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

本研究旨在阐明氢环境对TiNi薄膜微驱动器断裂强度和循环形状记忆变形的影响。采用滚压和溅射两种方法制备了厚度为4~16μm的TiNi薄膜微致动器。用酸性溶液进行阴极电解充氢。氢含量随着充电时间的延长而增加，在2000质量ppm左右达到饱和。在相同的充电时间下，薄膜越薄，氢含量越大。利用自行研制的微材料测试系统，用SSRT方法测量了两种TiNi薄膜微致动器的延迟断裂强度，发现随着充电时间的延长和薄膜厚度的减小，微致动器的延迟断裂强度降低。这些结果表明，延迟断裂强度与TiNi薄膜微致动器中的氢含量有关。在充氢环境下进行了循环形状记忆变形试验。充氢显著降低了合金的回复应力和应变恢复率。还进行了充氢环境下的疲劳试验。得到的S-N曲线(应力幅值与失效循环数之间的关系)表明，应力频率越低，疲劳寿命越短。最大应力与断裂时间之间的关系与应力频率无关。这一结果意味着TiNi薄膜在氢环境中的疲劳断裂是时间相关的断裂。此外，在本研究中，开发了TiNi丝增强复合材料驱动器，并对其作为一种使用小尺寸TiNi形状记忆合金的高功率驱动器进行了评估

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