Random strains and strain glass transformations in NiTiHf and NiTiZr systems: An NMR study

In this study, we investigated Hf and Zr substituted nitinol shape memory alloys using Nuclear Magnetic Resonance (NMR) spectroscopy, focusing on alloys tuned by thermal processing for martensite-transforming and non-transforming behavior. Ni_50.75Ti_29.25Zr₂₀ and Ni_50.3Ti_29.7Hf₂₀ non-transforming samples were found to exhibit a glass transition, shown very clearly in NMR by the development of large random local strains, despite no evidence for transformation in X-ray crystallography or calorimetry. These non-transforming samples maintain electronic similarity to the B2 cubic phase, indicating that the strain nano-domains are not locally equivalent to martensite. Using NMR relaxation measurements as a highly sensitive probe into kHz and MHz scale dynamical strains, in a NiTiHf sample tuned for strain glass behavior we identified non-Vogel Fulcher dynamical behavior, with rapidly fluctuating local strains developing in the high temperature strain-liquid regime, accompanied by the gradual disappearance of quasi-static dynamics above and below the glass freezing point. Notably, the results also show a clear difference between the dynamical behavior of the strain glass and a NiTiZr non-transforming sample with greater defect density, which exhibits frozen random strain below a well-defined transformation point but without the appearance of any strain-glass dynamics. This demonstrates the possibility for defect driven nanodomain distortions with no strain-glass behavior. These findings highlight the ability of NMR as a powerful tool for investigating the dynamics and local structures in these materials.