Loop Punching Research Articles

The design of Pd-containing high-entropy alloys and their hardening behavior under He ion irradiation

Imparting additional chemical heterogeneities of high-entropy alloys (HEAs) is considered as a novel strategy to improve their irradiation resistance on dislocation loop growth; however, there is still a lack of knowledge about the effects of this strategy on bubble evolution and irradiation-induced hardening. In this study, a series of Pd-containing FeCrNiCo HEAs with proven potential for chemical short-range order were designed and irradiated by 400 keV He ions to the fluences of 5 × 1016 cm−2, 1 × 1017 cm−2, and 5 × 1017 cm−2 at 723 K. Microscopic characterization demonstrates a remarkably inhibited growth of dislocation loops and promoted bubble coarsening in the Pd-containing HEAs compared to the HEA without Pd alloying. The underlying mechanism is that although the alloying of Pd can suppress the evolution of dislocation loops through severe local lattice distortion, it also significantly reduces the vacancy formation energy of the entire HEA system, leading to the encouraged bubble evolution. Pd-containing HEAs exhibit improved resistance to irradiation-induced hardening at low doses, but this advantage is unexpectedly absent with increasing dose. The hardening mechanism is explained by identifying the loop-dominated hardening at low dose and the bubble-induced hardening which makes an increased contribution at high dose. In addition, the Ni segregation around bubbles and loop punching mechanism are also discussed to provide essential evidence. These insights can help to shed light on the effects of Pd alloying on bubble evolution and irradiation-induced hardening behavior in HEAs, and thus contribute to the design of irradiation-resistant HEAs.

The incorporation of xenon at point defects and bubbles in uranium mononitride

Uranium mononitride (UN) has been proposed as an accident tolerant fuel for nuclear fission reactors and offers enhanced performance during accident scenarios relative to the current fuel, uranium dioxide. However, its performance in reactor is significantly less well understood than for the oxide. Therefore, this work explores incorporation of Xe into UN using density functional theory to understand the early stages of fission gas evolution. These results are used to derive a new potential for Xe in UN, which is then employed to simulate the growth of xenon bubbles in spherical voids of various sizes at 300 K and 1200 K. At sufficiently high gas densities, the xenon was found to mainly crystallise in an fcc arrangement. Loop punching was observed at 10.2 GPa and above for larger bubbles of 4.8 nm radius, significantly so for higher temperatures. This work suggests that no Xe undergoes thermal resolution at temperatures up to 1200 K and that the UN lattice prefers to undergo deformation instead.

Equation of state for He bubbles in W and model of He bubble growth and bursting near W{100} surfaces derived from molecular dynamics simulations

Molecular dynamics (MD) simulations are performed to derive an equation of state (EOS) for helium (He) bubbles in tungsten (W) and to study the growth of He bubbles under a W(100) surface until they burst. We study the growth as a function of the initial nucleation depth of the bubbles. During growth, successive loop-punching events are observed, accompanied by shifts in the depth of the bubble towards the surface. Subsequently, the MD data are used to derive models that describe the conditions that cause the loop punching and bursting events. Simulations have been performed at 500, 933, 1500, 2000, and 2500 K to fit the parameters in the models. To compute the pressure in the bubble at the loop punching and bursting events from the models, we derive an EOS for He bubbles in tungsten with an accompanying volume model to compute the bubble volume for a given number of vacancies (N_text {V}), He atoms (N_text {He}), and temperature (T). To derive the bubble EOS, we firstly derive the EOS for a free He gas. The derived free-gas EOS can accurately predict all MD data included in the analysis (which span up to 54 GPa at 2500 K). Subsequently, the bubble EOS is derived based on the free-gas EOS by correcting the gas density to account for the interaction between He and W atoms. The EOS for the bubbles is fitted to data from MD simulations of He bubbles in bulk W that span a wide range of gas density and sizes up to about 3 nm in diameter. The pressure of subsurface bubbles at the loop punching events as calculated using the bubble-EOS and the volume model agrees well with the pressure obtained directly from the MD simulations. In the loop punching model, for bubbles consisting of N_text {V} vacancies and N_text {He} helium atoms, the N_text {He}/N_text {V} ratio that causes the event, the resulting increase in N_text {V}, and the associated shift of the bubble depth are formulated as a function of N_text {V} and T. In the bursting model, a bubble must simultaneously reach a certain depth and N_text {He}/N_text {V} ratio in order to burst. The burst depth and N_text {He}/N_text {V} are also modeled as a function of N_text {V} and T. The majority of the loop punching events occur at bubble pressures between 20 and 60 GPa, depending on the bubble size and temperature. The larger the bubble and the higher the temperature, the lower the bubble pressure. Furthermore, our results indicate that at a higher temperature, a bubble can burst from a deeper region.

