Diffusion processes during heat treatment of Al–Cr–Fe thin films

Complex metallic alloys (CMAs) are a group of materials that have a giant unit cell and a high density of point defects. Among the more interesting phases is the hexagonal Al4Cr, containing 574 atoms, which can also be alloyed with iron to form a ternary Al4(Cr, Fe). One possibility to deposit such thin films is by sequential deposition of constituent elements and subsequent heat treatment in order to homogenize the depth profile. For this purpose bi- and trilayers were deposited in a triode sputtering apparatus. Heat treatment was performed in vacuum in the temperature range of 300–600 °C and annealing times up to 5 h. The evolution of depth profiling was evaluated by Auger electron spectroscopy (AES). It was also observed by indirect methods, such as X-ray diffraction (XRD), nanoindentation and electric resistivity measurement. A major change in depth profile was found to be accompanied by a change of present phases, a change in nanohardness and in electrical resistivity. These indirect methods are simple yet effective ways to follow the heat treatment. In simple bilayer systems, the quickest process is the diffusion of chromium into aluminum (occurs already at 400 °C), to be followed by the couple Al/Fe at 500 °C.