In the current research, the effect of thermomechanical processing on the microstructural evolution, transformation behavior, superelastic and shape memory properties of the Ti 50 Ni 50 and Ti 50 Ni 48 Co 2 shape memory alloys was investigated. In view of drawbacks in vacuum induction melting (e.g. carbon contamination of the ingot from graphite crucible and formation of TiC precipitates in the microstructure) and vacuum arc remelting (e.g. the lack of enough stirring and as a result chemical inhomogeneity and require to several remelting), in this study, Ti 50 Ni 50-x Co x ingots (x=2,4,8) were produced by a new process called copper boat induction melting (CBIM). In CBIM, similar to VIM, electromagnetic stirring results in excellent chemical homogeneity but graphite crucible is replaced with water-cooled copper mold (copper boat). Hence, the reaction between Ti and C is eliminated and a clean melt is achieved. The as-cast ingots were then homogenized, hot rolled and annealed to prepare the suitable initial microstructure. Thereafter, annealed specimens were cold rolled up to 70% thickness reduction at room temperature. Post deformation annealing was conducted at 400 ? C for 1h. Differential scanning calorimetry (DSC) results showed that the addition of Cobalt higher than 2 at.% can dramatically decrease the martensitic transformation temperatures so that the martensitic transformation temperature of the ternary alloys containing 4 and 8 at.% Co was lower than -60 ° C and -150 ° C, respectively. Hence, the optimum content of Co-addition was selected as 2 at.%. The homogenized Ti 50 Ni 48 Co 2 alloy had a two-stage transformation during cooling including the austenite to R phase and the R phase to martensite. Transmission electron microscopy (TEM) observations revealed that the initial deformation mechanism of Ti 50 Ni 50 and Ti 50 Ni 48 Co 2 alloys during cold rolling was stress-induced martensitic transformation followed by plastic deformation of martensite via dislocation slip and subsequent martensite to austenite transformation via the reverse transformation after unloading. Microstructural investigations showed that by increasing the cold deformation, a high density of dislocations is accumulated, leading gradually to nanocrystallization and amorphization. With increasing thickness reduction during cold rolling, the ultimate tensile strength of Ti 50 Ni 50 and Ti 50 Ni 48 Co 2 alloys increased, but elongation decreased. It is noteworthy that the 70% cold rolled specimens had a record strength level up to 1.8 GPa. After annealing at 400 ° C for 1h, the amorphous phase formed in the cold rolled specimens was completely crystallized and a fully nanocrystalline structure with the grain size between 10 and 70 nm was achieved in Ti 50 Ni 50 and Ti 50 Ni 48 Co 2 alloys. The non-isothermal DSC analysis results suggest that the crystallization mechanism is governed dominantly by a three-dimensional interface-controlled growth in which retained nanocrystallites embedded in the amorphous phase can act as inhomogenous nucleation sites. Results showed that the stress level of plateau (? SIM ) and its length in the stress–strain curve of the Ti 50 Ni 50 and Ti 50 Ni 48 Co 2 alloys could be significantly improved through the formation of the nanocrystalline structure. It is noteworthy that in the 70% cold rolled-annealed specimens, the value of ? SIM was measured to be 615 and 730 MPa, respectively for the Ti 50 Ni 50 and Ti 50 Ni 48 Co 2 alloys. With increasing thickness reduction during cold rolling, the irrecoverable strain of Ti 50 Ni 50 and Ti 50 Ni 48 Co 2 alloys was decreased during superelastic experiments so that the 70% cold rolled-annealed specimens exhibited about 12 % of recoverable strain. Observed improvement in the superelastic behavior of the Ti 50 Ni 50 and Ti 50 Ni 48 Co 2 alloys can be attributed to increase of the critical dislocation-induced slip stress as a result of thermomechanical treatment and formation of nanocrystalline structure. Comparing the superelastic and shape memory behavior of Ti 50 Ni 50 and Ti 50 Ni 48 Co 2 alloys showed that the addition of Co resulted in higher plateau stresses and recoverable strain when compared to the binary alloy.