Nanocomposites and nanomaterials

Radiation resistance of steel T91 after deformation in different structural states
Rostova H.Yu., Tolstolutskaya G.D., Tikhonovsky М.А., Kalchenko А.S.
National Science Center "Kharkiv Institute of Physics and Technology". 
1, Akademicheskaya St., Kharkiv, 61108, Ukraine 
E-mail: veg-annie@ukr.net

Ferritic-martensitic steels are considered as promising structural materials for future generations of reactors. Their undoubted advantage is their high sensitivity to the conditions of thermal and thermomechanical treatment (TMT), which makes it possible to modernize their structure, mechanical characteristics and increase radiation tolerance.
In this work, as a TMT, we used the multi-cycle “upsetting-extrusion” (MUE) method developed at the NSC KIPT, which has proven itself in the production of ultrafine-grained materials, both in laboratory and industrial conditions [1].
MUE of T91 steel was carried out in two temperature ranges: in the austenitic region and with a sequential decrease in the deformation temperature in the ferrite region. Subsequent heat treatment was carried out at temperatures of 730 ... 550 °C for 1 ... 50 h to stabilize the structure and precipitate carbide phases. Irradiation was carried out with Ar ions with energy of 1.4 MeV up to a dose of 120 dpa.
It was found by optical and transmission electron microscopy that the average grain size of T91 steel as a result of MUE decreases from 20 μm in the initial state to 140 nm after 5 cycles of MUE, which led to the achievement of record values for this steel of the yield strength (1279 MPa at 25 °C and 502 MPa at 600 °C) and strength (1340 MPa at 25 °C and 511 MPa at 600 °C). With an increase in the number of cycles and a decrease in the "upsetting-extrusion" temperature, simultaneously with a decrease in the average grain size, an increase in the degree of uniformity of their size distribution occurs. The creation of a high density of traps for the recombination of radiation defects (grain boundaries, nanosized carbides) during such processing led to a decrease in swelling by a factor of 1.3.
A study of the development of void structure after irradiation of steel T91 in various structural states was carried out. It was shown that after irradiation with Ar ions with an energy 1.4 MeV to a dose 120 dpa at T = 460 °C, the swelling in the ultrafine-grained ferritic structure is 0.65% compared with 0.26% for the initial martensitic structure, which confirms the influence of structural-phase state of steel T91 on its swelling. For comparing, the swelling of steel with an austenitic structure is ~ 15.6%, which is 24 times more than the level of swelling of steel with a ferritic structure.
Argon stabilizes cavity formation by a pressure effect. Numerous prior studies have shown that when the argon concentration is low, the cavity swelling initially increases with increasing Ar concentration. For moderate to high Ar concentrations, relatively small pressurized bubbles and relatively large underpressurized voids lead to bimodal cavity size distributions. The observed cavity swelling temperature regime for MeV Ar ion irradiated T91 steel is much broader than values suggested by ion irradiation studies using low ion energies. This is attributed to several well-known artifacts for ion irradiation research: cavity denuded zones with an adjacent enhanced swelling region and diffusional broadening of injected ions can reduce the breadth of the temperature regime for cavity swelling, and generally shift the cavity swelling regime to higher temperatures with increasing dose rate.

1.	Чорний О.В. та ін. Патент 42487 А Україна, МПК 7 Н01В12/00 «Спосіб виготовлення ніобій-титанового надпровідника». Бюл. №9. Заявл. 12.03.2001; Опубл. 15.10.2001.