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A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 9 4 5 NEWUNDERSTANDING Until recently, the specific mechanism for the hardening response of Ni-rich NiTi alloys was a mystery. Optical and SEM images of Nitinol 60 before and after hardening suggested that the only observable difference was the presence of a relative- ly minor amount of Ni 3 Ti platelets in the annealed material. However, x-ray diffraction revealed that the hardened materi- al contains a Ni 4 Ti 3 as a major phase, though it is not revealed by SEM. Further microstructural details were revealed using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) techniques. Figure 5 shows a hard- ened cross section of NiTi. What appeared to be a monolithic field of NiTi under optical and SEM imaging is more of a com- posite made up of Ni 4 Ti 3 platelets surrounded by a cohesive matrix of NiTi. Enlightened with this new understanding, at- tention has been focused on alloy additions to enhance prop- erties and improve processability. ALLOY ADVANCEMENTS Though the nickel-rich NiTi alloys likeNitinol 60 do not exhibit shape-mem- ory behavior with the adopted heat treatment, they share much of their metallurgy with their extraordinary shape-memory cousins. This led to using alloy chemistry additions along similar lines taken by the shape-memory alloy research community; i.e., for example, adding other elements like Pd, Pt, and Ag to tailor transition temperatures. In the case of NiTi bearing alloys, researchers focused on improvingmicro- structural homogeneity to improve fa- tigue life and to reduce processing chal- lenges such as high residual stresses that result from severe quenching required to achieve high hardness. One success- ful approach is adding a third element to the Buehler’s original Nitinol 60 to form a ternary alloy. A small amount (1 at.%) of Hf is very beneficial [6] . Figure 6 compares the microstructure of Nitinol 60 with a comparably processed NiTi-Hf sample after heat treatment to achieve high hardness. In addition to generally more uni- form, cleaner microstructures, Hf con- taining alloys, unlike Nitinol 60, can achieve high hardness levels using less severe quenching rates and are easier to machine. Hardening has even been achieved in NiTi-Hf using air cooling fol- lowed by aging, resulting in a product with very low residual thermal stress. As in the case of the baselined Nitinol 60, high magnification STEM confirms that NiTi-Hf alloys exhibit the very same hardening mechanism; i.e., nanoscale platelets of Ni 4 Ti 3 in a NiTi-type matrix. A re- cently awarded patent describes these phenomena and intro- duces additional alloying elements such as Zr, Ta, and others, indicating that there remains more potential to the NiTi al- loy system [6] . LOOKING AHEAD When other beneficial properties like electrical conduc- tivity, nonmagneticbehavior, andcorrosion resistanceare con- sidered, the use of NiTi alloys for bearings makes a compelling case compared with traditional and even high-performance steels. In fact, no current class of structural bearing alloys provides such high levels of load capacity and such a combi- nation of enabling properties. In this sense, dimensionally sta- Fig. 3 — Comparison of dent depth versus mean contact stress for 13-mm diameter Si 3 N 4 ball pressed on flat plate specimens of different materials. Fig. 4 — Data from Fig. 3 plotted based on indentation load shows that using Nitinol 60 signifi- cantly improves bearing static load capacity. 7 FEATURE