Structure and properties of severely cold-rolled and annealed Ti-Ni shape memory alloys

The substructure and structure formation as well as the mechanical and functional properties of thermomechanically treated Ti-50.7 at.%Ni and Ti-50.0 at.%Ni alloys were studied using transmission electron microscopy and mechanical testing. A low-temperature thermomechanical treatment is carried out by rolling at room temperature in a true strain range e = 0.3-1.9. It is shown that the severe plastic deformation of Ti-Ni alloys results in a partial material amorphization and in the subsequent formation of a nanocrystalline austenite structure during post-deformation annealing. As a result, the completely recoverable strain and recovery stress become much higher than the values reachable using traditional low-temperature thermomechanical treatment with post-deformation annealing which creates a polygonized dislocation substructure.