Exploring Deformation and Failure in Nano-Solids: Flaw-Driven Fracture in Nanocrystalline Pt and Grain boundary Sliding in Bi-crystalline Al
Julia Greer (Caltech), X Gu (Caltech), Zachary Aitken (Caltech), Yong-Wei Zhang (IHCP Singapore), Zhaoxuan Wu (IHCP Singapore)
Materials for Extreme Environments: Multiscale Experiments and Simulations
Mon 2:40 - 4:00
Salomon 203
We present results of experiments and computations on tensile, fracture, and compressive properties of nano solids, which contain grain boundaries - (1) multiple or (2) individual – on mechanical response of nano structures. 1. We explore fracture in nanomaterials by testing nano-scale samples with pre-fabricated surface flaws and by molecular dynamics (MD) simulations on identical structures. Nanocrystalline Pt nanopillars with ~120nm diameters and ~12nm grain sizes contained intentionally introduced surface notches and were uniaxially tensed in an in-situ SEM. Experiments demonstrated that 40% of samples failed at pre-existing flaws, with tensile failure strengths independent of failure locations. MD simulations revealed atomistic mechanisms of nanoscale fracture to be driven by competing effects of microstructural and structural flaws. Findings imply that failure in nanomaterials is driven by the weakest link within internal and external local stress fields. Compressed Pt samples exhibited weakening at pillar diameter of 60 nm (~5 grains across); strengths of larger samples did not deviate significantly from bulk yield strength. MD simulations reveal deformation mechanism to be a combination of grain boundary sliding at surface grains and emission of partial dislocations from triple junctions at interior grains. 2. We also describe room-temperature uniaxial compressions of 900 nm-diameter Al bi-crystals, each containing a high-angle grain boundary oriented at 24° to loading direction. Deformation occurred via frictional sliding along boundary plane, where top crystallite sheared off as a single unit instead of crystallographic slip and extensive dislocation activity. Compressive stress strain data was continuous, in contrast to stochastic in single crystals, and displayed a peak in stress of ~176 MPa followed by gradual softening and a plateau. Energetics-based physical model, which may explain observed room-temperature grain boundary sliding, in presented.