Radiation induced amorphization of carbides in additively manufactured and conventional ferritic-martensitic steels: In-situ experiments on extraction replicas

In-situ irradiations using 1 MeV Kr²⁺ ions in a transmission electron microscope were performed on extraction replica samples containing M₂₃C₆ carbides and MX carbonitrides from two ferritic-martensitic (FM) steels: (i) 9%Cr-1%W-TaV based Eurofer97 and (ii) 9%Cr-1%Mo-VNb (all in wt.%) based additively manufactured (AM) Grade91 steel. The irradiations were performed between 100 and 773 K up to a maximum dose of ∼2.4 displacements per atom (dpa). The M₂₃C₆ carbides are highly susceptible to radiation induced amorphization (RIA), while the MX type nanoprecipitates are highly amorphization resistant across the entire irradiation temperature range. Between 100 and 423 K, RIA of M₂₃C₆ carbides occurs very rapidly with critical amorphization doses ranging between ∼0.35 and 0.9 dpa, increasing to higher doses at 573 K. Complete amorphization of the M₂₃C₆ carbides up to doses of ∼2.4 dpa is not possible at 773 K under the present irradiation conditions. The critical amorphization dose of M₂₃C₆ carbides increases nearly exponentially with irradiation temperature. The critical temperature for the crystalline-to-amorphous phase transformation (T_c→a) of M₂₃C₆ carbides irradiated as replica samples, i.e. without the surrounding metal matrix, was estimated to be ∼812 K. Comparing the present results with neutron irradiation data on bulk samples reveals a decrease in the critical amorphization dose for M₂₃C₆ and an increase of T_c→a, highlighting the effect of dose rate on the amorphization behavior that is qualitatively consistent with literature on other non-metals. Changes in the minor chemistry of M₂₃C₆, such as presence or absence of W, V, Nb, Mo, seems to have little effect on the amorphization behavior.