Theoretical models for irradiation hardening and embrittlement in nuclear structural materials:a review and perspective

The study of irradiation hardening and embrittlement is critically important for the development of next-generation structural materials tolerant to neutron irradiation,and could dramatically affect the approach to the design of components for advanced nuclear reactors.In addition,a growing interest is observed in the field of research and development of irradiation-resistant materials.This review aims to provide an overview of the theoretical development related to irradiation hardening and embrittlement at moderate irradiation conditions achieved in recent years,which can help extend our fundamental knowledge on nuclear structural materials.After a general introduction to the irradiation effects on metallic materials,recent research progress covering theoretical modelling is summarized for different types of structural materials.The fundamental mechanisms are elucidated within a wide range of temporal and spatial scales.This review closes with the current understanding of irradiation hardening and embrittlement,and puts some perspectives deserving further study.

A novel terminal structure for total dose irradiation hardened of a P-VDMOS

Using positive surface charge instead of traditional γ-ray total dose irradiation, the electric field distribution of a P-channel VDMOS terminal has been analyzed. A novel terminal structure for improving the total dose irradiation hardened of P-channel VDMOS has been proposed, and the structure is simulated and demonstrated with a -150 V P-channel VDMOS. The results show that the peak current density is reduced from 5.51 × 10^3 A/cm^2 to 2.01 × 10^3 A/cm^2, and the changed value of the breakdown voltage is 2.5 V at 500 krad（Si）. Especially, using 60Co and X-ray to validate the results, which strictly match with the simulated values, there is not any added mask or process to fabricate the novel structure, of which the process is compatible with common P-channel VDMOS processes. The novel terminal structure can be widely used in total irradiation hardened P-channel VDMOS design and fabrication, which holds a great potential application in the space irradiation environment.

In-situ TEM investigation of dislocation loop reaction and irradiation hardening in H_(2)^(+)-He^(+) dual-beam irradiated Mo

Through in-situ TEM observation during 30 keV H_(2)^(+)-He^(+) dual-beam irradiation at 723 K,the reaction and transformation of dislocation loops in pure Mo were investigated,especially for<100>loops.Irradiation could directly cause the formation of 1/2<111>loops and<100>loops,but 1/2<111>loops were dominant.In-situ observation confirmed the formation mechanism of<100>loops,including direct irradiation induced mechanism,1/2<111>loop direct conversion mechanism,and reaction mechanism of two 1/2<111>loops.Meanwhile,the reaction of two 1/2<111>loops to produce<100>loop should not require the strict size similarity condition.The reaction between 1/2<111>loops could also produce 1/2<111>loop,which was essentially a process in which one loop absorbed another one.The yield strength increment caused by irradiation-induced loops was analyzed,and its saturation value reached0.48 GPa at 0.06 dpa.Compared with single He+irradiation,the number density and average diameter of loops increased significantly and more serious damage was caused under the synergistic effect of hydrogen and helium.The mechanism based on in-situ experimental observation was discussed in depth.

Effect of tellurium(Te^(4+))irradiation on microstructure and associated irradiation-induced hardening

The GH3535 alloy samples were irradiated using 15-MeV Te^(4+) ions at 650℃to a dose of 0.5,3.0,10,and 20 dpa,respectively.The Te atoms distribution and microstructure evolution were examined by electron probe microanalysis(EPMA)and transmission electron microscopy(TEM).The nano-indenter was then used to measure the nano-hardness changes of samples before and after irradiation.TEM results showed the formation of dislocation loops in the irradiated samples.Their mean diameters increase with the increase of irradiation dose and tends to be saturated when irradiation dose exceeds 10 dpa.The ratio of yield strength increments calculated by dispersed barrier hardening(DBH)model is basically consistent with that of nano-hardness increments measured by nano-indenter.In addition,the relationship between the nano-hardness increments and dpa for the GH3535 alloy irradiated by Te ions has been revealed in the study.

Microstructure and Nano-Hardness of 10 MeV Cl-Ion Irradiated T91 Steel

Hardening and elemental segregation of T91 martenstic steel irradiated by 10 MeV Cl ions to doses from 0.06 dpa to 0.83 dpa were investigated with the nanoindentation technique and transmission electron microscopy（TEM）.The results demonstrated that the irradiation hardening was closely related with irradiation dose.By increasing the dose,the hardness increased rapidly at first from the initial value of 3.15 GPa before irradiation,and then tended to saturate at a value of 3.58 GPa at the highest dose of 0.83 dpa.Combined with TEM observation,the mechanism of hardening was preliminary attributed to the formation of M（Fe,Cr）23C6 carbides induced by the high energy Cl-ion irradiation.

SIMULATION OF THE IN-PILE BEHAVIORS EVOLUTION IN NUCLEAR FUEL RODS WITH THE IRRADIATION DAMAGE EFFECTS

Based on the commercial computational software, a three-dimensional finite ele- ment model to simulate the thermo-mechanical behaviors in a nuclear fuel rod is established; By taking into consideration irradiation-swelling of the pellet and the irradiation damage effects in the cladding together with the coupling effects between the temperature field and the mechanical field, the user subroutines to define the special material performance and boundary conditions have been developed independently and validated. Three-dimensional numerical simulation of the thermo-mechanical coupling behaviors in a nuclear fuel rod is carried out, and the evolution rules of the important thermal and mechanical variables are obtained and analyzed. The research re- sults indicate that： （i） the fuel pellets will be in contact with the cladding at high burnup, which will induce a strong mechanical interaction between them; （2） the irradiation creep effect plays an important role in the mechanical behavior evolution in the nuclear fuel rod.

