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| Atomistic study of equilibrium pressure of helium bubble in irradiated FCC copper and its abnormal size effect |
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Abstract Helium density inside helium bubble and its corresponding internal pressure have a significant influence on the mechanical properties of materials with radiation-induced helium bubble. The equilibrium internal pressure, size effect, temperature effect and stress field of FCC copper containing helium bubble are systematically explored by means of molecular simulations in this paper. Based on energy principle and stress criterion, the method to determine the equilibrium internal pressure is proposed. It is found: 1) The Equilibrium pressures of the bubble obtained by means of the criterion of the minimum energy principle and stress criterion are self-consistent. 2) An abnormal size effect of helium bubble on the equilibrium pressure is found, i.e. when the size of the He bubble is less than a critical one, the equilibrium pressure does not increase but drops slightly with the decrease of bubble size, which is different from the prediction of Young-Laplace equation. The critical size for He bubble is about 3 nm. 3) Compared with molecular simulation results, the traditional Young-Laplace equation overestimates the equilibrium internal pressure of helium bubbles, and the error increases significantly with the decrease of the size of helium bubbles. For instance, when the aperture is 3nm, the relative error exceeds 63%.4) Due to the polyhedron characteristics of the nanoscale helium bubble and the anisotropy of the material, there exists an additional stress field in the surrounding copper matrix of the bubble under the action of the equilibrium internal pressure, which implies that helium bubble-induced stress cannot be eliminated by the equilibrium pressure completely. However, this residual stress decreases rapidly as the increase of the distance from the center of the bubble. In addition, a method to reduce the effect of thermal disturbance or fluctuation on the stress field is proposed by means of the superposition of time average and spatial average. The method adopted in this paper can be extended to investigate the effects of helium bubbles on the mechanical properties of irradiated materials and the interaction between irradiated defects.
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Received: 30 December 2019
Published: 28 December 2020
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2020, Vol. 41 
