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Plastic deformation of the brittle C14-Fe$_2$Nb Laves phase occurs mostly bybasal slip due to the complex crystal structure. Here, we compare the barriersfor basal slip for the known mechanisms of direct slip, synchroshear andundulating slip using density functional theory calculations. According to ourcalculated generalized stacking fault (SF) energies, the most favorablemechanisms are synchroshear and undulating slip. Both mechanisms lead to stableSF with a formation energy of 50 $mJ/m^2$ through the same unstable SFconfiguration at the transition. The energy barrier of approximately 3 $J/m^2$indicates a low dislocation mobility as expected from the brittle character. Wealso determine the influence of vacancies and antisite defects on the formationenergy of stable and unstable SF. Both kinds of point defects tend to lower theenergy barrier on both sides of 2:1 stoichiometry. This explains theexperimentally observed off-stoichiometric softening of C14-Fe$_2$Nb. The smallenergy differences between the Fe$_2$Nb Laves phase polytypes raises thequestion if there are further deformation mechanisms with low barrier.Therefore, we additionally consider transformations between C14, C15 and C36Laves phases as further deformation mechanism. Our calculations for polytypictransformations by successive synchroshear steps show that the correspondingenergy barriers are in fact very similar to the energy barrier for basal slipin C14. This suggests that the energy needed to create a stable SF in C14 bysynchroshear is also sufficient to initiate polytypic transformations whereexisting SFs in C14 are further transformed to form C15 or C36 Laves phases.

Off-stoichiometric softening and polytypic transformations in the plastic deformation of the C14 Fe$_2$Nb Laves phase