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富氢环境下镍钛形状记忆合金弹簧变形行为的实验和理论研究

EXPERIMENTAL AND THEORETICAL STUDY OF THE DEFORMATION BEHAVIORS FOR NITI SHAPE MEMORY ALLOY SPRINGS UNDER HYDROGEN-RICH ENVIRONMENT

  • 摘要: 镍钛形状记忆合金器件在某些特定服役环境下不可避免地与氢接触, 导致其力学性能发生改变. 实验研究方面, 通过对两种构型(弹簧指数为8.5和11.7)的镍钛合金弹簧进行异位电解充氢, 随后对充氢和未充氢镍钛合金弹簧进行不同幅值的拉伸−卸载实验, 揭示了氢对超弹性镍钛合金弹簧变形行为的影响. 结果表明, 氢会显著影响弹簧的相变硬化行为, 并且使马氏体相变临界力下降地更为迅速. 理论研究方面, 基于实验结果, 在不可逆热力学框架下构建了镍钛形状记忆合金力−扩散耦合本构模型. 在该本构模型中, 考虑了与弹性, 马氏体相变和氢致膨胀相关的应变和氢浓度场对马氏体相变的影响. 通过建立的Helmholtz自由能推导出相变行为的热力学驱动力. 基于质量守恒定律和Fick定律, 得到氢浓度场的演化方程. 为了准确地描述充氢和未充氢镍钛合金弹簧的变形行为, 在建立的本构模型基础上, 进一步发展了描述弹簧力−扩散耦合变形行为并考虑其扭转、弯曲变形模式的半解析模型. 通过与实验结果的对比, 可以发现, 提出的半解析理论模型能够很好地预测富氢环境下超弹性镍钛合金弹簧的变形行为.

     

    Abstract: In some specific service environments, NiTi shape memory alloys (SMAs) are inevitably in contact with hydrogen, which can change their mechanical properties and service performance. In this work, the effect of hydrogen charging on the deformation behavior of the superelastic NiTi SMA helical springs is investigated experimentally and theoretically. In the experimental aspect, the ex-situ electrochemical hydrogen charging is performed for the superelastic NiTi SMA helical springs with two different spring indexes (8.5 and 11.7). Then, a series of tension-unloading tests under different loading amplitudes are performed for the superelastic NiTi SMA helical springs with and without hydrogen charging. Experimental results show that the hydrogen can significantly affect the hardening behavior of NiTi SMA springs and cause more decrease in the critical force of martensite transformation. In the theoretical aspect, a diffusional-mechanically coupled constitutive model is constructed in the framework of irreversible thermodynamics based on the experimental results. In this model, the strains associated with elasticity, martensite transformation and hydrogen expansion are taken into account, and the effect of the hydrogen concentration field on the martensitic transformation is considered. The thermodynamic driving force of martensite transformation is derived from the newly established Helmholtz free energy. The evolution of the hydrogen concentration field is obtained by combining the mass conservation equation and Fick’s law. To accurately describe the deformation behavior of the superelastic NiTi SMA helical springs with and without hydrogen charging, a semi-analytical model of helical spring is developed by simultaneously considering the torsion and bending deformation modes, and the coupling effect between the deformation and hydrogen diffusion. The proposed theoretical model is verified by comparing the predicted results with the experimental ones. It is found that the proposed model is able to predict the deformation behaviors of the superelastic NiTi SMA helical springs under the hydrogen-rich environment in a reasonable manner.

     

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