Fabrication of lotus-type porous iron and its mechanical properties

Abstract

Porous iron whose long cylindrical pores are aligned in one direction has been fabricated by unidirectional solidification of the melt in a mixture gas of hydrogen (nitrogen) and argon. Both hydrogen and nitrogen saturated in the molten iron are rejected at the solid–liquid interface during the solidification due to the difference of solubility between the liquid and the solid. The gas pores are evolved from the hydrogen (nitrogen) insoluble in the solid iron, which grow unidirectionally. The porosity is controlled by the partial pressures of hydrogen (nitrogen) and argon during melting and solidification. By increasing the partial pressure of argon gas the pore formation is suppressed, since the pressure and, therefore, the density of the hydrogen (nitrogen) gas in the growing pore are increased with the total pressure of the atmosphere.The nitrogen concentration in solid iron fabricated under nitrogen atmosphere increases linearly with partial pressure of nitrogen, leading to the improvement of mechanical properties of the porous iron. The ultimate tensile strength and the yield strength of the porous iron with the pore orientation parallel and perpendicular to the tensile direction are about twice as high as those under hydrogen atmosphere. Such superior strength is attributed to the solid-solution hardening due to solute nitrogen atoms in iron matrix.