NiSn4 formation during the solidification of Sn–Ni alloys

The growth of NiSn4 is studied during the solidification of Sn–Ni alloys containing 0–0.45wt%Ni. Where past research has found metastable NiSn4 after solid state ageing of Ni–Sn couples, it is shown here that NiSn4 also forms during solidification as both a primary and a eutectic phase over a wide range of solidification conditions. The NiSn4 phase has crystal structure isomorphous to PtSn4 (oC20) and no evidence is found for NiSn4 being stable at solidification temperatures. Primary NiSn4 crystal growth was promoted by high cooling rates and a comparison of growth ledges on the facets of primary NiSn4 and Ni3Sn4 crystals suggests easier interface attachment kinetics for NiSn4. The Sn–NiSn4 eutectic grew under all solidification conditions used, with an orientation relationship that produces a low lattice disregistry at the interface plane and the same NiSn4 growth direction as that selected during free growth of primary NiSn4. In hyper-eutectic compositions, both the Sn–NiSn4 and Sn–Ni3Sn4 eutectics grew with the latter growing from primary Ni3Sn4 crystals.