Effect of annealing processes on microstructure and properties of Ti-Ni-Co shape memory alloy

The microstructure and mechanical properties of Ti-49.8Ni-1.0Co (at% ) shape memory alloy (SMA) were investigated by optical microscopy,X-ray diffraction and tensile test.The results show that the microstructure consists of fibrous and equi-axed grains for Ti-49.8Ni-1.0Co alloy wire annealed at 400 ℃ and 600 ℃ ,respectively.The A ←→ R ←→ M two-stage and A ←→ M one-stage type transformations occur in the alloys annealed at 400 ℃ and 600 ℃ during cooling and heating,respectively.The temperature hysteresis of the martensitic transformation in 400 ℃ annealed alloy is larger than that in 600 ℃ annealed alloy.The effect of annealing time ta on transformation type in Ti-49.8Ni-1.0Co alloy is not serious,but the alloy's transformation temperature and transformation peak area increase with increasing ta.The Ti-49.8Ni-1.0Co alloy annealed at 400 ℃ and 600 ℃ shows shape memory effect (SME ) and superelasticity (SE) at room temperature.The stress induced martensitic transformation stress (SIMTS) and plasticity of the alloy annealed at 600 ℃ are higher than those of the alloy annealed at 400 ℃ ,and the SIMTS of the alloy decreases with increasing ta.The Ti49.8Ni-1.0Co alloy spring annealed at 400 ℃ exhibits SME and SE for testing temperature Td ≤ 20 ℃ ,and it shows SE for testing temperature Td≥30 ℃ .The loop area of shear stress-shear strain for the alloy spring decreases and its SE enhances with increasing Td.