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金属玻璃高温变形过程中的结构年轻化研究

STUDY ON STRUCTURAL REJUVENATION OF A METALLIC GLASS DURING HIGH-TEMPERATURE DEFORMATION

  • 摘要: 金属玻璃高温均匀变形行为与其玻璃转变、老化、年轻化等关键过程密切关联, 是理解玻璃态本质和完善非晶力学理论体系的重要支撑. 探索均匀变形过程中老化与年轻化的动态竞争有利于实现金属玻璃能量状态的定量调控. 本研究基于扩展指数经验方程和自由体积理论, 通过动态力学分析仪对Pd20Pt20Cu20Ni20P20金属玻璃恒定/递减应变速率条件下的拉伸-应力松弛循环过程进行研究. 高温均匀流变过程中金属玻璃总变形由弹性、粘弹性和粘塑性分量构成, 应力松弛过程中金属玻璃变形由弹性、粘弹性分量向塑性分量转换. 通过真应力应变曲线峰值应力、应力松弛弛豫强度和自由体积缺陷浓度三个物理参量, 阐明了变形过程中的能量状态演化规律. 自由体积理论可以准确描述金属玻璃的均匀流变行为, 粘弹性分量对应于结构年轻化, 而粘塑性应变对应于结构老化. 金属玻璃稳态流变阶段能量状态仅受温度和应变速率调控, 与宏观热力学加载历史无关. 提高应变速率可以实现更高程度的结构年轻化, 进而扩展其动力学非均匀性并加速弛豫速率.

     

    Abstract: The homogenous deformation behavior of metallic glasses at high temperature is closely related to key processes such as glass transition, aging and rejuvenation, which is an important support for understanding the nature of glassy state and improving the theoretical system of amorphous mechanics. Exploring the dynamic competition between aging and rejuvenation in the process of homogenous deformation is conducive to the quantitative control of the energetic state of metallic glasses. Based on the stretched exponential equation and free volume model, the tension-stress relaxation cycle of Pd20Pt20Cu20Ni20P20 metallic glass under constant/decreasing strain rate was studied by dynamic mechanics analyzer. The total deformation in high-temperature homogenous deformation process is composed of elastic, anelastic and viscoplastic components, and the deformation completely changes from elastic and anelastic components to plastic components in stress relaxation process. Three physical parameters, i.e., peak stress of true stress-strain curve, stress relaxation strength and defect concentration, are used to elucidate the evolution of energetic state during deformation. The free volume theory can accurately describe the homogenous deformation behavior of metallic glasses, the anelastic component corresponds to the structural rejuvenation, and the viscoplastic strain corresponds to the structural relaxation. The energetic state of metallic glasses in steady state deformation is only controlled by temperature and applied strain rate, and is independent on their macroscopic thermomechanical loading history. Increasing the strain rate can achieve a higher degree of structural rejuvenation, thus extending the dynamic heterogeneity and accelerating the relaxation rate.

     

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