Public Abstract

DE-SC0019096: Dislocation dynamics across interfaces in polycrystals with high resolution x-ray imaging at diffraction limited light sources - Renewal - 1

Award Status: Inactive

Institution: Carnegie Mellon University, Pittsburgh, PA
UEI: U3NKNFLNQ613
DUNS: 052184116

Most Recent Award Date: 09/08/2021
Number of Support Periods: 4
PM: Vetrano, John

Current Budget Period: 08/15/2021 - 08/14/2022
Current Project Period: 08/15/2021 - 08/14/2022
PI: Rollett, Anthony

Supplement Budget Period: N/A

Public Abstract

Mesoscale science can be defined as the frontier where a constructionist approach to ascending the length and time scales fails. In the context of understanding the plastic deformation of polycrystals, the inability of continuum level simulations to describe strain and orientation fields at the grain scale and below is symptomatic of such a frontier. Such a failure is important because quantitative prediction of damage initiation and growth depends on calculating correctly the extremes of stress and strain and their relationship to microstructural features such as grain boundaries. A logical next step is to simulate deformation at a deeper scale using tools such as discrete dislocation dynamics (DD), which has the potential to inform coarser scale problems. Regardless of the technical issues that must be addressed in order to achieve realistic DD simulations of polycrystals, validation will require data from experiments that combine well defined boundary conditions with detailed probes of the response of individual grains and the dislocations contained therein. The proposed combination of x-ray dark field imaging, Bragg coherent diffractive imaging (BCDI) and high energy diffraction microscopy (HEDM) provides an opportunity to perform precisely such experiments. Near-field HEDM will be used to map out a volume of a polycrystal sample and coherent scattering will provide phase maps within individual grains from which displacement fields can be inferred. Given low enough dislocation densities, the displacement fields will be segmented to infer how the dislocation segments move around in each grain in response to varying mechanical load. The grain map along with the initial strain field will instantiate a continuum polycrystal plasticity model that will be used to perform concurrent simulations of the experimental loading sequence. With our collaborations at National Laboratories and universities, we will also perform dislocation dynamics simulations with the aim of directly comparing against the experimental results and advancing the technique. This project will strengthen US synchrotron capabilities for the materials sciences. Suitable x-ray beams are available at the Advanced Photon Source where our collaborators already have performed BCDI experiments. With current coherence properties, these measurements are restricted to x-ray energies around 10 keV, to low Z elements, and to micron sized grains in wires of roughly 100 micron diameter. Dark field microscopy, with intermediate resolution, allows nano-scale imaging in larger diameter samples with larger grains and possibly more complex defect structures. The demonstration of combined dark field, BCDI and HEDM measurements points forward to the APS Upgrade that will provide x-ray beam properties including higher coherence at higher energies and enhanced flux. The Upgrade will loosen restrictions and allow more diverse systems to be studied with the developed techniques. The development of these capabilities will enable applications to other materials and systems such as polymers and biomaterials. The proposed developments will also provide important characterization capability for answering questions such as “how do local stress states affect material stability,” which was cited as a central question by a recent DOE Workshop on Basic Research Needs.

Mesoscale science can be defined as the frontier where a constructionist approach to ascending the length and time scales fails. In the context of understanding the plastic deformation of polycrystals, the inability of continuum level simulations to describe strain and orientation fields at the grain scale and below is symptomatic of such a frontier. Such a failure is important because quantitative prediction of damage initiation and growth depends on calculating correctly the extremes of stress and strain and their relationship to microstructural features such as grain boundaries. A logical next step is to simulate deformation at a deeper scale using tools such as discrete dislocation dynamics (DD), which has the potential to inform coarser scale problems. Regardless of the technical issues that must be addressed in order to achieve realistic DD simulations of polycrystals, validation will require data from experiments that combine well defined boundary conditions with detailed probes of the response of individual grains and the dislocations contained therein. The proposed combination of x-ray dark field imaging, Bragg coherent diffractive imaging (BCDI) and high energy diffraction microscopy (HEDM) provides an opportunity to perform precisely such experiments. Near-field HEDM will be used to map out a volume of a polycrystal sample and coherent scattering will provide phase maps within individual grains from which displacement fields can be inferred. Given low enough dislocation densities, the displacement fields will be segmented to infer how the dislocation segments move around in each grain in response to varying mechanical load. The grain map along with the initial strain field will instantiate a continuum polycrystal plasticity model that will be used to perform concurrent simulations of the experimental loading sequence. With our collaborations at National Laboratories and universities, we will also perform dislocation dynamics simulations with the aim of directly comparing against the experimental results and advancing the technique.

This project will strengthen US synchrotron capabilities for the materials sciences. Suitable x-ray beams are available at the Advanced Photon Source where our collaborators already have performed BCDI experiments. With current coherence properties, these measurements are restricted to x-ray energies around 10 keV, to low Z elements, and to micron sized grains in wires of roughly 100 micron diameter. Dark field microscopy, with intermediate resolution, allows nano-scale imaging in larger diameter samples with larger grains and possibly more complex defect structures. The demonstration of combined dark field, BCDI and HEDM measurements points forward to the APS Upgrade that will provide x-ray beam properties including higher coherence at higher energies and enhanced flux. The Upgrade will loosen restrictions and allow more diverse systems to be studied with the developed techniques. The development of these capabilities will enable applications to other materials and systems such as polymers and biomaterials. The proposed developments will also provide important characterization capability for answering questions such as “how do local stress states affect material stability,” which was cited as a central question by a recent DOE Workshop on Basic Research Needs.