NMR Signature of Evolution of Ductile-to-Brittle Transition in Bulk Metallic Glasses

The mechanical properties of monolithic metallic glasses depend on the structures at atomic or subnanometer scales, while a clear correlation between mechanical behavior and structures has not been well established in such amorphous materials. In this work, we find a clear correlation of ${}^{27}\mathrm{Al}$ NMR isotropic shifts with a microalloying induced ductile-to-brittle transition at ambient temperature in bulk metallic glasses, which indicates that the ${}^{27}\mathrm{Al}$ NMR isotropic shift can be regarded as a structural signature to characterize plasticity for this metallic glass system. The study provides a compelling approach for investigating and understanding the mechanical properties of metallic glasses from the point of view of electronic structure.

Figure 1

Characterization of brittle-to-plastic transition in ${\mathrm{Zr}}_{61}{\mathrm{Cu}}_{18.3-x}{\mathrm{Ni}}_{12.8}{\mathrm{Al}}_{7.9}{\mathrm{Sn}}_x$ MGs with $0\le x\le 2.5$. Black squares represent prenotched toughness; red circles represent plastic process zone size; green triangles represent plastic strain; The inset displays the evolution of fracture morphology as a function of Sn concentration.

Figure 2

(a) Representative ${}^{27}\mathrm{Al}$ NMR central line shape of powder spectra of ${\mathrm{Zr}}_{61}{\mathrm{Cu}}_{18.3-x}{\mathrm{Ni}}_{12.8}{\mathrm{Al}}_{7.9}{\mathrm{Sn}}_x$ MGs with $x=0$, 0.5 and 2.5. The ${}^{27}\mathrm{Al}$ NMR spectra are normalized to the maximum intensity for various Sn concentrations and taken at $H=9.39\mathrm{Tesla}$ and $T=298\mathrm{K}$. The solid lines are fitted with Gaussian distribution. (b) The correlation of ${}^{27}\mathrm{Al}$ NMR line shift (left coordinate) and fracture energy (right coordinate) with Sn concentration; The solid lines and arrows are for guiding eyes. (c) Frequency dependence of the ${}^{27}\mathrm{Al}$ line width. The solid line is a fitting by the form ${\nu }_0\Delta \nu =a+b\nu _0{}^{\wedge 2}$.

Figure 3

(a) A schematic illustration of the overlap of nonpolarizable $s$-like and $pd$ hybrid atomic orbitals during shear; (b) the overlap of atomic orbital versus the atomic shear displacement. The overlap of $s$-like atomic orbital remains almost constant during shear; $pd$ hybrid atomic orbitals subjected to shear overcome a balance position in which the overlap between bond orbitals reduces to much less than $s$-like orbital since $p\mathrm{\text{−}}d$ hybrid orbital extends within one atomic distance, while $s$ orbital extends much further with a significant overlap beyond second nearest neighbors.