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.