Microstructure assessment of the low activation ferritic/martensitic steel F82H

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Abstract

The microstructure of the low activation F82H ferritic/martensitic steel has been investigated in the cases of the heat treated, irradiated and deformed material. The irradiation has been achieved with 590 MeV protons in the Proton IRradiation EXperiment (PIREX) facility to a dose of 0.5 dpa at a temperature of 523 K. The unirradiated material was deformed in tension to failure at room temperature. The dislocations character as well as the dislocation density are determined. The carbide chemical composition and the size distribution of the carbides are assessed.

Introduction

The ferritic/martensitic steels are candidates for the future fusion reactor structural components where doses above 100 dpa are expected. Even though these steels have proved to be a good alternative to austenitic steels, with respect to swelling resistance, there are a number of problems related to the changes in mechanical properties and to the activation under irradiation. In response to the latter point the chemical composition of the ferritic/martensitic steel F82H has been designed to obtain a reduced long term radioactivity. Owing to the complexity of the microstructure the mechanical properties are expected to depend on the chemical composition, the pre-austenite grain (PAG) sizes, the martensitic lath sizes, the carbide size distribution and composition, and the dislocation microstructure.

Irradiation is known to drive the microstructure to both the formation of He bubbles 1, 2, and to a change in the dislocation configuration 2, 3, 4, 5. The He production rate for fusion 14 MeV neutron is 13 appm/dpa [6]and about 130 appm/dpa for 590 MeV protons, but the effect of the He on the mechanical properties is believed to be small 7, 8. H production is about 800 appm/dpa for 590 MeV protons [9]but rapidly escapes the material. The relative resistance of ferritic/martensitic steels to swelling which increases at a rate of about 1% for 100 dpa [10]as compared to the 1% for 10 dpa for austenitic steels has to be explained by the fact that the irradiation induced vacancies are impeded to form voids. Several models were proposed in that sense on the basis of (1) a trapping mechanism of the interstitial and vacancies to impurities and alloying elements [10]that increases recombination, (2) the trapping of both type of point defects on dislocations with a Burgers vector of a/2 〈1 1 1〉, and (3) the point defect bias to dislocations being inherently lower in the bcc than in the fcc structure, the effect of the presence of dislocations with a Burgers vector equal to a 〈1 0 0〉 which trapping of interstitials leads to vacancy accumulation [4]is reduced. In the ferritic/martensitic steels the dislocation structure after irradiation develops dislocations with both a/2 〈1 1 1〉 and a 〈1 0 0〉 Burgers vectors, the latter being predominant when the Cr content is below the one of Fe–6Cr [2]. The a/2 〈1 1 1〉 is the most common Burgers vector in the bcc structure [10]whilst irradiation induced dislocations with the a 〈1 0 0〉 Burgers vector arises from the growth of a 〈1 0 0〉 interstitial loops [5].

A TEM study of the F82H microstructure is presented here. The mechanical properties and the chemical behavior of the F82H steel are presented elsewhere 11, 12.

Section snippets

Experimental

The ferritic/martensitic steel denominated F82H [13]have a composition of about 7.65 wt% Cr, 2 wt% W, and Mo, Mn, V, Ta, Ti, Si and C below 1 wt% in sum total, and Fe for the balance. The samples were submitted to the heat treatment (0.5 h at 1313 K for normalization and 2 h at 1013 K for tempering) that allows to obtain a fully martensitic structure.

The irradiation has been performed in the PIREX facility located in the Paul Scherrer Institut of Villigen, Switzerland, with protons of 590 MeV

Results and discussion

The micrographs of the general microstructure of the F82H heat treated (Fig. 1(a)), deformed (Fig. 1(b)) and irradiated (Fig. 1(c)) show common features characterized by both PAG boundaries and martensite laths. The martensite laths are about 1 μm wide and can reach more than 5 μm in length as it is clearly visible on Fig. 1(a). The lattice parameter of the heat treated F82H was measured with the diffraction patterns and the value averaged on three patterns obtained on different TEM sessions is

Conclusion

The dislocation structure of the F82H ferritic/martensitic steel consists of 1/2a0 〈1 1 1〉 Burgers vector dislocations. It appears that there is a strong tendency to screw character orientation in the unirradiated and specimens irradiated to 0.5 dpa, which suggests a Peierls mechanism. No differences in either the carbide microstructure, composition or the size distribution were found between unirradiated and irradiated specimens.

Acknowledgements

Dr David Gelles is acknowledged for his advices on the observation in TEM of magnetic specimens.

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