Fig. 1. Macro and micro morphologies of the as-spun Ni50Ti50 ribbons. (a) optical image of the ribbons; (b) schematic of the ribbon; (c, d) TEM bright-field micrographs of the free surface and the copper roller surface, respectively; (e) SEM micrograph of the cross section of the etched NiTi ribbon; (f) line and map scanning micrographs of the chemical composition of the cross section.

Fig. 2. Micro-morphologies of the annealed ribbon and comparison of phase transformation and mechanical behaviors between the as-spun and annealed ribbons. (a, b) TEM bright-field micrographs of the free surface and the copper roller surface of the annealed ribbon, respectively; (e) SEM micrograph of the cross section of the annealed ribbon after etching; (d) DSC curves (P1 represents peak 1 and so on), (e) XRD patterns and (f) tensile stress-strain curves.

Fig. 3. Phase transformation behaviors of the annealed ribbons after tensile deformation with different strains. (a) DSC curves (M1 and M2 represent the first and second martensitic transformation peaks); (b) evolutions of As, Af, Ms, Mf, M1P, M2P as a function of the deformation strains.

Fig. 4. Optical images showing shape evolution of 7% tensile deformed NiTi ribbons upon (a) heating and (b) cooling. RS and FS denote copper roller surface and free surface, respectively.

Fig. 5. Schematic of the curvature changes with two-step phase transformation upon heating and cooling.

Fig. 6. Effect of temperature on the evolutions of curvature upon the 1 st and 2nd thermal cycles of (a) 3%, (b) 5% and (c) 7% tensile deformed NiTi ribbons, respectively.