Lattice modulation and superelasticity in oxygen-added β-Ti alloys

The microstructure and martensitic transformation behavior of (Ti-23Nb)-1.0O (at.%) alloy have been closely and systematically investigated by transmission electron microscopy (TEM). Diffuse streaks along the 〈110〉β∗ directions with intensity maxima at 1/2 positions between basal spots are observed in selected area diffraction patterns obtained from the β phase. The streaks correspond to six variants of {110}β〈11¯0〉β transverse lattice modulation caused by randomly distributed oxygen atoms and their local strain fields. Nanosized modulated domains (nanodomains) in the β phase are confirmed in dark field micrographs. These nanodomains act as local barriers to martensitic transformation, thereby suppressing long-range martensitic transformation in the (Ti-23Nb)-1.0O alloy. We suggest a new mechanism for superelasticity in oxygen-containing β-Ti-Nb alloys based on the results of in situ X-ray diffraction measurements and in situ TEM observations.