ks to 3.46 Ms). Due to isothermal annealing, the intermetallic layer composed of the θ, η₂, δ, γ₁ and α₂ phases is produced at the original Cu/Al interface in both the HCA and ACA wires. The total thickness of the intermetallic layer is proportional to a power function of the annealing time. The exponent of the power function is 0.23–0.44 for the HCA wire and 0.33–0.53 for the ACA wire. Thus, boundary diffusion as well as volume diffusion contributes to the layer growth. Furthermore, the overall growth rate of the intermetallic layer is slightly greater for the HCA wire than for the ACA wire. Since the exponent is smaller for the HCA wire than for the ACA wire, the contribution of boundary diffusion is greater for the former than for the latter. The greater contribution of boundary diffusion may be the reason why the overall layer growth takes place faster in the HCA wire than in the ACA wire.

Fig. 5 The total thickness l of the IMC layer versus the annealing time t shown as cross, open rhombuses, triangles, squares and circles for T = 423, 453, 483, 513 and 543 K (150, 180, 210, 240 and 270°C), respectively: (a) HCA and (b) ACA.