Dual-beam Irradiation Research Articles

Effect of pre-existing dislocations and precipitates on microstructure evolution in W-0.5ZrC alloy during in-situ He+ & H2+ dual-beam synergistic irradiation

Bubble evolution in different defect sinks has a crucial effect on the properties of materials for fusion reactor. Here, transmission electron microscopy (TEM) was used to in-situ investigate bubble evolution in different defect sinks, including pre-existing dislocations and precipitates in W-0.5ZrC alloy during He+ & H2+ dual-beam synergistic irradiation at 900 °C. The average size and volume number density of bubbles with increasing irradiation dose were obtained, revealing not only the significant effect of different types of defect sinks, but also the obvious impact of the same type of defect sinks in different states. It was found that linear pre-existing dislocations promoted bubble growth while inhibited nucleation. Pre-existing dislocations characterized by curved form exhibited strong inhibition effect on bubble nucleation, while pre-existing dislocations with staggered-mesh form, not only inhibited bubble growth but also suppressed nucleation compare to the linear pre-existing dislocations. ZrC precipitates had minimal effect on bubble growth, but could decrease the bubble nucleation. Dispersed small precipitates could better reduce the bubble nucleation sites in the early stage of irradiation. Once the absorption capacity of small precipitate was saturated, bubble growth might accelerate sharply. The presence of hydrogen made the linear pre-existing dislocation maintain high mobility. The current research results are of great significance for understanding the irradiation behavior of tungsten alloys and optimizing the preparation process.

The effect of stress state and He concentration on the dislocation loop evolution in Ni superalloy irradiated by Ni+ & He+ dual-beam ions: In-situ TEM observation and MD simulations

In-situ TEM observation was conducted during Ni+&He+dual-beam irradiation to monitor the evolution of dislocation loops accompanied by He bubbles in the Ni-based alloy GH3535. Two distinct evolutions of dislocation loops, driven by residual stresses, were observed within the monitored grains. Hence, molecular dynamics (MD) simulations were employed to reveal the effects of stress magnitude and direction on loop evolution, including size, number density, type and variation. The simulations revealed that the presence of compressive stress reduced the formation energy of perfect dislocation loops, thus promoting their formation. Stress state was found to influence the preferential orientation of the loops, and compressive stress resulted in a decreased number density of dislocation loops but an increase in their size. This establishes a clear relationship between stress state and magnitude and the evolution of dislocation loops during ion beam irradiation. Additionally, the nature and characteristics of dislocation loops were quantified to explore the effects of He concentrations on their evolution. The higher He concentration not only promotes the nucleation of dislocation loops, leading to their higher number density, but also facilitates the unfaulting evolution by increasing the stacking fault energy (SFE). Moreover, the accumulation of He in the lower-He-concentration sample led to the growth of dislocation loops in multiple stages, explaining their nearly identical average sizes when compared to the higher-He-concentration sample.

Effect of hydrogen, helium and dose rate on the evolution of dislocation loop in pure iron during in-situ ion irradiation

Simulating neutron irradiation with ion irradiation requires considering the comprehensive effects of hydrogen, helium and dose rate to fully understand the microstructure evolution of material in an irradiation environment. Here, the characterization and evolution of dislocation loops in pure iron were investigated under heavy ion irradiation with four different injected gas ion types (Fe+, Fe++H2+, Fe++He+and Fe++H2++He+) and under dual-beam (Fe++He+) irradiation with four different dose rates using in-situ transmission electron microscopy at 723 K. Comparing to the single-beam (Fe+) irradiation, the additional injection of H or He, or co-injected H and He, mainly promoted the nucleation of dislocation loops at the early stage of irradiation and enhanced loop growth after that. Among them, the simultaneous irradiation of triple beams (Fe++H2++He+) was most pronounced. Moreover, triple-beam (Fe++H2++He+) irradiation resulted in the highest ratio of the rotation of loop habit-plane. Under dual-beam (Fe++He+) irradiation, the results demonstrated that as the dose rate increased, the loop density increased and the loop size decreased. Moreover, the fraction of 〈100〉 loops at low dose rate was noticeably greater than that at high dose rate. The results of the current work provide important references for a better understanding of the effectiveness of ion irradiation in simulating neutron irradiation.

