Investigation of the Effect of Foaming Agent on the Fabrication of NiTi Foams Using the Self-Propagating, High-Temperatu
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TRODUCTION
THE NiTi is considered as one of the most important intermetallic compounds due to its unique combination of properties, which makes it distinct from other intermetallic compounds; among these, the shape memory effect as well as superelasticity, good corrosion resistance, superior damping capability, and biocompatibility are well known.[1–6] NiTi is used in external and internal biomedical applications such as actuators, orthodontic arch wires, catheters, and dental implants.[1, 7–11] In addition, porous NiTi alloy has the potential to be used in bone implantation, since the porous structure facilitates the ingrowth of new bone tissue as well as body fluids transport and ensures the establishment of a bond between the body and the implant.[12,13] HELIA KALANTARI, MANDANA ADELI, and M. REZA ABOUTALEBI are with the School of Metallurgy and Materials Engineering, Iran University of Science and Technology, Narmak, Tehran 1684613114, Iran. Contact email: [email protected] Manuscript submitted February 28, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
The fabrication of porous NiTi using powder metallurgy (PM) routes, such as conventional pressing and sintering, combustion synthesis, hot isostatic pressing, metal injection molding, spark plasma sintering, and electroassisted PM, has been the subject of numerous research works, and different levels of pore size, shape, and distribution have been obtained thereof.[3,9,13–19] Among these, combustion synthesis is a novel method, which offers great savings of time, energy, and cost in comparison to conventional methods of fabrication of porous parts. The production of NiTi via combustion synthesis is based on the exothermic reaction between Ti and Ni. The process can be performed either in the thermal explosion (TE) mode by applying high heating rates to the entire sample, which results in simultaneous initiation of the reaction throughout the mixture, or in the self-propagating high-temperature synthesis (SHS) mode by initiating the reaction at one surface and permitting the reaction wave to traverse the sample.[2,12] Considering the enthalpy of formation of NiTi from the starting elemental powders ( 67.8 kJ/mol), the heat released as a result of the reaction between Ni and Ti is large enough for the resultant NiTi to reach its melting point (~ 1310 C); therefore, a low-viscosity NiTi melt is
usually obtained during the reaction. Incorporation of an endothermic agent in the Ni + Ti mixture leads to a decrease in combustion temperatures and, therefore, an increase in the viscosity of molten NiTi, which in turn results in decreased rupture of cell walls and less coarsening of the pores.[20] The effects of different parameters, such as preheating temperature,[18,21] heating rate,[18,22] application of pressure,[18] particle size of reactants,[2,15,20,23] mechanical activation,[21] and combustion conditions,[15] on combustion synthesis of NiTi have been studied. Different ignition methods, such as hot tungsten wire, exothermic mixture of Ti and C, laser, and microwav
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