Thickness effect on thermally induced phase transformations in sputtered titanium-nickel shape-memory films
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The effect of the film thickness on the phase transformations encountered in sputtered titanium-nickel (TiNi) shape-memory films due to thermal cycling in the temperature range of −150 to 150 °C was examined in the context of electrical resistivity (ER) measurements. A hysteresis in the ER response was observed for film thickness greater than 300 nm. This phenomenon is characteristic of shape-memory materials and is attributed to the rhombohedral (R) phase produced during cooling from the high-temperature cubic austenite phase to the low-temperature monoclinic martensite phase. The decrease of the TiNi film thickness below 300 nm resulted in a smaller ER hysteresis, leading eventually to its disappearance for film thickness less than ∼50 nm. The results indicate that spatial constraints introduced by the film surface and film/substrate interface generate a resistance force, which prevents lattice distortion and twinning. The inhibition of these mechanisms, which control self-accommodation R-phase transformation, leads to the suppression and eventual disappearance of the shape memory effect for film thickness less than ∼100 nm.
I. INTRODUCTION
Since the discovery of the shape memory effect (SME) in binary titanium-nickel (TiNi) alloys,1 a major research effort has been devoted to the investigation of the deformation behavior and associated phase transitions in shape memory alloy (SMA) materials. In view of the unique mechanical properties of sputtered TiNi films, significant attention has been given to the fabrication of various microdevices (e.g., valves, actuators, and grippers) and bioimplants (e.g., stents) using micrometerthick TiNi films. The SME is a manifestation of diffusionless phase transitions occurring during cooling from the parent cubic (B2) austenite phase to the twinned monoclinic (B19⬘) martensite phase involving the formation of an intermediate rhombohedral (R) phase and vice versa upon heating, but without the formation of the Rphase. In these phase transitions, atoms are cooperatively rearranged or shifted to another crystal structure by a shear-like self-accommodation mechanism.2–7 Because atom migration does not occur, these phase transformations take place at the speed of sound in the material.8 During cooling of austenitic TiNi alloys, the R-phase emerges before the onset of martensite transformation.9
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0209 1606
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 6, Jun 2005 Downloaded: 13 Mar 2015
Hence, TiNi alloys generally exhibit a two-stage phase transformation during cooling,10 i.e., B2 → R → B19⬘. The formation of the R phase is important because it characterizes the SME.11–14 Previous phase transformation studies examined bulk TiNi alloys or sputtered TiNi films of several micrometers in thickness.15 Nanoindentation experiments with sputtered austenitic TiNi films of thickness 1–2 m demonstrated a transition from pseudoelastic to elastic-plastic deformation, depending
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