Irradiation behaviors of two novel single-phase bcc-structure high-entropy alloys for accident-tolerant fuel cladding

High-entropy alloys(HEAs)are potential alternative materials for accident-tolerant fuel cladding due to their excellent irradiation resistance and high-temperature corrosion resistance.In this work,two novel body-centered cubic(bcc)structured Mo_(0.5)Nb Ti VCr_(0.25)and Mo_(0.5)Nb Ti V_(0.5)Zr_(0.25)HEAs were fabricated.Helium-ion irradiation was performed on the two HEAs to simulate neutron irradiation,and the crystal structure,hardness,and microstructure evolution were investigated.The crystal structure of the Mo_(0.5)NbTiVCr_(0.25)HEA remained stable at low fluences,while amorphization may occur at high fluences in the two HEAs.The irradiation hardening value of the Mo_(0.5)NbTiVCr_(0.25)was 0.77 GPa at fluences of 1×10^(17)ions/cm^(2)and 1.49 GPa at fluences of 5×10^(17)ions/cm^(2),while the hardening value of the Mo_(0.5)NbTiV_(0.5)Zr_(0.25)was 1.36 GPa at ion fluences of 5×10^(17)ions/cm^(2).In comparison with most of the conventional alloys,the two HEAs showed slight irradiation hardening.The helium bubbles and dislocation loops with small size were observed in the two HEAs after irradiation.This is the first time to report the formation of a dislocation loop in bcc-structure HEAs after irradiation.However,voids and precipitates were not observed in the two HEAs which could be ascribed to the high lattice distortion and compositional complexity of HEAs.This research revealed that the two HEAs show outstanding irradiation resistance,which may be promising accident-tolerant fuel cladding materials.

Unusual He-ion irradiation strengthening and inverse layer thickness-dependent strain rate sensitivity in transformable high-entropy alloy/metal nanolaminates:A comparison of Fe_(50)Mn_(30)Co_(10)Cr_(10)/Cu vs Fe_(50)Mn_(30)Co_(10)Ni_(10)/Cu

In this work,we prepare transformable HEA/Cu nanolaminates(NLs)with equal individual layer thick-ness(h)by the magnetron sputtering technique,i.e.,Fe_(50)Mn_(30)Co_(10)Cr_(10)/Cu and Fe_(50)Mn_(30)Co_(10)Ni_(10)/Cu,and comparatively study He-ion irradiation effects on their microstructure and mechanical properties.It ap-pears that the as-deposited HEA/Cu NLs manifest two size h-dependent hardness regimes(i.e.,increased hardness at small h and hardness plateau at large h),while the He-implanted ones exhibit monotonically increased hardness.Contrary to the fashion that smaller h renders less irradiation hardening in bimetal NLs,the Fe_(50)Mn_(30)Co_(10)Cr_(10)/Cu NLs manifest the trend that smaller h leads to greater irradiation hard-ening.By contrast,the Fe_(50)Mn_(30)Co_(10)Ni_(10)/Cu NLs exhibit the maximum irradiation hardening at a critical h=50 nm.Below this critical size,smaller h results in lower radiation hardening(similar to bimetal NLs),while above this size,smaller h results in greater radiation hardening(similar to Fe_(50)Mn_(30)Co_(10)Cr_(10)/Cu NLs).Moreover,these transformable HEA/Cu NLs display inverse h-dependent strain rate sensitivity(SRS m)before and after He-ion irradiation.Nevertheless,compared with as-deposited samples,the irradi-ated Fe_(50)Mn_(30)Co_(10)Cr_(10)/Cu NLs display reduced SRS,while the irradiated Fe_(50)Mn_(30)Co_(10)Ni_(10)/Cu NLs dis-play enhanced SRS.Such unusual size-dependent irradiation strengthening and inverse h effect on SRS in irradiated samples were rationalized by considering the blocking effects of He bubbles on dislocation nucleation and motion,i.e.,dislocations shearing or bypassing He bubbles.

Effects of MeV Fe Ions Irradiation on the Microstructure and Property of Nuclear Grade 304 Stainless Steel:Characterized by Positron Annihilation Spectroscopy,Transmission Electron Microscope and Nanoindentation

Nuclear grade 304 stainless steel was irradiated by 3.5 MeV Fe ions,with fluxes of 3.05E+15 ions/cm^(2)and 1.55E+16 ions/cm^(2).Irradiation effects were studied by positron annihilation spectroscopy(PAS),transmission electron microscope(TEM)and nanoindentation techniques.PAS results showed that different types of defects were produced after irradiation and that there was significant variance in defects formed when the samples were subjected to different irradiation doses.TEM characterization showed that the irradiation-induced dislocation loops enlarged in average size,but decreased in number density at higher irradiation doses.Nanoindentation test showed obvious irradiation hardening phenomenon,which was in good agreement with the PAS and TEM results.Irradiation hardening effect increased with an increase in irradiation dose and saturation occurred with an increase in irradiation dose from 3.2 to 16 dpa.Further statistical analysis showed that barrier strength of the Frank loop depends on the loop size and density produced by the ion irradiation.