Influence of Hf solute additions on the precipitation and hardenability in Ni-rich NiTi alloys

• The initial hardening for all alloys was due to the precipitation of Ni4Ti3.
• Extended aging resulted in H-phase precipitation for all the alloys.
• Its presence yielded a slight secondary increase in hardness for all alloys.
• The 56Ni–40Ti–4Hf (at.%) alloy yielded the highest hardness with a value of 679 VHN.
• H-phase also appeared to alter the decomposition sequence of Ni4Ti3 in this alloy.

Very Ni-rich NiTi alloys have recently been shown to have an unusually high hardness, comparable to tool steels, and other attributes that make them promising candidates for bearing and related applications. This high hardness has been associated with the precipitation of a large volume fraction of Ni4Ti3 platelets, resulting in a matrix that consists of narrow B2 matrix channels. In this work, a series of Ni-rich ternary alloys with dilute solute additions of Hf (54Ni–45Ti–1Hf, 55Ni–44Ti–1Hf, 54Ni–44Ti–2Hf, and 56Ni–40Ti–4Hf (at.%)) have been investigated. Transmission electron microscopy confirmed a B2 NiTi matrix phase containing nanoscale Ni4Ti3 platelets, H-phase precipitates, and R-phase; however, the H-phase and R-phase were not present initially but only after aging for a period of time. At aging times greater than ∼100 h at 400 °C, all ternary alloys showed a slight secondary increase in hardness, which was attributed to H-phase precipitation and growth within the B2 channels. In the particular case of the 56Ni–40Ti–4Hf alloy, hardness increased with aging to a maximum value of 679 VHN, which was greater than all other binary or ternary alloys examined. Additionally the H-phase appeared to alter or delay the typical breakdown sequence of the metastable Ni4Ti3 strengthening phase by removing the excess Ni needed for its decomposition. The collective results provide new material insights for creating a next-generation NiTi based bearing alloy.