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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 theas-machined RCF rods, prepared using focus ion beam, revealed some damage veryclose to the surface. TEM results after initial RCF cycling showed thatadditional damage was mainly confined to deformation bands that propagatedseveral microns into the sample. These bands formed via localized dislocationslip, possibly on multiple slip systems, within the B2 matrix and/or withintransformed B19 prime martensite phase under repeated applied contact stress.Further cycling of Ni55Ti45 and Ni54Ti45Hf1 led to shearing and dissolution ofthe strengthening precipitates within the deformation bands, followed byformation of nanocrystalline grains and finally amorphization of the remainingmatrix material within the bands. The Ni56Ti36Hf8 alloy exhibited a notableincrease in RCF performance and a smaller damage zone (1.5 microns) compared tothe Ni55Ti45 and Ni54Ti45Hf1 alloys (over 6 microns). This was attributed tothe low fraction of B2 matrix phase (less than or equal to 13 %) in theNi56Ti36Hf8 alloy, leading to formation of narrow deformation bands (less than11 nm) that were incapable of dissolving the much larger precipitates. Instead,the deformation bands were restricted to narrow channels between the densecubic NiTiHf and H-phase precipitates. In contrast, broad deformation bandsaccompanied by shearing and eventual dissolution of the Ni4Ti3 precipitateswere observed in the Ni55Ti45 and Ni54Ti45Hf1 alloys due to the high areafractions of B2 matrix phase (~49 %).

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