SUBJECT: Ph.D. Proposal Presentation
BY: Shuozhi Xu
TIME: Thursday, September 17, 2015, 11:00 a.m.
PLACE: MRDC Building, 4211
TITLE: Concurrent atomistic-continuum simulations of sequential slip transfer across grain boundaries in FCC metals
COMMITTEE: Dr. David L. McDowell, Chair (ME)
Dr. Ting Zhu (ME)
Dr. Chaitanya S. Deo (ME)
Dr. Laurent Capolungo (ME)
Dr. Surya Kalidindi (ME)
Dr. Thomas H. Sanders, Jr. (MSE)
Dr. Josh Kacher (MSE)


Sequential slip transfer across grain boundaries (GB) plays an important role in size-dependent propagation of plastic deformation in polycrystalline metals. For example, the Hall-Petch effect, which states that a smaller average grain size results in a higher yield stress, can be rationalized in terms of dislocation pile-ups against GBs. In spite of extensive studies in modeling individual phases and grains using atomistic simulations, well accepted criteria of slip transfer across GBs are still lacking, as well as models of predicting irreversible GB structure evolution. Slip transfer is inherently multiscale since both the atomic structure of the boundary and the long range fields of the dislocation pile-up come into play.

Due to their high computational cost, most atomistic simulations employ only an isolated, short, straight dislocation segment associated with a periodic image in a quasi 2-D specimen, excluding curved dislocations of mixed character. On the other hand, continuum models do not naturally incorporate the necessary degrees of freedom associated with the GBs and other evolving internal state variables that relate to slip transfer criteria. While partitioned-domain multiscale modeling has been pursued to investigate sequential slip transfer, they either can't accommodate long range dislocation pile-ups in the continuum domain or haven't been extended to 3-D. Therefore, we turn our attention to the recently developed concurrent atomistic-continuum (CAC) method which (1) describes interface reactions using fully resolved atomistics, (2) preserves the net Burgers vector and associated long range stress fields of curved mixed character dislocations in a sufficiently large continuum domain in a fully 3-D model, and (3) employs the same governing equations and interatomic potentials in both domains to avoid the usage of phenomenological parameters/criteria and ad hoc procedures for passing dislocation segments between atomistic and continuum domains.

In the proposed work, zero temperature CAC simulations with new types of elements will be performed to study sequential slip transfer of a series of curved dislocations from a given pile-up on a wide range of GBs in two FCC metals Cu and Al. It is anticipated that our finding will improve physical understanding of the slip transfer across a wide range of GBs as a function of dislocation line length, applied shear stress, and nanoscale GB structure.