Molecular dynamics study on low temperature brittleness in tungsten single crystals

This study employed a numerical model combining molecular dynamics and micromechanics to study the low temperature fracture of tungsten. In the simulations a pre-crack was introduced on the (110) planes and cleavage was observed along the (121) planes. Cleavage along (121) planes has also been observed in experiments. Simulations were performed with three sizes of molecular dynamic regions at 77 K, and it was found that the results were independent of the size. Brittle fracture processes were simulated at temperatures between 77 K and 225 K with the combined model. The fracture toughness obtained in the simulations showed clear temperature dependency, although the values showed poor agreement with experimental results. A brittle fracture process at 77 K was discussed considering driving forces for dislocation emissions and cleavage in an atomic scale region of the crack tip. The driving force for dislocation emissions was saturated after the first dislocation emission, whilst the driving force for cleavage gradually increased with the loading K-field. The increased driving force caused cleavage when it reached a critical value. The critical values of driving force, which were close to the theoretical strength of the materials, were not influenced by temperature. This indicates that the temperature dependency of fracture toughness is not caused by the temperature dependency of dislocation emissions, but by that of dislocation mobility.