TiNi-Based Shape Memory Foams Produced by Self-Propagating High Temperature Synthesis

The crystalline structure, martensitic transformations and shape memory effects were studied in the TiNi-based shape memory foams. The TiNi foams of different compositions were produced by self-propagating high temperature synthesis (SHS) at different pre-heating temperatures. It was shown that a pre-heating temperature hardly influenced the properties of TiNi-based foams. The main parameter influencing the structure and properties of TiNi-based foams was Ni concentration in the Ti+Ni powder mixture using for SHS. It was found that an increase in the Ni concentration from 45 to 52 at % resulted in a decrease in volume fractions of precipitates (Ti₂Ni, Ti₃Ni₄, TiNi₃) and an increase in volume fraction of TiNi phase. However, as concentration of Ni in TiNi phase increased, volume fraction of TiNi phase undergoing a martensitic transformation decreased. In this case TiNi-based foams demonstrated poor functional properties. To improve a martensitic transformation kinetics and shape memory effects TiNi-based foams were subjected to annealing at a temperature range of 300 – 600 ^oC for duration of 30 – 240 min. It was found that maximum of volume fraction of TiNi phase undergoing a martensitic transformation was observed in TiNi-based foams annealed at 500 ^oC for 2 hours. It resulted in observation of good shape memory and pseudoeleasticity effects. It was shown that Ti₅₅Ni₄₅ foam demonstrated functional properties like a Ti₅₀Ni₅₀ cast alloy and properties of Ti₅₀Ni₅₀ and Ti₄₈Ni₅₂ foams were closed to the properties of Ti_49.3Ni_50.7 cast alloy. It was shown that variation in Ni concentration and annealing parameters allowed one to obtain a TiNi foams with a different temperatures of martensitic transformations and various functional properties which will be used for different applications.