Characterization of nanocrystalline TiNi powder Yongqing Fu, Christopher Shearwood * School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Received 28 July 2003; received in revised form 29 September 2003; accepted 16 October 2003 Abstract Nanocrystalline TiNi powder, prepared by the electro-explosion of wire method, was characterized in this work. Analysis using differential scanning calorimetry (DSC) clearly indicated the shape memory effect for this material. An activation energy of 113 ± 10% kJ/mol for sintering was obtained using DSC with variable heating rates. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: TiNi alloy; Nanocrystalline material; Shape memory alloy (SMA); Sintering 1. Introduction Titanium–nickel shape memory alloy (SMA) offers a combination of novel properties, such as the shape memory effect, super-elasticity, biocompatibility and high damping capacity, which enable them to be widely used in numerous applications, especially for biomedi- cal engineering and micro–electro-mechanical-system (MEMS) [1–4]. These alloys are conventionally pro- duced by arc or induction melting followed by hot or cold working or machining to their final dimensions. Ideally, to produce TiNi components on an industrial scale, near net shape fabrication routes are preferred with limited machining of the material. Recently, there has been significant interest in the sintering of TiNi materials using powder metallurgy (PM) processes [5–7]. These PM process can avoid problems associated with casting, like segregation or extensive grain growth [8,9]. Other advantages include precise control of composition and easier achievement of complex shape with minimal post-machining treatment [10,11]. At present, most studies on the PM process of TiNi have been restricted to the use of TiNi powder of micron size [5–12]. The refinement of TiNi particle size can be beneficial for the sintering process. In particular, nano- crystalline powders are known to show very rapid den- sification kinetics in the initial stage of sintering during heating [13]. Despite these advantages it is presently unclear whether a nanocrystalline powder of NiTi would retain its shape memory effect and if so, would the phase transformation temperatures alter due to the size re- duction of the powder. However, so far, there are few reports on the characterization of TiNi nanopowder, due to problems of high chemical reactivity, ease of oxygen and carbon contamination, as well as pyro- phoric property. This present study will focus on the behavior of TiNi nano-powders prepared by the electro-explosion of wire (EEW) process. In this process, a continuous wire is fed into a reactor and is electroexploded into aerosolized particles that are continuously collected. The pulse du- ration is extremely short (about 1 ls) and peak tem- peratures are in excess of 15,000 K. The resulting explosion causes aerosols of metal to be dispersed at a few km/s through cold argon. This results in extraordi- nary quenching rates producing nanoparticles with crystalline and often complex and metastable micro- structures [13,14]. 2. Experimental Titanium–nickel nanopowder was prepared using the EEW process (as described above) with TiNi wire (equiatomic 50/50 Ni/Ti atomic ratio) in an Ar gas (99.999% purity, pressure of 2 MPa) by Argonide Nanomaterials, USA. Prior to characterization, the TiNi powder was placed into a vacuum chamber to re- move the hexane liquid, used for storage and transport of the pyrophonic powder. The microstructure of the * Corresponding author. Tel.: +65-67905524; fax: +65-67911859. E-mail address: mcshearwood@ntu.edu.sg (C. Shearwood). 1359-6462/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2003.10.018 Scripta Materialia 50 (2004) 319–323 www.actamat-journals.com