On the electronic and mechanical instabilities in Ni50.9Ti49.1

Both the superelastic and the shape memory properties of the cubic NiTi alloy known as Nitinol arise from two-phase transitions; from a simple cubic austenite phase to a trigonal R-phase and to a lower symmetry martensite phase. In this article, we speculate that two competing forces drive these transitions. One is “mechanical ” in nature and it takes the cubic austenite structure of NiTi to a denser packing arrangement. The other one appears to be “electronic ” in nature and it modifies the cubic austenite structure by correlated “Jahn–Teller ” type distortions, and perhaps a fluctuation of charge density. © 2004 Elsevier B.V. All rights reserved. Keywords: NiTi; Nitinol; R-phase; Synchrotron; XANES; Martensit