Microstructure of Fragile Metallic Glasses Inferred from Ultrasound-Accelerated Crystallization in Pd-Based Metallic Glasses

By utilizing ultrasonic annealing at a temperature below (or near) the glass transition temperature $T_{\mathrm{g}}$, we revealed a microstructural pattern of a partially crystallized Pd-based metallic glass with a high-resolution electron microscopy. On the basis of the observed microstructure, we inferred a plausible microstructural model of fragile metallic glasses composed of strongly bonded regions surrounded by weakly bonded regions (WBRs). The crystallization in WBRs at such a low temperature under the ultrasonic vibrations is caused by accumulation of atomic jumps associated with the $\beta$ relaxation being resonant with the ultrasonic strains. This microstructural model successfully illustrates a marked increase of elasticity after crystallization with a small density change and a correlation between the fragility of the liquid and the Poisson ratio of the solid.

Figure 1

High-frequency internal friction as a function of temperature for $\beta$ relaxation in Pd-based metallic glasses. (a) Temperature dependence of high-frequency internal friction measured for ${\mathrm{Pd}}_{40}{\mathrm{Ni}}_{40}{\mathrm{P}}_{20}$ metallic glass ($T_{\mathrm{g}}\approx 300°\mathrm{C}$). Shear vibration modes were used for the internal-friction measurements, and the rate of temperature scan was about $1°\mathrm{C}/\min$. The resonance-frequency shifts $\Delta f$ are also displayed for reference. The sudden drop of $Q^{-1}$ and marked increase in $\Delta f$ near $T_{\mathrm{g}}$ are caused by abrupt crystallization. (b) $Q_{\beta }^{-1}$-$T$ curves calculated using Eq. (2) with $\Delta E\sim 1.0\mathrm{eV}$ and ${\tau }_0\sim 1.0\times {10}^{-14}\mathrm{s}$ taken from the literature [11].

Figure 2

Typical HREM image of ${\mathrm{Pd}}_{42.5}{\mathrm{Ni}}_{7.5}{\mathrm{Cu}}_{30}{\mathrm{P}}_{20}$ metallic glass after annealing at 290 °C (below or near $T_{\mathrm{g}}$) for 10 h under ultrasonic vibrations of 0.35 MHz. The remaining amorphous fraction was estimated to be about 80% from the crystallization heat measured by differential scanning calorimetry. It can be clearly recognized that the amorphous domains are surrounded by crystallized walls that show lattice fringes. Ultrasonic vibrations are considered to accelerate crystallization of WBRs via $\beta$ relaxation. The crystallized walls would be thin if annealing time is shortened. The sample for HREM was prepared by ion milling with cooling using liquid ${\mathrm{N}}_2$ to prevent crystallization due to ion irradiation. For the sake of conspicuity of the boundaries, the amorphous-like regions are colored grey.

Figure 3

(a) Microstructural model of fragile metallic glasses inferred on the basis of the microstructure of the ultrasonic-annealed sample (Fig. 2). (b) Simplified model for computing the macroscopic elastic moduli. In this model, the glassy substances are separated into two parts: SBRs and WBRs. Atomic mobility in WBRs is higher than that in SBRs (the long and short arrows denote the atomic mobility). The symbol $\xi$ represents the characteristic length.

Figure 4

Macroscopic elastic constants of glasses composed of SBRs and WBRs as a function of the volume fraction of SBRs. The elastic constants of WBRs are $c_{11}=5\mathrm{GPa}$ and Poisson’s ratio $\nu$ is 0.49. (a) Poisson’s ratio in SBR is large compared with that in WBR. The elastic constants in SBRs are $c_{11}=200\mathrm{GPa}$ and $\nu =0.33$. (b) Poisson’s ratios in SBRs and WBRs differ greatly. The elastic constants in SBRs are $c_{11}=200\mathrm{GPa}$ and $\nu =0.09$. In the present calculations, although the elastic constants are arbitrarily chosen for SBRs and WBRs, as long as the magnitude relationship is maintained, changing the values does not affect the qualitative discussion.