Micro-XRF and micro-XAFS studies of an Al matrix Fe-Ni composite

We report on the distribution and local coordination of Fe and Ni in an Al matrix Fe-Ni composite, by means of X-ray Fluorescence mapping (XRF), micro- (μ-) and conventional Extended X-ray Absorption Fine Structure (EXAFS) spectroscopies. The μ-XRF maps reveal that Fe segregates and forms Fe-rich islands which are depleted of Ni. The combined μ-EXAFS and EXAFS results reveal that both metals are bonded only to Al. More specifically, the Fe-rich islands are identified as FeAl3 microcrystallites while in the Fe-poor regions, Fe belongs to an intermetallic FeAl phase. The bonding environment of Ni is also modified due to the variations in the distribution of the metals. In the region with high Ni concentration, i.e. Fe-poor regions, the Ni atoms are bonded to 10.5 ± 1.1 Al atoms that substitute Ni in fcc Ni. On the contrary, in the Ni-poor regions, where the Fe concentration exhibits maxima, the number of Al atoms in the first nearest shell of Ni is equal to 7.8 ± 0.9, i.e. in this region, Ni forms intermetallic NiAl. Finally, the atomic percentage of the Fe and Ni atoms that belong to the Fe-rich and Fe-poor islands is determined from the Fe-K and Ni-K edge EXAFS analysis. The majority of the Fe atoms (∼80 at%) belongs to the FeAl3 microcrystallites, embedded into a FeAl matrix. On the contrary, the same atomic percentage of Ni atoms (∼50 at%), occupies sites in both the NiAl regions as well as in the matrix of the (Ni, Al) solid solution.