Surface blistering and void swelling of α-Al2O3 irradiated with H2+ ions followed by 1000, 1200 °C annealing

Surface morphology and defect structures in α-Αl2O3 irradiated with 106 keV H2+ and post-irradiation-annealed at 1000, 1200 °C were investigated. The ion irradiations and annealing cause modifications to the Raman scattering peaks such as the peak disappearance and peak shifts. Surface blisters with several micrometers in diameter are visible. The mean blister sizes increase with increasing annealing temperatures and decrease with increasing irradiation fluences. Transmission electron microscopy characterizations reveal large numbers of spherical or oval shaped voids around the peak-damage zone. Faceted voids are also observed. The irradiation-induced dislocation loops are mostly annealed out with only some dislocation lines left inside. Nevertheless, small dislocation loops with sizes of ∼20–30 nm are characterized under different diffraction conditions, which are found to be associated with the void structures in the 1.5 × 1017 H2+/cm2 irradiated and annealed sample. These loop structures are considered to be induced through the ‘loop punching’ mechanism due to a high gas pressure inside during the bubble growth period.

Punching of arbitrary face prismatic loops from hydrogen nanobubbles in copper

When a metal surface is exposed to prolonged irradiation with energetic H−, the ions are expected to penetrate into bulk and dissolve in the matrix. However, the irradiated surfaces exhibit dramatic morphological changes in the form of “blisters” covering the surface exposed to irradiation. Blistering is usually explained by accumulation of implanted gas in the bubbles near surface. However, the exact mechanism of continuous growth of a bubble after it reaches the measurable size is still not fully clear. Commonly such growth is related to prismatic loop punching, which is a short time scale process not easily accessible by experimental techniques. Even atomistic modelling of loop punching in FCC metals is somewhat cumbersome. Since the void surfaces in these metals yield easily through shear loops, these were debatably suggested to explain the plastic growth of a bubble in copper, without demonstrating the detachment of these loops from the void.We address the mechanisms of fast bubble growth in Cu which is associated with blistering of Cu surface exposed to H− irradiation. We observe the emission of a complete prismatic loop enclosed within the number of shear loops with the Burgers vectors aligned with the gliding direction of the prismatic loop. We show that the prismatic loops punched from the bubble surface do not need to be smaller than the bubble cross-section. These simulations capture the general trend of dislocation emission in the condition of hydrostatic pressure exerted by the accumulated gas on the wall of the bubble.

Helium bubble formation in nickel under in-situ krypton and helium ions dual-beam irradiation

Helium has profound effects on the microstructure and mechanical property of materials used in nuclear power systems. To understand the interaction between helium and irradiation-induced defects and its effect on bubble formation, single-beam and dual-beam irradiations on nickel were performed with 16 keV helium ions and 1 MeV krypton ions under in-situ TEM observation. For 1 MeV krypton single-beam irradiation, voids were observed at 600 °C but not at 500 °C and 700 °C. For helium single-beam irradiation at 500 °C, helium moved freely and formed bubble-loop complexes. For dual-beam irradiation at 500 °C, helium was trapped by vacancies and formed bubbles homogeneously. The structure of dislocation loops was also affected by the presence of helium. For krypton single-beam irradiation at 500 °C, the microstructure was {100} periodic wall of faulted and perfect dislocation loops. For dual-beam irradiation at 500 °C, periodic wall was not observed, and dislocation loops were predominantly perfect. For single-beam helium at 500 °C, dislocation loops were produced by athermal process of SIAs ejection or loop punching mechanisms. The interplay between helium and vacancies played an important role on the evolution of helium bubbles and defect microstructures.

Dissociated prismatic loop punching by bubble growth in FCC metals.

Materials performance can be significantly degraded due to bubble generation. In this work, the bubble growth process is elaborated in Cu by atomistic modeling to bridge the gap of experimental observations. Upon continuous He implantation, bubble growth is accommodated first by nucleation of dislocation network from bubble surface, then formation of dissociated prismatic dislocation loop (DPDL), and final DPDL emission in langle 110rangle directions. As the DPDL is found capable of collecting He atoms, this process is likely to assist the formation of self-organized bubble superlattice, which has been reported from experiments. Moreover, the pressurized bubble in solid state manifests the shape of an imperfect octahedron, built by Cu {111} surfaces, consistent with experiments. These atomistic details integrating experimental work fill the gap of mechanistic understanding of athermal bubble growth in Cu. Importantly, by associating with nanoindentation testings, DPDL punching by bubble growth arguably applies to various FCC (face-centered cubic) metals such as Au, Ag, Ni, and Al.

