Amorphous-Crystalline TiNi-Based Shape Memory Ribbons

A special method for production of amorphous-crystalline TiNi-based thin ribbons were developed on the base of partial isothermal crystallization. It allowed one to prepare TiNi-based (Ti₅₀Ni₂₅Cu₂₅, Ti_40.7Hf_9.5Ni_44.8Cu₅ and Ti_40.7Hf_9.5Ni_39.8Cu₁₀) samples with different volume fraction of crystalline phase from 0 to 100 %. It was observed that crystalline phase of  Ti₅₀Ni₂₅Cu₂₅ alloy underwent a B2 ↔ B19 phase transformation and a crystalline phase of  TiHfNiCu alloys underwent a B2 ↔ B19’ transition. An increase in volume fraction of crystalline phase hardly influenced the kinetics of B2 ↔ B19 transformation and resulted in a significant variation in temperatures of B2 ↔ B19’ transition. It was assumed that different sensitivity of B2 ↔ B19 and B2 ↔ B19’ transformations to variation in crystalline structure was due to difference in shear strain accompanying the transformations. It was shown that amorphous-crystalline alloys demonstrated one-way shape memory effects. As expected an increase in volume fraction of crystalline phase resulted in an increase in fully recoverable strain. The maximum of recoverable strain was 3% in  Ti₅₀Ni₂₅Cu₂₅ alloy and 6% in TiHfNiCu alloys. The TiHfNiCu amorphous-crystalline alloys demonstrated a two-way shape memory effect. The dependence of two-way shape memory effect value of the volume fraction of crystalline phase was no-monotonic and the maximum of the reversible strain was observed in the samples with 50% of crystalline phase. It was assumed that the interface between amorphous and crystalline phase was a preferable place for formation of oriented internal stresses that were responsible for observation of two-way shape memory effects. In Ti₅₀Ni₂₅Cu₂₅ alloy an internal stresses was very small due to a small shear strain accompanying the B2 ↔ B19 transformation and the value of stress was not enough for inducing a two-way shape memory effect.