Processing and Properties of Porous NiTiPublic Deposited
Increasing interest in long-life bone implants with reduced mechanical properties reducing the stress-shielding effect and a structure mimicking bone porous architecture has encouraged study and development of fabrication methods for porous NiTi. The main objective of this work was to advance these goals by developing new processing procedures for porous NiTi that offer the following advances over existing technology: (i) controllable pore characteristics (porosity, pore size/shape, connectivity) to stimulate and accommodate a high level of bone ingrowth; (ii) mechanical properties (yield strength, stiffness, recovery strain) matching those of bone, to minimize stress-shielding effects; (iii) dense metal struts and walls, to prevent crack initiation and increase ductility; (iv) simplicity and low cost of production; and (v) capability for either shape-memory or superelasticity. A new strategy to fabricate porous NiTi with the above characteristics is presented. This strategy is based on densification by hot isostatic pressing (HIP) of a mixture of pre-alloyed NiTi powders and salt space holders, followed by removal of the salt. Using this strategy, the porosity of the NiTi is controllable by varying the volume fraction of the salt, and the pore size and shape can be tailored in accordance with the geometry of the salt used. The effects of using different space holders, i.e. sodium fluoride (NaF) and sodium chloride (NaCl), on the final porous product are examined. The influence of HIP temperature and further post-HIP sintering on foam properties is also presented. The high cost associated with HIP processing drove subsequent work towards new strategies to simplify porous NiTi synthesis while maintaining high-quality microstructures and mechanical properties. The strategy followed here involves combining the NaCl space holder technique with in situ transient liquid phase sintering of pre-alloyed NiTi and Nb powders, thereby integrating the densification of NiTi struts and the removal of space holders into a single step. The effect of Nb addition on structural and physical properties of porous NiTi is discussed. Some extension of the work towards understanding the mechanical behavior and microstructure development of porous NiTi is included.