薄膜与小尺度材料及力学性能研究组

Shape memory behaviour of annealed Ti_48.5Ni_(51.5-x)Cu_x (x = 6.2-33.5) thin films

A. Ishida ^a; M. Sato ^a

^a National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan

Abstract

The shape memory behaviour of (Ni,Cu)-rich Ti-Ni-Cu thin films (Ti_48.9Ni_44.9Cu_6.2, Ti_48.5Ni₄₀Cu_11.5, Ti_48.6Ni_35.9Cu_15.5, Ti_48.3Ni_28.4Cu_23.3, Ti_48.3Ni_23.9Cu_27.8 and Ti_48.5Ni₁₈Cu_33.5) annealed at 773, 873 and 973 K for 1 h was investigated. The films with 6.2, 11.5-15.5 and 23.3-33.5 at% Cu showed a single-stage deformation due to a B2 ↔ B19' transformation, a two-stage deformation due to the B2 ↔ B19 ↔ B19' transformation and a single-stage deformation due to the B2 ↔ B19' transformation, respectively. The martensitic transformation start temperature (M_s) increased with increasing Cu content and then levelled off for more than 15 at% Cu, indicating a high Ms temperature of 345 K. Temperature hystereses were almost 15 K for all films with more than 10 at% Cu. The critical stress for slip increased with increasing Cu content and increased significantly for the Ti_48.5Ni₁₈Cu_33.5 film, whereas the maximum recoverable strain significantly decreased for the Ti_48.5Ni₁₈Cu_33.5 film. With decreasing annealing temperature, the critical stress for slip increased, but the M_s temperature decreased. It was found that films with 11.5 at% Cu or more, annealed at 873 K, showed a high martensitic transformation temperature and a high critical stress for slip.

Keywords: martensitic transformation; mechanical behaviour; shape memory alloys; shape memory effect; sputtering; thin films; Ti-Ni-Cu

Microstructure of annealed Ti_48.5Ni_(51.5-x)Cu_x (x = 6.2-33.5) thin films

Ishida ^a;  M. Sato ^a; K. Ogawa ^a

^a National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan

Abstract

(Ni, Cu)-rich Ti-Ni-Cu amorphous films with a Cu content of 6.2-33.5 at. % formed by sputtering were annealed at 773, 873 and 973 K for 1 h and their microstructures investigated. Two types of precipitate were observed in the annealed films: a Ti(NiCu)₂ phase for the Ti_48.5Ni₄₀Cu_11.5, Ti_48.6Ni_35.9Cu_15.5, Ti_48.3Ni_28.4Cu_23.3 and Ti_48.3Ni_23.9Cu_27.8 films, plus a TiCu phase for the Ti_48.5Ni₁₈Cu_33.5 films. These precipitates were found to have coherency with the B2 matrix in the films annealed at 773 K and were densely distributed within the grains. However, in the films annealed at 873 K, their size increased 10-fold and their density decreased. Annealing at 973 K promoted grain-boundary precipitation and, accordingly, the density of the precipitates in the grain interiors decreased. On the other hand, the annealed Ti_48.9Ni_44.9Cu_6.2 films showed no precipitates in their grain interiors, but the number of grain-boundary precipitates increased with increasing annealing temperature. It was also found that grain size decreased with increasing Cu content and was significantly decreased for the Ti_48.5Ni₁₈Cu_33.5 films.

Keywords: annealing; microstructural characterization; shape memory alloys; sputtering; thin films; Ti-Ni-Cu