The microstructure and mechanical properties of unidirectionally solidified NiSn eutectic alloy

Abstract

Ni-33.5wt.%Sn (the starting materials were high purity (99.96%) nickel and high purity (99.95%) tin) eutectic alloy was unidirectionally solidified at rates of 0.1−1.8 cm h −1. The microstructure consisted of continous alternating lamellae of a nickel solid solution in an Ni 3Sn matrix. The orientation relationship between the phases was determined by electron diffraction. The interlamellar spacing λ obeyed the relation λ 2R = constant where R is the solidification rate. A transmission electron microscopy study showed that the nickel phase was rich in dislocations concentrated in the vicinity of the phase boundary and contained fine precipitates. The Ni 3Sn phase was free of any precipitation and contained a low density of dislocation pairs. The microstructure remained stable up to 1173 K. The tensile strength is proportional to λ −1 2 and follows a Hall-Petch type of relation. No correlation was found between the compressive properties and R. It is shown that the Griffith brittle fracture theory may be applied to the failure in tension which initiates from cracks in the Ni 3Sn matrix.