Mechanical Behavior at the Nanoscale VI: Small Scale and In Situ Testing
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

Wednesday 8:30 AM
March 2, 2022
Room: 262B
Location: Anaheim Convention Center

Session Chair: Mohsen Zaeem, Colorado School of Mines; Shraddha Vachhani, Iowa State University

8:30 AM  Invited
In Situ Nanoscale Mechanical Testing to Isolate the Effect of Grain Boundary and Linear Complexions: Timothy Rupert1; 1University of California, Irvine
    Complexions, local equilibrium states associated with defects such as grain boundaries and dislocations, can enable unique pathways for the control of mechanical behavior. In this talk, we explore the use of in situ nanomechanical testing inside of electron microscopes to understand how these features affect macroscale response such as plasticity, failure, and strain hardening, as well as nanoscale defect physics. Deformation of nanocrystalline Cu and Al alloys demonstrates that amorphous grain boundary complexions can be toughening features and that hierarchical nanostructures comprised of different complexions can be used to tailor plasticity. In these cases, dislocation plasticity is modified by tuning the boundaries with which these defects eventually interact. Uniaxial experiments in transmission imaging modes show that individual dislocation motion can be directly modified with linear complexions, providing an even more direct pathway for microstructure design.

9:00 AM  
Understanding the Role of Surface Faceting in Metallic Nanoparticles via In Situ TEM Compression: Soodabeh Azadehranjbar1; Ruikang Ding1; Andrew Baker1; Ingrid Padilla Espinosa2; Ashlie Martini2; Tevis Jacobs1; 1University of Pittsburgh; 2University of California Merced
    The mechanical behavior of metals is size-dependent, with dislocation-starvation creating a “smaller is stronger” trend, and Coble-creep-like diffusion creating a “smaller is weaker” trend. However, ultra-small nanoparticles tend to form facets with various surface energies that may play a role in the mechanical behavior of nanoparticles (NPs). Here, we explore the effect of surface faceting on the deformation behavior of metallic nanoparticles. Platinum and gold nanoparticles down to single-digit-nanometer sizes were compressed in a transmission electron microscope, with high-resolution measurements of force, deformation, and particle shape transitions. Deformation mechanisms were investigated as a function of the size and shape of nanoparticles. Differences in deformation behavior of Pt and Au NPs were linked to physical properties of nanoparticles such as the homologous temperature, surface diffusion rates, and surface energy. Finally, experimental results were compared with matched atomistic simulations for further insight into driving forces and atomic-scale mechanisms.

9:20 AM  
Nanostructured Metallic Glasses from Colloidal Nanoparticles: Melody Wang1; Wendy Gu1; Mehrdad Kiani1; 1Stanford University
    Metallic glasses possess superior mechanical properties, but often deform via shear bands leading to near-zero ductility. To overcome this issue, we use iron boride amorphous nanoparticles made via colloidal synthesis to form bulk nanoglasses, which are analogous to nanocrystalline materials except with amorphous grains and grain boundaries. The individual nanoparticles are compressed in an SEM, and stress-strain curves and videos show that ductile deformation occurs through the slow formation of multiple cracks. We compact nanoparticles into millimeter sized nanoglasses and measure elastic modulus and hardness values of up to 160 GPa and 8 GPa, respectively, via nanoindentation. TEM is used to characterize the amorphous grains and interfaces. Large (~12 um) nanoglass micropillars are compressed and found to have plastic strain of 3.5%, which is higher than most iron-based glasses that are brittle. We relate this plasticity to deformation at the amorphous interfaces. The effect of nanoparticle size is also explored.

9:40 AM  
Direct Observation of Intermittent Dislocation Motions and Deformation Mechanisms in Nanocrystalline Molybdenum: Haw-Wen Hsiao1; Jia-Hong Huang2; Jian Min Zuo1; 1University of Illinois; 2National Tsing Hua University
    We report on a direct observation of intermittent dislocation activities in nanocrystalline molybdenum (Mo) in fabricated nanopillars under compression. Mo is selected as a representative refractory metal with the body-centered-cubic (BCC) structure. The size dependent plastic behaviors of BCC Mo differ significantly from these of face-centered-cubic (FCC) metals. Various mechanisms have been proposed to explain the differences, but they remain largely unverified experimentally. Here, using results obtained from in-situ electron imaging, we show that multiple mechanisms operate during the compression of nanocrystalline Mo nanopillars, starting initially from grain boundary deformation and followed by dislocation hardening before yielding. After yielding, intermittent dislocation avalanches and grain boundary dislocation transmission are observed, with dislocation intermittency dominating the plastic deformation. Dislocations of both screw and edge characters are also observed in the BCC crystal. Together, our findings here provide critical insights into the strengthening via grain refinement for a refractory metal.

10:00 AM Break

10:20 AM  Invited
In Situ Nanomechanical Testing to Understand the Role of Grain Boundary Structure in Materials: Nan Li1; Saryu Fensin1; Abigail Hunter1; Darby Luscher1; 1Los Alamos National Laboratory
    Grain boundaries(GBs) are one of the most important planar defects in materials. Correlated mechanical response under loading leads to localized strain accumulation and structural changes in the vicinity of boundaries. This can determine whether dislocations can nucleate, transmit, or just be blocked at a boundary. In order to understand this process, we have performed in-situ pillar compression tests in a SEM coupled with electron backscatter diffraction(EBSD). During uniaxial compression, the indentation was paused at different strains to perform the EBSD scanning. Depending on the boundary structure, various interactions with dislocations at the boundary were captured. Correlated with the EBSD mapping at different strains, we could quantify lattice rotation and local strain tensor along the loading axis. Such information has helped us better understand the role of GB structure in determining its interactions with dislocations and provided a unique dataset for validation of modeling at both atomic and mesoscales.