Peculiarities of Fe-Ni alloy crystallization and stability inside C nanotubes as derived through electron microscopy

Alloys of the Fe-Ni system encapsulated into C nanotubular fibers during hydrogen plasma enhanced chemical vapor deposition on Invar and Inconel alloy substrates are analyzed in detail using electron microscopy. High-resolution lattice imaging, electron diffraction, energy dispersion X-ray analysis and energy filtering electron microscopy are all utilized to get insights into the structure, chemistry and stability of tip-end cone-like nanoparticle- or nanowire-like fillings. It is documented that the alloys exhibit highly inhomogeneous chemical compositions ranging from Ni-rich to Fe-rich. The Fe/Ni atomic ratios vary widely between various fillings and even within domains of individual nanostructures. Fe-depletion and Ni-enrichment are typically observed as the filling penetrates deeper into the tube body. As a rule, the alloys crystallize in a standard face-centered cubic (fcc) lattice, a = 3.59 A, most common for the Fe-Ni system within the composition range studied, albeit several fragments exhibit long-period fcc lattices, a = 5.43 A, additionally possessing superstructures. The peculiarities of filling crystallization and intra-stability under heating are thoroughly explored and discussed in the light of morphology, chemistry and structure variations.