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

Abstract

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 (Tc→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 Tc→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. In-situ irradiations on extraction replica samples provide a novel pathway to explore radiation tolerance of nanoprecipitates in nuclear structural materials