Revealing extreme variations in local deformation via 4D-STEM-based strain mapping in nanocrystalline gold

Abstract The distinctly different mechanical behavior of nanocrystalline materials compared to their coarse-grained counterparts has been extensively studied using atomistic simulations and experimental works. In-situ TEM deformation has revealed multiple mechanisms, including grain rotation and deformation twinning. Yet the effect of elastic strain concentrations from mechanical anisotropy or dislocation pile-ups remains less understood, as precise measurements were previously limited to specifically oriented grains. Here, we show that combining precession nanobeam electron diffraction with advanced orientation and strain-mapping algorithms allows measurement of transient elastic strain fields across all grains in a sputter-deposited nanocrystalline Au thin film. During nominally elastic loading, we observe an increase in tensile elastic strain and substantial grain rotations exceeding 6°, with pronounced heterogeneity among individual grains. Analysis reveals that these rotations arise from a combination of grain size, initial strain state, and dislocation density. The importance of the initial microstructural state is further highlighted in annealed samples: reducing defect density delays yielding and triggers deformation twinning at similar length scales. These results demonstrate the feasibility of mapping elastic strains with high accuracy and nanometer resolution across arbitrary grain boundaries, and reveal the critical role of local strain heterogeneities in determining the deformation behavior of nanocrystalline materials.