Abstract Magnesium (Mg) and its alloys usually show relatively low strength and poor ductility at room temperature due to their anisotropic hexagonal close-packed (HCP) crystal structure that provides a limited number of independent slip systems. Here we report that unique combinations of strength and ductility can be realized in bulk polycrystalline pure Mg by tuning the predominant deformation mode. We succeeded in obtaining the fully recrystallized specimens of pure Mg having a wide range of average grain sizes, of which minimum grain size was 650 nm, and clarified mechanical properties and deformation mechanisms at room temperature systematically as a function of the grain size. Deformation twinning and basal slip governed plastic deformation in the conventional coarse-grained region, but twinning was suppressed when the grain size was refined down to several micro-meters. Eventually, grain boundary mediated plasticity, i.e., grain boundary sliding became dominant in the ultrafine-grained (UFG) specimen having a mean grain size smaller than 1 μm. The transition of the deformation modes led to a significant increase of tensile elongation and breakdown of Hall-Petch relationship. It was quantitatively confirmed by detailed microstructural observation and theoretical calculation that the change in strength and ductility arose from the distinct grain size dependence of the critical shear stress for activating different deformation modes.

中文翻译：

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通过超细晶粒细化至亚微米，块状多晶纯镁的主要变形模式转变

摘要 镁 (Mg) 及其合金在室温下通常表现出相对较低的强度和较差的延展性，这是由于其各向异性的六方密堆积 (HCP) 晶体结构提供了有限数量的独立滑移系统。在这里，我们报告说，通过调整主要变形模式，可以在块状多晶纯镁中实现强度和延展性的独特组合。我们成功地获得了具有广泛平均晶粒尺寸的纯镁完全再结晶试样，其中最小晶粒尺寸为 650 nm，并系统地阐明了室温下机械性能和变形机制作为晶粒尺寸的函数。变形孪晶和基底滑移控制了常规粗晶区域的塑性变形，但是当晶粒尺寸细化到几微米时，孪晶被抑制了。最终，晶界介导的塑性，即晶界滑动在平均晶粒尺寸小于 1 μm 的超细晶 (UFG) 试样中占主导地位。变形模式的转变导致拉伸伸长率的显着增加和霍尔-佩奇关系的破坏。通过详细的显微组织观察和理论计算定量证实，强度和延展性的变化是由激活不同变形模式的临界剪切应力的明显晶粒尺寸依赖性引起的。晶界滑动在平均晶粒尺寸小于 1 μm 的超细晶 (UFG) 试样中占主导地位。变形模式的转变导致拉伸伸长率的显着增加和霍尔-佩奇关系的破坏。通过详细的显微组织观察和理论计算定量证实，强度和延展性的变化是由激活不同变形模式的临界剪切应力的明显晶粒尺寸依赖性引起的。晶界滑动在平均晶粒尺寸小于 1 μm 的超细晶 (UFG) 试样中占主导地位。变形模式的转变导致拉伸伸长率的显着增加和霍尔-佩奇关系的破坏。通过详细的显微组织观察和理论计算定量证实，强度和延展性的变化是由激活不同变形模式的临界剪切应力的明显晶粒尺寸依赖性引起的。