  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 %).
