Fabrication of lotus-type porous NiTi shape memory alloys using the continuous zone melting method and tensile property

Lotus-type porous NiTi alloys with unidirectionally elongated cylindrical pores were fabricated using the continuous zone melting method in a high-pressure mixture of hydrogen and helium gases. The lotus-type porous NiTi alloy with elongated pores was successfully fabricated under a hydrogen partial pressure less than 1.75 MPa. However, under a hydrogen partial pressure greater than 2.0 MPa, the pores formed during solidification rapidly combined and finally did not grow homogenously. Since the increase in the pore volume due to pore combination reduced the area of the solid-liquid interface where supersaturated hydrogen was released from the solid, hydrogen was not provided in sufficient amounts to the pores. The stress-strain curves of porous and nonporous NiTi with tensile direction parallel to the pore orientation exhibit similar tendency while the tensile strength decreases with increase of the porosity.