Effects of Nb additions on the precipitate morphology and hardening behavior of Ni-rich Ni₅₅Ti₄₅ alloys

Due to their high hardness, comparable to that of tool steels, as well as other properties, Ni-rich NiTi alloys have recently been considered as a promising alloy system for using in bearing and related applications. The large volume fraction of nanoscale Ni₄Ti₃ precipitates is the main origin of the high hardness of these alloys. This study investigated the effects of Nb content (1, 3, 5 at.%) on the precipitation and the hardening behavior of the Ni-rich Ni₅₅Ti₄₅ (at.%) alloys. The microstructures of the quenched and aged alloys are mainly composed of B2-NiTi matrix, nanoscale Ni₄Ti₃ precipitates, and a small amount of Ti₂Ni and Ni₃Ti₂ phases, and no Nb-containing phase has been detected. A few R phases are also observed in the Ni₅₅Ti₄₅ base alloy and are eliminated by Nb additions. With the Nb additions, the Ni₄Ti₃ precipitates transform from lens morphology to block morphology and show enhanced hardness. After aging at 400 °C for 3 h, the hardness increases from 643 ± 12.3 Hv for the base alloy to 745 ± 9.2 Hv for the 5Nb alloy. A linear arrangement of the sphere or ellipsoid Ni₃Ti₂ particles is also observed in the present alloys, with the number density and size of the Ni₃Ti₂ precipitates increasing with Nb addition. The present work provides important new insights for creating a novel NiTi-based bearing alloy.