Dynamic evolution of He bubble and its effects on void nucleation-growth and thermomechanical properties in the spallation of aluminum
TT Zhou and AM He and P Wang, JOURNAL OF NUCLEAR MATERIALS, 542, 152496 (2020).
Using molecular dynamics simulation, we investigate the dynamic evolution characteristics of He bubble and its effects on void nucleation-growth and thermomechanical properties in single crystal Al during spallation. The evolution process of bubble includes four stages: slow and independent expansion, rapid and independent expansion, deceleration expansion due to inter-bubble interaction and merging, and fast expansion after merging. The bubble radius-time relation is approximately linear both before and after merging and the corresponding expansion rates are obtained. The mechanism of bubble expansion (growth) in solid metal under dynamic tension involves plastic deformation via dislocation nucleation and movement and temperature rising around the bubble. The growth toward neighboring bubble leads to the overlap of plastic deformation regions that promotes local melting. The flows of melted atoms along both normal and tangential directions of the two bubbles result in the thinning and breaking of the inter-bubble ligament, and thus bubble merging occurs. In melted metal, the tension deformation under velocity gradient leads to bubble growth and coalescence. He bubble impedes the nucleation and growth of nearby voids and the expanded bubble absorbs grown voids when they contact. Such suppression enhances with the increase of bubble size or density while attenuates with the increase of shock strength. The number of nucleated voids is much lower and the average radius of voids is smaller for the sample with He bubbles in solid state. After melting, the reduction in quantity gets smaller and the radius becomes a little higher due to the suppression on continuing nucleation of voids. The bubble behavior and its influence on void evolution result in the variation of spall mechanism: the expansion and merging of He bubble play the leading role, independent on shock strength, bubble density, or bubble size. The effects of He bubble on temperature and stress are also addressed. (C) 2020 Elsevier B.V. All rights reserved.
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