Nanocrystallization of the Ti₅₀Ni₄₈Co₂ Shape Memory Alloy by Thermomechanical Treatment

The microstructural evolution during nanostructuring of the Ti₅₀Ni₄₈Co₂ shape memory alloy by thermomechanical processing is investigated. The high purity ingots were fabricated by a copper boat vacuum induction melting technique. The differential scanning calorimetry measurements showed that the homogenized Ti₅₀Ni₄₈Co₂ specimen have two-stage transformation during cooling including the austenite to R phase and the R phase to martensite. The homogenized specimens were then hot rolled and annealed to prepare the initial microstructure. Thereafter, annealed specimens were subjected to cold rolling with various thickness reductions up to 70 %. Transmission electron microscopy revealed that the severe cold rolling led to the formation of a mixed microstructure consisting of amorphous and nanocrystalline phases in Ti₅₀Ni₄₈Co₂ alloy. After annealing at 400 °C, the amorphous phase formed in the 70 % cold-rolled specimen was completely crystallized and an entire nanocrystalline structure with the grain size between 10 and 60 nm was achieved. The nanocrystalline Ti₅₀Ni₄₈Co₂ alloy exhibited about 12 % of recoverable strain and very high plateau stress (about 730 MPa) which was significantly higher than that of the coarse-grained state.