The specific microstructure in the Ni5Zn21 layer was successfully characterized using FIB (Focused Ion Beam) microscopy system. Ni5Zn21 was the only reaction product formed in Sn-9 wt%2n/Ni interfacial reactions at 170 degrees C and 250 degrees C and various reaction times. In the initial stage of liquid-state reactions the Ni5Zn21 layer was composed of three sublayers with different microstructures. The middle sublayer (Layer II) with regular and columnar-grains was dominant and its thickness increased with reaction time. At the interface adjacent to Ni, one thin sublayer (Layer I) consisted of monolayer of column-shaped grains. The sublayer near the solder exhibited equiaxed grain structure. A new sublayer (Layer III) containing a small amount of Sn was formed between Layer II and Layer D after aging for 6 h. It had a columnar-grained structure. The thickness of the Layer III increased gradually and became dominant in the Ni5Zn21 layer. In the solid-state reactions two sublayers (Layers I and II) were present in the Ni5Zn21 layer. Layer I was consumed to form Layer II. Series of complex microstructure transformations were revealed and discussed in this study. (C) 2013 Elsevier B.V. All rights reserved.