Coupling effect of stretch-bending deformation and electric pulse treatment on phase transformation behavior and superelasticity of a Ti-50.8 at.% Ni alloy

In order to improve the functional stability of Ti-50.8 at.% Ni alloy, repeated stretch-bending deformation (SBD), electric pulse (EP), and age-hardening were combined. Phase transformation behavior and superelastic prop-erties of NiTi alloy subjected to different treatments were compared and analyzed in terms of average grain size, dislocation morphology, and nanoscaled Ni4Ti3 precipitates. The results indicate that the average grain size was refined from the annealing state of 9.1 mu m down to 5.4 mu m, and that a certain amount of dislocations or dislocation networks were introduced via 7 SBD passes. As EP-processed, both local stress relaxation and the nonuniform temperature field facilitated the microstructure to be a more homogeneous and stable state. With the increase of pulse frequency from 150 Hz to 400 Hz, a two-stage martensitic transformation occurred to replace the one-stage phase transformation and the combined multi-process induced multi-stage martensitic transformation behavior. Nanoscaled Ni4Ti3 precipitates with sizes in the range of 20-40 nm were homogeneously distributed in the matrix of NiTi alloy after aging at 250 degrees C for 48 h. As a result, based on the coupling effect of "Annealing-SBD-150 Hz EP-aging", excellent functional stability was obtained. The accumulative residual strain was reduced from the annealing state of 3.6%-1.1%.