Deformation at triple junctions : dislocation plasticity and strain distribution

Abstract

This study aims to gain better understanding of plastic deformation accommodation near triple junctions (TJs) where three grain boundaries meet in polycrystalline metals with a face-centered-cubic crystal structure, using Nickel as a model material. In the literature, TJs are often found to be stress concentration or crack nucleation sites, which might be related to how plastic strain is accommodated in TJ regions. To minimize three-dimensional grain constraints, experiments are conducted on nickel specimens with a columnar grain structure. In-plane strain fields were measured by the Digital Image Correlation (DIC) technique. Slip trace analysis was carried out by utilizing results from scanning electron microscopy (SEM) and atomic force microscopy (AFM). TJ regions generally exhibit a high degree of strain heterogeneity as well as spatial heterogeneity in lattice rotation. The activation of relatively compatible slip systems is different from single crystal response in some grains. The dislocation slip intensity changes when approaching some TJs. Significant out-of-plane relative rotation among grains is also observed as an important accommodation mechanism in the present columnar grain structure. Deformation at twin boundary-grain boundary junctions is studied as well. Lower strains are seen along the twin boundary if slip bands that formed on {111} planes are in parallel configuration with the annealing twin.

Furthermore, the deformation behaviors in adjacent TJs sharing two common grains are compared; local rotations, strain distributions, and activated slip systems within the same grain changes as it approaches a different TJ. Triple junctions consisted of three low-energy grain boundaries with special character yield lower triple line energy, and the tensile strain and slip activity may be reduced at such junctions. The understanding gained in my research will be important for developing predictive models for deformation in polycrystalline materials.