Electron energy loss spectroscopy of carbon in dissociated dislocations in tantalum carbide

Abstract

The carbon concentration in individual stacking faults in dissociated dislocations in tantalum carbide (TaCx) was analyzed using electron energy loss spectroscopy. Although the faulted region is less than 10 nm wide, the small diameter electron beam (0.5 nm) of a dedicated scanning transmission electron microscope allowed the carbon K x-ray excitation edge from the faulted region to be distinguished from the corresponding signal from the unfaulted region. The 50-nm thick foil was prepared by grinding, polishing, and ion milling a specimen sawed from a single crystal of TaC0.78. The analysis showed a significantly lower value for the carbon concentration in the fault, in accordance with crystallographic and energy considerations. The stacking fault in NaCl-structure TaCx must exhibit hcp symmetry, but the appropriate hcp phase, Ta2C, contains less carbon. Hence diffusion of carbon away from the moving dislocation must accompany plastic deformation. However, in view of the high melting point and high activation energy for carbon migration, diffusion is slow below approximately 1600 °C. This temperature corresponds approximately with the brittle-ductile transition for TaC. The isomorphic compound TiC does not exhibit dissociated dislocations, and hence this form of Suzuki hardening should not occur. Indeed, gross plastic deformation in TiC can occur at temperatures as low as 800 °C.