Insights into the anomalous hardness of the tantalum carbides from dislocation mobility

The tantalum carbides, TaCx, have been repeatedly shown to harden dramatically with some loss of carbon content, then soften with further decarburization. First observed in 1963, this anomalous hardness behavior has been reproduced for decades without satisfactory explanation. Prior attempts to characterize this phenomenon using elastic stiffnesses have failed to reproduce the anomalous hardness behavior. In this work, we demonstrate a change in slip system preference from {111}B1 to {110}B1 in TaCx as x decreases, while no such transition is observed in TiCx. We find this to be the primary mechanism of the anomalous hardness, arising from reduced energetic favorability of dissociation of dislocations on {111}B1 into Shockley partials at lower carbon contents. We also present experimental hardness measurements for bulk and thin-film TaCx at different carbon contents. An anomalous hardness peak is observed in the bulk samples, but not in the thin films, due to loss of dislocation plasticity in the nanocrystalline films.