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Due to their combination of good thermomechanical properties, corrosion resistance, and biocompatibility, nearstoichiometric NiTi alloys are the shape memory alloys (SMAs) that have had the greatest commercial success. [1][2][3][4][5] Their shape memory effect is derived from a diffusionless transformation in the solid state known as the martensitic transformation. On cooling from high temperatures, the material undergoes a transformation from the austenite phase to the martensite phase that begins at the martensite start temperature (M S ) and is completed at the martensite finish temperature (M F ). Upon reheating, the reverse transformation occurs between the austenite start temperature (A S ) and the austenite finish temperature (A F ). Martensitic transformations are typically associated with a thermal and/or mechanical hysteresis: [6][7][8][9] The forward transformation starts at a lower temperature (in the case of a thermal transformation) or at a higher stress (in the case of a stress-induced transformation) than the reverse transformation. Upon heating or unloading, the martensite begins to transform back to austenite at a relatively higher temperature than was required to induce the transformation upon cooling or mechanical loading of the material. [6][7][8][9] Ball and James [6,7] and others [8,9] have shown that the width of the hysteresis depends on the crystallographic compat- COMMUNICATION[*] Dr.

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Transformations from austenite to martensite or bainite in ferrous alloys have great technological importance, but some aspects remain elusive. The orientation relationship (OR), morphology and habit plane can vary considerably from one system to another. Much published work considers these OR in terms of their variation from named relationships such as Kurdjumov-Sachs and Nishiyama-Wassermann. We discuss here, instead, the use of a set of angular parameters based on the classic work of Kurdjumov and Sachs in the 1930s, that provide a unified and elegant description facilitating extraction and detailed statistical treatment of OR from large electron backscatter diffraction datasets, as well as straightforward comparison with named OR and with the predictions from the phenomenological theory. Spatially correlated mappings of OR parameters obtained using this approach suggest that the observed variations in OR are related to the martensitic morphology rather than being an experimental artefact.

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