The effect of hydrogen-helium synergism on dislocation loops in RAFM steel at different temperatures by dual-beam ion irradiation

Simultaneous Fe+H ion irradiation (denoted as Fe+H) and Fe+H ion irradiation with pre-implantation of He (denoted as He/Fe+H) were carried out on Chinese Low-Activation Martensitic (CLAM) steels at 350 ℃-550 ℃ to investigate the synergistic effect between H and He. The peak dose was 15 dpa, the He concentration in the observation area was 11 appm/dpa and the H concentration was 44 appm/dpa. The microstructure and hardening of the irradiated samples were evaluated by TEM and nanoindentation, respectively. Bubbles were not observed in irradiated CLAM steels at all temperatures, which implies that the promotion of bubbles by H may not be as strong as that by He and that H attenuates the promotion of bubbles by He. The hardening induced by He/Fe+H irradiation at 450 °C is about 1.6 times that of Fe+H irradiation, the synergistic effect of H and He is most significant at this temperature. Comparison with Fe+He irradiation and single Fe irradiation reveals that Fe+H irradiation at 350 °C induces lower interstitial defect damage than Fe+He irradiation, but the level of interstitial defects induced by the synergistic effect of both H and He with displacement damage is comparable at higher temperatures. The average size of dislocation loops induced by Fe+H irradiation is between that of single Fe irradiation and Fe+He irradiation, but the number density of dislocation loops induced by Fe+H irradiation is higher than that of Fe+He irradiation. The synergistic effect of He with displacement damage tends to promote the growth of dislocation loops, while the synergistic effect of H tends to promote the nucleation of dislocation loops.

In-situ TEM investigation of zirconium alloy under Kr+ single-beam and Kr+-He+ dual-beam synergetic irradiation

The in-situ TEM irradiation experiments of zirconium alloy were conducted at 573 K, 673 K, and 773 K utilizing a 400 keV Kr + single beam and a 400 keV Kr+ and 30 keV He + dual beam. The results show that a large number of dislocation loops have been characterized in the matrix of the zirconium alloy under irradiation. With increasing the irradiation damage dose, some dislocation loops have reacted with one another to form a larger dislocation loop, which has finally formed dislocation lines or other defect structures. In zirconium alloys irradiated with Kr + single beam and Kr+ and He + dual-beam radiation, the proportion of <a> type dislocation loops with different Burgers vectors is essentially the same at low damage doses, but the proportion of interstitial type dislocation loops with the same Burgers vectors is obviously different. The amorphization of the second phase and the dissolution of the small-sized second phase were also pointed out. With the increase in temperature, the density of the dislocation loop in zirconium alloy gradually decreases, and the size of dislocation loop first increases and then decreases. Kr+ and He + dual beam irradiation increases the size of dislocation loops but decreases their density as compared with Kr + single beam irradiation.

Effect of He/dpa ratio on bubble characteristics in Fe9Cr1.5W0.4Si F/M steel during irradiation and annealing

The He production rate (i.e., He/dpa) in nuclear reactors strongly affects the degradation of material properties. This is an important but not yet fully understood issue. Here, the effect of He/dpa on bubble characteristics in Fe9Cr1.5W0.4Si ferrite-martensitic (F/M) steel was in situ studied during 400 keV Fe+ and 30 keV He+ dual-beam irradiation at 723 K with three ratios of 100, 500, and 2500 appm He/dpa and subsequent stepwise annealing using transmission electron microscopy (TEM). He/dpa strongly affected the bubble characteristics. During irradiation, the higher the He/dpa, the smaller the size of irradiated bubbles, but the higher their density. However, He/dpa didn't affect the final saturation size of irradiated bubbles for all three cases, which was ∼2.2 nm. During annealing, high He/dpa caused large, immobile, dense polyhedral bubbles with a wider bubble size distribution, while low He/dpa caused small, low-mobility, and relatively low-density spherical bubbles. It was found that the higher the He/dpa ratio, the greater the swelling during irradiation and annealing, and annealing further enhanced the swelling. Moreover, the tunnel structure was first found in body-centered cubic (BCC) F/M steel during in-situ irradiation. The current work provides valuable and potential insights for further understanding the He/dpa effects in materials serving in different nuclear reactors.