Material migration in W and Mo during bubble growth and fuzz formation

Growth of helium (He) induced bubbles and fuzz in tungsten (W) and molybdenum (Mo) is investigated using samples of W films on Mo substrates and Mo films on W substrates exposed to He-containing plasma in the temperature range of 340 to 1075 K, fluence range of 1.0–14 × 1025 He·m−2, and incident ion energy of <50 eV. No fuzz (only up to 2 nm diameter bubbles) and no material transport occur in W films at ⩽750 K, while precursors-of or fully-developed fuzz and material mixing occur in W and Mo films at ⩾800 K. This suggests that fuzz forms in multi-material systems as long as one material meets the conditions for fuzz formation, namely T s/T m ∼ 0.27–0.5 where T s and T m are the sample exposure and material melting temperatures, respectively. Larger He bubbles, more material mixing, and further-developed fuzz occur at higher temperature due to increased mobility of He atoms and small He clusters. Accumulation of substrate material at the surface of fuzzy W and Mo thin-film (<80 nm) samples suggests fuzz growth by material transport from the bubble layer in the bulk up to the fiber tip, likely by a two-step process: (i) diffusion of punched dislocation loops in the bulk toward the fuzz base and (ii) diffusion of adatoms along the fuzz base and fiber surface (with effective transport of adatoms upwards due to trapping of adatoms at curved surfaces of fiber tips and/or due to the continuous generation of adatoms at the fuzz base). While the bubble size and fuzz thickness increase with reduced W concentration in Mo thin-film samples at 838 K likely due to an increase in trap mutation and dislocation loop punching in Mo compared to W, the fuzz thickness decreases with reduced W concentration at 1075 K despite an increase in the bubble size likely due to slower diffusion of interstitial loops in Mo.

Panorama of “fuzz” growth on tungsten surface under He irradiation

Nanostructure formation on tungsten (W) surfaces under helium (He) irradiation is a unique and interesting phenomenon that directly impacts the performance and lifetime of W-based materials as plasma-facing materials (PFMs) in nuclear fusion reactors. The evolution of He bubbles beneath the W surface and its effect on surface morphology are essential for understanding the deterioration of the surface properties of PFMs. In this work, the morphology of the W surface under different irradiation conditions was investigated using molecular dynamics in order to extract essential factors that determine the nanostructure of the W surface. The design concerned with investigating the W surface morphology encompassed: (1) the morphology of the W-free surface without He bubbles vs. recoil energy; (2) the morphology of the W surface in the presence of He bubbles vs. recoil energy; and (3) sputtering vs. surface roughness in the absence of He bubbles. A mass of adatoms generated primarily by loop punching of sub-surface He bubbles slightly roughens the surface. Bubble bursting further heightens surface protrusions. In this case, the fiber-like structure of the surface is attributable to two processes involved in He bubble evolution: He diffusion plus aggregation and bubble expansion plus bursting. A high temperature window expedites He diffusion and aggregation beneath the protruded surface. Once the He bubbles burst regardless of spontaneous bursting (when the bubble pressure is beyond the critical value), or bursting due to external irradiation, the protrusions will be further heightened, thus serving as precursors of the fiber-like structure. Continuous aggregation, followed by bursting, should constitute the recursive process of protrusion upgrowth. The panoramic view of the fiber-like structure growth described here is expected to provide a clue for designing tungsten-based materials with high radiation-resistance performance.

Atomistic simulations of helium clustering in copper

The formation and growth of helium (He) clusters in copper (Cu) bulk were studied using molecular dynamics simulations. During the growth of a single He cluster in a Cu box, stacking faults were found to occur around the He cluster. Loop punching occurred along the 〈110〉 crystal direction, which promoted further growth. The He-to-vacancy ratio was calculated as ranging between 1.5 and 2. When He atoms were randomly introduced into a Cu box, multiple He clusters were formed. Thermal diffusion promoted the formation of large He clusters. With the generation of more He clusters, more dislocations were formed, most of which were in the form of Shockley dislocations.

