Development of Nickel-Rich Nickel–Titanium–Hafnium Alloys for Tribological Applications

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 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

Mechanisms of Shock Strength Exhibited by a Nickel‐Rich Nickel‐Titanium‐Hafnium Alloy

“…Figure documents HRTEM analysis that shows the atomic nature of an interface, including the presence of dislocations, whereas Figure shows electron back‐scattered diffraction (EBSD) analysis that revealed an ≈2° misorientation of the lattice within the microband relative to the parent grain. While the spacing of interfacial dislocations is consistent with previous observations under quasi‐static deformation in which martensite was retained within the bands, [ 20 ] suggesting that perhaps they are geometrically necessary austenite–martensite interfacial dislocations formed during phase transformation, the crystallographic misorientation must result from some irreversible processes that occur due to the shock loading, such as higher amounts of dislocation activity due to slip interactions that result in dissolution of Ni 4 Ti 3 precipitates.…”

“…[ 18,19 ] One reason is that the small Hf addition suppresses brittle Ni 3 Ti intermetallic precipitation. [ 20 ] Here, we show that this material also enhanced characteristics in response to shock loading when compared with previously studied NiTi SMAs; analysis of the microstructure and mechanical response of this alloy after undergoing 300–650 m s −1 shock loading reveals the mechanistic understanding as to why.…”

“…There is great promise for future investigations of Ni‐rich SMAs with small Hf additions, as recent work has shown that dent resistance and rolling contact fatigue performance can be attained through more densely precipitated microstructures and other precipitate phases. [ 20,22,23 ] The activation of such high‐rate impact resistance mechanisms has not been previously reported in studying shock‐loading of NiTi and CuAlNi SMAs. However, published work on shock loading of NiTi alloys to date has documented nonprecipitate‐strengthened alloys of lower Ni‐composition.…”

“…The samples used in this study were taken from the same lots of material used in published studies of the quasi-static compression behavior and deformation mechanisms, [21] microstructure responses to heat treatments, [20,22] and rolling contact fatigue behavior [20,23] of the alloy Ni 54 Ti 45 Hf 1 . Two sets of plate impact shock experiments were conducted to interrogate the shock response of the Ni 54 Ti 45 Hf 1 SMA using the light gas gun facility at Combat Capabilities Development Command Army Research Laboratory.…”

Experimental investigation of the effects of aging and cryogenic treatments on the mechanical properties of superelastic nickel-titanium shape-memory alloys

“…43 In similar studies, researchers have noted that nanoscale Ni 4 Ti 3 precipitation is inevitable in Nirich NiTi alloys after aging heat treatment and that the hardness value increases in nanoscale Ni 4 Ti 3 precipitates with high-volume ratios. 44,45 In this study, the low-value hardness measured in the samples after the tensile test was attributed to changes in their Ni and Ti ratios. In order to reach maximum hardness in NiTi alloys, 55-56% Ni…”