title: Crystal structures and shape-memory behaviour of NiTi.
abstract: Shape-memory alloys (SMAs) are a unique class of metal alloys that after a large deformation can, on heating, recover their original shape. In the many practical applications of SMAs, the most commonly used material is NiTi (nitinol). A full atomic-level understanding of the shape-memory effect in NiTi is still lacking, a problem particularly relevant to ongoing work on scaling down shape-memory devices for use in micro-electromechanical systems. Here we present a first-principles density functional study of the structural energetics of NiTi. Surprisingly, we find that the reported B19' structure of NiTi is unstable relative to a base-centred orthorhombic structure that cannot store shape memory at the atomic level. However, the reported structure is stabilized by a wide range of applied or residual internal stresses. We propose that the memory is stored primarily at the micro-structural level: this eliminates the need for two separate mechanisms in describing the two-way shape-memory effect.
authors.affiliation: Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854-8019, USA.
authors.forename: Xiangyang
authors.initials: X
authors.lastname: Huang
authors.forename: Graeme J
authors.initials: GJ
authors.lastname: Ackland
authors.forename: Karin M
authors.initials: KM
authors.lastname: Rabe