Microstructural evolution of compositionally complex solid-solution alloys under in-situ dual-beam irradiation

This work attempts to link the microstructural evolution of single-phase compositionally complex alloy (CCA) compositions under dual-beam irradiation to their Mn-content via the stacking-fault energy (SFE) and vacancy migration energies. Two alloys, Cr18Fe27Mn27Ni28 and Cr15Fe35Mn15Ni35, along with less compositionally complex pure Ni and Fe56Ni44 binary were irradiated at 500 and 600 °C under dual-beam 1 MeV Kr2+ and 16 keV He+ heavy-ions up to 7 displacements per atom (dpa) with a He/dpa ratio of 0.75 %/dpa using in situ transmission electron microscopy (TEM). Due to the bubble-stabilizing effect of implanted He, bubbles were observed in all irradiations, and populations of faulted interstitial loops were characterized in Cr18Fe27Mn27Ni28 and Cr15Fe35Mn15Ni35. A reduction in swelling was observed in the two CCAs compared to pure Ni and Fe56Ni44. Although swelling increased from 500 to 600 °C in Fe56Ni44, Cr15Fe35Mn15Ni35 and Cr18Fe27Mn27Ni28 both swelled slightly more at 500 °C. This was attributed to the difference in vacancy mobility, stronger pinning effect of vacancies on He, and the sink strength of faulted dislocation loops. Faulted interstitial loops nucleated with a higher number density and dislocation line density in Cr15Fe35Mn15Ni35 at both temperatures, and at 600 °C in both materials. The differences in faulted loop population and temperature effect on swelling are correlated to the Mn-content and the measured SFE (20.2 ± 6.7 mJ/m2 for Cr18Fe27Mn27Ni28 and 9.2 ± 3.4 mJ/m2 for Cr15Fe35Mn15Ni35).

Study on the mechanism of He effect on dislocation loop formation in three types of dual ion beam irradiations

A comprehensive investigation was conducted to explore the impact of helium on dislocation loops in RAFM steel. This was achieved by employing three types of dual-beam irradiations, which include simultaneous dual-beam irradiation with Ar++He+, sequential dual-beam irradiation with Ar+/He+ and He+/Ar+. In addition, single-beam irradiation with Ar+ and He+ were also performed for reference. The irradiation temperature is 723 ± 5 K. The evolutions of dislocation loop were characterized by transmission electron microscopy (TEM). After He+ irradiated to 0.005 dpa, high-density loops with an average size of 4.3 nm were observed. The main mechanism for the formation of the loops at such a low damage dose should be attributed to the process of the ejection of self-interstitial atoms triggered by He clusters. Significantly, in the RAFM steel exposed to Ar++He+ irradiation, the dislocation loops exhibit a significantly larger average size compared to those resulting from Ar+/He+ and He+/Ar+ irradiation. The loop density produced by He+/Ar+is the largest, and the loop density produced by Ar+/He+ is the smallest. The order of the amount of self-interstitial atoms (loops) caused by three types of dual ion beam irradiation is Ar++He+ > He+/Ar+ > Ar+/He+.

Ultrafast Dynamics of Extraordinary Optical Transmission through Two-Slit Plasmonic Antenna.

We have theoretically investigated the spatial-temporal dynamics of extraordinary optical transmission (EOT) through a two-slit plasmonic antenna under femtosecond laser dual-beam irradiation. The dynamic interference of the crossed femtosecond laser dual-beam with the transiently excited surface plasmon polariton waves are proposed to characterize the particular spatial-temporal evolutions of EOT. It is revealed that the dynamic EOT can be flexibly switched with tunable symmetry through the respective slit of a two-slit plasmonic antenna by manipulating the phase correlation of the crossed femtosecond laser dual-beam. This is explained as tunable interference dynamics by phase control of surface plasmon polariton waves, allowing the dynamic modulation of EOT at optimized oblique incidences of dual-beams. Furthermore, we have obtained the unobserved traits of symmetry-broken transient spectra of EOT from the respective up- and down-slit of the antenna under crossed femtosecond laser dual-beam irradiation. This study can provide fundamental insights into the ultrafast dynamics of EOT in two-slit plasmonic antennas, which can be helpful to advance a wide range of applications, such as ultrafast plasmonic switch, ultrahigh resolution imaging, the transient amplification of non-linear effects, etc.

