An atomistic-to-mesoscale prediction of the complex reaction between the plastic flow and the interfaces in heterogeneous materials
Date
2023-05
Authors
Peng, Yipeng
Major Professor
Advisor
Xiong, Liming
Bastawros, Ashraf
Collins, Peter
Cui, Jun
Levitas, Valery
Committee Member
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Abstract
The microstructure of many engineering materials is intrinsically heterogeneous because they are embedded with a high density of interfaces, such as grain boundaries (GBs), phase boundaries (PBs), twin boundaries (TBs), and several others. When subjected to deformation, especially severe plastic deformation, the performance of those materials is largely dictated by: (I) the reaction between the dislocation-mediated plastic flow and the interfaces; and (II) the subsequent structure changes including phase transformations (PTs), crack initiation, twinning nucleation, atomic diffusion, and so on. However, it remains a challenge to use existing single-scale techniques or methodologies to simultaneously resolve the microscale plastic flow and the atomic-level structure changes near the buried interfaces. To meet this challenge, the goal of this dissertation research is to develop a data-driven multiscale computational framework that can: (a) accommodate the micrometer-level dislocation slip together with the atomic-level nucleation of twinning/PTs within one single model which needs the empirical interatomic potential or the machine learning-based interatomic potential trained from ab initio data is the only constitutive relation; (b) characterize the complexity of the local stresses induced by the reaction between a microscale plastic flow and an atomically resolved interface; (c) create a mechanism-based database that can be used to guide the formulation/selection of local stress-based PT criterion and slip transfer metrics for being used in higher length-scale models through machine learning.
As a preliminary attempt, in this dissertation, Firstly, we performed a series of atomistic simulations to study the phase transformations in graphite under pressure. In graphite-to-diamond phase transformations, we found that: 1) the phase transformation path depends on the compression orientation, 2) when dislocations are introduced into the model, the
critical pressure for the phase transformation is reduced, and 3) the PT is controlled by local stresses from which a criterion can be formulated for detecting the graphite lattice instability. A series of follow-up simulations also include multi-axial loading of the perfect single-crystal graphite using LCBOP and ReaxFF potential to form the lattice instability of graphite.
Then we took a view of the dislocations pileup. Taking a two-phase material, we perform concurrent atomistic-continuum simulations to (i) characterize the internal stress induced by the microscale dislocation pileup at an atomically structured interface (ii) decompose this stress into two parts, one of which is from the dislocations behind the pileup tip according to the Eshelby model and the other is from the dislocations at the pileup tip according to a super-dislocation model; and (iii) assess how such internal stresses contribute to the atomic-scale phase transformations (PTs), reverse PTs, and twinning. Our major findings are: (a) the interface dynamically responds to a pileup by forming steps/ledges, the height of which is proportional to
the number of dislocations arriving at the interface; (b) the stress intensity factors are linearly proportional to the number of the dislocations in a nanoscale pileup, but upper bends to a high level when tens of dislocations are involved in a microscale pileup; (c) when the pre-sheared sample is compressed, a direct square-to-hexagonal PT occurs ahead of the pileup tip and
eventually grows into a wedge shape. The two variants of the hexagonal phases form a twin with respect to each other; (d) upon a further increase of the loading, part of the newly formed hexagonal phase transforms back to the square phase. The square product phase resulting from this reverse PT forms a twin with respect to the initial square phase. All phase boundaries (PBs) and twin boundaries (TBs) are stationary and correspond to zero thermodynamic Eshelby driving
forces; and (e) the stress intensity induced by a pileup consisting of 16 dislocations reduces the stress required for initiating a PT by a factor of 5.5, compared with that in the sample containing no dislocations. The gained knowledge will advance our understanding of how the multi-phase material behaves in many complex physical processes, such as the synthesis of multi-phase high-entropy alloys or superhard ceramics under high-pressure torsion, deep mantle earthquakes in geophysics, and so on, which all involve dislocation slip, PTs, twinning, and their interactions across from the atomistic to the microscale and beyond.
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dissertation