Phase formation of B2-RuAl during annealing of Ru/Al multilayers

The formation of B2-RuAl from Ru/Al multilayers (MLs) with an average MLs composition of Ru47Al53 and modulation periods Λ up to 22.4 nm was studied by in-situ X-ray diffraction (XRD), differential scanning calorimetry, scanning electron microscopy and transmission electron microscopy. The as-deposited MLs with Λ < 4.5 nm grow epitaxially with relatively small roughness of the atomic layers. At higher Λ values, the epitaxy is lost and polycrystalline MLs with strongly distorted atomic layers develop during deposition. In-situ high-temperature XRD demonstrated that Λ influences the phase evolution and kinetics during annealing. At annealing temperatures TA < 673 K Al diffuses into the Ru layers leading to the formation first of Ru(Al) solid solution. At TA > 823 K the ordered B2-RuAl phase is formed via a diffusion-controlled nucleation. The RuAl grain growth kinetics accelerates with increasing Λ.