In-situ TEM bubble to cavity evolution due to annealing post helium and dual ion irradiation in Cu-10Ta and Cu-3Ta

Materials have been essential to every major invention in the history of mankind, e.g. satellites, nuclear reactors, or space shuttles would not exist without advancements in materials withstanding temperature and radiation environments. In this work, we investigate room-temperature implantations of helium and concurrent He and Au via in-situ transmission electron microscopy (TEM) followed by isochronal annealing up to 450 °C to understand the microstructural stability and bubble to cavity evolution in advanced nanocrystalline (NC) Cu-3 at.%Ta and Cu-10 at.%Ta alloys. The room temperature irradiation to high helium concentration and displacements per atom (dpa) levels did not lead to significant coarsening of grains in both Cu10Ta and Cu3Ta. Overall grain sizes remained <150 nm indicating extraordinary microstructural stability in both alloys. Post-irradiation, incubation for a few days resulted in the observation of ~1 nm diameter bubbles homogenously distributed in both the alloys and both irradiation conditions. Further evolution of these bubbles to cavities with annealing to 450 °C indicated Brownian motion migration and coalescence to be the dominant coarsening mechanism involved. A comparison of bubble to cavities evolution in Cu3Ta and Cu10Ta under similar conditions showed smaller bubble size and density in Cu3Ta indicating an overall enhanced resistance of Cu3Ta to swelling under both helium and dual-beam irradiation at high temperatures. This is primarily attributed to the high fraction of essential coherent/incoherent precipitates that act as effective defect sinks in Cu3Ta.

Helium role in Fe9Cr1.5W0.4Si F/M steel during Fe++He+ dual-beam irradiation

In materials exposed to neutron irradiation, helium atoms produced by transmutation reactions are prevalent and play a significant role in the evolution of dislocation loops, causing mechanical property degradation. However, systematic investigation on the helium role is lack and possible origin of helium effects is unclear. Here, by performing 400 keV Fe ion and 30 keV He in-situ irradiation, this study found that about 9.2×1012 cm−3 loops nucleated for every increase of 1 appm He at 0.9 dpa in Fe++He+ dual beam irradiation and 87% loop density was produced with the help of helium at 2262 appm/dpa. The different loop evolution in single He irradiation and Fe++He+ irradiation indicated the presence of high-density TEM-invisible (≤ 2 nm) small loops after Fe+ irradiation at high temperatures. The large number of loops produced by helium irradiation were possibly related with the small bubbles.

In-situ analysis on defect evolution in potassium-doped tungsten during synergistic He+ and H2+ dual-beam irradiation

Potassium-doped tungsten (KW), as an advanced candidate for first wall material, was developed to mitigate intrinsic embrittlement of bulk W. The addition of gaseous potassium bubbles (KBs) was known to act as effective barrier to the motion of irradiation damage and grain boundaries (GBs). The irradiation response to the He/H synergistic effect and the role of KBs on damage defects were two key issues when evaluating the performance of KW in future fusion reactors. In this work, KW was in-situ irradiated by 30 keV He+ and H2+ with He/H ratios of 2.5, 0.5, and 0.1 at 823 K. The synergy of He, H, and damage defects were discussed. The number density and the average size of irradiation-induced loops and bubbles nearby KBs, nearby GBs, and within the matrix was quantified. Increasing the ratio of H to He enhanced the nucleation sites of loops and promoted the accumulation rates of bubbles. The formation of a loop denuded zone nearby a KB was related to the KB size. The difference between the loop denuded zones nearby KBs and GBs was found. The estimated sink strengths of KBs and GBs were in line with our experimental results.

In-situ TEM investigation on the evolution of He bubbles and dislocation loops in Ni-based alloy irradiated by H2+ &amp; He+ dual-beam ions

