Influence of nanocrystalline Ni–Ti reaction agent on self-propagating high-temperature synthesized porous NiTi

Nanocrystalline Ni–Ti was used in self-propagating high-temperature synthesis (SHS) to fabricate porous NiTi. The SHS of porous NiTi using elemental powders was also prepared for comparison. Results showed that the main phase was NiTi with unreacted Ni when using elemental powders, which is detrimental to medical use. A large amount of Ti2Ni secondary phase was also detected. By employing mechanically alloyed nanocrystalline Ni–Ti as a reaction agent, the secondary intermetallic phase (i.e. Ti2Ni) was significantly reduced and the unreacted Ni was eliminated. The addition of 25wt% nanocrystalline Ni–Ti reaction agent produced porous NiTi with an average porosity of 52–55vol% and a general pore size of 100–600μm under preheating temperatures of 200 and 300°C. This general pore size in the range of 100–600μm is beneficial to biomedical application for osseointegration. By further increase of the reaction agent to 50wt% in the reactant, a porous NiTi part was produced at ambient temperature (i.e. no preheating was necessary) and a dense part was formed at preheated temperature of 200°C due to the large amount of energies in the nanocrystalline reaction agent. This revealed that the use of nanocrystalline reaction agent effectively lowered the activation barriers for combustion synthesis reaction.