Thermodynamic effects in the ion-beam mixing of Fe-Al and Mo-Cr multilayers

Ion-beam mixing of Fe-Al and Mo-Cr multilayers was studied by alternately sputtering with 5 keV Ar+ and then analyzing with Auger electron spectroscopy. The target temperature was variously room temperature, 450, 525, or 600 K. The two systems were chosen on the basis of having heats of mixing (Delta H-m) which were opposite in sign, namely H-m(1:1)=-0.25 eV for solid Fe-Al and Delta H-m(1:1)=+0.075 eV for solid Mo-Cr. (Delta H-m(1:1) applies to a 1:1 composition.) With Fe-Al a well-defined peak broadening was observed as the profiling temperature increased, while with Mo-Cr there was a slight narrowing of the peaks with increasing temperature. The results were analyzed in terms of numerical solutions of a diffusion equation which contained terms appropriate not only to ballistic motion but also to the motion of bombardment-induced residual defects. The latter were assumed to be in part ''chemically guided,'' as quantified with the parameter ''q(p)'' (q(p) similar to proportional to Delta H-m). A major difference between Fe-Al and Mo-Cr was found. The peak changes with Fe-Al scaled with what we have termed the effective diffusion coefficient D-eff = D-t(1 - q(p)/4). Here D-t is the total diffusion coefficient, i.e., it describes the total diffusivity for all mixing processes. With Mo-Cr, on the other hand, the peak changes scaled approximately with 1/q(p). The mixing behavior of Fe-Al, with negative Delta H-m, could thus be said to be mainly kinetic (with D-eff playing a major role), while that of Mo-Cr, with positive Delta H-m, is more nearly thermodynamic (with q(p) playing a major role). (C) 1996 American Institute of Physics.