• Synergistic evolution mechanism of He bubbles and loops in Ni-based alloy irradiated by dual-beam ions were investigated via in-situ TEM. • Several evolution stages of the loop-punching mechanism and its effect on evolution of neighboring bubbles and loops were revealed. • The details of the unfaulting processes of the Frank loops and the formation mechanism of rhombic loops were discussed. • Four Frank loop variations were identified, and their different evolution behaviors were studied. The synergistic evolution mechanisms of He bubbles and dislocation loops under 30 keV H 2 + & H e + dual-beam ions irradiation at 650°C in the Ni-based alloy GH3535, which is the most promising candidate structure material for molten salt reactors (MSRs), were revealed via in-situ TEM. The nucleation, merging, and change in the size of the dislocation loops and He bubbles were characterized in detail to study the influences of irradiation fluence and pre-existing dislocation loops on their evolutions. The number density of both the He bubbles and dislocation loops increases rapidly and subsequently saturates, whereas their size continuously increases with the increasing ion fluence. Pre-existing dislocation loops with strong absorption characteristics grow preferentially and suppress the nucleation of dislocation loops during the dual-beam ions irradiation. Moreover, the bubbles tend to nucleate within the dislocation loops to form bubble-loop complexes, and then decrease in their number density. The details of the unfaulting processes of the Frank loops were discussed, where the energy difference between the two types of loops as well as the evolution of the inside Shockley dislocation loops dominates the unfaulting behavior. The several evolution stages of the loop-punching mechanism are revealed, and the emitted loops can directly form perfect loops as well as unfault the neighboring Frank loops. The He bubbles inside the loops provide corresponding stress for the formation of rhombic loops, which can achieve rapid growth and sweep ability by merging with the neighboring loops. Additionally, its dissociation to Shockley dislocation can unfault the Frank loops along their slip direction. Among the four Frank loop variations, the edge-on Frank loop variations have the highest growth rate, followed by the perfect loop. The related mechanisms based on in-situ experimental observation are discussed in depth. Synergistic evolution mechanism of He bubbles and dislocation loops, including loop-punching mechanism and bubble-loop complexes, in Ni-based alloy irradiated by dual-beam ions were investigated via in-situ TEM.

Development of the Dual-Beam Ion Irradiation Facility for Fusion Materials (DiFU).

The Dual-beam ion irradiation facility for Fusion materials (DiFU) has been developed and installed at the Ruđer Bošković Institute with the purpose to perform irradiation of samples of fusion materials by one or two ion beams. Ion beams are delivered to the DiFU chamber by a 6 MV EN Tandem Van de Graaff and a 1 MV HVE Tandetron accelerator, enabling irradiation of areas up to 30 × 30 mm2. The sample holder enables the three-dimensional positioning of samples that can be irradiated while being heated, cooled, or kept at room temperature. Ion fluxes are measured indirectly by the insertion of two large Faraday cups. Besides, the ion flux is monitored continuously by two sets of horizontal and vertical slits, which, in turn, define the limits of the irradiation area on the sample. Sample temperature and conditions during irradiation are additionally monitored by a set of thermocouples, an IR camera, and a video camera. Particular care is dedicated to the mitigation of carbon contamination during ion irradiation.

In-situ TEM study on the evolution of dislocation loops and bubbles in CeO2 during Kr+ single-beam and Kr+-H2+ dual-beam synergetic irradiation

CeO2 is widely used as the nonradioactive surrogate fuel of UO2 when studying the irradiation performance of UO2. The evolution and characteristics of dislocation loops and bubbles in CeO2 foils were studied by in-situ transmission electron microscopy (TEM) observation during 400 keV Kr+ & 30 keV H2+ dual-beam synergetic irradiation and 400 keV Kr+ single-beam irradiation at 1073 K. The rotation of the habit plane of dislocation loops induced by above ion irradiation was found in the CeO2 for the first time, such as from [2¯11¯] to [3¯11¯] and then to [1¯00]. The rafted loops were first observed under Kr+ & H2+ dual-beam synergetic irradiation, which not only had similar [1¯11¯] direction, but also belonged to perfect dislocation loops (PDLs). The rafted loops were formed not only by the growth of loops that absorbed irradiation defects, but also by the combination of loops. But, this phenomenon of loop rafting was not obvious during Kr single-beam irradiation. It was first found that Kr+ irradiation induced the change of Burgers vectors of PDLs. The absorption of large PDLs to small PDLs was also observed. The average size and areal number density of dislocation loops and gas bubbles as a function of irradiation dose were constructed, which showed that the addition of H2+ obviously affected the characteristics of dislocation loops and gas bubbles and the swelling of CeO2.

Modification of the high fluence irradiation facility at the University of Tokyo: Assessment of radiation-induced amorphization of Zr(Cr,Fe)2 Laves phase under 180 keV-He+ irradiation

The High Fluence Irradiation Facility at the University of Tokyo, which had provided a platform for dual-beam irradiation in Japan, was severely damaged by the 2011 earthquake. This paper reports a modification at this facility, i.e., installing a low-energy ion implanter instead of repairing the previous Van de Graaff accelerator. The platform of the dual-beam irradiation and/or s gas-ion beam irradiation was reconstructed, and the newly installed ion implanter was a stable, uniform, and high-density light element ion beam. The radiation-induced amorphization (RIA) behavior in Zr(Fe,Cr)2 Laves phase was investigated in 180 keV-He+ beam irradiation to assess the performance of this ion implanter, and the irradiation temperature and dose-dependent RIA behavior were confirmed. Implying that this modification ensures the capability in terms of gas-ion beam irradiation and dual-beam irradiation in this facility, which will promote the research progress on radiation effects in nuclear material.

