Estelle Meslin

Effect of C on Vacancy Migration in α-Iron

Carbon atoms are always present in Fe-based materials, either as impurities even in high purity samples or as an intrinsic constituent in steels. Density Functional Theory calculations have been performed to study the interaction between C atoms and vacancies (V) in α-Fe. We find that the formation of VCn complexes is energetically favourable for n ≤ 3, with VC2 being the most stable one. The energy gain corresponding to the clustering reaction VCn-1 + C → VCn depends mainly on the strength of C-C covalent bonds. The vacancy diffusivity is shown to be significantly modified by the formation of vacancy-carbon complexes, exhibiting non-Arrhenius behaviour. Effective vacancy diffusion coefficients in α-Fe are calculated as a function of temperature and carbon content using a simplified thermodynamic model. The results are discussed in detail in the limiting case of excess of C with respect to vacancies.

Influence of injected interstitials on α′ precipitation in Fe–Cr alloys under self-ion irradiation

ABSTRACT Depending on the particle used to irradiate Fe–Cr alloys, at the same dose and temperature, α′ precipitation may occur or not. This paper aims at explaining the origin of the absence of α′ precipitation under ion irradiation. A Fe–15at.%Cr alloy was irradiated with 2 MeV Fe2+ ions at 300°C and analysed using atom probe tomography. For the first time to our knowledge, α′ particles were observed under ion irradiation. The characterisation of the α/α′ decomposition with depth showed that injected Fe strongly reduced α′ precipitation. They might have played a major role in the absence of α′ precipitation under ion irradiation. GRAPHICAL ABSTRACT IMPACT STATEMENT This paper reports the first observation of α′ precipitation in Fe–Cr alloys after heavy ion irradiation and demonstrates that the injected interstitial effect drastically modifies the precipitation behaviour.

Dramatic reduction of void swelling by helium in ion-irradiated high purity α-iron

ABSTRACT Effect of helium on void swelling was studied in high-purity α-iron, irradiated using energetic self-ions to 157 displacements per atom (dpa) at 773 K, with and without helium co-implantation up to 17 atomic parts-per-million (appm) He/dpa. Helium is known to enhance cavity formation in metals in irradiation environments, leading to early void swelling onset. In this study, microstructure characterization by transmission electron microscopy revealed compelling evidence of dramatic swelling reduction by helium co-implantation, achieved primarily by cavity size reduction. A comprehensive understanding of helium induced cavity microstructure development is discussed using sink strength ratios of dislocations and cavities. IMPACT STATEMENT Reduction of void swelling by helium co-implantation is reported, highlighting that it’s not always true that swelling will be higher in metals when helium is present along with irradiation damage. GRAPHICAL ABSTRACT