Ductile processes at aluminium crack tips: Comparison of orbital-free density functional theory with classical potential predictions

We compare behaviour of quasi-two-dimensional aluminium crack tips undergoing mode I loading using orbital-free density functional theory (OFDFT) and the classical embedded atom method (EAM). Low-index crack orientations are compared in the context of the Griffith, Rice and Tadmor-Hai continuum criteria, using values from Kohn-Sham DFT (KSDFT). All orientations are predicted to be ductile, and twinning is expected to occur only in certain orientations of low-dimensional or low-temperature Al. OFDFT and the EAM predict similar values to KSDFT for the relevant properties. In simulations of two crack orientations, the critical stress intensity factor in EAM simulations is close to continuum predictions while crack tips modelled by OFDFT do not exhibit plasticity until loaded at least 13% over the continuum prediction. The EAM and OFDFT give qualitatively similar results for a crack orientation that emits edge dislocations. For a twinning orientation, OFDFT simulations emit partial dislocations in the same order, even with different pseudopotentials. However, EAM simulations predict that a partial is emitted along a different slip plane from OFDFT. Differences between EAM and OFDFT simulations suggest that methods that give accurate stacking fault energies, elastic constants and surface energies may not necessarily reproduce all important physical processes at crack tips.