Fabrication of lotus-structured porous iron by unidirectional solidification under nitrogen gas

A novel technique has been developed to fabricate lotus-structured porous iron in which long cylindrical pores are aligned in one direction. The iron is melted and unidirectionally solidified in a pressurized gas mixture of nitrogen and argon. The process involves the dissolution of nitrogen in molten iron and the evolution of nitrogen pores due to the decrease in solubility of nitrogen during solidification. The porosity is controlled by adjusting the partial pressures of nitrogen and argon during melting and solidification. The nitrogen concentration in solid iron increases with increasing partial pressure of nitrogen at a given total pressure, leading to improvement of the mechanical properties of the porous iron.