| Structural metals are widely used in nuclear reactors and nuclear waste storage,thus they are required to serve under extreme conditions known to date,such as high-energy particle irradiation,high temperature,corrosive medium and stress application.Nanolayered metal composites are known to have excellent radiation resistance,owing to the copious interplay between their densely populated internal boundaries,which can serve as sinks for irradiation-induced defects.Therefore,they have been considered as promising candidates for structural applications in advanced nuclear reactors.However,nanolayered metal composites usually rely on low throughput fabrication strategies such as magnetron sputtering,atomic layer deposition,accumulative roll bonding,etc.,so it is necessary to develop scalable fabrication route for the production of bulk metals and components with nanolayered structure.In the meantime,nanocarbon materials(graphene,carbon nanotubes,etc.)are often added to metals as novel reinforcement.In addition to playing a role in the mechanical properties and conduction properties of materials,nanocarbon materials can also introduce a large number of heterogeneous interfaces due to their high specific surface area.The research on the irradiation effect of nanocarbon reinforced metal matrix composites is still in its infancy.Specifically,most of these materials are prepared by small-scale processes,and there is a lack of research on direct uniaxial tensile properties testing,deformation mechanism investigation,defect evolution and its interaction with composite interfaces of nanocarbon reinforced metal matrix composites after irradiation.To solve the above bottleneck issues,the reduced graphene oxide(RGO)reinforced aluminum(Al)matrix composite with nanolaminated microstructure was used as the model material in this study and was subject to high-energy helium ion irradiation to simulate the environment in a reactor.The mechanical properties,deformation mechanisms and defect evolution of the composites were studied afterwards.The main research results were listed as follows:(1)Uniaxial tension and compression properties and deformation modes of samples irradiated at room temperature.The uniaxial tension and compression tests on the irradiated zone showed that the RGO-Al composites had lower irradiation-induced strengthening magnitude(reduced from～300 MPa in pure Al to～150 MPa in the RGO-Al composite),a considerable total elongation and a completely different deformation mechanism compared to the unreinforced matrix.This is a result of the addition of RGO layers,which played a role in absorbing and storing helium bubbles and reducing the crystal defects in the matrix.However,the RGO layers were also prone to act as a path for crack to initiate and propagate during the fracture process,forming stepped fracture morphology.(2)Impact of irradiation temperature on mechanical properties and deformation mechanisms.The composites were irradiated with high-energy helium ions at room temperature and high temperature(200℃),respectively.Through the precise control of the indentation depth,the influence of the substrate effect on the indentation results is eliminated,and we obtained the nanoindentation hardness and modulus of as-fabricated,as-annealed,room temperature irradiated,high temperature irradiated and post-irradiation annealed composites.The results showed that the composite had good thermal stability,and the hardness and modulus of the high temperature irradiated and post-irradiation annealed composites recovered to some extend compared with room temperature irradiated composite.Moreover,the size and morphology of the helium bubbles also changed.Transmission electron microscopy(TEM)of deformed area showed that the arrays of helium bubbles and the nanoscale deformation twins formed in the room temperature irradiated composite,while the deformation of the high temperature irradiated composites was dominated by dislocation slip and accompanied by the rheology of helium bubbles.According to the indentation creep test,the fitted creep stress exponents,n,and the extracted apparent activation volumes,V~*,reflected a mechanistic shift,from a dislocation-mediated creep for as-fabricated composite,to a diffusion-assisted mechanism in the irradiated samples.(3)Evolution of irradiation-induced helium bubbles during annealing process and the effect of composite interface on the helium bubble growth.The evolution of helium bubbles in the composite was observed by a combination of in situ and ex situ TEM annealing.Specifically,the size of the helium bubbles increases with Al layer thickness,while the density of helium bubbles showed the opposite trend.In the same Al layer,the size of helium bubbles in the grain interior is larger than that in the vicinity of composite interface,while the density of helium bubbles was much higher near the interface as compared to the aluminum grain interior.It is likely that the amorphous alumina layer in the composite interface can act as an effective sink for interstitial atoms and vacancies,and also act as an emission source for interstitial atoms.After heating,the mobility of interstitial atoms is much higher than that of the vacancies,which results in the retention of the vacancy in the center of the Al layer.However,the interstitial atoms are simultaneously emitted from the interface and recombine with the vacancies around the interface,inhibiting the growth of helium bubbles near the interface.Combined with the theoretical model of helium bubble growth,it is concluded that the growth mechanism of helium bubbles is dominated by vacancy dissolution controlled Oswald ripening(OR/v)together with migration and coalescence(MC)in the temperature range of 423 K to 523 K,while the growth mechanism of helium bubbles is mainly dominated by MC from 523 K to 623 K.In the experiment,anomalous large helium bubbles were also observed at the end of composite interface,the results of Raman spectra and molecular dynamics simulation indicated that irradiation and annealing can cause structural destruction(i.e.,amorphization)and self-healing of RGO layers,respectively.The defective graphene can act as a diffusion channel for helium atoms,promoting helium diffusion along the interface.Therefore,the increase in helium concentration at the end of the interface would promote the helium bubble to further absorb vacancies and form an anomalous large bubble.In summary,RGO-Al composite was used as model material in the present work,we took high-energy helium ions irradiation as simulated reactor environment.The irradiation effects of RGO-Al composite were investigated in detail by using a variety of micro-/nano-mechanical tests,microstructure characterization and molecular dynamics simulation.In particular,the tensile property and the effects of irradiation temperature on the mechanical properties and deformation mechanisms of irradiated nanocarbon materials reinforced metal matrix composites,as well as the effects of composite interface on the defect evolution,were systematically studied.This work provides a new design idea for the development of nuclear structural materials and a new method for the evaluation of the irradiation effect of nanocarbon reinforced metal matrix composites.Moreover,the methodology and the scalable fabrication route developed in this study are not limited to any particular composite systems,but can be readily extended and applied in those that are heavily used in nuclear applications,such as the steel-,nickel-,and zirconium-based systems. |
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