NMR study of Ni50+xTi50-x Strain-Glass

TL;DR

This study uses NMR and x-ray diffraction to investigate strain-glass formation in Ni50+xTi50-x alloys, revealing the microscopic freezing process of nano-domains without structural phase change.

Contribution

It demonstrates that NMR effectively probes strain-glass freezing and clarifies the microscopic dynamics and domain behavior in Ni-Ti alloys.

Findings

NMR line-shape changes indicate strain glass freezing.

No structural change detected in x >= 1.2 samples.

Relaxation times suggest thermally activated domain dynamics.

Abstract

We studied Ni50+xTi50-x with compositions up to x = 2, performing 47Ti and 49Ti nuclear magnetic resonance (NMR) measurements from 4 K to 400 K. For large x in this system, a strain-glass appears in which frozen ferroelastic nano-domains replace the displacive martensite structural transition. Here we demonstrate that NMR can provide an extremely effective probe of the strain glass freezing process, with large changes in NMR line-shape due to the effects of random strains which become motionally narrowed at high temperatures. At the same time with high-resolution x-ray diffraction we confirm the lack of structural changes in x >= 1.2 samples, while we show that there is little change in the electronic behavior across the strain glass freezing temperature. NMR spin-lattice relaxation time (T1) measurements provide a further measure of the dynamics of the freezing process, and indicate a predominantly thermally activated behavior both above and below the strain-glass freezing temperature. We show that the strain glass results are consistent with a very small density of critically divergent domains undergoing a Vogel-Fulcher-type freezing process, coexisting with domains exhibiting faster dynamics and stronger pinning.