An In Situ Study of Sintering Behavior and Phase Transformation Kinetics in NiTi Using Neutron Diffraction

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NICKEL titanium (NiTi) alloys have been receiving significant interest for a wide range of applications such as couplings, actuators, and medical devices, because they have a unique shape memory effect, superelastic properties with large energy absorption, good corrosion resistance, and good biocompatibility.[1] However, the high production cost of NiTi components limits their applications. Powder metallurgy, particularly near-net shaping technologies, offers a promise to reduce the manufacturing cost of NiTi products. Among PM manufacturing techniques, conventional press-and-sinter is a simple, cost-effective and most common powder metallurgical route.[2] Recently, there have been a large number of investigations into the influences of chemical compositions,[3,4] pressing pressure,[5,6] dimensional change,[7–10] sintering,[3,5,11,12] particle size,[3,13] and the use of titanium hydride powder[6,11,14–20] on the microstructures and mechanical properties of the sintered NiTi alloys. In GANG CHEN, Research Fellow, is with the Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand, and also with the State Key Laboratory of Porous Metal Materials, Northwest Institute for Nonferrous Metal Research, Xi’an, 710016 Shaanxi, P.R. China. KLAUS-DIETER LISS, Senior Scientist, is with the Australian Nuclear Science and Technology Organisation, New Illawarra Road, Lucas Heights, NSW 2234, Australia, and also with the Quantum Beam Science Directorate, Japan Atomic Energy Agency, 2-4 Shirakata-Shirane Tokai-mura, Naka-gun, Ibaraki-ken 319-1195, Japan. PENG CAO, Associate Professor, is with the Department of Chemical and Materials Engineering, the University of Auckland. Contact e-mail: [email protected] Manuscript submitted January 22, 2015 Article published online September 15, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A

most cases, powder sintering is undertaken in a solid state, i.e., at a temperature below 1173 K (900 C) to avoid the occurrence of liquid phases.[21] However, a lengthy solid-state sintering time, often up to 30 hours, is required in order to obtain a single B2 phase—a prerequisite of shape memory effect.[21] Liquid-phase sintering is deemed to help accelerate chemical homogenization and to achieve a single B2 phase in the sintered NiTi alloys, but possibly at the expense of dimensional integrity. In practice, a single B2 phase is difficult to obtain during Ni/Ti powder sintering, regardless of how high the sintered density is achieved: some minor intermetallic phases persistently exist in the sintered compact.[17] Another equally important aspect in the Ni/ Ti powder sintering is the formation of porosity. In the Ni-Ti system there exists a eutectic reaction of b-Ti + NiTi2 fi L at Teu-b = 1215 K (942 C).[22] Above Teu-b, the melting event occurs and subsequent capillary flow of the melt leaves a pore behind the prior b-Ti particle.[17] This eutectic reaction plays two roles in the sintering process[17,23,24]: (i) achieving a higher alloying de

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