On formation of intermetallic compounds at the Copper /Niobium-Titanium interface

Temperature dependence of the formation of harmful, very brittle Cu-Ti intermetallic phases at the Cu/Nb-Ti interface was studied by annealing the samples for 2 h. Lamellar structured Cu/Nb-Ti diffusion couples were created and subjected to hot upsetting at 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, 800 °C, 825 °C, 850 °C, 875 °C and 900 °C. It was found that at and below 800 °C, the Cu/Nb-Ti interface is free of intermetallic compounds, at least with a micron resolution. However, at 825 °C, the first intermetallic phases are formed with a thickness above 1 µm, partly covering the original Cu/Nb-Ti interface. It is shown that the thickness and amount of intermetallic phases rapidly increase with increasing temperature. From extrapolating the measured thickness–temperature data to zero thickness, it was found that the intermetallics appear with a size of at least a micron at about 820 °C within 2 h of holding time. However, diffusional calculations provided a much lower thickness than expected. Thermodynamic calculations show that the first liquid phase appears in the ternary Cu-Nb-Ti system at about 819 °C. Above this temperature, a catastrophic formation of intermetallics is expected. This result agrees with the 820 °C value found empirically as the starting temperature of micron-sized intermetallic formation. [Display omitted] •Diffusion couple experiments in Cu/Nb-Ti / 2 h / 300–900 °C.•No intermetallic found with a micron resolution at and below 800 °C.•Diffusion calculation predicted intermetallics of 200 nm thickness / 800 °C / 2 h.•At and above 825 °C intermetallics are formed above micron thickness.•This is due to the formation of the first liquid phase in Cu-Nb-Ti at 819 °C.