Abstract. In this paper we introduce a stacking-fault based model to understand the energetics of formation of the nanolamellar-based metal carbide and nitride structures. The model is able to reproduce the cohesive energies of the stacking fault phases from Density Functional Theory calculations by fitting the energy of different stacking sequences of metal layers. The model demonstrates that the first and second nearest metal-metal neighbor interactions and the nearest metal-carbon/nitrogen interaction are the dominant terms in determining the cohesive energy of these structures. The model further demonstrates that above a metal to non-metal ratio of 75%, there is no energetic favorability for the stacking faults to form a long-range ordered structure. The model's applicability is demonstrated using the Ta-C system as its case study from which we report that the interfacial energy between ζ-Ta 4 C 3 and TaC or Ta 2 C is negligible. Our results suggest that the closed packed planes of these phases should be aligned and that precipitated phases should be thin, which is in agreement with experiments.
Predicting
the properties of grain boundaries poses a challenge
because of the complex relationships between structural and chemical
attributes both at the atomic and continuum scales. Grain boundary
systems are typically characterized by parameters used to classify
local atomic arrangements in order to extract features such as grain
boundary energy or grain boundary strength. The present work utilizes
a combination of high-throughput atomistic simulations, macroscopic
and microscopic descriptors, and machine-learning techniques to characterize
the energy and strength of silicon carbide grain boundaries. A diverse
data set of symmetric tilt and twist grain boundaries are described
using macroscopic metrics such as misorientation, the alignment of
critical low-index planes, and the Schmid factor, but also in terms
of microscopic metrics, by quantifying the local atomic structure
and chemistry at the interface. These descriptors are used to create
random-forest regression models, allowing for their relative importance
to the grain boundary energy and decohesion stress to be better understood.
Results show that while the energetics of the grain boundary were
best described using the microscopic descriptors, the ability of the
macroscopic descriptors to reasonably predict grain boundaries with
low energy suggests a link between the crystallographic orientation
and the resultant atomic structure that forms at the grain boundary
within this regime. For grain boundary strength, neither microscopic
nor macroscopic descriptors were able to fully capture the response
individually. However, when both descriptor sets were utilized, the
decohesion stress of the grain boundary could be accurately predicted.
These results highlight the importance of considering both macroscopic
and microscopic factors when constructing constitutive models for
grain boundary systems, which has significant implications for both
understanding the fundamental mechanisms at work and the ability to
bridge length scales.
The local atomic structure, local
chemistry, and stoichiometry
of grain boundaries control in part the strength and fracture toughness
of silicon carbide components. The predictions of the structure and
properties of these grain boundaries are generally limited to their
ground-state configurations. We investigated the tensile strength
behavior of metastable grain boundaries in silicon carbide using high-throughput
atomistic simulations combined with machine learning techniques. We
analyzed and compared the ∑5 ⟨100⟩{120} and ∑9
⟨110⟩{122} tilt grain boundary metastable configurations
to identify structural and chemical attributes that dominate their
tensile strength. We characterized these metastable grain boundaries
using a set of microscopic descriptors representing the local grain
boundary atomic structure and the local grain boundary stoichiometry
and chemical-bound types. We
used a boosted regression tree surrogate model for the successful
prediction of metastable grain boundary strength as a function of
these descriptors. Our results show that the tensile strength of generic
(i.e., any random grain boundary from the entire grain boundary population),
metastable grain boundaries is primarily dominated by the grain boundary
excess free volume, closely followed by the type of structure composing
the boundary and the amount of C–C bonds. The 5% strongest
metastable grain boundaries have particular characteristics with a
low amount of free volume and the highest density of C–C bonds.
Our results reveal that the 5% strongest and weakest metastable grain
boundaries are most sensitive to the local stoichiometry, regardless
of the local atomic structure composing the grain boundary as compared
to any other generic metastable grain boundaries. We show that the
strongest and weakest metastable grain boundary configurations can
be identified as specific regions in a low-dimensional-representation
space of their microscopic descriptors. Taken together, these findings
showcase the effectiveness and validity of using a low-dimensional
representation of the grain boundary structure and machine-learned
surrogate models to rapidly assess metastable grain boundary strength
without the need to perform actual tensile simulations.
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