Structure and properties of amorphous and nanocrystalline NiTi prepared by severe plastic deformation and annealing

The formation of homogeneous nanocrystalline structure by nanocrystallization of amorphous NiTi subjected to high pressure torsion is demonstrated. Structural evolution during annealing was investigated and homogeneous nanocrystalline structures with different grain sizes have been obtained by controlled annealing. Nanocrystallization results in the record value of room temperature strength for this material equal to 2650 MPa with an elongation to failure of about 5%. At elevated temperatures of (0.4…0.5)Tm nanocrystalline nitinol showed a high ultimate strength with sufficient elongation (up to 200%). The observation that the shape and the size of grains after deformation remain close to that of the initial state suggests that in nanocrystalline NiTi such mechanism as grain boundary sliding and grain rotation are active and the generation and motion of dislocations play the role of accommodation of stress concentration.