Abstract
Summary A band of coarse subsurface particles was generated by annealing Al-3%Li at 550°C in dry air, dry hydrogen, or wet air. The particles were typically facetted, and they delineated grain boundaries. These particles were tentatively identified as LiH (NaCl structure, a° = 0.4085nm), and a mechanism for their formation was proposed. A PFZ was observed in this work, in agreement with other studies, and the proposed mechanism predicted this phenomena. Subsurface porosity was not observed in samples annealed in dry air, but mild and severe hydrogen porosity was observed in samples annealed in dry hydrogen and wet air, respectively. LiH particles formed preferentially to the the formation of H2 gas bubbles, and the bubbles appeared to nucleate in or near the LiH particles closest to the metal/scale interface. The coalesence of excess vacancies to form empty voids appeared to be suppressed in environments containing slight amounts of moisture. This was attributed to the mutual trapping of excess vacancies and atomic H. Ongoing work will address the effects of alloying additions, very dry environments, temperature, and the resulting scale characteristics on the formation of LiH and H2 porosity.
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