Partial Amorphization of B2 Type Shape Memory Alloys by Cold Rolling

摘要

The amorphization by cold rolling in Ti-Ni (Ti_(50.0)Ni_(50.0), Ti_(49.8)Ni_(50.2)) and Zr_(50)Co_(37)Ni_(13) alloy was investigated and compared. In the case of Ti-Ni alloy, the occurrence of amorphization was always from the B2 parent phase. If the starting structure was the B19' martensite, the martensitic phase transformed to parent B2 phase then became amorphous. This amorphization associated with the shear band formation at over 40 % reduction, because the amorphization of Ti-Ni B2 phase required heavy deformation and high strain rate. Therefore, these amorphous regions formed in the form of band inclined to the rolling direction. On the other hand, Zr_(50)Co_(37)Ni_(13) alloy became amorphous in the B_f martensitic phase and B2 parent phase did not become amorphous. The amorphous phase laid parallel to the rolling direction. This difference might be caused by the crystal symmetry. The Bf phase has a lower crystal symmetry than B2 phase, so that the defect density can increase than in a higher crystal symmetry phase at the same reduction.