Dissociated prismatic loop punching by bubble growth in Cu

Dissociated prismatic loop punching by bubble growth in Cu

Clustering and dislocation loop punching induced by different noble gas bubbles in tungsten: a molecular dynamics study

Different behaviours of noble gas X (X = He or Ne) atoms in the bulk and on the surface of tungsten (W) have been studied with molecular dynamics simulations to explore the fuzz formation induced by different noble gas bubbles. The formation of X clusters and bubbles in bulk W were simulated at temperatures from 400 to 2000 K. The results showed that higher temperature promotes the nucleation of X bubbles, and the sizes of self-interstitial atoms and X-vacancy clusters of Ne in W were larger than those of He in W at nano-scale, which indicates that Ne atoms can be trapped more easily than He atoms in W. The continuous nucleation processes of He/Ne bubbles near the surface of bulk W were also simulated. When the sizes of the X bubbles were large enough, the behaviour of dislocation loop punching was observed for both He and Ne clusters. From the comparison of the pressure of He/Ne bubble, it is found that the He-bubble can induce the loop-punching with lower critical pressure than the case of Ne-bubble. These results indicate a new understanding of noble gas effects in W in fusion reactors.

Temperature dependent He-enhanced damage and strain in He-implanted AlN

He implantation was performed at RT and 550 °C into AlN epitaxial films. The resulting microstructural damage and elastic strain were investigated using TEM and XRD. Basal stacking faults, clusters of point defects and bubbles aligned along (0001) planes are observed at both temperatures. Apart from slight variations of size and densities of defects, no significant difference is observed on the microstructure after implantation at RT and 550 °C. In particular, He bubbles are observed with similar size, showing that the long-range migration of vacancies is not triggered at 550 °C. An important effect of He on the enhancement of damage and strain build-up during implantation is evidenced. He bubbles are observed to strongly favor the nucleation of extended defects, both basal stacking faults and point defect clusters, which results from the increase of free interstitial concentration due to stabilization of vacancies by He that could be enhanced by the bubble growth by loop punching or trap mutation to relieve the gas pressure. At an earlier stage of implantation-induced damage, the presence of He is also seen to induce a strong increase of strain with increasing implantation temperature, whilst efficient point defect recombination takes place in the regions free of He, or with low concentration of He, resulting in a decrease of strain. The strong increase of strain with increasing temperature in the deep implanted region, concomitant with a strong reduction of scattered intensity, is ascribed to the growth of He–V complexes, probably induced by a thermally enhanced mobility of He, and only occurs for sufficient He concentration.

Helium bubble growth under strain fields in tungsten investigated by atomic method

The evolution of helium (He) bubble and dislocation loop from loop punching process affected by external strain field have been studied at atomic scale with hydrostatic strains of around −2% to 2%. The 1st maximal displacements of tungsten atoms around HenV, the formation energy of HenV clusters and the binding energy between the clusters and He are calculated, indicating that the number of He atoms needed to form the 1st self-interstitial atom (SIA) increases from tensile state to compressive state, and the configurations of the 1st SIAs are all in the form of 〈1 1 1〉 crowdion, independent of applied strains. Comparing to the case without external strain effect, the tensile and compressive strains affect not only the critical number of He atoms to punch the 1st loop in loop-punching process but also the Burgers vectors of formed loops. All these results provide a new understanding to radiation damages related with He bubble growth in nuclear materials.

Dislocation climbing mechanism for helium bubble growth in tungsten

Molecular dynamics simulations were used to study the growth process of a He bubble on an edge dislocation in W, and the results indicated that the growth was controlled by a dislocation climbing mechanism. Unlike the well-known loop punching mechanism, the bubble grew by punching out interstitial atoms, which were immediately absorbed into the dislocation core, causing the edge dislocation to climb. Moreover, the dislocation climbing mechanism was shown to be more energetically favorable than the loop punching mechanism for the He bubble growth.

A new loop-punching mechanism for helium bubble growth in tungsten

Growth of helium (He) bubbles with different initial sizes in tungsten (W) has been investigated by performing molecular dynamics simulations. Based on the simulation results a new loop punching mechanism for the large helium bubble growth is proposed. Different from the growth of small-size He bubbles by pushing out self-interstitial atoms and then rearranging into a prismatic dislocation loop, a large-size bubble grows by pushing out a dislocation, subsequently cross-slipping of its screw components and finally evolving into a prismatic dislocation loop. Such dislocations may react with each other to form a dislocation net around the bubble rather than to convert to prismatic dislocation loops.

