Figure 4: Three
efficiently packed, inter-penetrating, self-consistent clusters
in the
Al25La55Ni20 BMG.
(September 15, 2015)
Abstract
Great progress has been made in understanding the atomic
structure of metallic glasses, but there is still no clear connection between
atomic structure and glass-forming ability. Here we give new insights into
perhaps the most important question in the field of amorphous metals: how can
glass-forming ability be predicted from atomic structure? We give a new
approach to modelling metallic glass atomic structures by solving three
long-standing problems: we discover a new family of structural defects that
discourage glass formation; we impose efficient local packing around all atoms
simultaneously; and we enforce structural self-consistency. Fewer than a dozen
binary structures satisfy these constraints, but extra degrees of freedom in
structures with three or more different atom sizes significantly expand the
number of relatively stable, ‘bulk’ metallic glasses. The present work gives a
new approach towards achieving the long-sought goal of a predictive capability
for bulk metallic glasses.
Introduction
From the moment metallic glasses were discovered in 1960
(ref. 1), questions arose about their atomic structure. The dense random
packing (DRP) model was introduced independently to describe the structure of
monatomic liquids2, 3, 4. The metallic glass community adopted the DRP model,
even though it consisted of single-sized atoms and metallic glasses always had
atoms of different sizes. Attempts to put smaller atoms in the natural gaps of
the DRP model5, 6 were abandoned since the gaps were too small and too few to
agree with metallic glasses7. In a dramatic break from the DRP model, the
stereo-chemically defined (SCD) model used efficiently packed, solute-centred
clusters with total coordination of 9 as structural building blocks for
metal-metalloid glasses8. This model included atoms of unequal size and gave a
physical basis for chemical short-range order (SRO) known to exist in metallic
glasses. However, it could not explain the medium-range order (MRO) found soon
after the SCD model was introduced9, there was never a satisfying description
of how efficiently packed clusters were arranged to avoid packing
frustration10, and it could not explain the full range of atom sizes and
concentrations that produced metallic glasses. Studies clearly showed that
metallic glass structures were, indeed, efficiently packed11, 12, 13, 14, but
none were able to explain how glass structures accomplished this feat. Reviews
of the first 30 years of metallic glass structural modelling are available15,
16, 17.