Thermo-mechanical response and plastic deformation behavior of Hastelloy X at elevated temperatures — role of dynamic strain aging

In this work, the plastic behavior of Hastelloy X subjected to thermo-mechanical loading is investigated, and the phenomenon of dynamic strain aging is studied under isothermal and temperature jump experiments, using the technique of digital image correlation (DIC). The material of interest, a Ni-based superalloy, is widely used in high temperature applications, and it is important to understand its plastic response in such loading regimes. It is seen that Hastelloy X exhibits serrated or jerky flow behavior at higher temperatures, which manifests as oscillations on the uniaxial tension stress strain curve. This is a consequence of the Portevin-Le Chatelier (PLC) effect, and is characterized by strain bands propagating through the length of the specimen, at specific time intervals, leading to a local increase in strain as the band propagates. Although the existence of a PLC effect in Hastelloy X has been observed before, the current work elucidates the effect of PLC bands on the deformation under uniaxial and biaxial states of stress, and under both isothermal and thermal cycling conditions. Variability in the response at different temperatures is studied locally, through the DIC measurements, and globally through far field measurements, with a relation between the two measurement scales being established. The DIC results, when correlated with the global load-displacement measurements, illustrate the contribution of the PLC effect to the plastic response of the specimen. A section of experimental work is devoted to characterization of the material behavior, and understanding the phenomenon of dynamic strain aging. A qualitative assessment of PLC band characteristics is also discussed. The effects of dynamic strain aging on the reduction of flow stress are investigated under both uniaxial and biaxial loading conditions. Cyclic thermal experiments under the influence of stress concentrations lead to certain counterintuitive phenomena such as greater strain accumulation at lower flow stresses