Hardening And Embrittlement Of Fe Based Alloys Irradiated By High Energy Heavy Ions

In active and advanced nuclear systems including fast breeder reactor and fusion reactors,structure components such as pressure vessel,fuel cladding and first wall/blanket have to endure harsh conditions of high temperature,high pressure and strong irradiation.The intensive neutron irradiation will generate high concentration of defects in materials and result in performance degradation,seriously affecting the service safety of the nuclear reactor.Hardening/embrittlement caused by long-term irradiation of high-energy neutrons is the main form of material performance degradation.Therefore,clarifying the irradiation hardening/embrittlement behavior and its physical mechanism is of guiding significance for evaluating the safe service life of the reactor internals and developing new candidate structural materials.Heavy ion beams have long been used to simulate neutron irradiation damage in materials,due to the advantages of high atom displacement damage rate,controllable irradiation parameters,low radioactivity and low cost.In addition,energetic heavy ions can produce similar initial cascade damages as by fast neutrons in materials.In this paper,the irradiation hardening and embrittlement of three representative iron-based alloys（Reactor Pressure Vessel steel,Reduced Activation Ferritic/Martensitic steel and Oxide Dispersion Strengthened ferritic steel）for the active Pressurized Water Reactor（PWR）and advanced nuclear power system were studied under high energy heavy ion irradiation.The ion beam was provided by Heavy Ion Research Facility in Lanzhou（HIRFL）.An energy degrader was used in the irradiation experiment to generate a quasi uniform distribution of atomic displacement damage plateau within the thickness of about 23μm in the material sample.The mechanical properties,such as hardening and ductility loss,were characterized by nano-indenter,Vickers microhardness tester and small punch test teconology.The microstructure was analyzed by positron annihilation lifetime spectrum and transmission electron microscope.At the same time,the defect reaction rate equation and dispersion barrier hardening model were used to calculate and discuss.The results were compared with the experimental.This paper focused on the influence mechanism of defects on the irradiation hardening and embrittlement of materials.The main work is described as follows:1.Irradiation hardening of RPV steel:The RPV steel is a key component of PWR in active service.The A508-3 RPV steel was irradiated with 352.8 MeV ⁵⁶Fe ions to two damage levels of 0.15 and 0.21 displacements per atom（dpa）.The sample temperature was kept at 173 K.The Vickers micro-hardness of the damaged layer was measured from the specimens after stepwise thermal annealing at temperatures of 300,573,623 and 673 K,respectively.A Nix–Gao model was used to fit the obtained data,the results show that the specimens irradiated to both two doses exhibit obvious hardening.With the increase of annealing temperature,the hardness decreases monotonously.Fitting of the Arrhenius plots gives an apparent activation energy of0.10±0.01 eV for the temperature regime from 573 to 673 K.The results of positron annihilation lifetime spectra show that a large number of vacancy produced by irradiation obviously recombined with the increase of annealing temperature.The vacancy then agglomerates with each other and grows up gradually at the temperature higher than 573 K.The results of TEM show that dislocation loops in a high density were generated in the irradiated specimens after annealed at 673 K,and no voids with size greater than 1 nm existed.Assuming that the evolution of dislocation loops is controlled by the migration and coalescence process of self interstitial atom（SIA）clusters,an average activation energy E^m（0.55±0.05 e V）was deduced for the migration of SIA clusters.2.Irradiation hardening and embrittlement of ODS ferritic steel:ODS steel has important application prospects in fault-tolerant fuel technology and fusion reactor cladding due to its excellent high temperature creep resistance and irradiation swelling resistance.Three ODS ferritic steels containing different oxide dispersoids were irradiated with 357.9 MeV ⁵⁸Ni ion to 0.8 dpa at 223 K.The nano-indentation and Vickers micro-hardness were measured.The Nix-Gao model taking account of the indentation size effect（ISE）was used to fit the hardness data.The small punch test was carried out to obtain the change data of material elongation.A linear relationship between nano-hardness and micro-hardness was found.Three ODS steels show different hardening/embrittlement phenomenon after irradiation,among which a higher number density of oxide dispersoids with a smaller average diameter corresponds to an increased resistance to irradiation hardening/embrittlement.This can be attributed to the high absorption intensity of point defects at the oxide/matrix interface,which effectively inhibits the nucleation and growth of dislocation loops.The ductility loss rate induced by irradiation is mainly relate to irradiation hardening.There is a power law correlation between the sink strength of oxide dispersoids and the irradiation hardening amount.The improved irradiation resistance is primarily attributed to the increased sink strength.3.Irradiation hardening and embrittlement of RAFM steel:RAFM steel is the main candidate material for the fusion reactor cladding.The CLF-1 steels were irradiated with high-energy ¹⁴N and ⁵⁶Fe ions to three successively increasing damage levels of 0.05,0.1 and 0.2 dpa at 223 K.The results of nanoindentation and micro-hardness test results show that hardening was observable at the lowest damage level,and increased with increasing irradiation dose.A power-law relationship between hardness and damage level is proposed,which is similar to the dose-dependence of irradiation hardening in other RAFM steels（CLAM,JLF-1,F82H,EUROFER97,etc.）under neutron or charged particle irradiation conditions.The ductility loss rate of the material varies with the dose has the same trend as the hardness.The results of positron annihilation lifetime spectroscopy and TEM show that the size of vacancy group was still less than 1 nm when the damage level reached to 0.2 dpa,which contributed less to irradiation hardening.Assuming that interstitial dislocation loops are the main reason for irradiation hardening,the dose dependence of irradiation hardening given by the defect reaction rate equation combined with the dispersion barrier hardening model is consistent with the experimental results.