Microstructure evolution of Ni5Zn21 intermetallic compound at Sn–9 wt%Zn/Ni interface

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%Zn/Ni interfacial reactions at 170 °C and 250 °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.