X. L. Meng, M. Sato, A. Ishida, National Institute for Materials Science, Ibaraki, Japan
Ti-Ni-Cu ternary alloy thin films have attracted considerable attention as a kind of fast response actuator materials due to the narrow temperature hysteresis. Up to now, the annealing effect, precipitation behaviors and shape memory properties have been investigated extensively. After proper annealing, Guinier-preston (GP) zones precipitate in Ti-Ni-Cu thin films. However, the results about the effect of GP zones on the martensite structure in the Ti-Ni-Cu thin films have not been reported. In the present paper, three kinds of Ti-Ni-Cu thin films were prepared with a carrousel-type magnetron sputtering apparatus. After annealed at 773K for 1h, three Ti-Ni-Cu thin films show the B19 martensite structure without precipitate, with a few GP zones and a large amount of GP zones respectively. Using those films, the effect of GP zones on the martensite structure in Ti-Ni-Cu thin films has been studied by transmission electron microscopy (TEM) observation. It was found that for the thin film without precipitates, the martensite variants mainly show (011) compound twin relation, whereas for that with a few GP zones, the martensite variants mainly exhibit (111) type I twin relation. When the amount of GP zones increases greatly in a Ti-Ni-Cu thin film, the (011) compound twins become dominant again. The change in the martensite structure can be primarily attributed to the variation of the stress field induced by the GP zones with different densities.
Summary: The effect of precipitates on martensite structure in Ti-rich Ti-Ni-Cu thin films has been studied by TEM. It was found that when the precipitation changes from no precipitate to a few GP zones and finally to a large amount of GP zones, the dominant martensite consquently shows (011) compound twins, (111) type I twins and (011) compound twins. And the related reasons were also discussed.
