PNNL

Abstract

Estimation of the melting point of a material is important not only for materials applied under a normal condition but also for cases under an energetic particle implantation condition. In this paper, a coupled effect of high-energy proton implantation and radiation defects on the melting point of aluminum (Al), as a beam dump used in a proton accelerator, is extensively investigated using molecular dynamics (MD) method. The results reveal that interstitial hydrogen impurities can induce lower melting point than hydrides containing the same number hydrogen atoms. Elastic modulus calculation clearly indicates that interstitial hydrogen impurities could induce lower (C11-C12)/2 and C44 than that of hydrides, resulting in a stronger instability and a lower melting point. Furthermore, formation of radiation defect-hydrogen complexes, e.g. dislocation loop/line-hydrogen and vacancy-hydrogen clusters, could also induce a lower melting point than cases containing the same radiation defects without the presence of hydrogen atoms. Therefore, a synergistic effect is observed between these radiation defects and hydrogen atoms in lowering the melting point. The lowest melting point induced by the above complexes is ~87 % of the melting point of pure Al. Microstructure evolution of these complexes indicates the absorption of hydrogen atoms by defects could change the potential energy and the stress state of the initial defects, resulting in larger atomic displacements of Al atoms around such a defect-hydrogen complex (e.g. dislocation/loop-hydrogen, vacancy-hydrogen clusters), lowering the melting point. However, an opposite effect is observed when a void interact with hydrogen atoms, in this case, the void-hydrogen complexes limits the atomic displacements of Al atoms, resulting in a higher melting point. All these results suggest that when an Al alloy is used as a beam dump in a proton accelerator, the proton energy and current should be controlled appropriately to avoid the melt of the alloy.