Abstract
Hydrogen diffusivity in Al-(2-14mass%) Si alloy ingots melted in nitrogen gas flow and solidified unidirectionally were measured by a vacuum hot extraction method. The diffusivity of D1 for as-cast samples decreased to D2 for hydrogen-charged and extracted samples, under an influence of the heating effects. The differences between D1 and D2 increased at the lower temperatures and with amounts of eutectics. The heating effects attended with the decreasing of diffusivity were due to two phenomena. The first was a loss of a long-range enhanced diffusion paths along the surfaces on the thin plate-like particles of eutectic silicon in as-cast samples, and the second was a grow-up of hydrogen trapping at the large granular silicon particles precipitated in α-aluminum at around 673K. The hydrogen solubilities in Al-Si alloys increased with the concentration of silicon.
