| The effects of adding Co and heat treatment on the microstructure, transformation, shape memory effect(SME)and superelasticity(SE)of Ti-Ni shape memory alloys (SMA), and the transformation and deformation characterisics of Ti-Ni-Co SMA were investigated by optical micrometry, thermogravimetric, X-ray diffraction, differential scanning calorimetry and tensile test using Ti-49.8Ni and Ti-49.8Ni-1.0Co as the samples in this paper. The results are as follows.The composition phase of Ti-49.8Ni alloy is martensite (M) (monoclinic structure) and shows SME at the room temperature. The composition phases of Ti-49.8Ni-1.0Co alloy are parent phase (A) (CsCl structure) and shows SE at the room temperature. The deforming microstructure of two alloys is fibrous. With increasing the annealing temprature (Ta), the fibrous microstructure evolves gradually to equiaxed grain. The oxidation of the alloys speeds up when Ta is over 600℃.With increasing Ta, the transformation types of Ti-49.8Ni-1.0Co alloy are from A→R / R→A to A→R→M / M→R→A to A→R→M / M→A then to A→M / M→A upon cooling / heating, and the R phase transformation temperature decreases, the M transformation temperature increases firstly then decreases, the R transformation temperature hysteresis (ΔTR ) is nearly not change (about 4℃), and the M transformation temperature hysteresis (ΔTM) decrease firstly then turns to stable. With increasing the aging time (tag), the transformation types of 300℃aged Ti-49.8Ni-1.0Co alloy are from A→R→M/M→R→A to A→R/R→A, the ones of 400℃aged alloy are from A→R→M/M→R→A to A→R→M/M→A, and the one of 500℃aged alloy is A→R→M /M→A constant upon cooling and heating. With increasing the tag, both R and M transformation temperatures of the Ti-49.8Ni-1.0Co alloy increases first then turns to stable, theΔTM decreases sharply first then turn to stable, and theΔTR alters a little.The platform stress of the tensile curves in Ti-49.8Ni-1.0Co alloy is higher than that in Ti-49.8Ni alloy. With increasing Ta, the alloys'platform stress decrease first and then increase. With increasing the Td, the fraction of shape memory effect decreases, that of superelasticity increases, and the platform stress increases in the alloys. With increasing the tag, the stress of the stress inducing martensite transformation decreases and increases for 400℃and 500℃aged Ti-49.8Ni-1.0Co alloy, respectively.The strain recovery ratio of Ti-49.8Ni and Ti-49.8Ni-1.0Co alloy springs decrease firstly then turn to stable with increasing the cyclic times. The strain recovery ratio of 500℃aged alloy is lower than that of 400℃aged alloy. The high strain recovery ratio can be obtained in the alloy aging at low temperature. The prior-cyclic training can enhance the stability of the Ti-Ni alloy springs.
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