Shape memory behavior and internal structure of Ti–Ni–Cu shape memory alloy thin films and their application for microactuators

Abstract

Internal structure and shape memory behavior of Ti–38.3Ni–9.3Cu (at.%) thin films heat-treated at 873 K, 923 K, 973 K and 1023 K were investigated by TEM observation and thermal cycling tests under various constant stresses. The thin film heat-treated at 873 K contained two types of precipitates, i.e., fine platelets and Ti 2Ni particles. The density of the platelets decreased with increasing heat-treatment temperature and annihilated completely when the heat-treatment temperature reached 973 K. The Ti 2Ni precipitates increased in volume fraction with increasing heat-treatment temperature from 873 K to 923 K, then their volume fraction was almost kept constant above 923 K. The recoverable strain decreased and the M s increased with increasing heat-treatment temperature from 873 K to 923 K. Both the recoverable strain and the M s became almost constant when the heat-treatment temperature was above 923 K. A diaphragm-type microactuator utilizing a Ti–38.0Ni–10.0Cu (at.%) thin film was fabricated. The diaphragm was square with the width of 500 μm. The actuation properties were investigated under conditions of both quasi-static and dynamic actuation. The M s and the transformation temperature hysteresis of the microactuator were determined to be 352 K and 6 K, respectively. The microactuator operated at a working frequency of 100 Hz.