Hastelloy X, a nickel-based superalloy, has been extensively used for high temperature applications. In this work, Hastelloy X notched samples were used to investigate fatigue crack growth and fatigue crack closure both at room and elevated temperatures. Digital Image Correlation (DIC) experiments at two different scales were performed: 2 µm/pixel, and 400 nm/pixel. Isothermal, thermal jump, and thermal overload experiments at varying temperatures, were performed and images were collected at various stages of crack growth. Competing mechanisms were investigated (including crack tip blunting and change in temperature, yield stress, elastic modulus, and plastic zone size) that are thought to be responsible for the changes in fatigue crack growth rates and closure levels following the thermal jumps and during the thermal overload. Isothermal conditions were shown to have no effect on crack closure, while thermal jumps and thermal overloads created significant changes in closure levels following the change in temperature.