  The effect of cooling on the brittleness of glasses in general, and bulk
metallic glasses (BMGs) in particular, is usually studied with continuously
varying cooling rates; slower cooling rates lead to stiffer, harder, and more
brittle glasses than higher cooling rates. These protocols obscure any
potential discontinuity that a glass might experience depending on whether its
microstructure resembles that of a fragile or a strong glass-forming liquid.
Here, we use large-scale molecular dynamics to simulate the nanoindentation
behavior of model BMGs (Zr$_{0.6}$Cu$_{0.3}$Al$_{0.1}$) obtained by rapidly
quenching equilibrium melts from temperatures above and below the
fragile-to-strong transition temperature $T_\textrm{fst}$, leading to fragile
and strong glasses, respectively. While the contact modulus deduced from the
indentation simulation evolves smoothly with the temperature $T_\text{q}$ from
which the melt is quenched, the plastic response changes quasi-discontinuously
as $T_\text{q}$ passes through $T_\textrm{fst}$. In particular, strong glasses
develop highly asymmetric flow profiles with mature shear bands, in contrast to
fragile glasses. Quantitative differences reveal themselves not only through a
formal von Mises localization parameter analysis but also through image
analysis of flow patterns using pre-trained artificial intelligence models.
Moreover, seemingly erratic flow profiles for our indentation geometry produced
surprisingly reproducible and, thus, deterministic features. It remains to be
determined to what extent other classes of glass formers follow our observation
that the degree of brittleness is significantly influenced by whether the melt
is fragile or strong when it falls out of equilibrium at the glass transition
temperature.
