Digital image correlation study of mechanical response of nickel superalloy Hastelloy X under thermal and mechanical cycling: Uniaxial and biaxial stress states

The cyclic plastic strain accumulation of a dynamic strain aging Ni-based superalloy, Hastelloy X, is investigated using digital image correlation under both uniaxial and biaxial thermomechanical loading conditions. Thermal cycling and uniaxial mechanical tensile loading demonstrate that this material exhibits a Cottrell-Stokes-type response in which yield stress is not fully recovered after a mechanical loading cycle followed by a second isothermal loading cycle at elevated temperature (between room temperature and 600 °C in this work). The Portevin-Le Chatelier dynamic strain aging effect at higher temperatures, which has been seen to occur in this material, may also contribute to this reduction in flow stress. Additionally, under biaxial stress states introduced by means of a double-notch experimental sample, various cyclic thermal and mechanical loading combinations demonstrated a complex thermomechanical plastic response of this material. For example, cyclic thermal loading experiments at fixed displacement amounts for each cycle show greater plastic strain accumulation after three cycles when cycling is performed at a combination of 300 °C and 600 °C than that for three isothermal 600 °C cycles.