Mechanical Behavior at the Nanoscale VI: Deformation Mechanisms I
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Computational Materials Science and Engineering Committee, TMS: Mechanical Behavior of Materials Committee, TMS: Nanomechanical Materials Behavior Committee
Program Organizers: Matthew Daly, University of Illinois-Chicago; Douglas Stauffer, Bruker Nano Surfaces & Metrology; Wei Gao, University of Texas at San Antonio; Changhong Cao, McGill University; Mohsen Asle Zaeem, Colorado School of Mines

Monday 8:30 AM
February 28, 2022
Room: 262B
Location: Anaheim Convention Center

Session Chair: Matthew Daly, University of Illinois at Chicago; Christopher Weinberger, Colorado State University

8:30 AM Introductory Comments

8:40 AM  
Deciphering the Deformation Mechanisms of Nanocrystalline-nanotwinned Metals in Hall-Petch Breakdown Regime: Yinmin (Morris) Wang1; 1University of California, Los Angeles
    Although the mechanisms of Hall-Petch breakdown in nanocrystalline (average grain size <100 nm) materials have been extensively investigated, it remains a challenge to fully decipher the origins of Hall-Petch breakdown in nanotwinned (average twin spacing <100 nm) metals. An interesting question arises when a material has both nanocrystalline grains and nanotwinned structures. This presentation will discuss both experimental and simulation results on a class of nanocrystalline-nanotwinned materials, where new deformation mechanisms could start to emerge at nanoscales. We show that defects in experimental synthesized materials may play a significant role in triggering Hall-Petch breakdown. Molecular dynamics simulations with defective and ideal materials suggest drastically different deformation mechanisms that may help to reconcile some literature controversies. The presentation will further discuss the implications of these defect structures on other physical properties such as electrical conductivity and thermal stability.

9:00 AM  Invited
Deformation Twins in BCC Metals - Atomic Level Origins: Christopher Weinberger1; Anik Faisal1; 1Colorado State University
    It is well known that BCC transition metals deform via deformation twinning at sufficiently low temperatures or high strain rates. Furthermore, as crystallite size approaches the nanoscale, deformation twining has been shown to be competitive with dislocation slip. However, there are still open questions regarding the structure of deformation twins and how they compete with dislocation slip. Here, examine the atomic level structure of twins in BCC metals using density functional theory, demonstrating that twin structure depends on chemistry. Using DFT, classical atomistics, and continuum models, we examine the nucleation processes and energetics of twin nucleation. This provide insight into how the structure affects nucleation, the stresses required for nucleation as well as provides insight into how twin nucleation stresses compare to the bulk stresses. These results are further compared with similar calculations for dislocation nucleation, which provides insight into the competition between twinning and slip at the nanoscale.

9:30 AM  
The Complexity of Deformation Twinning: Huseyin Sehitoglu1; Sameer Mohammed1; Gorkem Gengor1; Orcun Celebi1; 1University of Illinois
    Our understanding of deformation twinning relies on fcc metallic materials where the boundaries are classified as compound type. However, in bcc and hcp crystals and lower symmetry crystals the twinning motion is rather complex and requires understanding of atomic motion (shears, shuffles and offsets) that require atomic simulations. Furthermore, the presence of external stress could cause deviations from the twin plane determination relying on classical twinning theory. In this presentation, we will start with the reflective twins observed in fcc, then transition to bcc crystals and then cover hcp and monoclinic crystals. In many cases, combined MD and DFT calculations are needed in conjunction with anisotropic elasticity and topological model to gain insight into twin nucleation and twin variant motion. It is through combination of these tools in conjunction with high resolution experimental measurements that one can develop a better scientific understanding.