Void-swelling in irons and ferritic steels: II. An experimental survey of materials irradiated in a fast reactor

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The void-swelling response of a wide range of ferritic alloys irradiated in the Dounreay Fast Reactor to displacement doses up to 30 dpa (N/2) and covering the temperature range 380–615°C have been compared. The materials selected included high purity irons, together with commercial mild and low alloy steels, high chromium (12–14%) ferritic and martensitic stainless steels and a range of high purity binary iron-chromium alloys containing chromium contents up to 15%. The pure irons and binary iron-chromium alloys exhibited measurable but relatively low swellings (<1%) whilst all the commercial ferritic steels appeared to be void-swelling resistant, with swellings below the experimental detection limit (0.1%). The pattern emerging is thus one of overall swelling resistance in ferritic materials as a general class. Void-swelling in the pure iron peaked at two irradiation temperatures (~420, ~510°C), and the low magnitude of the swelling was rationalized in terms of the operation of solute-controlled swelling suppression mechanisms involving residual interstitial impurities. The complex functional dependence of peak-swelling on chromium content in the binary iron-chromium alloys was explained in terms of void-swelling suppression based on the presence of weak interactions between chromium atoms in solution and vacancies, modified by depletion of chromium from solid solution by α' precipitation at chromium contents exceeding 10%. The validation of the high swelling resistance of the 12% Cr martensitic stainless steels in a fast reactor environment provides confidence in the selection of these alloys as alternative core component materials for commercial fast reactor systems.