  The effects of various heat treatments on the microstructure and hardness of
new Ni56Ti41Hf3 and Ni56Ti36Hf8 (atomic %) alloys were studied to evaluate the
suitability of these materials for tribological applications. A solid-solution
strengthening effect due to Hf atoms was observed for the solution annealed
(SA) Ni56Ti36Hf8 alloy (716 HV), resulting in a comparable hardness to the
Ni56Ti41Hf3 alloy containing 54 vol.% of Ni4Ti3 precipitates (707 HV). In the
Ni56Ti41Hf3 alloy, the maximum hardness (752 HV), achieved after aging at 300C
for 12 h, was attributed to dense, semi-coherent precipitation of the Ni4Ti3
phase. Unlike the lenticular morphology usually observed within binary NiTi
alloys, a blocky Ni4Ti3 morphology formed within Ni56Ti36Hf3 due to a smaller
lattice mismatch in the direction normal to the habit plane at the
precipitate/matrix interface. The maximum hardness for Ni56Ti36Hf8 (769 HV) was
obtained after applying an intermediate aging step (300C for 12 h) followed by
normal aging (550C for 4 h). This two-step aging treatment induces dense
nanoscale precipitation of two interspersed precipitate phases, namely H-phase
and a new cubic Ni-rich precipitate phase, resulting in the highest hardness
exhibited yet by this family alloys. The composition of cubic Ni-rich
precipitates was measured using atom probe tomography to be approximately
Ni61.5Ti31Hf7.5, while HAADF-STEM revealed a 54 atom motif cubic structure (a=
8.87 Angstroms), and electron diffraction showed that the structure belongs to
the pm-3m (No. 221) space group.
