Letter to the editor

The yield-stress temperature relation for iron at low temperature

During the past two decades, twinning and slip in hexagonal close-packed structures have been extensively studied using molecular dynamics. However, the simulation methods and corresponding results have rendered different conclusions regarding the active twin modes and their mechanisms for nucleation and growth. The nucleation mechanisms for twinning in hexagonal close-packed polycrystalline materials are known to depend strongly on grain boundary orientations, but little is known of the exact mechanisms that occur. The variability in the experimental behavior of single crystals reported in early literature may result from the extreme sensitivity of twinning and slip to heterogeneities in the crystals and their complex dislocation cores. Therefore, both the boundary conditions and loading directions are likely to have profound effects on the mechanisms of deformation captured by molecular dynamics simulations. In an effort to rationalize the inconsistencies reported in literature and guide future molecular dynamics studies, we have performed a comprehensive molecular dynamics study on magnesium that encompasses effects of crystal orientation, boundary conditions, initial defects, and interatomic potentials. The general trends of the results support theories which advocate heterogeneous nucleation for both twinning and slip. In fact, the behavior of perfect crystals with periodic boundary conditions deviated substantially from previous experimental observations. Additionally, strong non-Schmid effects were identified and consistently correlated to non-Schmid stresses. Deviations from Schmid's law were strikingly reduced by introducing defects such as free surfaces and voids, but twinning was still influenced by non-Schmid stresses. Twin–twin interactions led to secondary twinning and nanovoids, which encouraged stress localization and damage.

In order to estimate the tribological properties of structural materials for superconducting magnet at cryogenic temperatures, we investigated Vickers hardness from 4.2 K in liquid helium to 293 K. The clear change of hardness property was obtained around 77 K. We considered that it would be caused by a transformation of the deformation mechanism from dislocation slip at higher temperature to deformation twinning at lower temperature from the thermal activation process. We also considered that the transition temperature changing hardness property was related to the intersection temperature at which slip stress became equal to twinning stress. The change of deformation mechanism was confirmed experimentally by changing the loading rate, and by the TEM observation of the indentation scar.

Investigation was carried out to study the microstructural changes (if any) in medium carbon steel (En-8) deformed at low strain rate (quasistatic) and very high strain rates employing different temperatures. A dynamic compression test rig was used for firing a flat ended cylindrical tool steel projectile against disc shaped specimen placed upon a comparatively rigid anvil. Metallographic specimens were prepared from as-received material as well as from the specimens deformed at low and very high strain rates. The mode of deformation was revealed by metallographic examination of all the deformed specimens. When the specimens were compressed, deformed grains were obtained. No deformation twins were observed in the steel used in the present study which deformed at various strain rates and at sub-zero, room and warm temperatures. It is envisaged that the initiation of twinning during impact was affected by the presence of alloying elements and inclusions in the medium carbon steel, despite its high strain rate sensitivity.

This paper is entirely dedicated to the subject of cleavage microfracture in crystalline solids at non-elevated temperatures. The significance of microcracking in determining the mechanical and fracture characteristics of crystalline solids is outlined. The micromechanics of stress-induced microcracking in polycrystals is reviewed, which occurs as a result of heterogeneous plasticity, due to dislocation pile-up at the grain boundaries. Alternative mechanisms of microcracking mostly pertinent to the cases of fracture of single crystals by the intersecting slip or twin bands, or through splitting of the dislocation walls, are also considered. Included is also a discussion of the fracture of polyphase crystalline solids.