TEM-EELS analysis reveals the W-atom mediated radiation-induced amorphization in M23C6

Abstract

To gain a mechanistic understanding of the phase stability of M₂₃C₆ upon irradiation, the bulk W-doped M₂₃C₆ (Cr-W-C system) in the range of 0–12 at.% W concentration was prepared and subjected to helium beam irradiation, following with a thorough electron energy loss spectroscopy (EELS) analysis. Radiation-induced amorphization (RIA) was observed only at the 4 W sample with a W concentration of ∼12 at.%. Analysis of the low-loss spectrum showed that the inelastic mean free path (λ) could be applied an effective indicator of the presence of an amorphous phase. The white line ratio of the carbon K-edge spectrum showed that the chemical bonding state in the crystalline state is mainly 2p^3/2 bonding, and it changes to dominantly 2p^1/2 bonding accompanying with the crystal-to-amorphous (c-a) transition. Discussion on the relationship between the change in λ (Δλ) and the lattice parameter (Δa) due to irradiation reveals that Δa is not dependent on Δλ, indicating that Δλ is mainly caused by the volume expansion due to the c-a transition. In addition, a crystalline state is remained even after a lattice parameter change of ∼1.5 % in 0 W and 1W-samples, whereas, a lattice expansion of ∼0.2 % would trigger the occurrence of crystal-to-amorphous transition in the 4W-sample. The detailed EELS analysis demonstrated that the constitutional W atoms play an important role in facilitating the occurrence of RIA in M₂₃C₆, that is, the phase instability accompanying the lattice expansion due to irradiation was emphasized by the addition of W in M₂₃C₆. The insights obtained here suggest that a higher W concentration in M₂₃C₆ is more susceptible to RIA, and therefore the resistance to amorphization is achievable by decreasing the W concentration in the steels.