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Indentation-based rate-dependent plastic deformation of polycrystalline pure magnesium

机译：基于压痕的速率依赖性多晶纯镁塑性变形

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摘要

Using a Berkovich pyramidal indenter and an instrumented nanoindentation platform, dual stage nanoindentation creep tests, including loading to a pre-set load and holding at the constant load then unloading, were performed on polycrystalline pure magnesium at 300K. Indentation tests were performed at four indentation loading rates of 0.05, 0.5, 5, and 50mN/s at constant load of 45mN and holding time of 400s. These were performed to assess indentation force-displacement response along with average indentation stress, indentation strain rate sensitivity and ambient temperature rate dependent plastic deformation response of the material. To this end, activation volume, a kinetic characteristic of plastic deformation, and density of mobile dislocations in plastically deforming material in the plastic zone around the indenter are discussed in detail. Uncertainties and sources of error, i.e. indentation size effect, surface roughness, and thermal drift, and pile-up/sink-in measuring creep response through nanoindentation are provided in the current paper as well. The microstructure of the material was also studied through optical and scanning electron microscopy to further investigate the microstructure/property correlations in the tested polycrystalline pure magnesium. The results show the dependency of indentation stress, strain rate sensitivity, and activation volume upon depth and loading rate. According to the creep stress exponent measurements, the dominant mechanism of rate dependent plastic deformation for polycrystalline pure Mg at ambient temperature is attributed to obstacle limited dislocation glide. Finally, the contribution of mechanical twins in the plastic zone around the indenter in the studied polycrystalline pure magnesium is briefly discussed.
机译：使用Berkovich金字塔压头和仪器化的纳米压痕平台，在300K的多晶纯镁上进行了双阶段纳米压痕蠕变测试，包括加载至预设负载并保持恒定负载然后卸载。压痕测试以0.05、0.5、5和50mN / s的四种压痕加载速率在45mN的恒定负载和400s的保持时间下进行。进行这些测试以评估压痕力-位移响应以及材料的平均压痕应力，压痕应变速率敏感性和环境温度速率依赖性塑性变形响应。为此，详细讨论了压头周围塑性区内塑性变形材料的活化体积，塑性变形的动力学特征和活动位错的密度。本文还提供了不确定性和误差源，即压痕尺寸效应，表面粗糙度和热漂移，以及通过纳米压痕测量堆积/沉入的蠕变响应。还通过光学和扫描电子显微镜研究了材料的微观结构，以进一步研究所测试的多晶纯镁中的微观结构/性质相关性。结果表明压痕应力，应变速率敏感性和激活体积对深度和加载速率的依赖性。根据蠕变应力指数测量，环境温度下多晶纯Mg与速率相关的塑性变形的主要机理归因于障碍物有限的位错滑移。最后，简要讨论了在所研究的多晶纯镁中压头周围塑性区内的机械孪晶的贡献。

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