Despite much experimental and simulation effort, the existence of a Hall-Petch to inverse Hall-Petch transition in nanocrystalline ceramics remains elusive. By employing molecular dynamics simulations, we unambiguously reveal a transition from strengthening to softening in the shear deformation of nanocrystalline silicon carbide ceramics as a function of grain size. Results show a well-defined maximum in the shear strength for grain sizes in the range 6.2 to 7.7 nm. Further decrease in grain size leads to diminishing strength, consistent with an inverse Hall-Petch behavior. As grain size is reduced the increasing grain boundary (GB) regions lead to homogenization of shear stresses across the microstructure, allowing for lower local shear stress levels at higher macroscopically applied stresses. This delays shear localization within GB regions, preventing cavitation, nanocracking, and premature failure, and is responsible for the observed Hall-Petch behavior. In contrast, at grain sizes <6.2 nm, the rather compliant nature of the structurally disordered GB regions dominates the mechanical response, reducing the shear strength and triggering a transition into the inverse Hall-Petch behavior. A composite model delineating the transition between Hall-Petch and inverse Hall-Petch behavior is successful at describing the mechanical behavior of nanocrystalline silicon carbide as a function of grain size.

中文翻译：

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纳米晶碳化硅中霍尔-佩奇行为和逆霍尔-佩奇行为之间的转变

尽管进行了大量的实验和模拟工作，但纳米晶陶瓷中霍尔-佩奇到逆霍尔-佩奇跃迁的存在仍然难以捉摸。通过采用分子动力学模拟，我们明确地揭示了纳米晶碳化硅陶瓷在剪切变形中作为晶粒尺寸的函数从强化到软化的转变。结果表明，对于 6.2 至 7.7 nm 范围内的晶粒尺寸，剪切强度具有明确的最大值。晶粒尺寸的进一步减小导致强度降低，这与反向霍尔-佩奇行为一致。随着晶粒尺寸的减小，增加的晶界 (GB) 区域会导致整个微观结构的剪切应力均匀化，从而在较高的宏观施加应力下允许较低的局部剪切应力水平。这会延迟 GB 区域内的剪切定位，防止空化、纳米裂纹和过早失效，并负责观察到的霍尔-佩奇行为。相比之下，在晶粒尺寸 <6.2 nm 时，结构无序的 GB 区域相当顺从的性质主导了机械响应，降低了剪切强度并触发了向反向霍尔-佩奇行为的转变。描述霍尔-佩奇行为和逆霍尔-佩奇行为之间转变的复合模型成功地将纳米晶碳化硅的机械行为描述为晶粒尺寸的函数。降低剪切强度并触发向反向霍尔-佩奇行为的转变。描述霍尔-佩奇行为和逆霍尔-佩奇行为之间转变的复合模型成功地将纳米晶碳化硅的机械行为描述为晶粒尺寸的函数。降低剪切强度并触发向反向霍尔-佩奇行为的转变。描述霍尔-佩奇行为和逆霍尔-佩奇行为之间转变的复合模型成功地将纳米晶碳化硅的机械行为描述为晶粒尺寸的函数。