NiTiCr is used in medicine for retriever devices, dental-and guide wires [1 -3]. Very little has been published in literature about this ternary NiTi-alloy. In the present work, the mechanical properties of NiTiCr shape memory alloy (SMA) wires (0.25 wt.% Cr) in the as-received condition and after a number of different heat treatments were studied. The surface quality of the wires was examined, because it is known that the surface condition has an influence on fatigue life. Mechanical uniaxial loading-unloading tensile and cyclic tests were performed. Bending-rotation fatigue (BRF) tests at higher rpm-levels were conducted in air and in oil. This is important with regard to possible applications, where NiTiCr might be used as a structural component.

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.

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.

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