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

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 M\"ossbauer 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 Fe₅₅{Al}₄₅$. $⁵⁷Fe$ 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.