Thermomechanical tensile characterization of Ti–Ni shape memory alloy films for design of MEMS actuator

Abstract

This paper investigates thermomechanical deformation behaviors of titanium–nickel (Ti–Ni) shape memory alloy (SMA) films in order to derive their constitutive relationships for a reliable design of Ti–Ni SMA microelectromechanical systems (MEMS) actuators. dc magnetron sputtering with block-on-disk target was employed to deposit composition-controlled Ti–Ni SMA films, characterized by uniaxial tensile testing with in situ X-ray diffraction (XRD) measurements. The XRD tensile tests enable us to measure the Poisson's ratio, as well as the Young's modulus of Ti–Ni films with various film compositions. At room temperature (RT), Ti–Ni films having Ti contents less than 50 atomic% (at.%) exhibit superelastic behavior, and those having Ti contents greater than 50 at.% exhibit shape memory behavior. The Ni–53.2 at.% Ti film, however, failed in a brittle manner at mere 0.8%, because precipitation hardening occurred with increasing Ti content. At elevated temperatures, Ti–Ni films having Ti contents of 50.2–52.6 at.% undergo phase change from martensite to austenite. The Young's modulus depends on temperature at each phase, regardless of film composition. But the Poisson's ratio is independent of both temperature and composition. However, film composition does affect the measured material constants, b A , b M , c A , and c M , that are used to construct the constitutive relationships. Stress–strain curves calculated from the constructed constitutive relationships are in close agreement for both the martensite and austenite phases with those obtained from tensile tests, so that the constitutive relationships are expected to find great utility in the design of Ti–Ni film-actuated MEMS.