Experimental mechanistic investigation of the nanostructuring of tungsten with low energy helium plasmas

Helium ion bombardment of tungsten at temperatures between approximately one third and one half of its melting point has shown growth of nanostructures colloquially referred to as “fuzz”. The nanostructures take the form of thin tendrils of diameter about 30 nm and grow out of the bulk material. Tungsten will and does compose one of the key materials for plasma facing components (PFCs) in fusion reactors. The formation of nanostructured fuzz layers on PFCs would be detrimental to the performance of the reactor, and must therefore be avoided. Previous experiments have shown evidence that tungsten fuzz is initially grown by loop punching of helium bubbles created in the bulk. However, once the tendrils grow to sufficient length, the tendrils should intercept the entire helium flux, halting the production of fuzz. Fuzz continues to grow though. To increase the understanding of the mechanisms of tungsten fuzz formation, and thereby aid the avoidance of its production, a series of tests were performed to examine the validity of several theories regarding later stage tungsten fuzz growth. Tests showed that the fuzz formation was dependent solely on the bombardment of helium ions, and not on electric fields, or adatom diffusion. Experiments employing a tungsten coated molybdenum sample indicate the presence of a strong mixing layer and strongly suggest that tungsten fuzz growth continues to occur from the bottom up even as the tendrils grow in size. Tests also show a similarity between different metals exposed to helium ion fluxes where the ratio of bubble diameter to tendril diameter is constant.

In situ Observation of Microstructure Evolution in 4H–SiC under 3.5 keV He+ Irradiation

4H–SiC was irradiated with 3.5 keV He+ ions using the MIAMI facility at University of Huddersfield. The evolution of microstructure and gas bubbles during the irradiation at 700 °C, 800 °C and 900 °C was observed by in situ transmission electron microscopy. Under irradiation, isolated bubbles and bubble discs formed in the SiC matrix. Bubble discs lying on {0001} and {10-10} crystal planes were beginning to form at ion fluence above 2.3 × 1020 He+/m2 at 700 °C. The density of bubble discs increased with increasing irradiation fluence. However, growth rates were different at different of the implantation periods and temperature holding periods. The nucleation and growth of the bubble discs were attributed to be coalescence of the adjacent He vacancies and combination of loop punching and trap mutation, respectively.

Microstructural modifications in tungsten induced by high flux plasma exposure: TEM examination

We have performed microstructural characterization using transmission electron microscopy (TEM) techniques to reveal nanometric features in the sub-surface region of tungsten samples exposed to high flux, low energy deuterium plasma. TEM examination revealed formation of a dense dislocation network and dislocation tangles, overall resulting in a strong increase in the dislocation density by at least one order of magnitude as compared to the initial one. Plasma-induced dislocation microstructure vanishes beyond a depth of about 10 μm from the top of the exposed surface where the dislocation density and its morphology becomes comparable to the reference microstructure. Interstitial edge dislocation loops with Burgers vector a0/2〈111〉 and a0〈100〉 were regularly observed within 6 μm of the sub-surface region of the exposed samples, but absent in the reference material. The presence of these loops points to a co-existence of nanometric D bubbles, growing by loop punching mechanism, and sub-micron deuterium flakes, resulting in the formation of surface blisters, also observed here by scanning electron microscopy.

Stick–slip behavior identified in helium cluster growth in the subsurface of tungsten: effects of cluster depth

We have performed a molecular dynamics study on the growth of helium (He) clusters in the subsurface of tungsten (W) (1 0 0) at 300 K, focusing on the role of cluster depth. Irregular ‘stick–slip’ behavior exhibited during the evolution of the He cluster growth is identified, which is due to the combined effects of the continuous cluster growth and the loop punching induced pressure relief. We demonstrate that the He cluster grows via trap-mutation and loop punching mechanisms. Initially, the self-interstitial atom SIA clusters are almost always attached to the He cluster; while they are instantly emitted to the surface once a critical cluster pressure is reached. The repetition of this process results in the He cluster approaching the surface via a ‘stop-and-go’ manner and the formation of surface adatom islands (surface roughening), ultimately leading to cluster bursting and He escape. We reveal that, for the Nth loop punching event, the critical size of the He cluster to trigger loop punching and the size of the emitted SIA clusters are correspondingly increased with the increasing initial cluster depth. We tentatively attribute the observed depth effects to the lower formation energies of Frenkel pairs and the greatly reduced barriers for loop punching in the stress field of the W subsurface. In addition, some intriguing features emerge, such as the morphological transformation of the He cluster from ‘platelet-like’ to spherical, to ellipsoidal with a ‘bullet-like’ tip, and finally to a ‘bottle-like’ shape after cluster rupture.