Process of nanocrystallization and partial amorphization by cold rolling in TiNi

TiNi alloy is vulnerable to solid-state crystalline-amorphous transformation by many means, such as particle irradiation, shot peening, compression and cold rolling. In the present study a process of structural refinement and partial amorphization by cold rolling was studied on Ti-50.2 at%Ni. Crystal structures and microstructures were characterized by X-ray diffractometry, optical microscopy and electron microscopy. Differential scanning calorimetry revealed that the introduction of high density of dislocations and resultant structural refinement by cold rolling suppressed martensitic transformation. In the transversal section of cold rolled samples, microscopic and macroscopic shear band formation was observed after 40-50% rolling. Transmission electron microscopy observations revealed that amorphous bands exist in the sample with rolling reduction over 40%. The amorphous bands form about 35-45 degrees with respect to the rolling direction. This suggests that a part of the shear bands becomes amorphous.