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Nanocrystalline Al thin films have been strained in situ in a transmission electron microscope using two separate nanomechanical techniques involving a push-to-pull device and a microelectromechanical system (MEMS) device. Deformation-induced grain growth was observed to occur via stress-assisted grain boundary migration with extensive grain growth occurring in the necked region, indicating that the increase in local stress drives the boundary migration. Under applied tensile stresses close to the ultimate tensile strength of 450 MPa for a nanocrystalline Al specimen, measured boundary migration speeds are 0.2 – 0.7 nm s⁻¹ for grains outside necked region and increases to 2.5 nm s⁻¹ for grains within the necked region where the local estimated tensile stresses are elevated to around 630 MPa. By tracking grain boundary motion over time, molecular dynamics simulations showed qualitative agreement in …