Interfacial reactions between nickel–chromium alloys and aluminum

We have studied the interface structure and reactions of Ni–Cr alloy and Al thin film bilayers using glancing‐angle x‐ray reflectivity and extended x‐ray‐absorption fine structure techniques. The Al/Ni–Cr bilayers were prepared by evaporation in ultrahigh vacuum with a clean interface (Clean) or an interface exposed to 600 Langmuir oxygen (O‐exposed). No interfacial reaction is observed at room temperature in either the Clean or the O‐exposed samples. Upon annealing for 10 minutes, initial reaction occurs at about 250 °C in the Clean sample and 310 °C in the O‐exposed sample. The oxygen impurity at the interface retards the initial reaction and also modifies the diffusion behavior. In both Clean and O‐exposed systems, the reaction starts between Ni and Al forming NiAl3. The reaction results in a Cr‐rich region near the interface which transforms to the bcc structure. Only at higher temperatures do Cr and Al react to form CrAl7. The role of various diffusion channels and oxygen impurity at the interface in...