The effect of the combustion channels on the compressive strength of porous NiTi shape memory alloy fabricated by SHS as implant material Mehmet Kaya a, * , Nuri Orhan b , Gül Tosun c a Adıyaman University, Vocational School, 02040 Adıyaman, Turkey b Fırat University, Technical Education Faculty, Metal Education Department, 23119 Elazig, Turkey c Fırat University, Technical Vocational School, 23119 Elazig, Turkey article info Article history: Received 7 May 2009 Accepted 24 July 2009 Keywords: Porous NiTi shape memory alloy SHS Implant material Compressive strength abstract Porous NiTi shape memory alloy (SMA) with ideal porosity and high compressive strength as an implant material was fabricated by self-propagating high-temperature synthesis (SHS). In this study, a new igni- tion technique ‘‘high voltage electric arc” was used to ignite the green specimens and control the orien- tation of combustion channels which effect compressive strength. It was determined that the compressive strength of specimens was increased when the combustion channels were parallel along the specimen axis, and the compressive strength was decreased when the combustion channels were perpendicular to specimen axis. The desired phases such as B2(NiTi) and B19 0 (NiTi) were dominant while the second phases (Ni 4 Ti 3 and NiTi 2 ) in small amount. The undesired phases (such as pure Ni and Ni 3 Ti) for biocompatibility are not found in the structure. The transformation temperatures were higher for medical applications by heat treatment and partly decreased at every next thermal cycle where the heat- ing rate of the specimen was increased. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction NiTi shape memory alloys (SMAs) have been successfully used in engineering, medical and orthopaedic applications due to their excellent properties, such as shape memory effect (SME), super- elasticity (SE), corrosion resistant and biocompatibility which make these alloys suitable for surgery and brackets, hard tissue replacement and implantation [1–3]. Recently, as a promising bio- material, porous NiTi shape memory alloys (SMAs) have brought new advantages to biomedical applications for hard tissue replace- ment, 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 struc- ture allowing in-growth of the human tissue, nutrition exchange and medicament transportation within human bodies [4]. In addi- tion, lightweight, superelasticity and adjustable mechanical prop- erties of the porous NiTi SMAs can decrease the stiffness mismatches between implant and human bones [5,6]. Superelastic behavior of NiTi can recover up to 8% strain in uni- axial deformation by a reversible stress-induced transformation. Human bone also recovers high strains (up to 2%) and NiTi can thus match this mechanical property as well [7]. The ideal implant material should have the porosity in the range of 30–90%, the optimal pore size (100–500 lm) for bone tissue ingress, elastic modulus should have close values that of bone (20 GPa) and higher compressive strength that of bone. The compressive strength of a compact human bone can reach 170–193 MPa when the test direc- tion is parallel to bone axis and 133 MPa when the test direction is normal to bone axis [8,9]. For this reason, a porous structure of similar compression strength as the bone is desired for implant. On the other hand, it requires controlling pore size, morphology and orientation. Currently, there are several powder metallurgy (PM) methods for fabrication of porous NiTi SMA, such as element powder sinter- ing [10], SHS [11–15], hot isostatic pressing (HIP), capsule-free hot isostatic pressing (CF-HIP) [16], spark plasma sintering (SPS) [17], metal injection moulding (MIM) [18], and mechanical alloying (MA) [19]. Evidence in the literature [13–17,20–23] indicates that the SHS method results in higher levels of porosity and a higher de- gree of NiTi formation. In addition, SHS offers more advantages compared to conventional powder metallurgy (PM). This process is simple and avoids the need for expensive processing facilities and equipment. Furthermore, the rapid exothermic reaction pro- cess leads to time and cost savings. The SHS process is associated with a combustion wave and has the capability to expel volatile impurities as a wave propagates through the sample. As a result, it leads to high purity of products [24]. The process practically con- sists of the mixing powder pressing into pellets and then ignition by an external source locally [13]. Ignition plays role in the reaction 1359-0286/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2009.07.002 * Corresponding author. Tel.: +90 4162232125; fax: +90 4162232129. E-mail address: mkaya@adiyaman.edu.tr (M. Kaya). Current Opinion in Solid State and Materials Science 14 (2010) 21–25 Contents lists available at ScienceDirect Current Opinion in Solid State and Materials Science journal homepage: www.elsevier.com/locate/cossms