“Smaller is Stronger” is a very common feature observed in crystalline metallic materials. The yield strength of crystalline metallic materials is closely associated with the grain size and the sample size. The classical Hall-Petch relation is widely used to describe the connection between the yield strength of the crystalline metallic materials and grain size, i.e., the yield strength increases as the grain size decreases. Micro-testing of crystalline metallic materials shows that the yield strength decreases with increasing sample size until it reaches the bulk strength. However, the underlying mechanism and unified description of sample size-dependent yield strength remains an unsolved problem.
Recently, Huiling Duan’s group from College of Engineering, Peking University, found that the transition from size-dependent to size independent yield strength of crystalline metallic materials is controlled by the competition between the stresses required for dislocation source activation and dislocation motion. Moreover, the classical Hall–Petch relation was extended to the micro-size polycrystals based on the evolution of grain boundary (GB) density. On this basis, a unified model that could characterize the yield strength for crystalline metallic materials with sample size from microscales to macroscales is established.
This proposed model successfully interprets the physical mechanism of the yielding of miniaturized crystalline metallic materials, and reveals the internal connection between yield strength and sample size. Various reported experimental results have been accurately captured by this unified model, including the pre-strained nickel, irradiated copper, ultrafine grain tungsten (Fig. 2), and so on.
This research work was published in Physical Review Letters in the form of "Editors’ Suggestion" on June 9, 2020, entitled“Unified Model for Size-Dependent to Size-Independent Transition in Yield Strength of Crystalline Metallic Materials” [Phys. Rev. Lett. 124, 235501, (2020)].
Fig. 1 Editors’ Suggestion (Published 9 June 2020)
Fig. 2. (a) Schematic of yield strength sy as a function of the sample size D; (b) Size-dependent yield stresses sy for the single crystal and ultrafine grain W.
Graduate student Wenbin Liu from Peking University is the first author of this work, and Professor Huiling Duan is the corresponding author. This work was financially supported by National Natural Science Foundation of China.
Huiling Duan’s group has been devoted to the study of mechanical properties of metallic materials under the extreme conditions (high temperature, irradiation, small-scale samples, etc) for many years, and has a series of important results (including J. Mech. Phys. Solids, 2020; 2019; 2015a; 2015b; Int. J. Plasticity, 2020; 2018; 2015; Int. J. Solids Struct., 2016; Phys. Rev. Mater., 2019; Proc. R. Soc. A, 2017).