Melting point decrease induced by synergistic effect of proton implantation and radiation defects in aluminum

A coupled effect of high-energy proton implantation and radiation defects on the melting point of aluminum (Al) is investigated using molecular dynamics (MD) method. The results reveal that interstitial hydrogen impurities can induce lower melting point than hydrides. Calculation of three independent elastic constants C₁₁, C₁₂, and C₄₄ of Al before and after proton implantations 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. Formation of radiation defect-hydrogen complexes could also induce a lower melting point than cases without the presence of hydrogen atoms. The lowest melting point is ~ 87% of the melting point of pure Al, which is observed in a dislocation loop-hydrogen complex system with a hydrogen concentration around 1% and radius of a dislocation loop around 15 Å. Microstructure evolution indicates the absorption of hydrogen atoms by defects could change the potential energy and the stress state of defects, resulting in larger atomic displacements of Al atoms around dislocation/loop-hydrogen and vacancy-hydrogen clusters, lowering the melting point. However, an opposite effect is observed when a void interacts with hydrogen atoms, in which the atomic displacements of Al atoms are limited, resulting in a higher melting point than that induced only by a void. 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.