Microstructure and mechanical properties of as-cast quasibinary NiTi–Nb eutectic alloy

Abstract

Ni40Ti40Nb20 is reported in the literature as a eutectic composition in the quasi-binary NiTi–Nb system. Eutectic and near-eutectic alloys find applications in various fields, including hydrogen permeation, energy damping, and liquid phase bonding of NiTi structures. In this study, we examine a cast Ni–Ti–Nb alloy with the above eutectic composition. Prealloyed, near-stoichiometric NiTi powders and pure Nb powders are blended and heated above the eutectic temperature to create a melt with the eutectic composition. After solidification, the ingot shows some elemental segregation at its top and bottom, but a large middle region exists with a homogeneous eutectic structure. The eutectic has average composition of Ti–40.1Ni–19.6Nbat% and consists of Nb-rich lamellae (Nb–19Ti–10Ni) dispersed within a NiTi-rich matrix (Ti–41Ni–15Nb). Under monotonic compressive deformation, the eutectic alloy yields at 630MPa, and then shows a linear hardening region, where both plastic and superplastic deformation are active, until a stress of 1080MPa is reached at an applied strain of 14.7%. On unloading, some superelastic strain is recovered. Upon unloading during compressive load–unload cycling, the eutectic alloy exhibits approximately twice as much superelastic recovery as elastic strain recovery, regardless of the maximum cycle strain.