The grain size, and therefore the grain boundary density, is known to play a major role in the flow stress of metallic materials. A linear relationship to the inverse of the square root of the grain size was identified about 70 years ago giving rise to the well-established Hall-Petch grain refinement strengthening effect. Nevertheless, grain refinement softening is known to take place at high homologous temperatures and both effects have been given separate treatments. A recent model showed that a general relationship can explain both the Hall-Petch strengthening effect at low temperatures and superplasticity at high temperatures. The present review discusses recent advances in structural and mechanical characterization to provide an updated analysis of trends observed in the relationship between the grain size and the flow stress. The model of grain boundary sliding is evaluated using multiple sets of data in the literature and a general description is provided for the transition between grain refinement hardening and grain refinement softening. The analysis incorporate data from over 30 different metals and alloys with different grain sizes and after testing at different strain rates and temperatures. Data from molecular dynamic simulations are also included and show supporting evidence to the model of grain boundary sliding. The thermal contribution of the grain size strengthening and threshold stress is discussed including the trends observed in the strain rate sensitivity of fine-grained materials.

众所周知，晶粒尺寸以及晶界密度在金属材料的流动应力中起着重要作用。大约 70 年前确定了与晶粒尺寸平方根倒数的线性关系，从而产生了公认的 Hall-Petch 晶粒细化强化效应。然而，已知晶粒细化软化发生在高同系温度下，并且这两种效应已被单独处理。最近的模型表明，一般关系可以解释低温下的 Hall-Petch 强化效应和高温下的超塑性。本综述讨论了结构和机械表征方面的最新进展，以提供对晶粒尺寸和流变应力之间关系观察到的趋势的最新分析。使用文献中的多组数据评估晶界滑动模型，并为晶粒细化硬化和晶粒细化软化之间的转变提供了一般描述。该分析结合了来自 30 多种具有不同晶粒尺寸并在不同应变率和温度下进行测试后的不同金属和合金的数据。来自分子动力学模拟的数据也包括在内，并显示了对晶界滑动模型的支持证据。讨论了晶粒尺寸强化和阈值应力的热贡献，包括在细晶粒材料的应变率敏感性中观察到的趋势。