NMR Study of Nickel-Titanium Strain-Glass and Shape Memory Alloys

We studied Ni_50+xTi_50-x with compositions between x = 0.1 and 2.0, by performing ⁴⁷Ti and ⁴⁹Ti nuclear magnetic resonance (NMR) measurements over a temperature range from 4 K to 400 K. This work demonstrates the value of NMR as a tool for detecting local electronic configurations and phase transitions in these shape memory and stain glass materials. NMR line shapes for the near-equiatomic sample clearly show signatures of the classic martensite transition, while for x ≥ 1.2 we detect a different behavior characteristic of strain glass transformations caused by the freezing of dynamical ferroelastic nanodomains. The spin-lattice relaxation time (T₁) measurements show how the undoped and doped materials exhibit distinct metallic characteristics, indicating a large enhancement of fermi-level density of state via doping. As the temperature increases, we discover a fluctuation-induced NMR T₁ in the austenite phase for x = 0.1, suggesting the configuration's dynamic disorder configuration. The T₁ results also provide a measure of the dynamical slowing down process in strain glasses. We also describe density functional theory (DFT) calculations modeling the electric field gradients which provide a sensitive probe to detect the local strain distortion in NMR.