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Simultaneous 4D-STEM / EDX dataof Ni80Cr20alloy corroded by EuF3in molten FLiNaK salt.4D-STEM scan "4DSTEM_EDS_scan_300kV_Diffraction SI.dm4"taken of the same region coincident with the EDX measurement "4DSTEM_EDS_scan_300kV.hspy" and "4DSTEM_EDS_scan_300kVhits.hspy". "Calibration_Al_300kV_Diffraction SI.dm4"contains calibration and diffractionstandard data using a polycrystalline Al standard sampleforthe analysis.

Nickel-rich (54–56 at.% Ni) NiTi-based alloys have gained increased attention for their high hardness, corrosion resistance, strength, and wear resistance, leading to their development for high-performance bearings and other wear applications. An investigation of a broader compositional range of NiTiHf alloys, in terms of Ni and Hf content, is presented in this study. Their Vickers micro-hardness, 3 ball-on-rod rolling contact fatigue, and compression performances are benchmarked against early compositions identified for bearing applications, e.g., Ni55Ti45 and Ni54Ti45Hf1. The results show that by varying heat treatments and alloy composition, certain NiTiHf alloys can exhibit long life (~ 107 cycles) at 10–20% greater contact stress levels (up to 2.2–2.4 GPa), together with up to 20% greater hardness (up to 770 HV) and 30% larger compressive yield strengths (up to 3.4 GPa) than the original bearing compositions. These improvements are attributed to the ability to develop alloys with high-volume fractions of fine precipitate phases. Interestingly, a variety of combinations of different precipitate strengthening phases, depending on alloy composition and heat treatment, were able to achieve improvements relative to Ni55Ti45 and Ni54Ti45Hf1, demonstrating the versatility of the NiTiHf system.

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 300°C 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 (300 °C for 12 h) followed by normal aging (550 °C 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 of alloys. The composition and structure of this new precipitate phase was characterized using atom probe tomography and transmission electron microscopy techniques to be cubic with a lattice parameter of a = 8.87 A and symmetry belonging to one of the primitive subgroups of m 3 ¯ m with approximately Ni61.5Ti31Hf7.5 composition .

The deformation mechanisms that dictate the tribological performances of heat treated Ni55Ti45, Ni54Ti45Hf1 and Ni56Ti36Hf8 alloys are revealed through rolling contact fatigue (RCF) testing and transmission electron microscopy (TEM) analyses. Analysis of worn samples that passed a 5×108 cycle RCF runout condition shows that damage is primarily confined to deformation bands that propagate several hundred nanometers – to – several microns beneath the surface. These bands nucleate via localization of dislocation slip within the B2 austenite phase of the alloys. For the Ni55Ti45 and Ni54Ti45Hf1 samples, further damage and eventual spall failures occur by shearing and dissolution of the strengthening Ni4Ti3 nanoprecipitates within the deformation bands, followed by nanocrystallization that sometimes includes stress-induced nucleation of B19′ nanocrystals. Eventually, complete amorphization occurs prior to fracture. The relative 15 – 25% increase in RCF contact stress performance of the Ni56Ti36Hf8 alloy correlates with a more limited depth of damaged material beneath the wear surface; heavily damaged material beneath spall failure sites only extends 1.5 µm into the sample, compared to > 6 µm for Ni55Ti45 and Ni54Ti45Hf1 alloys. This superior RCF damage resistance of the Ni56Ti36Hf8 alloy results from a lower fraction of B2 matrix phase (≤ 13 %) that is highly confined by both cubic Ni-rich NiTiHf and H-phase nanoprecipitates. Specifically, this microstructure limits the widths of deformation bands within the B2 austenite phase to less than the size of the strengthening nanoprecipitates, which in turn inhibits precipitate shearing and dissolution processes that precede nanocrystallization and amorphization.

A "cubic Ni3Ti2" precipitate phase recently reported to exist within NiTi and NiTiNb alloys is documented to also form within a Ni56Ti36Hf8 alloy upon aging. Using transmission electron microscopy and ab intio density functional theory techniques, the phase is more precisely identified to be Ni16Ti11 with symmetry of the pm3m space group and a lattice parameter a = 8.816 A. In this structure, nickel atoms preferentially segregate to the perimeter lattice sites of the 54-atom unit cell.