A X-ray view on solutal melting in Cu-Pd

Kurzfassung

We present solute-driven, i.e. solutal melting, experiments on isomorphous Cu-Pd melting couples to better understand the kinetics of the solid-liquid (s-l) interface evolution. In-situ monitoring of the melting process with a sub-second time-resolution by X-radiography (XRR) is key to identify the evolution of the concentration fields during the process. The samples used, were concentric. They have been fabricated by casting Cu into a BN-coated steel-mold under an inert Argon atmosphere. The mold contained a short 3 mm diameter Pd-rod at its center around which the Cu did freeze. For the XRR experiments thin samples of 200 μm and 500 μm thickness, respectively were manufactured by slicing 1 mm discs from the as-cast Cu-Pd rod and grinding. The samples have been processed in an isothermal furnace by rapid heating with approximately 19.5 ℃⁄min followed by annealing at holding temperatures slightly above the melting point of Cu (1085 ℃) but far below the melting point of Pd (1555 ℃). Consequently, the thermally melted liquid Cu was in contact with solid Pd. Since the liquid and solid compositions at the s-l interface were out of equilibrium, solutal melting of Pd was uniformly observed in real-time. At the same time fast diffusion of Cu into solid Pd was observed. We compare our data with observations made on Cu-Ni and Ag-Au isomorphous systems and solutal melting experiments. In our study particular attention was paid to the effect of temperature and specimen height on the s-l interface velocity. Furthermore, post-mortem analysis was performed using SEM to investigate the radial composition gradients of Cu in the re-solidified Cu-Pd alloy for comparison with the in-situ measurement results and to serve for calibration of the real-time experiments.