R. Zarnetta, A. Savan, M. Ehmann, A. Ludwig, Ruhr-University Bochum, Bochum, Germany
Thin film shape memory alloys (SMA) are of high interest for miniaturized actuators due to their high energy density and tailorable transformation temperatures. For a successful application of the SMA in micro-actuators several requirements have to be fulfilled simultaneously: high actuation effect (switchable stress value, Δσ), high transformation temperatures (here Af), low thermal hysteresis width (ΔT) and a small temperature interval of transformation (ΔI). In literature most of the time these requirements were optimized individually and only for very restricted sets of alloys. Here we present our combinatorial approach towards fabrication and characterization of shape memory alloys thin film actuators and introduce a figure of merit (FOM) in order to determine the compositions with optimal shape memory properties.Ti-Ni-Cu shape memory thin films within a broad composition range, i.e. Ti64‑xNi27+xCu9 (x= 0 – 22 at. %) and Ti49-xNi37Cu14+x (x= 0 – 9.5 at. %) were investigated by the cantilever deflection method. The transformation temperatures and the thin film stress change were determined. A clear dependency of the actuator response on the thin film composition was found. The optimal shape memory alloy composition with improved functional properties, i.e. high stress change, high transformation temperatures, low thermal hysteresis width and small temperature interval of transformation, were identified using a figure of merit defined as (Δσ*Af)/(ΔT*ΔI). Ti50Ni41Cu9 and Ti45Ni46Cu9 exhibit the best shape memory properties for compositions showing a B2-->B19 and a B2-->R-phase transformation, respectively. Further, data for binary and other ternary shape memory alloy systems, e.g. Ti-Ni-Pd, -Ag from other groups were analyzed with respect to their FOM and compared with the Ti-Ni-Cu thin film actuators.
Summary: Ti-Ni-Cu shape memory thin films within a broad composition range were investigated by the cantilever deflection method using a combinatorial materials science approach. Optimal compositions with improved functional properties, i.e. high stress change, high transformation temperatures, low thermal hysteresis width and small temperature interval of transformation, were identified using a newly defined figure of merit. Out of the investigated alloys, Ti50Ni41Cu9 and Ti45Ni46Cu9 exhibit the best shape memory properties for compositions showing a B2-->B19 and a B2-->R phase transformation, respectively.
