Control of porosity and superelasticity of porous NiTi shape memory alloys prepared by hot isostatic pressing

Porous Ni-rich NiTi SMAs have been successfully prepared from elemental Ni and Ti powders, using capsule-free hot isostatic pressing processing. The fabricated alloys have isotropic and uniform pore distributions of spherical pores ranging from 50 to 200 νm. The porosities of these NiTi alloys range from 30 to 40% by volume. The microstructure, martensitic transformation behaviors and mechanical properties of the fabricated porous NiTi SMAs aged under different conditions were studied. It has been found that the phase transformation behaviors of porous NiTi SMAs are similar to those of conventionally cast Ni-rich NiTi alloys. It is encouraging that the porous NiTi SMAs exhibit excellent superelasticity at room temperature after the appropriate thermomechanical treatment. Experimental results reveal that the superelasticity, which is controlled by the pore characteristics, such as pore shape, pore size and pore distribution, is important for improving the toughness, elastic properties and compression strength of the porous NiTi SMAs. © 2005 IOP Publishing Ltd.