Property of Amorphous/Nanocrystalline Hybrid Wires of TiNi-Base Shape Memory Alloys

The microstructures and mechanical properties of amorphous/nanocrystalline hybrid TiNi wires produced by severe cold drawing were investigated. Annealed wires of Ti-50.9 mol%Ni and Ti-41 mol%Ni-8.5 mol%Cu were subjected to severe cold drawing of 50-70% reduction. The as-drawn TiNi wires were composed of the mixture of amorphous phase and predominantly B2 nanocrystalline phase. Young's modulus increased with the drawing reduction which can be attributed to the increase in the amount of amorphous phase. For the binary TiNi wires, the volume fraction of amorphous phase was estimated to be about 60% from Young's modulus and electrical resistivity. The wires drawn over 60% exhibited peculiar large linear elastic strain which is quite different from superelasticity. Aging at 573 K led to an increase in tensile elongation as well as in the recoverable strain. The amorphization by cold drawing was also confirmed for Ti-41 mol%Ni-8.5 mol%Cu in 62% drawn wires.