Achieving high fracture toughness and maintaining high strength at the same time are main goals in materials science. In this work, scale-bridging fracture experiments on ultrafine-grained chromium (UFG, Cr) are performed at different length scales, starting from the macroscale over the microscale (in situ SEM) down to the nanoscale (in situ TEM). A quantitative assessment of the fracture toughness yields values of ∼3 MPa m1/2 in the frame of linear elastic fracture mechanics (LEFM) for the macrosamples. The in situ TEM tests reveal explicitly the occurrence of dislocation emission processes involved in energy dissipation and crack tip blunting serving as toughening mechanisms before intercrystalline fracture in UFG body-centered cubic (bcc) metals. In relation to coarse-grained Cr, in situ TEM tests, in this work, demonstrate the importance of strengthening grain boundaries as promising strategy in promoting further ductility and toughening in UFG bcc metals.

Nanoscale models are very small but involve multiple physics, multiscales, confinement, high rates, etc. These make the numerical simulation of intrinsic and extrinsic size effects difficult for the ferrite/cementite layered structure of carbon steel. In this work, a “new” simulation approach is proposed with “hypothesis” as the key to make the on-going simulation simple, physically sound, and the related atomistically based simulation productive. Using this refreshed approach, which is based on the traditional scientific philosophy, size effects that are related to interface structure and the layer thickness on failure due to thermomechanical coupling are investigated. Among interesting findings, it proves that the peak stress in the strain–stress curve as the characteristic parameter to describe the size effects on failure of thermomechanical coupling is not fully accurate, and a multiplication form of energy barrier with fast decaying function of thermal activation energy should be used in the size-dependent failure initiation criterion. This makes developing guidelines available for designing interface sizes at nanoscale.

Grain size effect on twin thickness has been rarely investigated, especially when the grain size is less than 1000 nm. In our previous work (Mater. Sci. Eng.A527, 3942, 2010), different severe plastic deformation techniques were used to achieve a wide range of grain sizes from about 3 μm to 70 nm in a Cu–30% Zn alloy. Transmission electron microscopy (TEM) revealed a gradual decrease in the deformation twin thickness with decreasing grain size. In the present work, high-resolution TEM was used to further identify deformation twins and measure their thickness, especially for grain sizes below 70 nm. The twin thickness was found to gradually reduce with decreasing grain size, until a critical size (20 nm), below which only stacking faults were observed. Interestingly, the relationship between twin thickness and grain size in the ultrafine/nanocrystalline regime is found similar to that in the coarse-grained regime, despite the differences in their twinning mechanisms. This work provides a large set of data for setting up a model to predict the twin thickness in ultrafine-grained and nanocrystalline face-centered cubic materials.