Computational Studies of the NiTi Alloy System: Bulk, Supercell, and Surface Calculations
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Computational Studies of the NiTi Alloy System: Bulk, Supercell, and Surface Calculations Amanda C. Stott,a,b Phillip B. Abel,b Christopher DellaCorte,b Stephen V. Pepper,b and David A. Dixona a
Department of Chemistry, The University of Alabama, Shelby Hall, Box 870336, Tuscaloosa, AL 35487-0336 b NASA Glenn Research Center, Tribology and Mechanical Components Branch, Cleveland, OH 44135 ABSTRACT Plane wave ab initio density functional theory (DFT) calculations of the B2 NiTi (100), (110), and (111) surfaces, the B2 and B19´ phases of NiTi, and the supercell structures of NiTi, Ni4Ti3 and Ni3Ti are reported. Electronic energies from the electronic structure calculations are used to assess relative stability of the different surface and supercell geometries. INTRODUCTION NiTi is important as a shape memory alloy material. Equimolar NiTi undergoes a thermally induced martensitic phase transformation between the high temperature, high symmetry rigid austenitic cubic phase (B2) and the low temperature, low symmetry ductile martensitic monoclinic phase (B19’) during deformation and heat treatment. Upon cooling below the martensitic transformation temperature, unstrained shape memory NiTi has a twinned microstructure. Upon deformation, the twins reorient along the direction of applied stress. When heated above the austenite transition temperature, the alloy reverts to the original shape in the austenite phase.[1,2] Additionally, this alloy possesses other interesting physical properties which are only just being realized. Tailoring the microstructure of NiTi by increasing the Ni atomic content leads to alloys with increased dimensional stability. For example, Ni-rich NiTi alloys have excellent corrosion resistance and relatively high ductility,[3] as well as increasing wear resistance with increasing Ni content.[4] NiTi has also been demonstrated as a potential bulkphase alloy material for rolling contact bearing applications, maintaining tribological performance equal to that of traditional tool steels in friction tests.[5] Unlike traditional tool steels, which are magnetic and may corrode in harsh environments, Ni55Ti45 is non-corrosive and non-magnetic, both of which are desirable properties currently not found in any bearing material. To fully understand the atomic behavior of this alloy in rolling contact applications, a detailed analysis of the surface electronic structure is needed. We present ab initio density functional theory (DFT) calculations of the B2 NiTi (100), (110), and (111) surfaces, the B2 and B19´ phases of NiTi, and the supercell structures of NiTi, Ni4Ti3 and Ni3Ti. THEORY First principles total energy calculations were performed using plane wave DFT as implemented in the Vienna Ab-Initio Simulation Package (VASP 5.2).[6,7] The calculations were performed at 0K using the projector augmented wave (PAW) method [8,9] with the PW91 generalized gradient approximation (GGA) exchange-correlation functional.[10,11] Atomic
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positions and unit cell dimensions were optimized for each crystal structure w
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