Theory of Pressure-Induced Rejuvenation and Strain Hardening in Metallic Glasses

We theoretically investigate high-pressure effects on the atomic dynamics of metallic glasses. The theory predicts compression-induced rejuvenation and the resulting strain hardening that have been recently observed in metallic glasses. Structural relaxation under pressure is mainly governed by local cage dynamics. The external pressure restricts the dynamical constraints and slows down the atomic mobility. In addition, the compression induces a rejuvenated metastable state (local minimum) at a higher energy in the free-energy landscape. Thus, compressed metallic glasses can rejuvenate and the corresponding relaxation is reversible. This behavior leads to strain hardening in mechanical deformation experiments. Theoretical predictions agree well with experiments.

Figure 1

Logarithm of structural relaxation time of several metallic glasses as a function of $1000/T$ at ambient pressure. Open points are experimental data in Refs. [33, 37] and solid curves correspond to ECNLE calculations.

Figure 2

The dynamic free energy as a function of reduced particle displacement for a hard-sphere fluid of packing fraction $\mathrm{\Phi }=0.60$ at several pressures in units of $k_{B}T/d^3$. Key length scale of barriers for the local dynamics are indicated.

Figure 3

The theoretical pressure dependence of structural relaxation time of ${\mathrm{Pd}}_{30}{\mathrm{Ni}}_{50}{\mathrm{P}}_{20}$ (solid curves) and ${\mathrm{Pd}}_{40}{\mathrm{Ni}}_{10}{\mathrm{Cu}}_{30}{\mathrm{P}}_{20}$ (dash-dotted curves). Red, orange, yellow-green, and green curves correspond to calculations at $T=635.33$, 618.67, 602, and 595.33 K, respectively.

Figure 4

The dynamic free energy as a function of reduced particle displacement for a hard-sphere fluid of packing fraction $\mathrm{\Phi }=0.53$ at several stresses in the case of (a) strain hardening and (b) strain softening. The yield stresses ${\sigma }_y$ of strain-hardening ($\sim 26$ $k_{B}T/d^3$) and strain-softening ($\sim 4$ $k_{B}T/d^3$) calculations correspond to the minimum stress to suppress the secondary and primary localization in $F_{\mathrm{dyn}}(r)$, respectively.