Met. Mater. Int., doi: 10.1007/s12540-015-5098-y Effect of High-Pressure Torsion on the Microstructure and Wear Behavior of NiTi Alloy Mohammad Farvizi 1, * , Mohammad Reza Akbarpour 2 , Eun Yoo Yoon 3 , and Hyoung Seop Kim 4 1 Ceramic Division, Materials and Energy Research Center, Tehran, P.O. Box 14155-4777, Iran 2 Department of Materials Engineering, Faculty of Engineering, University of Maragheh, Maragheh, Iran 3 Korea Institute of Materials Science (KIMS), Changwon 642-831, Korea 4 Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea (received date: 17 February 2015 / accepted date: 4 May 2015) The wear property of NiTi is one of the most important properties of this alloy. In the current study, the effect of high-pressure torsion (HPT) process on the wear properties of an austenitic NiTi shape memory alloy is investigated. Full density NiTi samples with a composition of Ti-56 wt% Ni are fabricated using hot isostatic pressing (HIP), followed by the HPT process at room temperature, with an applied pressure of 6 GPa for 10 turns. The microstructural analysis reveals that the HIP-processed samples with a B2-NiTi phase evolve into significant grain refinement after HPT process and an interwoven B2-B19′ nanocrystalline/amorphous structure formed, leading to increased hardness in these samples. The results of the wear tests using a ball-on-disc configuration at room temperature demonstrate that the wear performance of the samples is improved after the HPT process. This is due to greater hardness and better pseudo-elasticity in the HPT-processed samples. Keywords: shape memory alloys, severe plastic deformation, phase transformation, wear, scanning/transmission electron microscopy (STEM) 1. INTRODUCTION Because of the interesting and unique properties they pro- duce, severe plastic deformation (SPD) methods have recently been favored for manufacturing bulk nanomaterials. High- pressure torsion (HPT) is one of the most successful SPD methods. In this method, disc-shaped samples are subjected to high levels of applied pressure and torsion. This imposes extremely large strains on the target materials and yields ultrafine- grained structure, compared with other SPD methods [1]. Recently, various aspects of NiTi shape memory alloys, such as pseudo-elasticity (PE) and shape memory effect (SME), have been investigated [2]. Both PE and SME effects originate from a martensitic transformation between austenite (high- temperature) and martensite (low temperature) phases. The wear property of NiTi is one of the most important proper- ties of this alloy [3]. Different hypotheses have been proposed to describe its extraordinary wear resistance. Some researchers [3,4] believe that the PE property is the main factor enhancing the wear resistance of the alloy under low loads. Some evi- dences show that this property cannot be very effective under high loads and long travel distances (during wear testing) where heavy plastic deformations induce in worn surfaces [5-7]. Under such a condition, other properties, for example hardness and strain hardening, play major roles in enhance- ment of wear resistance of the NiTi alloy. There are numerous reports about the successful applica- tion of the HPT process for metals, intermetallics, and compos- ites [8-10]. This method has also been employed for the production of bulk nanostructured NiTi. While different aspects of the HPT-processed NiTi, such as microstructure [11,12], mechani- cal properties [13], and corrosion behavior [14], have been well investigated, to the best of our knowledge, there is no report about the effect of employing the HPT process on the wear behavior of this alloy. In this study, in order to understand the effects of the HPT process on the wear resistance of NiTi, first, full density NiTi samples with an austenitic structure at room temperature were consolidated using a hot isostatic pressing (HIP) method. Then they were subjected to the HPT process. The wear behavior of the samples before and after the HPT process was investigated. 2. EXPERIMENTAL PROCEDURES High-purity pre-alloyed NiTi powders with a composition *Corresponding author: mmfarvizi@yahoo.com, mmfarvizi@merc.ac.ir KIM and Springer