ION-BEAM MIXING WITH CHEMICAL GUIDANCE .4. THERMODYNAMIC EFFECTS WITHOUT INVOKING THERMAL SPIKES

Various studies on ion-beam mixing suggest that the extent of mixing is sensitive to the sign and magnitude of the heat of mixing DELTAH(m). This suggests a role, not only for random motion, but also for chemical driving forces such that the total diffusion flux includes a modi ed Darken factor: 1 - alpha(i)(1 - alpha(i))[2h(m)p/RT(1 + p)] = - 1 - alpha(i)(1 -alpha(i))q. Here alpha(i) is the atomic fraction of component i, alpha(i)(1 - alpha(i))h(m) is the heat of mixing of a regular solution, and p is the ratio of the diffusivities for chemically guided defect motion to those for random motion of all types. We wish to evaluate the parameter p from a variety of recent experiments. We first consider the work of Marton, Fine, and Chambers on profiling multilayers of Ni-Ag and conclude that the narrowing of the Ag profiles, as temperature increases, is connected to a strongly reduced solubility. By fitting the profiles to solutions of the continuity equation corresponding to the above total diffusion flux, values of q are obtained. Provided T can be taken as the ambient temperature, values of p then follow: 0.04 +/- 0.01 at 300 K, 0.15 +/- 0.02 at 400 K, and 0.24 +/- 0.04 at 600 K. We then consider ion-beam mixing experiments based in some cases on solubility changes (11 examples), in some cases on interface broadening versus DELTAH(m) (40 examples), and in still other cases on interface broadening in the presence and in the supposed absence of defect-induced transport (5 examples). Considering all results, reasonably consistent values of p are obtained for temperatures ranging from 77 to 603 K. It is thus possible to understand a variety of experimental results relating to profiling and to ion-beam mixing in terms of chemical driving forces and, moreover, to do so without invoking thermal spikes. In our opinion this approach constitutes a more satisfactory framework for describing mixing (and related bombardment-induced solid-state processes) than that in which thermal spikes are assumed.