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 ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer spectroscopy (CEMS). A concept of interface-sensitive CEMS which exploits the deposition of a thin (∼50-Å) layer of iron enriched to 95.45.% of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ at the interface between the aluminium substrate and an overlayer of natural iron (containing only 2.2% of ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$) 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 ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ nuclei show that not all the ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ 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 ${\mathrm{Ar}}^{+}$ ions at a dose of ∼${10}^{16}$ ions/${\mathrm{cm}}^2$ into a random metastable alloy having an average composition of ${\mathrm{Fe}}_{55}$ ${\mathrm{Al}}_{45}$. ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ 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 ${\mathrm{Fe}}_3$Al phases is seen. On further annealing of the sample at 600 °C, ${\mathrm{Fe}}_3$Al 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.