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

The effects of Co microalloying and annealing technology on the transformation,microstructure and shape memory effect (SME) of Ti-Ni shape memory alloy were investigated by differential scanning calorimetry,optical microscopy,X-ray diffraction and tensile test for Ti-49.8Ni and Ti-49.8Ni-1.0Co alloys annealed at 400 and 600℃,respectively.The results are described as follows.The A→R→M (A-parent phase,R-R phase,M-martensite) two-stage transformations takes place in Ti-49.8Ni and Ti-49.8Ni-1.0Co alloys during cooling from 400℃ annealing,while during heating the M→A one stage and M→R→A two-stage transformations takes place in Ti-49.8Ni and Ti-49.8Ni-1.0Co alloys,respectively.The 400℃ annealed Ti-49.8Ni and Ti-49.8Ni-1.0Co alloys show fiber microstructure and poor plasticity,and the 600℃ annealed alloys shown equiaxed grains and good plasticity.Since Ti-49.8Ni alloy at room temperature consist of a single martensite,the alloy exhibits SEM,while Ti-49.8Ni-1.0Co alloy consist of a single parent phase B2,and thus shows superelasticity (SE).The effect of the annealing time on microstructure and properties of Ti-Ni alloy is not obvious.The transformation temperature of the Ti-Ni alloy increases slowly with increasing annealing time.The deforming temperature obviously affects the SE of Ti-Ni alloy.The SE stress increases and the SE loop area decreases in Ti-Ni alloys with increasing deformation temperature.