Microstructure evolution during solid-state reactions in polycrystalline Nb/Al and Ti/Al multilayer thin films

摘要

The microstructural changes that occur during the reaction of sputter-deposited Nb/Al and Ti/Al multilayer thin-films with bilayer thicknesses ranging from 10 nm to 333 nm have been studied. The films were deposited with an overall stoichiometryof XAl{sub}3 (X = Nb,Ti) and subsequently annealed to different stages of the reaction in a differential scanning calorimeter (DSC). Data obtained from cross-sectional transmission electron microscopy (XTEM), and in situ synchrotron X-ray diffraction(XRD) experiments have provided evidence for a two-stage reaction mechanism for the formation of NbAl{sub}3. Microscopy results from a film with a bilayer period of 333 nm showed a microstructure that was consistent with two-dimensional growth in theplane of the interface. A uniform, 10 nm thick continuous layer of the product phase was formed followed by growth normal to the interface that initially consisted of larger, faceted grains. By the end of the reaction, an equiaxed NbAl{sub}3 grainstructure was observed. High resolution elemental mapping using a scanning transmission electron microscope (STEM) revealed penetration of Nb into the Al layer and enhanced growth in regions where Al grain boundaries intersected the interface.Characterization of microstructure evolution in the Ti/Al system was complicated by the formation of two metastable structures consisting of cubic L1{sub}2 followed by tetragonal DO{sub}23, and finally the equilibrium, tetragonal DO{sub}22 structure.However, the metastable phase transition temperatures were clearly isolated using the in situ XRD technique.