The evolution and interaction of irradiation defects in CeO2 during H2+ & He+ dual-beam synergistic irradiation investigated by in-situ TEM

As a surrogate material of UO2, non-radioactive CeO2 was investigated by in-situ transmission electron microscopy during 30 keV H2+ & He+ dual-beam synergistic irradiation at 1073 K. The relationships between the size/density of dislocation loops and irradiation fluence were established, as well as for gas bubbles. Based on the analysis of the Burgers vectors, the formation of perfect dislocation loop (PDL) through the reaction between PDLs and the direct transformation from Frank dislocation loop (FDL) to PDL were obtained. The shrinkage of FDLs at high dose was observed. It was not only found that the number density and size of bubbles near the grain boundary were lower than those in the crystal, but also found that no loop-bubbles complex was formed. The mechanism of irradiation-induced excess oxygen vacancies should be the reason for the shrinkage of dislocation loops at high dose and the depletion of bubbles in the dislocation loops.

Irradiation-induced microstructural transformations in UO2 accelerated upon electronic energy deposition

The combination of electronic and nuclear energy deposition can have an effect on the defect production and evolution that is not simply the sum of both independent contributions. In nuclear reactors, the fuel is exposed to the irradiation of several particles such as the fission fragments (FF) whose electronic to nuclear energy loss ratio evolves along their path. To understand the impact of this energy-loss coupling on the fuel microstructure, single and dual-beam ion irradiations of uranium dioxide (UO2) were carried out. The damage evolution was investigated by Raman spectroscopy analysis correlated with Transmission Electronic Microscopy observations. It was found a significant acceleration effect of the electronic energy dissipation on the formation and evolution of defects induced by the nuclear energy deposition. The magnitude of this acceleration was shown to depend on both the electronic energy loss level and the microstructure at the moment when the coupling occurs.

Synergies between H, He and radiation damage in dual and triple ion irradiation of candidate fusion blanket materials

Three ferritic/martensitic alloys were studied to understand the synergistic effect between single ion beam (Fe2+), dual ion beam (Fe2++He2+ and Fe2++H+), and triple ion beam (Fe2++He2++H+) irradiations on cavity evolution. A commercial alloy, F82H, a castable nanostructured alloy, CNA3, and a high purity model alloy, Fe8Cr2W, were irradiated at 400°C to 600°C to a damage level of 50 dpa at a damage rate of 1 × 10−3 dpa/s with He and H injection rates of 10 and 40 appm/dpa, respectively. Post-irradiation characterization via bright field transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy was performed on all irradiated conditions to characterize the cavity size distribution and determine the effects of H/He injection on cavity microstructure. In all three alloys, hydrogen co-injection with helium resulted in an increased cavity number density and maximum cavity size, producing an increase in swelling over that from helium injection alone. Swelling in F82H appears to peak between 450°C and 500°C. At 600°C, swelling was minimal and cavities of high density and small size were confined to grain boundaries and dislocations while at 400°C, swelling is also low with a nearly homogeneous, high density, distribution of very small cavities throughout. Swelling was least in the commercial alloy F82H due to the high sink strength. The CNA3 alloy underwent dissolution of precipitates that lowered the sink strength and resulted in higher swelling than F82H, but less than the model alloy. Electron energy loss spectroscopy (EELS) elemental mapping revealed hydrogen forming a halo-like structure about the periphery of the cavities and helium residing within the cavities themselves. This observation suggests that hydrogen reduces the surface energy of helium-filled cavities which results in both increased cavity number density and cavity size in triple beam irradiation over dual beam irradiation.

In-situ TEM investigation of dislocation loop reaction and irradiation hardening in H2+-He+ dual-beam irradiated Mo

• Irradiation can directly cause the formation of 1/2<111> loops and <100> loops. • Irradiation can directly induce 1/2<111>loop to transform into <100> loop. • Two 1/2<111> loops react to form <100> loop without strict size similarity condition. • Yield strength increment caused by irradiation-induced loops is analyzed. • Loop evolution and its relationship with irradiation dose are established. 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 reached 0.48 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. Graphical Abstract .