Suppression of He bubbles at the graphene interface

As outlined above, He bubble formations are of concern in maintaining the mechanical stability in nuclear structural materials. He is known to rapidly combine with vacancy clusters to form bubbles that can migrate and agglomerate to result in intergranular failure by embrittlement and crck formation41,42,43. However, atomistic simulations on Cu-Nb nanolayers showed that the interfaces can act as sinks for high concentrations of vacancies due to the lower formation energies of vacancies at the Cu-Nb interfaces that can lead to lower density of He bubbles in Cu-Nb nanolayers in comparison to that in bulk Cu or Nb29,44,45,46. The V-graphene nanolayers are expected to also be excellent at terminating He gas migration in comparison to the nanolayers composed of only metal layers due to the impermeability of the He gas through the graphene layer35. Cross-section TEM images of V-graphene with 300 nm repeat layer spacing was taken at different locations as marked in Fig. 3 using 400 nm under-focus condition. He bubbles were observed at the top V surface region as shown in Fig. 3b, but most He bubbles were concentrated at the graphene interface that are shown as white dots in Fig. 3c. The graphene interface hindered the He bubbles from migrating and agglomerating to larger sizes, which can potentially penetrate through thickness of the layers to result in blister. It should be noted again that the chosen irradiation energy is sufficient for He ions to have penetrated through thickness of the nanolayers, but the graphene interface has effectively stopped migration of He bubbles.

Figure 3 (a) TEM analysis of the V-graphene nanolayer after He+ irradiation at 120 keV for V-graphene with 300 nm repeat layer. High magnification images are taken using 400 nm under-focus condition at (b) the top layer, (c) the graphene interface, and (d) the bottom layer. White dots are indicative of He bubbles, which are concentrated near the graphene interface shown in (c). Full size image

Role of graphene in reducing radiation induced hardening

The increase in yield strength after He implantation is commonly accepted to be due to the formation of radiation induced crystalline defects as well as He bubbles22,23,24,47,48. Our results indicate that the flow stress at 5% strain for V-graphene with 300 nm and 110 nm repeat layer spacing exposed to He+ irradiation were increased by 40% and 25%, respectively while pure V after irradiation resulted in increase in flow stress of 88%. Significant reduction in radiation hardening in V-graphene nanolayers is expected to be due to reduction in He bubble formation, as well as the ability of the V-graphene interface to absorb the crystalline defects. The total radiation hardening can be expressed as

where is the radiation induced hardening in the presence of He bubbles and is from other crystalline defects. Each of these contributions is individually considered for the case of V-graphene nanolayers.

Radiation induced hardening due to presence of He bubbles ( can be calculated from Friedel–Kroupa–Hirsch (FKH) model for weak obstacles49,50, where the increase in yield strength is given by

where M is Taylor factor, is the shear modulus, b is the Burgers vector, d is bubble diameter and N is bubble density. For the measured He bubble size from TEM images for V-graphene with 110 nm repeat layer spacing, the radiation induced hardening is calculated to be 260 MPa for typical values of M, , b for V metal. The calculated value is smaller than that of the experimentally measured radiation induced hardening of 1200 MPa. Therefore, is calculated to 940 MPa, which can be attributed to radiation induced hardening from other crystalline defects that are introduced from collision cascade. Orowan’s model can now be used to determine the approximate length scale for the existing pinning points within the V matrix.

where is the barrier strength, is average spacing between obstacles then given by = 10.1 nm. Similar calculations of for V-graphene with 300 nm repeat layer spacing and pure V are 7.6 nm and 4.8 nm, respectively. This analysis indicates that the length scale for separation distance between the crystalline defects is the largest for the case with V-graphene with 110 nm spacing, which is indicative of the V-graphene interface effectively absorbing the crystalline defects from the V matrix to minimize radiation induced hardening. In the work by Zhang et al., radiation induced hardening variation of Cu-V with 100 nm spacing under total ion fluence 6 × 1020 ions/m2 was reported to be 0.7 GPa, which again confirms that our V-graphene with 100 nm spacing is significantly more efficient with radiation induced hardening of 1.2 GPa under higher total ion fluence of 1 × 1022 ions/m2.

Molecular dynamics simulations: Analysis

The role of graphene in enhancing the radiation tolerance was investigated by employing molecular dynamics (MD) simulations on a model system of pure V and V-graphene nanolayers. We model the radiation event as a collision of a primary knock-on atom (PKA) with 2 keV (See Fig. 4a). Because the interaction between V and graphene is van der Waal’s type, there are no preexisting defects or misfit dislocations at the interface. The detailed interface structure and defect-interface characteristics before the collision is available in the Supplementary Information Note 3, 4. After the initial explosive collision cascade event, numerous vacancy and interstitial pairs are created. For the body-centered cubic (bcc) crystal such as V, a special type of delocalized mobile interstitial, called crowdion, is formed which can move along the [111] direction quickly without large thermal activation. Crowdions can be recognized as small chains of 3-4 high energy atoms in the Fig. 4c and the Supplementary Movie 2 and 3. The MD simulation results indicate that the radiation induced of V-graphene nanolayers is less severe than that of pure V since the number of remaining point defects from the collision cascade becomes significantly smaller in the presence of graphene layers, as compared in Fig. 4.

