Microstructural and deformation characteristics of a high strength 60NiTi alloy

NiTi alloys with higher than stoichiometric nickel content, typically in the range of 57 to 62 wt.% Ni, have shown to be promising materials for bearing and other tribological components for aerospace and mechanism applications.  In this study, microstructural characterization of the deformation behavior of a Ni-rich Ni₅₅Ti₄₅ (at. %) alloy, commonly known as  60NiTi (wt.%), has been analyzed using pre and post-test scanning and transmission electron microscopy (SEM/TEM), and  in situ neutron and synchrotron x-ray diffraction during isothermal tension and compression loading. The alloy was heat treated prior to testing to produce fine Ni₄Ti₃ precipitates for additional hardening.  The alloy was found to be very strong, capable of withstanding high stresses (~ 1.5GPa in tension and ~ 2.5GPa in compression) and still recover completely with no residual strain.  However, non-linear stress-strain behavior was observed in tension, but not in compression. This was analyzed via in situ neutron diffraction measurements and attributed to reversible Stress-Induced Martensite (SIM) that formed in tension to accommodate the stress.  TEM/SEM revealed stress-induced coarsening of Ni₄Ti₃ precipitates in both tension and compression, but in tension, precipitation and growth of the stable Ni₃Ti phase within grains and at grain boundaries were also observed. This led to an early failure in tension as compared to that in compression.  These microstructural changes are not desirable and likely to lead to a loss in strength, as well as become sources for fatigue, wear failure, and localized corrosion in 60NiTi components. The potential ramifications of these structural/microstructural changes are discussed.