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Ultrafine-grained (UFG) alloy Ti 49:4 Ni 50:6 possessing both nano-as well as submicrocrystalline structure has been successfully produced using two techniques of severe plastic deformation (SPD) processing: high pressure torsion and equal channel angular pressing. The features of microstructure, martensitic transformation and deformation behavior of the UFG alloy have been studied in details. The effects of grain size on the mechanical and functional properties of the alloy are discussed.

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Martensitic transformations in NiTi shape memory alloys (SMAs) strongly depend on the microstructure. In the present work, we investigate how martensitic transformations are affected by various types of ultra-fine grained (UFG) microstructures resulting from various processing routes. NiTi SMAs with UFG microstructures were obtained by equal channel angular pressing, high pressure torsion, wire drawing and subsequent annealing treatments. The resulting material states were characterized by transmission electron microscopy and differential scanning calorimetry (DSC). The three thermomechanical processing routes yield microstructures which significantly differ in terms of grain size and related DSC chart features. While the initial coarse grained material shows a well defined one-step martensitic transformation on cooling, two-step transformations were found for all UFG material states. The functional stability of the various UFG microstructures was evaluated by thermal cycling. It was found that UFG NiTi alloys show a significantly higher stability. In the present work, we also provide preliminary results on the effect of grain size on the undercooling required to transform the material into B19’ and on the related heat of transformation.

Severe plastic deformation (SPD) processes, such as equal channel angular pressing (ECAP) and high pressure torsion (HPT), are successfully employed to produce ultra fine grain (UFG) and nanocrystalline (NC) microstructures in a Ti–50.7 at% Ni shape memory alloy. The effect of grain size on subsequent Ni‐rich particle precipitation during annealing is investigated by transmission electron microscopy (TEM), selected area electron diffraction (SAD, SAED), and X‐ray diffraction (XRD). It is observed that Ni4Ti3 precipitation is suppressed in grains of cross‐sectional equivalent diameter below approximately 150 nm, and that particle coarsening is inhibited by very fine grain sizes. The results suggest that fine grain sizes impede precipitation processes by disrupting the formation of self‐accommodating particle arrays and that the arrays locally compensate for coherency strains during nucleation and growth.