Invited Lecture
Atomic diffusion and phase transformations in nanolayered films

Nanolayered films offer model systems for studying atomic diffusion and phase transformations over a range of temperatures and composition gradients. This presentation describes atomic intermixing and intermetallic phase formation in Ni/Al multilayer films by summarizing more than ten years of experiments and molecular dynamic simulations. Solid state diffusion was characterized using differential scanning calorimetry, nanocalorimetry, and uniform heating of Ni/Al multilayers; solid state phase formations were characterized using nanocalorimetry combined with in situ dynamic TEM (DTEM). As expected, solid state intermixing is dominated by grain boundary diffusion at low temperatures and by bulk diffusion at higher temperatures. A more novel finding is that solid state, intermetallic phase formations are inhibited by steep composition gradients. Liquid state diffusion was studied by varying the maximum temperature at which reactions self-propagate within 20mm thick Ni/Al nanolayered foils. Inert layers were added to the foils to lower the maximum reaction temperatures while not interfering with the Ni and Al intermixing. Liquid state phase formation was characterized using in situ XRD and in situ DTEM studies, as well as isothermal and isochronal simulations. Both the experiments and the simulations show that liquid state diffusion depends strongly on the temperature of mixing, primarily due to the formation of intermetallic phases at the Ni/Al interfaces. Only the NiAl phase is seen to nucleate from the Ni-Al liquid solutions, and as in the case of solid state intermixing, steep composition gradients are shown to inhibit nucleation.