The Structure and Functional Properties of Nanopatterned Ti-Ni Shape Memory Alloy

The papers are devoted to the research of the influence of isothermal and thermo-cycling treatment on the structure and functional properties of Ti-50.5at%Ni alloys. The methods of diffraction analysis, electron microscopy and differential scanning calorimetry (DSC) were used in the research.

The experiments were carried out on the plates cut out of the rod with diameter 6 mm after electro-plastic deformation (EPD) at the density of unipolar impulsive current I = (90-120)À/mm². The true strain was in the range l=ln^ds/df from 0.44 to 1.72.

The phenomenon of awful high sensitivity to brittle fracture after EPD was found out. The annealing at the temperature 400–450°Ñ during 1.5–3.0 hours stabilizes the alloy structure and facilitates to the phase yield development at the stresses 150-200 MPa. For the first time the phase transition “martensite ↔ austenite” through the intermediate phase of R-martensite at the martensitic and austenitic transformation was found out in the mentioned alloys.

The analyses of microstructure showed the presence of indistinct foliation in the rolling direction. The average grain diameter depends on the severe deformation value. It is shown that the grain diameter reduces from 800 nm to 30 nm at rolling with the reduction ratio increasing.

Thermo-cycling treatment of nanopatterned specimens facilitates to the intensive forming of two-way effect. At that the thermo-mechanical hysteresis loop shifts to the range of positive temperatures (M_s = 7°Ñ, M_f =45°Ñ). After 10–15 thermo-cycles through the interval 20 ÷ (–80)°Ñ with deformation (1,0%) the two-way shape memory effect stabilizes at the reactive stresses generating of 12 MPa. The obtained results seem to be awful important for practice.