Heat treatment - microstructure - hardness relationships of new nickel-rich nickel-titanium-hafnium alloys developed for tribological applications

TL;DR

This study investigates how different heat treatments affect the microstructure and hardness of new nickel-rich Ni-Ti-Hf alloys, revealing optimal processing conditions that enhance their tribological properties for industrial applications.

Contribution

It introduces new heat treatment protocols that significantly improve the hardness and microstructure of Ni-Ti-Hf alloys, including the discovery of a new cubic Ni-rich precipitate phase.

Findings

Maximum hardness of 769 HV achieved after two-step aging.

Dense nanoscale precipitation of H-phase and cubic Ni-rich phase.

Blocky Ni4Ti3 morphology differs from binary NiTi alloys.

Abstract

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.