Phase transformations after long-time annealing in metastable Fe-Cr alloy films prepared by pulsed laser deposition

摘要

Metastable Fe-Cr alloy films of various composition prepared by cross-beam pulsed laser deposition using two different procedures are investigated by wide-angle X-ray scattering. Depending on the Fe-Cr composition of the samples in an extended range, a body-centered cubic (bcc) phase or metastable phases with body-centered tetragonal (bct), face-centered orthorhombic (fco) or primitive orthorhombic (po) and primitive cubic (pc) lattices are formed in the films prepared by simultaneous co-deposition of Fe and Cr. In the films produced by layer-by-layer deposition of thin separate Fe and Cr layers (thickness of about 1 nm), only bcc and bct Fe-Cr phases were observed. A long-time annealing (approx 50 h) at a temperature of 425 deg C near the low-temperature existence limit of the sigma-phase under equilibrium conditions followed by slow cooling (rate approx 0.5 deg C/min) has been performed and various phase transformations were observed. In addition to known equilibrium and metastable Fe-Cr crystalline phases (mainly bcc and bct phases in the films prepared by layer-by-layer technique and bcc, bct and sigma-FeCr phases in co-deposited films), a new metastable Fe-Cr superstructure characterised by a primitive tetragonal lattice with parameters a and c of about 0.57 and 0.63 nm, respectively, has been identified. It is shown that the formation of alpha"-crystallites with preferred orientation in the metastable Fe-Cr alloy films during dedicated long-time annealing gives rise to a spatially periodic modulation of chemical composition resulting in the formation of multilayers with periods of one or a few atomic monolayers of individual Fe and Cr components.