Mechanism of instability of carbides in Fe–TaC alloy under high energy electron irradiation at 673 K

• Fe–TaC alloy was fabricated as a model alloy for F82H steel.
• Instability of TaC in Fe was observed under high energy electron irradiation at 673 K.
• The rate of shrinkage depended on energy, flux, degree of beam focus.
• Displacement of Ta in TaC, or radiation-enhanced diffusion of Ta are the mechanism of instability.

Reduced activation ferritic/martensitic steels (RAFMs), such as F82H steel, are designed to enhance the high-temperature strength by formation of MX-type nanometer-scale precipitates, mainly TaC. However, their instability under irradiation was recently reported. The purpose of this work, therefore, is to clarify the mechanism employing simultaneous observations under electron irradiation at elevated temperature in a high voltage electron microscope. In this work, Fe-0.2 wt.% TaC was fabricated as a model alloy of F82H steel. The instability of the precipitates was observed under electron irradiation at 1 MeV or above. The remarkable shrinkage and disappearance were clearly observed under irradiation with 1.5 MeV and above. On the contrary, the precipitates were mostly stable below 0.75 MeV. Two kinds of mechanism of the irradiation-induced instability were deduced from the electron-energy dependence. One is the dissolution and diffusion of tantalum from precipitates in ferrite matrix. The other is the displacements of tantalum in precipitates that introduce dissolution of Ta into matrix.