Rolling contact fatigue deformation mechanisms of nickel-rich nickel-titanium-hafnium alloys

The tribological performance and underlying deformation behavior of Ni55Ti45, Ni54Ti45Hf1 and Ni56Ti36Hf8 alloys were studied using rolling contact fatigue (RCF) testing and transmission electron microscopy (TEM). TEM results of the as-machined RCF rods, prepared using focus ion beam, revealed some damage very close to the surface. TEM results after initial RCF cycling showed that additional damage was mainly confined to deformation bands that propagated several microns into the sample. These bands formed via localized dislocation slip, possibly on multiple slip systems, within the B2 matrix and/or within transformed B19 prime martensite phase under repeated applied contact stress. Further cycling of Ni55Ti45 and Ni54Ti45Hf1 led to shearing and dissolution of the strengthening precipitates within the deformation bands, followed by formation of nanocrystalline grains and finally amorphization of the remaining matrix material within the bands. The Ni56Ti36Hf8 alloy exhibited a notable increase in RCF performance and a smaller damage zone (1.5 microns) compared to the Ni55Ti45 and Ni54Ti45Hf1 alloys (over 6 microns). This was attributed to the low fraction of B2 matrix phase (less than or equal to 13 %) in the Ni56Ti36Hf8 alloy, leading to formation of narrow deformation bands (less than 11 nm) that were incapable of dissolving the much larger precipitates. Instead, the deformation bands were restricted to narrow channels between the dense cubic NiTiHf and H-phase precipitates. In contrast, broad deformation bands accompanied by shearing and eventual dissolution of the Ni4Ti3 precipitates were observed in the Ni55Ti45 and Ni54Ti45Hf1 alloys due to the high area fractions of B2 matrix phase (~49 %).