Shape memory characteristics of as-spun and annealed Ti51Ni49 crystalline ribbons

The as-spun Ti51Ni49 ribbon is essentially crystalline and shows a two-stage B2 ⇒ R and R ⇒ B19′ martensitic transformation with a significant transformation enthalpy. The as-spun ribbon already has plate-like Guinier-Preston (GP) zones which are proposed to induce the R-phase in martensitic transformation. After being annealed at 200 °C for 1 h, the size of the GP zones increases but their number decreases simultaneously. The maximal recoverable strain of the as-spun and 200 °C annealed Ti51Ni49 ribbons reaches 6.2% and 6.4%, respectively. With the increase of the annealing temperature up to 500 °C and 600 °C, only Ti2Ni precipitates can be observed in the ribbons and the two-stage martensitic transformation tends to coalesce. Experimental results reveal that the ribbons with GP zones show a higher recoverable strain, a lower permanent strain and a lower Ms temperature than those with Ti2Ni precipitates. The reason why the GP zones are formed in as-spun Ti51Ni49 ribbon during melt-spinning procedure is also discussed.