Grain boundary (GB) diffusion and viscous flow play dominant roles in mechanical relaxation of polycrystalline materials. The pioneering work of Zener and Ke, by accounting for relaxation in GBs by viscous shearing, predicts a single peak in the internal friction spectrum. Later investigations show the existence of two to three peaks in the internal friction spectrum when taking into account both GB diffusion and viscous flow for dissipation. In this paper, we further identify the characteristic relaxation modes in polycrystalline materials. We illustrate that competitive viscous flow and diffusion for normal stress relaxation give rise to distinct dependence of relaxation time on grain size. We construct an internal friction spectrum mapping based on the competitive deformation mechanisms including viscous flow in both normal and tangential directions and GB diffusion. The essential features of internal friction spectrum of polycrystalline materials from our analysis are consistent with available experimental observations. These findings may also be applicable to study relaxation dynamics of other material systems such as metallic glasses and porous materials.

晶界扩散和晶界粘滞变形是多晶材料发生力学弛豫的重要原因, Zener和葛庭燧的开创性工作,分别解释和从实验上验证了晶界粘滞滑动引起的内耗峰.后续研究表明,当同时考虑晶界粘滞与扩散时,多晶体内耗谱上可能出现两个,甚至是三个内耗峰.本研究通过阐明晶界扩散与粘滞变形在晶界法向应力弛豫中的竞争关系,理论上给出了多内耗峰的物理机制、明确晶界变形的主导模式与内耗峰之间的关系,并揭示了多晶体弛豫时间的不同晶粒尺寸依赖性.本工作有助于金属玻璃、多孔介质等非均质材料力学弛豫行为的研究.