Interface-sensitive conversion-electron Mössbauer 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 $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy (CEMS). A concept of interface-sensitive CEMS which exploits the deposition of a thin (\ensuremath{\sim}50-A\r{}) 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 \ensuremath{\alpha}-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 \ensuremath{\sim}${10}^{16}$ 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 \ensuremath{\alpha}-Fe at the other. This alloy does not show any phase precipitation on vacuum annealing at 300 and 400 \ifmmode^\circ\else\textdegree\fi{}C for 20 min. However, on annealing at 500 \ifmmode^\circ\else\textdegree\fi{}C, a sudden precipitation of \ensuremath{\alpha}-Fe and ${\mathrm{Fe}}_{3}$Al phases is seen. On further annealing of the sample at 600 \ifmmode^\circ\else\textdegree\fi{}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.