Effect of nano Al 2 O 3 addition on mechanical properties and wear behavior of NiTi intermetallic M. Farvizi a,⇑ , T. Ebadzadeh b , M.R. Vaezi a , H.S. Kim c , A. Simchi d a Nanotechnology and Advanced Materials Division, Materials and Energy Research Center, P.O. Box 14155-4777, Tehran, Iran b Ceramic Division, Materials and Energy Research Center, P.O. Box 14155-4777, Tehran, Iran c Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, South Korea d Department of Materials Science and Engineering and Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O. Box 11365-9466, Tehran, Iran article info Article history: Received 22 January 2013 Accepted 29 March 2013 Available online 9 April 2013 Keywords: NiTi Nano Al 2 O 3 Phase stability Nanoindentation Wear abstract It has been found that the high wear resistance of NiTi alloy is mainly attributed to its pseudoelasticity which is only effective within a small temperature range. It is believed that pseudoelasticity becomes ineffective by applying high-load wear condition which yields plastic deformation and temperature increment during wear test. Therefore, the enhanced wear resistance can be obtained from the improve- ment of mechanical property of the alloy without much reduction of pseudoelasticity. In this study, a low weight percentage of hard Al 2 O 3 nanoparticles were added to NiTi atomized powders. The resultant pow- der mixture was homogenized by ball milling and sintered in a vacuum furnace in order to improve the wear property of the composite in comparison with the NiTi alloy. The results demonstrated that the addition of nanoparticles increased the stability of martensite phase. Nanoindentation test results showed that both hardness and elastic modulus were considerably increased in the composite samples in comparison with pure NiTi, while the pseudoelasticity was not significantly reduced. The worn sur- faces were investigated using a scanning electron microscope equipped with energy dispersive X-ray. The wear test results confirmed the improved wear performance of NiTi matrix after the addition of nanoparticles under both low and high loads can be mainly attributed to superior mechanical properties combined with pseudoelasticity effect of the composite samples. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Recent investigations have demonstrated that NiTi alloy exhib- its important wear property and can be used as a wear resistant material [1,2]. The high wear performance of the NiTi alloy is pri- marily attributed to its unique pseudoelasticity property as well as the intrinsically high strain-hardening and fatigue resistance of the NiTi alloy [3,4]. Pseudoelasticity results from the reversible martensitic trans- formation and a large amount of deformation recovered due its complete or partial occurrence but it is well-known that with increasing the applied load, plastic deformation with slip mecha- nism will precede the stress induced martensitic transformation. In this condition, NiTi will deform like normal alloys and pseudo- elasticity cannot be effective. Also, complete pseudoelasticity is only occurred in a narrow temperature range higher than A f (Aus- tenite finishing temperature) and applying long distances during wear test will cause surface temperature increment which yields to pulling away from this territory. So, improvement of mechanical properties of NiTi can help to increase the high-load wear resistance. Due to its unique properties, such as good ductility at room temperature, good vibration damping properties and pseudoelas- ticity, NiTi SMAs have a great potential to be used as matrix mate- rials of the wear resistant composites. There are few reported studies on wear behavior of NiTi-based composites reinforced using hard particles. Previous investigations conducted on TiC– NiTi composites by Li et al. [5,6] demonstrated that the addition of TiC microparticles had a positive effect on the wear resistance of NiTi, even though the TiC particles reduced the pseudoelasticity of the material. Interestingly, in the Li et al’s work, the optimum volume fraction of the reinforcements, which caused significantly less pseudoelasticity, was almost the same as that of the NiTi ma- trix which will cause brittleness of the composite. Also, it is proved that addition of reinforcement particles affects the phase stability due to formation of mismatch and misfit dislocations [7]. In the re- search by Li et al. [8], they demonstrated that adding pseudoelastic NiTi constituents to WC–Co composites can improve the wear per- formance of the composites. Also, they showed that using ZrW 2 O 8 and tungsten can assist in the adjustment of effective pseudoelas- tic temperature range and as a consequence enhance the wear resistance of the alloy [9]. 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.03.099 ⇑ Corresponding author. Tel.: +98 935 848 5439; fax: +98 21 88773352. E-mail addresses: mmfarvizi@yahoo.com, mmfarizi@merc.ac.ir (M. Farvizi). Materials and Design 51 (2013) 375–382 Contents lists available at SciVerse ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes