Crystal Refinement by Cold Rolling in NiTi Shape Memory Alloys

Repeated cold-rolling with intermediate folding of sheet samples represents a technique to obtain severe plastic deformation that avoids excessive heating at the internal interfaces and that proceeds without the simultaneous action of a high pressure in the range of several GPa. In the present work, bulk nanocrystalline Ni 5 0 Ti 5 0 samples with average grain sizes below 10 nm have been synthesized at ambient temperature using two different processing pathways that start from a layered array of individual elemental sheets and alternatively, starting from the prealloyed NiTi intermetallic compound. The development of the microstructure during the different deformation routes was investigated by X-ray diffraction and scanning and transmission electron microscopy as a function of the deformation level. The development of the microstructure during cold rolling - including the early and the late stages of deformation-induced nanocrystal formation - is analyzed in terms of the major thermodynamic and mechanical properties that govern the intermixing during the deformation treatment. Additionally, the microstructure refinement is discussed with regard to a crystal-to-amorphous transition that can occur during intense mechanical deformation.