Effect of impurity hydrogen on the deformation and fracture in an Al-5 mass pct Mg alloy

Air-melted and argon-melted Ai-5 mass pet Mg alloy specimens containing impurity hydrogen of 0.27 and 0.04 mass ppm, respectively, were tensile-tested at ambient temperature. The ductility and fracture processes were compared in the two specimens, and hydrogen evolution behavior during the test was also;compared using a special testing machine equipped with a mass spectrometer and ultra high vacuum chamber. The air-melted specimen, containing a higher amount of hydrogen, had less reduction in area (RA) and a higher amount of evolved hydrogen gas on fracture. This implied that the impurity hydrogen was in the transgranular voids, which appeared as dimples on the fracture surface. Fracture process analysis involving fractography, load-displacement curve analysis, and optical microscopy on a cross section of the deformed test piece demonstrated that the impurity hydrogen reduces nonuniform elongation by accelerating the nucleation of transgranular voids produced under triaxial tensile stress after necking. Hydrogen evolution was also detected corresponding to each load drop in the serrated flow of the air-melted specimen, supporting the idea that hydrogen atoms are transported with moving dislocations.