Grain-size stabilization by impurities and effect on stress-coupled grain growth in nanocrystalline Al thin films

Room-temperature tensile experiments on sub-micrometer freestanding thin films deposited at varied base pressures reveal two distinct classes of mechanical response. Samples that contain sufficient impurity concentrations to stabilize the microstructure against an applied stress show strong but brittle response. However, films that were deposited at lower vacuum base pressures that still allow for thermally stable nanostructures show remarkably different deformation response; namely, moderate strength and over 15% plastic strain to failure. Post-mortem transmission electron microscopy of deformed samples with different levels of impurity pinning atmospheres reveals stress-driven discontinuous grain growth that facilitates a fundamental change in the deformation behavior of these thin films. The results indicate a critical impurity concentration to sufficiently pin or immobilize grain boundaries against the coupling of applied stresses.