Ion beam mixing at high and low temperatures

The relevance of the Darken analysis to ion mixing was first pointed out by Johnson et al. To experimentally test this model, ion beam mixing in Cu bilayer systems which have little mutual solid solubility was measured at 6 K. The systems Cu-Mo, Cu-Nb and Cu-Bi were selected. Mo and Nb are particularly well suited for this comparison since they are adjacent in the periodic table, have similar melting temperatures and sublimation energies, and are the same crystal structure, but Mo has a high positive heat of mixing with Cu, whereas Nb has essentially zero heat of mixing with Cu (referred to liquid states). It was found that mixing in Cu-Mo is essentially absent whereas mixing is observed in Cu-Nb. Cu-Bi, which also has a small heat of mixing, mixes rather efficiently. The mixing in Cu-Nb and Cu-Bi is about 1/3 and 5 times as large as in pure Cu-Cu bilayer specimens, respectively. These relative magnitudes of mixing are consistent with the dependence of mixing on vacancy mobilities. Aside from the value the heat of mixing, it is noted that Cu forms miscible liquids with Nb and Bi, but Cu and Mo are immiscible