Splitting of fast relaxation in a metallic glass by laser shocks

Fast relaxation, a locally dynamic activation, universally occurs in metallic glasses below room temperature. Despite much attention, its structural origin has barely been solved and remains mysterious. In this work, the dynamic mechanical relaxations of a typical Zr-based metallic glass with different structural states are systematically studied from 135 to 748 K. We find that the single-peak (${\beta }^{\prime }$) fast relaxation in the annealed glass splits into two distinct peaks (γ and ${\beta }^{\prime }$) by multiple-pulse laser shocks, and this dynamics splitting will vanish after re-annealing. Slight structural rejuvenation of the laser-shocked sample is detected by thermodynamic measurements. Structural characterizations further reveal that the laser shocks lead to an increase in the edge-sharing medium-range orders (MROs), which is then recovered by re-annealing. The one-to-one changes in relaxation dynamics and atomic structures provide convincing evidence that the fast relaxation originates from the excitation of MRO structures, and its splitting is due to the reconstruction of MROs. We also find that the split fast relaxations contribute to the room-temperature plasticity of the studied glasses.

Figure 1

(a) Schematic illustration of the laser shock treatment on the bulk Vit105 MG sample. (b) XRD patterns of the annealed, laser-shocked, and re-annealed samples.

Figure 2

Plots of the normalized loss modulus $E{}^{″}/E_{\alpha }^{{}^{″}}$ versus $T/T_{\alpha }$ for the annealed, laser-shocked, and re-annealed samples. The inset is a magnification of the dashed box.

Figure 3

(a) Temperature dependence of $E^{″}$ for the annealed, laser-shocked, and re-annealed samples tested at driven frequencies of 1, 2, 4, and 6 Hz. The inset is a magnification of the dashed box. (b) Plots of ln($f$) versus $1000/T_i$ for the annealed, laser-shocked, and re-annealed samples. The black, blue, red, and green dots represent the γ, ${\beta }^{\prime }$, β, and α relaxations, respectively. The activation of the γ, ${\beta }^{\prime }$, and β relaxation follows the Arrhenius equation depicted by the solid lines, whereas that of the α relaxation adheres to the VFT equation depicted by the dashed lines.

Figure 4

(a) Specific heat capacity curves $C_{\mathrm{p}}$ of the annealed, laser-shocked, and re-annealed samples. The inset enlarges the changes of relaxation enthalpy before the glass transition. (b) Plots of $(C_{\mathrm{p}}-{\gamma }_{\mathrm{s}}T)/T^3$ versus $T$ between 1.9 and 100 K. The BPs’ changes are highlighted in the inset.

Figure 5

(a)–(c) HRTEM images of the annealed, laser-shocked, and re-annealed samples, respectively. The insets show the corresponding SAED patterns. (d) RDFs deduced from the SAED patterns. The dashed box highlights the second-neighbor position.

Figure 6

High-energy XRD results for the annealed, laser-shocked, and re-annealed samples. (a) Diffraction intensity curves $I$($q$). The inset shows the local magnification of the upper portion of the first peaks. (b) RDF $G$($r$) obtained by Fourier transformation of $I$($q$) in (a).

Figure 7

Enlarged view of the second RDF peaks in Fig. 6. The ordinate and abscissa are respectively scaled by the height $g$($r_1$) and position $r_1$ of the first RDF peak. The vertical dashed lines mark the position of different MROs’ types, which are interpenetrating ($1.57r_1$), face sharing ($1.66r_1$), edge sharing ($1.90r_1$), and vertex sharing ($2.10r_1$). The insets are the schematics of corresponding MROs’ types. The red spheres represent the two central atoms that are second neighbor to each other. The green spheres belong to the left atomic clusters and the blue to the right. The yellow spheres are the sharing atoms of two atomic clusters.

Figure 8

Mechanical responses of the annealed, laser-shocked, and re-annealed samples to nanoindentation. (a) Load- and unload-depth curves. (b)–(d) AFM images of indents after unloading and the corresponding profiles along the downward dashed arrows.