In-situ transmission electron microscopy observations of nucleation and growth of intermetallic phases during reaction of Ni(V)/Al multilayers

The Ni(V)/Al multilayers, known as NanoFoil® (bilayer thickness of λ ~ 50 nm, total thickness of 80 μm), were used for in-situ transmission electron microscopy heating experiments carried out in order to establish the nucleation sites and growth modes of the intermetallic phases formed during their reaction. The performed observations showed that while the intermediate Al3Ni and Al3Ni2 phases nucleate at Ni(V)/Al and Ni(V)/Al3Ni interfaces, respectively, the final NiAl nucleates throughout the intermixed Al3Ni and Al3Ni2 (original Al) layer. The growth of the Al3Ni crystallites proceeds by a moving planar front along the Al layer changing the Ni(V)/Al into Ni(V)/Al3Ni multilayers. The growth of Al3Ni2 crystallites resembles the standard coarsening of rounded precipitates. The growth of final NiAl phase is much faster than Al3Ni2 and results in formation of a string of cubical crystallites separated by a discontinuous layer of fine Al8V5 crystallites. The described series of phase transformations might be considered as a model for solid state diffusion controlled reaction of Ni/Al multilayers.