Research on Effect of Hydrogen Environment on Fracture and Shape Memory Deformation of SMA Thin Film Microactuators and Life Prediction

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This research was conducted for the purpose of clarifying the effect of hydrogen environment on fracture strength and cyclic shape memory deformation of TiNi thin film microactuators. Two kinds of TiNi thin film microactuators with a thickness of 4〜16 μm fabricated by the roll-bonding method and sputtering method were used in this research. Hydrogen charging was conducted by cathodic electrolysis using an acid solution. Hydrogen content increased with increasing charging time and saturated at about 2000 mass ppm. When compared at the same charging time, hydrogen content was larger for thinner film. For both kinds of TiNi thin film microactuators, delayed fracture strength of hydrogen-charged microactuators measured by the SSRT method using a developed micromaterial testing system decreased as the charging time increased and the film thickness got smaller. These results show that delayed fracture strength depends on the hydrogen content in TiNi thin film microactuators. The cyclic shape memory deformation tests were carried out in the hydrogen-charging environment. The recovery stress and strain recovery rate markedly decreased by hydrogen charging. Fatigue tests in the hydrogen-charging environment were also conducted. The obtained S-N curves (the relationship between the stress amplitude and number of cycles to failure) showed shorter fatigue life for lower stress frequency. The relationship between the maximum stress and time to rupture was independent of the stress frequency. This result means that the fatigue fracture of TiNi thin films in hydrogen environment is time-dependent fracture. Also, in this research, TiNi wire-reinforced composite actuators were developed and evaluated as a high-power actuator with small-size TiNi shape memory alloys