NMR Studies of NiTi Strain-Glass

We have studied Ni_50+xTi_50-x materials using ⁴⁷Ti and ⁴⁹Ti NMR, at temperatures up to 400 K. Measured compositions range from x = 0.1 featuring a martensitic transformation, to x = 1.2 exhibiting a strain glass transformation, which is believed to involve freezing of dynamical ferroelastic nanodomains. In the high-temperature phase we find a small enhancement of the NMR Knight shift for the strain glass composition, an indication of an enhanced electronic density of states at EF. For x = 0.1, the line shape changes observed across the austenite to martensite transformation (B2 to B19') are consistent with previous reports, and include a very small change of the Knight shift, along with an enhancement of the electric field gradients (EFGs) in the locally distorted B19' phase. The strain glass exhibits a significantly larger reduction in Knight shift at the freezing transformation near 180 K, along with a large increase in EFG in the frozen strain glass, corresponding to the anisotropic atomic arrangement within the frozen strain glass. These results are analyzed according to the reduced metallic behavior of the strain glass, and the local dynamics above the strain glass transition, and compared to the proposed R-phase structure for this system.