Relationship between hydrogen diffusivity and microstructure in Al-Si alloys ingots

Abstract

Hydrogen diffusivity in Al-(2-18 mass%)Si alloys melted in air were measured by a desorption technique which was an outgassing study from cylindrical samples. The diffusivity of D₁ for as-cast samples were decreased to D₂ for hydrogen-charged samples. The differences between D₁ and D₂ were enlarged for lower temperatures and for the more eutectic at higher silicon concentration. The decreasing of diffusivity from D₁ to Dv₂ by heating was due to a morphogy change of eutectic silicon, from two reasons, one of which is a loss of long-range enhanced diffusion path along surfaces on plate-like particles of eutectic silicon for as-cast samples and the other reason is a grow-up of hydrogentrapping at coagulated and isolated particles of silicon. A downward aberration of diffusivity at around 400°C was related to a hydrogen trapping by small silicon particles precipitated from a supersaturated alpha-aluminum solid solution.