Interface-sensitive conversion-electron Mossbauer study of ion-beam mixing at the Fe-Al interface

Abstract

Ion-beam-induced atomic mixing and the effect of thermally activated transformations at the Fe-Al interface have been studied for the first time with use of the technique of conversion-electron ⁵⁷Fe Mossbauer spectroscopy (CEMS). A concept of interface-sensitive CEMS which exploits the deposition of a thin (~50-A°) layer of iron enriched to 95.45.% of ⁵⁷Fe at the interface between the aluminium substrate and an overlayer of natural iron (containing only 2.2% of ⁵⁷Fe) has been introduced and used in the present investigations. CEMS spectra of the as-deposited sample, fitted in terms of the distribution of hyperfine fields at ⁵⁷Fe nuclei show that not all the ⁵⁷Fe atoms in the interface region see the environment as in α-Fe but have one or more aluminium neighbors. The interface layers are transformed on bombardment with 100-keV Ar⁺ ions at a dose of ~10¹⁶ ions/cm² into a random metastable alloy having an average composition of Fe₅₅Al₄₅. ⁵⁷Fe atoms in this alloy experience a variety of environments ranging from dimers in Al matrix at one end to that typically characteristic of α -Fe at the other. This alloy does not show any phase precipitation on vacuum annealing at 300 and 400 °C for 20 min. However, on annealing at 500°C, a sudden precipitation of α-Fe and Fe₃Al phases is seen. On further annealing of the sample at 600 °C, Fe₃Al phase is seen to decompose to give iron clusters. These results of CEMS measurements have been confirmed by small-angle x-ray-diffraction studies. A non-interface-sensitive CEMS study has also been performed to investigate the dose dependence of ion-beam mixing.