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 degrees 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. (C) 2017 Elsevier B.V. All rights reserved.