Thin
films of amorphous alumina (a-Al2O3)
have recently been found to deform permanently up to 100% elongation
without fracture at room temperature. If the underlying ductile deformation
mechanism can be understood at the nanoscale and exploited in bulk
samples, it could help to facilitate the design of damage-tolerant
glassy materials, the holy grail within glass science. Here, based
on atomistic simulations and classification-based machine learning,
we reveal that the propensity of a-Al2O3 to
exhibit nanoscale ductility is encoded in its static (nonstrained)
structure. By considering the fracture response of a series of a-Al2O3 systems quenched under varying pressure, we
demonstrate that the degree of nanoductility is correlated with the
number of bond switching events, specifically the fraction of 5- and
6-fold coordinated Al atoms, which are able to decrease their coordination
numbers under stress. In turn, we find that the tendency for bond
switching can be predicted based on a nonintuitive structural descriptor
calculated based on the static structure, namely, the recently developed
“softness” metric as determined from machine learning.
Importantly, the softness metric is here trained from the spontaneous
dynamics of the system (i.e., under
zero strain) but, interestingly, is able to readily predict the fracture
behavior of the glass (i.e., under
strain). That is, lower softness facilitates Al bond switching and
the local accumulation of high-softness regions leads to rapid crack
propagation. These results are helpful for designing glass formulations
with improved resistance to fracture.