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 < 1 1 0 >*(beta) directions with intensity maxima at 1/2 positions between basal spots are observed in selected area diffraction patterns obtained from the 13 phase. The streaks correspond to six variants of {110}(beta) < 1 (1) over bar0 >(beta) transverse lattice modulation caused by randomly distributed oxygen atoms and their local strain fields. Nano-sized modulated domains (nanodomains) in the beta 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 beta-Ti-Nb alloys based on the results of in situ X-ray diffraction measurements and in situ TEM observations. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.