Communication Shape Memory Behavior of Porous NiTi Alloy MEHMET KAYA and O ¨ MER C¸ AKMAK Shape memory behavior of porous NiTi alloy is dependent on the phases, and mechanical or thermal background. The phases change with solution heat treatment and aging. Fully reversible shape memory behavior was observed during thermal cycling, and recoverable strains increased with the increasing stress from 2 to 50 MPa. The porous NiTi sample shows recoverable transformation strain response under lower constant load. DOI: 10.1007/s11661-015-3318-1 Ó The Minerals, Metals & Materials Society and ASM International 2016 NiTi shape memory alloys (SMAs) have been suc- cessfully used in engineering, especially in medical applications due to their excellent properties, such as shape memory effect (SME), pseudoelesticity or supere- lasticity (SE), corrosion resistance, and biocompatibility which make these alloys suitable for surgery and brackets, implantation, and hard tissue replacement. [1–4] Shape memory and pseudoelesticity properties of these alloys occur due to phase transformations in their microstructures. Phase-transformation behaviors in nickel titanium alloys have been studied extensively with the most emphasis being placed on the near-equia- tomic Ni–Ti compositions concerning aforementioned material responses: ‘‘shape memory’’ and ‘‘pseudoelas- ticity.’’ The ‘‘shape memory’’ refers to the transforma- tion of the material from martensite to austenite upon heating to a temperature exceeding the austenite start temperature. ‘‘Pseudoelasticity’’ is the forward transfor- mation upon loading and the reverse transformation upon unloading at temperatures above the austenite finish temperature. Recently, porous NiTi SMAs have been fabricated as a promising bio-material for hard tissue replacement, in particular for hip implantation and femur repair. These porous alloys have the adjustable mechanical properties, reduced weight, and increased biocompatibility due to their porous structure allowing in-growth of the human tissue, medicament transportation, and nutrition exchange within human bodies. [3–5] Also, some proper- ties such as superelasticity, lightweight, and adjustable mechanical properties of the porous NiTi SMAs can decrease the stiffness mismatches between human bones and implant, and thus the wearing of bones is prevented. [6–8] Porous NiTi SMA used in this study was fabricated using self-propagating high-temperature synthesis (SHS). Several other powder metallurgy methods such as element powder sintering, [9] spark plasma sintering (SPS), [10] hot isostatic pressing (HIP), [11] capsule-free hot isostatic pressing (CF-HIP), [11] mechanical alloying (MA), [12] and metal injection molding (MIM) [13] have also been used for the fabrication of porous NiTi SMAs, and the details of these processes are readily available in the literature. These methods can be used to avoid the problems associated with casting, such as extensive grain growth or segregation, and have some advantages such as easy realization of complex part shapes and precise control of composition. [14] The SHS method offers more advantages compared to the aforementioned powder metallurgy methods during fabrication of porous NiTi SMAs. First, NiTi alloys with more porosity can be fabricated using SHS method (up to 70 pct). The SHS process is simple and does not require expensive facilities and equipment, and also, provides savings in terms of time and cost. The process practically consists of two following activities; pressing the mixed powder into pellets and then igniting it by an external source locally under argon atmosphere. Ignition plays role in the reaction and product morphology due to the high thermal conductivity of reactants and products. In our previous studies, [5,14] a different ignition technique, high-voltage electric arc, was used to ignite the green Ni-Ti specimens during fabrication. Considering the need for an implant, this ignition technique allows orienting the combustion channels, and therefore can be MEHMET KAYA, Associate Professor, and O ¨ MER C¸ AKMAK, Master Student, Research Assistant, are with the Metallurgy and Material Engineering Department, Engineering Faculty, Adıyaman University, Adıyaman 02040, Turkey. Contact e-mails: mkaya@ adiyaman.edu.tr, mehmetkaya75@hotmail.com Manuscript submitted April 23, 2015. Article published online January 19, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 47A, APRIL 2016—1499