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

摘要

Ni_(40)Ti_(40)Nb_(20) 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-19.6Nb at% 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 630 MPa, and then shows a linear hardening region, where both plastic and superplastic deformation are active, until a stress of 1080 MPa 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.