Study of decagonal approximant and γ-brass-type compounds in Al-Cr-Fe thin films

This paper reports the preparation conditions and structure characteristics of Al-Cr-Fe very thin films (10-30 nm) obtained by the flash evaporation technique. The films are either amorphous or crystallized, depending on the thickness of the sample and temperature of the substrate. Annealing of amorphous films leads to crystallization of intermetallic phases that are all linked with quasicrystals. In particular, we have identified by transmission electron microscopy the following structures: body-centered-cubic (bcc) gamma-brass phase, monoclinic lambda-Al-13(Cr,Fe)(4) phase, and orthorhombic O-1-phase, all of them already observed in this system, together with four new structures, i.e., a face-centered-cubic (fcc) gamma-brass phase (superstructure of the bcc phase), monoclinic lambda'-phase (related to the lambda-phase) and two orthorhombic phases (1/1/; 1/1) and (1/0; 2/1) approximants of the decagonal phase). In this study, we point out the occurrence of twin defects of the lambda-Al-13(Cr,Fe)(4) phase. Films prepared directly in the crystalline state comprise the O-1 approximant. Electron energy loss spectroscopy measurements show that all films are not oxidized except for the presence of a native oxide layer that forms in ambient atmosphere with a thickness that cannot exceed 0.3 nm. Optical properties were investigated and show that films need to be large enough (>30 nm) to reproduce the properties of bulk alloys. Finally, contact angle wetting measurements reveal that the presence of such films on a substrate, even at very low thickness, considerably decreases the wetting behavior by water.