Figure 4: Molecular dynamics simulations of the knock-on event. (a) Schematic of simulation. (b) Damage on graphene after the knock-on event. The damage on graphene reduces as the distance to PKA increases because large portion of the energy is released by the vacancy-interstitial formation as well as local heating in lattice. (c) The collision cascade after the knock-on event for d = 15 . The amount of cascade is significantly reduced by the graphene layer. Significantly more defects remain in the pure V. Atoms with high potential energy (above −4.4 eV) are visualized selectively. (d) The formation energies of vacancies are significantly lower at the graphene interface than in the bulk lattice of V. The migrated vacancies can then be annihilated with the crowdions to result in self-healing of radiation induced defects. Full size image

One of the main reasons for the reduction in crystalline defects in the presence of the graphene interface is due to the reduction in the vacancy-interstitial formation. The knock-on energy from the collision cascade is typically relaxed through local heating of the crystal lattice as well as by forming vacancy-interstitial pairs, but a large portion of the energy in the case of V-graphene nanolayers is consumed to break the covalent bonds in graphene due to the high atomization energy (7.4 eV) of the graphene. Thus, significant damage of graphene is observed when the knock-on event occurs close to graphene layers (see Fig. 4b), and this results in reduction of the size of the collision cascade in comparison to that in pure V.

Second reason for less defect formations is due to the V-graphene interface acting as an efficient sink for the interstitial defects or crowdions. Crowdions move along the [111] directions until they encounter the V-graphene interface, and form adatoms between the graphene and V. Vacancies also diffuse to the V-graphene interface and the arrived vacancies are then trapped at the interface because the vacancy formation energy is smaller at the graphene interface (1.2 eV) than in the bulk lattice (2.5 eV) (see Fig. 4d). The vacancy migration mechanism due to its lower formation energy at the interface was also observed in the atomistic modeling of Cu/Nb interface by Misra et al.24. Using the Fick’s first law, one can estimate the time scale (t) for the vacancy to arrive at the interface as

where is diffusion distance assumed to be half of the layer spacing (300 nm → 150 nm, 110 nm → 55 nm) and D is the vacancy diffusion coefficient, is Debye frequency of 1013Hz, is vacancy jump energy barrier calculated from nudged elestic band calculations51 as 0.187 eV, is Boltzmann constant and T is absolute temperature. The timescales for the vacancy to arrive at the graphene interface for V-graphene with 110 nm and 300 nm repeated layer spacings are 3.27 × 104 s and 2.43 × 105 s, respectively, at room temperature. However, during the ion irradiation testing and FIB processing to fabricate pillars for compression testing, the actual temperature can increase up to ~200 °C that could then enable vacancies to reach the interface in just a 0.26 seconds for V-graphene with 110 nm repeated layer spacing. Therefore it is expected that the interstitials in the form of crowdion and vacancies gathered at the interface to annihilate near the graphene interfaces. Higher density of the V-graphene interfaces (i.e. small repeat layer spacings) results in more effective annihilation of interstitials and vacancies, thereby increasing the self-healing ability of the nanolayers. Our analysis is consistent with the experimental observation in which the V-graphene with 110 nm repeat layer spacing showed significantly smaller radiation induced hardening than in 300 nm repeat layer spacing specimen. The self-healing ability as confirmed from MD simulations is expected to be one of the major causes for enhanced radiation tolerance in V-graphene nanolayers, while suppression of He bubbles at the graphene interface also suppress the brittle failure.

Summary

In this study, we developed and analyzed the radiation resistance V-graphene nanolayered composites with varying repeat layer spacings of 110 nm, 300 nm and compared the results to those of pure V. He+ irradiation with dosage of 13.5 dpa revealed that the V-graphene nanolayers had reduced formations of radiation induced crystalline defects compared with pure V, which agreed with the nanopillar compression results of irradiated specimen V-graphene showing reduction in radiation induced hardening and suppression of brittle failure. In-situ SEM compression tests showed that the graphene interface can hinder the crack propagation thus helping to suppress the brittle failure. The structural material degradation due to He bubble formations was shown to be significantly reduced in V-graphene nanolayers due to the impermeability of He through the graphene layer that prevents agglomeration of He gas into large bubbles. Orowan’s model was then used to determine the approximate length scale for the pinning points from the presence of radiation induced defects that indicated less accrual of defects in the V-graphene layers. Finally, MD simulations confirmed that the cause for less crystalline defects in V-graphene layers is due to the graphene interface being able to absorb the crystalline defects that are introduced from collision cascade. Therefore, inclusion of graphene in the form of V-graphene nanolayers can not only result in initially high strength material, but the graphene can self-heal the crystalline defects that are introduced during irradiation as well as terminating migration of He bubbles to result in extremely radiation resistance material.